Drought-induced changes in root biomass largely result from altered root morphological traits: Evidence from a synthesis of global field trials

Unraveling root and rhizosphere traits in temperate maize landraces and modern cultivars: Implications for soil resource acquisition and drought adaptation

A holistic understanding of plant strategies to acquire soil resources is pivotal in achieving sustainable food security. However, we lack knowledge about variety-specific root and rhizosphere traits for resource acquisition, their plasticity and adaptation to drought. We conducted a greenhouse experiment to phenotype root and rhizosphere traits (mean root diameter [Root D], specific root length [SRL], root tissue density, root nitrogen content, specific rhizosheath mass [SRM], arbuscular mycorrhizal fungi [AMF] colonization) of 16 landraces and 22 modern cultivars of temperate maize (Zea mays L.). Our results demonstrate that landraces and modern cultivars diverge in their root and rhizosphere traits. Although landraces follow a 'do-it-yourself' strategy with high SRLs, modern cultivars exhibit an 'outsourcing' strategy with increased mean Root Ds and a tendency towards increased root colonization by AMF. We further identified that SRM indicates an 'outsourcing' strategy. Additionally, landraces were more drought-responsive compared to modern cultivars based on multitrait response indices. We suggest that breeding leads to distinct resource acquisition strategies between temperate maize varieties. Future breeding efforts should increasingly target root and rhizosphere economics, with SRM serving as a valuable proxy for identifying varieties employing an outsourcing resource acquisition strategy.

Soil legacies of tree species richness in a young plantation do not modulate tree seedling response to watering regime

Trees have a strong and species-specific influence on biotic and abiotic properties of the soil. Even after the vegetation is removed, the effect can persist to form so-called soil legacies. We investigated the effects of soil legacies of tree species richness on the emergence and growth of tree seedlings, and how these legacy effects modulate the seedling responses to irrigation frequency. We used a 9-year-old tree plantation on former agricultural land in Belgium, which is part of a biodiversity-ecosystem functioning experiment (FORBIO). Soil originating from monocultures and four-species plots, with different species combinations, was translocated to a greenhouse. Five tree species (Betula pendula, Fagus sylvatica, Pinus sylvestris, Quercus robur, and Tilia cordata) were sown and grown for one growing season in these soils. We performed a watering treatment (low and high irrigation frequency) to measure any potential interaction effects between the soil legacies and irrigation frequency. There was no evidence for soil legacy effects of species richness on plant performance or their response to the irrigation frequency. However, the effect of irrigation frequency was dependent on species identity of the tree seedlings. Despite the lack of clear legacy effects, performance measures did show correlated responses that are likely due to species composition effects. We ascribe these patterns to the young age of the forest and the agricultural past land use. At this early stage in forest development, the land-use history likely has a more important role in shaping soil characteristics that affect plant growth and their response to drought, than species diversity.

Understanding role of roots in plant response to drought: Way forward to climate-resilient crops

Drought stress leads to a significant amount of agricultural crop loss. Thus, with changing climatic conditions, it is important to develop resilience measures in agricultural systems against drought stress. Roots play a crucial role in regulating plant development under drought stress. In this review, we have summarized the studies on the role of roots and root-mediated plant responses. We have also discussed the importance of root system architecture (RSA) and the various structural and anatomical changes that it undergoes to increase survival and productivity under drought. Various genes, transcription factors, and quantitative trait loci involved in regulating root growth and development are also discussed. A summarization of various instruments and software that can be used for high-throughput phenotyping in the field is also provided in this review. More comprehensive studies are required to help build a detailed understanding of RSA and associated traits for breeding drought-resilient cultivars.

Effect of sodium alginate-based superabsorbent hydrogel on tomato growth under different water deficit conditions

The main challenge facing agriculture today is water scarcity. At present, agriculture consumes around 70 % of the planet's freshwater, much of which is lost through evaporation, leaching and runoff. This wastage, combined with the increased frequency and severity of droughts linked to climate change, is having a considerable negative impact on crops. As a result, the food security of people living in regions with limited water resources is threatened. In this regard, efficient water management using water-saving materials and soil additives such as superabsorbent polymers (SAPs) are recognized as an effective strategy to boost water use efficiency by plants and improve agricultural productivity. The present study fits with this strategy and aims to investigate the effect of new sodium alginate-based hydrogel-treated sandy loam soil on seed emergence and growth of tomatoes as a crop model under different water-deficit stress levels. A set of pot experiments was conducted in a greenhouse chamber using sandy loam soil amended with two levels of hydrogel (0.1 % and 0.5 % by weight) along with untreated control, all under water-deficit stress at three levels: 30 % of the daily amount of required irrigation water (DARW) for different growing cycles (severe stress), 70 % DARW (mild stress), and 100 % DARW (normal irrigation conditions). The germination test showed the absence of phytotoxicity of the developed hydrogel and confirmed its suitability in protecting seedlings from drought stress. Greenhouse experiment results demonstrated that water stress and levels of applied hydrogel significantly (P < 0.05) affected plant growth parameters such as plant height, stem diameter, number of leaves, chlorophyll content, fresh weight, and dry weight compared with the treatments without SAPs. The developed sodium alginate-based SAPs showed relevant agronomical benefits under drought stress by retaining more water and nutrients, thus it had the potential to be used in agriculture for better water management along with significant environmental benefits.

Using the Aqueous Phase Produced from Hydrothermal Carbonization Process of Brown Seaweed to Improve the Growth of Phaseolus vulgaris

Seaweeds are considered a biomass for third-generation biofuel, and hydrothermal carbonization (HTC) is a valuable process for efficiently disposing of the excess of macroalgae biomass for conversion into multiple value-added products. However, the HTC process produces a liquid phase to be disposed of. The present study aims to investigate the effects of seed-priming treatment with three HTC-discarded liquid phases (namely AHL₁₈₀, AHL₂₄₀, and AHL₃₀₀), obtained from different experimental procedures, on seed germination and plant growth and productivity of Phaseolus vulgaris L. To disentangle the osmotic effects from the use of AHL, isotonic solutions of polyethylene glycol (PEG) 6000 have also been tested. Seed germination was not affected by AHL seed-priming treatment. In contrast, PEG-treated samples showed significantly lower seed germination success. AHL-treated samples showed changes in plant biomass: higher shoot biomass was recorded especially in AHL₁₈₀ samples. Conversely, AHL₂₄₀ and AHL₃₀₀ samples showed higher root biomass. The higher plant biomass values recorded in AHL-treated samples were the consequence of higher values of photosynthesis rate and water use efficiency, which, in turn, were related to higher stomatal density. Recorded data strongly support the hypothesis of the AHL solution reuse in agriculture in the framework of resource management and circular green economy.

Germination and seedling growth of Calluna vulgaris is sensitive to regional climate, heathland succession, and drought

The coastal heathlands of Northwest Europe are highly valued cultural landscapes, that are critically endangered due to land use and climatic changes, such as increased frequency and severity of drought events. Our study is the first to assess how the germination and early seedling growth of Calluna vulgaris respond to drought. In a factorial design field experiment, we exposed maternal plants to three in-situ drought treatments (control, 60%, 90% roof coverage), across three successional stages after fire (pioneer, building, mature), and two regions (60°N, 65°N). Seeds from 540 plants within the experiment were, weighed, and exposed to five water potentials, ranging from -0.25 to -1.7 MPa, in a growth chamber experiment. We recorded germination (percentage, rate), seedling growth (above- vs. belowground allocation), and seedling functional traits (specific leaf area [SLA], specific root length [SRL]). Overall variation in germination between regions, successional stages, and maternal drought treatments was largely mediated by variation in seed mass. Plants from the northernmost region had higher seed mass and germination percentages. This is indicative of higher investment in seeds, likely linked to the populations' absence of vegetative root sprouting. Seeds from the mature successional stage germinated to lower final percentages than those from earlier successional stages, especially when the maternal plants had been exposed to drought (60% and 90% roof coverage). Exposure to reduced water availability decreased germination percentage and increased the time to 50% germination. Seedlings fully developed in the range -0.25 to -0.7 MPa, with increased root:shoot and lower SRL during reduced water availability, suggesting a resource-conservative response to drought during the early stages of development. Our results thus suggest a sensitivity to drought during the germination and seedling life-history stages that may reduce Calluna's ability to re-establish from seeds as the incidence and severity of droughts are projected to increase under future climates.

Plant growth strategy determines the magnitude and direction of drought-induced changes in root exudates in subtropical forests

Root exudates are an important pathway for plant-microbial interactions and are highly sensitive to climate change. However, how extreme drought affects root exudates and the main components, as well as species-specific differences in response magnitude and direction, are poorly understood. In this study, root exudation rates of total carbon (C) and its components (e.g., sugar, organic acid, and amino acid) were measured under the control and extreme drought treatments (i.e., 70% throughfall reduction) by in situ collection of four tree species with different growth rates in a subtropical forest. We also quantified soil properties, root morphological traits, and mycorrhizal infection rates to examine the driving factors underlying variations in root exudation. Our results showed that extreme drought significantly decreased root exudation rates of total C, sugar, and amino acid by 17.8%, 30.8%, and 35.0%, respectively, but increased root exudation rate of organic acid by 38.6%, which were largely associated with drought-induced changes in tree growth rates, root morphological traits, and mycorrhizal infection rates. Specifically, trees with relatively high growth rates were more responsive to drought for root exudation rates compared with those with relatively low growth rates, which were closely related to root morphological traits and mycorrhizal infection rates. These findings highlight the importance of plant growth strategy in mediating drought-induced changes in root exudation rates. The coordinations among root exudation rates, root morphological traits, and mycorrhizal symbioses in response to drought could be incorporated into land surface models to improve the prediction of climate change impacts on rhizosphere C dynamics in forest ecosystems.

Macroalgal-derived alginate/wastepaper hydrogel to alleviate sunflower drought stress

MAIN CONCLUSION

Alginate/wastepaper hydrogel mitigated sunflower drought stress by increasing the water holding capacity of the soil and decreasing phosphate leaching. The hydrogel promoted sunflower growth and decreased stress related biomarkers. There is a growing interest in the development of biodegradable hydrogels for the alleviation of drought stress on plants. A novel hydrogel based on brown algal-derived alginate and wastepaper was developed and tested as a soil supplement for sunflower growth under moderate (75% field capacity (FC)) and extreme (50% FC) water-deficit stress. The hydrogel showed fast swelling in water, which obeyed the pseudo-first order kinetics. Besides, it increased the water holding capacity of the soil and exhibited a good phosphate adsorption (37 mg PO₄^- g^-1 hydrogel after 6 days) in the soil, and more than 67% of the adsorbed phosphate was desorbed after 20 days. Thus, the phosphate leaching from the hydrogel-amended soil in a column experiment was only 2.77 mg after 4 times of over-irrigation, compared to 11.91 mg without the hydrogel. The hydrogel application promoted various root traits such as fresh and dry biomass, area, and length by > 2-, > 1.6-, > 1.35-, and > 1.3-folds under both water regimes in relation to the no-hydrogel treatments at the same conditions. Furthermore, the sunflower shoots exhibited similar proline contents to the well-watered control (100% FC), with > 50% reduction in relation to the drought-stressed plants under the same conditions. Similarly, the malondialdehyde contents were lowered by > 15%. The analysis of the antioxidant enzymes also indicated a marked reduction in the specific activities of catalase and ascorbate peroxidase under both 75 and 50% FC compared to the respective controls. Additionally, the hydrogel promoted the uptake of phosphate by sunflower roots. These results implied that the developed biodegradable hydrogel could be effectively applied as a soil additive to alleviate drought stress on crops.

Cigarette: an unsung anthropogenic evil in the environment

The world's population is growing steadily, and this trend is mirrored by a sharp rise in the number of people who smoke cigarettes. Instead of properly disposing of their cigarette waste, most people simply toss them aside, leading to serious environmental consequences. According to previous statistics, in 2012 alone, 6.25 trillion cigarettes were consumed by 967 million chain smokers. Past studies have shown that up to 30% of global litter is made up of cigarette waste. These discarded cigarette butts are non-biodegradable and contain over 7000 toxicants such as benzene, 1,3-butadiene, nitrosamine ketone, N-Nitrosonornicotine, nicotine, formaldehyde, acrolein, ammonia, aniline, polycyclic aromatic hydrocarbons, and various heavy metals. These toxicants have a negative impact on the habitats of wildlife and can cause serious health problems such as cancer, respiratory disorders, cardiac issues, and sexual dysfunction. Although it is still unclear how littered cigarettes affect plant growth, germination, and development, it is clear that they have the potential to harm plant health. Just like single-use plastic, trashed cigarette butts are a critical new rising form of pollution that requires scientific attention for effective recycling and disposal management. It is important to properly dispose of cigarette waste to protect the environment and wildlife, as well as to prevent harm to human health.

Drought- and soil substrate-induced variations in root nonstructural carbohydrates result from fine root morphological and anatomical traits of Juglans mandshurica seedlings

BACKGROUND

Nonstructural carbohydrates (NSCs) reflect the carbon supply status and affect the construction and development of plants. Previous studies have focused on the dynamics of NSCs among plant organs, however, few studies have paid attention to the synergistic variations between fine root traits and NSCs under drought based on the perspective of branch order roots. This study aims to explore the responses of fine root traits and NSCs among root orders of Juglans mandshurica seedlings under different drought intensities and soil substrates. The 2-year-old J. mandshurica potted seedlings were planted in three different soil substrates (humus, loam and sandy-loam soil) and subjected to four drought intensities (CK, mild drought T1, moderate drought T2 and severe drought T3) for 60 days.

RESULTS

The root biomass of seedlings in sandy-loam soil under the same drought intensity was higher than that of seedlings in humus soil. With an increase in drought, the root biomass, average diameter, root tissue density and cortex thickness decreased significantly, and the specific root length, stele diameter and conduit density increased. The root NSC contents in humus soil were higher than those in sandy-loam soil. The fine root soluble sugar content in all soil substrates decreased with increasing drought intensity, while the root starch and total NSC contents varied among the different soil substrates. Compared with transportive roots, the morphological and anatomical traits jointly explained the higher variation in NSC contents of the absorptive roots. The anatomical traits explained the higher variation in the NSC content of first five order roots.

CONCLUSION

Our results suggest that coordinated adaptation of the root traits and NSCs of Manchurian walnut seedlings exposed to water gradients in different soil substrates.

Analysis of QTL mapping for germination and seedling response to drought stress in sunflower (Helianthus annuus L.)

Sunflower is an important oilseed crop across the world. It is considered as a moderately drought tolerant plant, however, its yield is still negatively affected by drought stress. Improving drought tolerance is of the outmost important for breeding. Although several studies have documented the relationship between the sunflower phenotype and genotype under drought stress, but relatively few studies have simultaneously investigated the molecular mechanisms of drought tolerance in the sunflower at different growth stages. In this study, we conducted quantitative trait locus (QTL) analysis for different sunflower traits during the germination and seedling stages. Eighteen phenotypic traits were evaluated under well-watered and drought stress conditions. We determined that the germination rate, germination potential, germination index, and root-to-shoot ratio can be used as effective indexes for drought tolerance selection and breeding. A total of 33 QTLs were identified on eight chromosomes (PVE: 0.016%-10.712% with LOD: 2.017-7.439). Within the confidence interval of the QTL, we identified 60 putative drought-related genes. Four genes located on chromosome 13 may function in both germination and seedling stages for drought response. Genes LOC110898128, LOC110898092, LOC110898071, and LOC110898072 were annotated as aquaporin SIP1-2-like, cytochrome P450 94C1, GABA transporter 1-like, and GABA transporter 1-like isoform X2, respectively. These genes will be used for further functional validation. This study provides insight into the molecular mechanisms of the sunflower's in response to drought stress. At the same time, it lays a foundation for sunflower drought tolerance breeding and genetic improvement.

Response strategies of fine root morphology of Cupressus funebris to the different soil environment

Understanding fine root morphology is crucial to uncover water and nutrient acquisition and transposition of fine roots. However, there is still a lack of knowledge regarding how the soil environment affects the fine root morphology of various root orders in the stable forest ecosystem. Therefore, this experiment assessed the response strategies of fine root morphology (first- to fifth -order fine roots) in four different soil environments. The results showed that fine root morphology was related to soil environment, and there were significant differences in specific root length (SRL), specific surface area (SRA), diameter (D), and root tissue density (RTD) of first- and second -order fine roots. Soil total nitrogen (TN), alkaline nitrogen (AN) and available phosphorus (AP) were positively correlated with SRL and SRA and negatively correlated with D and RTD. Soil moisture (SW) was positively correlated with the D and RTD of first- and second-order fine roots and negatively correlated with the SRL and SRA. Soil temperature (ST), organic carbon (OC), soil bulk density (SBD) and soil porosity (SP) were not significantly correlated with the D, SRL, SRA, and RTD of the first- and second -order fine roots. AN was positively correlated with SRL and SRA and negatively correlated with both D and RTD in the first- and second -order fine roots, and the correlation coefficient was very significant. Therefore, we finally concluded that soil AN was the most critical factor affecting root D, SRL, SRA and RTD of fine roots, and mainly affected the morphology of first- and second -order fine roots. In conclusion, our research provides support for understanding the relationship between fine root morphology and soil environment, and indicates that soil nutrient gradient forms good root morphology at intraspecific scale.

Responses of root system architecture to water stress at multiple levels: A meta-analysis of trials under controlled conditions

Plants are exposed to increasingly severe drought events and roots play vital roles in maintaining plant survival, growth, and reproduction. A large body of literature has investigated the adaptive responses of root traits in various plants to water stress and these studies have been reviewed in certain groups of plant species at a certain scale. Nevertheless, these responses have not been synthesized at multiple levels. This paper screened over 2000 literatures for studies of typical root traits including root growth angle, root depth, root length, root diameter, root dry weight, root-to-shoot ratio, root hair length and density and integrates their drought responses at genetic and morphological scales. The genes, quantitative trait loci (QTLs) and hormones that are involved in the regulation of drought response of the root traits were summarized. We then statistically analyzed the drought responses of root traits and discussed the underlying mechanisms. Moreover, we highlighted the drought response of 1-D and 2-D root length density (RLD) distribution in the soil profile. This paper will provide a framework for an integrated understanding of root adaptive responses to water deficit at multiple scales and such insights may provide a basis for selection and breeding of drought tolerant crop lines.

Fine-root functional trait response to nitrogen deposition across forest ecosystems: A meta-analysis

Nitrogen (N) deposition has complex effects on vegetation dynamics and nutrient cycling in terrestrial ecosystems. However, how N deposition alters fine root traits remains unclear in forest ecosystems. Here, we carried out a synthesis based on 890 paired observations of 14 fine root traits from 79 articles to assess the effects of N deposition on fine root traits. The results showed that N deposition mainly affected root nutrient content and stoichiometry. Specifically, N deposition increased the root N content, root carbon: phosphorus (C:P) and root nitrogen: phosphorus (N:P) ratio, but decreased the root P content and root C:N ratio. Moreover, N deposition increased fine root respiration, but had no significant effect on other root morphological and physiological traits. N deposition effects on fine root biomass, root tissue density and fungal colonization decreased with N deposition duration. Compared to fine root P content, N deposition effects on fine root C content and C:P ratio increased with N deposition level. Moreover, the interaction between N deposition level and duration significantly affected fine root biomass. N deposition effects on fine-root biomass decreased with greater N deposition duration, especially in high N deposition experiments. Moreover, the effect of N deposition on root diameter decreased with mean annual temperature and mean annual precipitation. N form, forest type and soil depth significantly affect the effect of N deposition on fine root C:P. Therefore, the effects of N deposition on fine root traits were not only determined by N deposition level, duration and their interactions, but also regulated by abiotic factors. These findings highlight the diverse responses of fine root traits to N deposition have strong implications for forest ecosystems soil carbon stocks in a world of increasing N deposition associated with decreased root-derived carbon inputs and increases in fine-root respiration.

Drought legacy effects on root morphological traits and plant biomass via soil biota feedback

Drought causes soil feedback effects on plant performance. However, how the linkages between conditioned soil biota and root traits contribute to explain plant-soil feedback (PSF) as a function of drought is unknown. We utilized soil inoculum from a conditioning experiment where grassland species grew under well-watered and drought conditions, and their soil fungi were analyzed. Under well-watered conditions, we grew 21 grassland species with those inocula from either conspecific or heterospecific soils. At harvest, plant biomass and root traits were measured. Negative PSF (higher biomass in heterospecific than in conspecific soils) was predominant, and favored in drought-conditioned soils. Previous drought affected the relationship between root traits and fungal groups. Specific root surface area (SRSA) was higher in heterospecific than in conspecific droughted soils and was linked to an increase in saprotroph richness. Overall, root diameter was higher in conspecific soils and was linked to mutualist and pathogen composition, whereas the decrease of root : shoot in heterospecific soils was linked to pathogenic fungi. Drought legacy affects biomass and root morphological traits via conditioned soil biota, even after the drought conditions have disappeared. This provides new insights into the role that soil biota have modulating PSF responses to drought.

Effect of drought and nutrient availability on invaded plant communities in a semi-arid ecosystem

Ecosystem functions are heavily dependent on the functional composition of the plant community, i.e., the functional traits of plants forming the community. This, on the one hand, depends on plant occurrence, but on the other hand, depends on the intraspecific variability of functional traits of the species, which are influenced by climate and nutrient availability and affected by plant–plant interactions. To illustrate that, we studied the effects of drought and nitrogen addition (+ N), two important abiotic variables which are changing with ongoing global change, as well as their combined effect on the functional responses of grassland communities in semi‐arid environments of Northern Africa comprising of natural and invasive species. We conducted an experiment where we planted three native species and one invasive plant species in artificial communities of five individuals per species per plot. We exposed these communities to four different treatments: a drought treatment, an N‐addition treatment, the combination between drought and N‐addition, as well as a control. To assess the performance of plants within treatments, we measured selected plant functional traits (plant height, specific leaf area [SLA], leaf dry matter content [LDMC], N content of the leaves [N_mass], specific root length [SRL], and root diameter) for all individuals occurring in our plots, and additionally assessed the above and belowground biomass for each plant individual. We found that the invasive species showed a higher performance (higher biomass accumulation, taller plants, higher SLA, N_mass, SRL, and root diameter as well as lower LDMC) than the native species under drought conditions. The invasive species was especially successful with the combined impact of drought + N, which is a likely scenario in ongoing global change for our research area. Thus, plant functional traits might be a key factor for the invasion success of plant species which will be even more pronounced under ongoing global change.

Limited Acclimation in Leaf Morphology and Anatomy to Experimental Drought in Temperate Forest Species

Simple Summary

Climate change shown to have a significant impact on the forest ecosystem due to increased and more frequent occurrence of extreme drought. However, in order to successfully adjust to the xeric environments, plants can usually adopt a variety of adaptation strategies. Here, we investigated the morpho-anatomical traits and biomass allocation patterns as acclimation mechanisms in drought conditions. We found that the interrelation between leaf morphological and anatomical traits were equally affected by drought conditions across all species. This suggests that there is no convincing evidence to classify taxa based on drought resistance vs. drought tolerance. However, based on the biomass allocation pattern, we found that P. koraiensis and F. mandshurica had the higher RMF and total PB, but lower LFM, suggesting higher drought tolerance than those of the other species. Therefore, our dataset revealed some easily measurable traits, such as LMF, RMF, and PB, which demonstrated the seedling’s ability to cope with drought and which could be utilized to choose drought-tolerant species for reforestation in the temperate forest.

Abstract

Drought is a critical and increasingly common abiotic factor that has impacts on plant structures and functioning and is a challenge for the successful management of forest ecosystems. Here, we test the shifts in leaf morpho-anatomical or hydraulic traits and plant growth above ground caused by drought. A factorial experiment was conducted with two gymnosperms (Larix gmelinii and Pinus koraiensis) and two angiosperms (Fraxinus mandshurica and Tilia amurensis), tree species grown under three varying drought intensities in NE China. Considering all the species studied, the plant height (PH), root collar diameter (RCD), and plant biomass (PB) were significantly decreased by drought. The leaf thickness (LT) increased, while the leaf area (LA) decreased with drought intensity. In the gymnosperms, the mesophyll thickness (MT) increased, and the resin duct decreased, while in the angiosperms the palisade mesophyll thickness (PMT), the spongy mesophyll thickness (SMT), and the abaxial (ABE) and adaxial epidermis (ADE) thickness were increased by drought. The correlation analysis revealed that P. koraiensis and F. mandshurica had the higher RMF and total plant biomass, but the least LMF, suggesting drought tolerance. In contrast, the L. gmelinii had the least RMF and higher LMF, suggesting vulnerability to drought. Similarly, T. amurensis had the higher leaf size, which increased the evaporative demand and depleted the soil water quickly relative to the other species. The interrelation among the morpho-anatomical leaf traits was equally affected by drought across all the studied species, suggesting that there is no clear evidence to differentiate the taxa based on drought resistance vs. drought tolerance. Thus, we have identified some easily measurable traits (i.e., LMF, RMF, and PB) which evidenced the seedling’s ability to cope with drought and which therefore could be used as proxies in the selection of drought tolerant species for reforestation in the temperate forest.

Acclimatization of Mediterranean Native Sages (Salvia spp.) and Interspecific Hybrids in an Urban Green Roof under Regular and Reduced Irrigation

Native-to-Greece sage species, namely, Salvia fruticosa, S. officinalis, S. pomifera ssp. pomifera, S. ringens, S. tomentosa and interspecific hybrids, were evaluated for their acclimatization in an extensive Mediterranean green roof during summer under regular and reduced irrigation (every 2–3 days with substrate moisture 16–22% v/v and 4–5 days with substrate moisture 7–11% v/v, respectively). A substrate (grape-marc compost:perlite:pumice, 3:3:4, v/v) that was 10 cm deep was used. Regardless of the irrigation frequency, S. pomifera ssp. pomifera × S. ringens and S. officinalis × S. pomifera ssp. pomifera showed the highest survival of all hybrids and species, along with satisfactory growth, while S. fruticosa showed the lowest survival. Reduced irrigation resulted in the reduction of aboveground and root biomass, with no damage to the photosynthetic apparatus. S. fruticosa showed the highest (53%) aboveground biomass reduction and S. officinalis, S. officinalis × S. ringens and S. pomifera ssp. pomifera × S. ringens showed the lowest (28, 23 and 3%, respectively), while S. officinalis × S. pomifera ssp. pomifera and S. pomifera ssp. pomifera × S. ringens showed the lowest reduction in root biomass (13 and 16%, respectively). With a reservation for S. fruticosa, Greek Salvia spp. and their interspecific hybrids studied in the present work are recommended for sustainable exploitation in extensive green roofs in arid regions and generally in xeriscaping.

Using the Halophyte Crithmum maritimum in Green Roofs for Sustainable Urban Horticulture: Effect of Substrate and Nutrient Content Analysis including Potentially Toxic Elements

The effect of substrate type and cultivation site in the urban fabric on growth, nutrient content and potentially toxic element (PTE) accumulation in tissues of the halophyte Crithmum maritimum was studied. Plantlets were cultivated for twelve months in containers with a green-roof infrastructure fitted and placed either on an urban second-floor roof or on ground level by the side of a moderate-traffic street. Two substrate types were used; one comprising grape marc compost, perlite and pumice (3:3:4, v/v) and one composed of grape marc compost, perlite, pumice and soil (3:3:2:2, v/v), with 10 cm depth. Plants grew well on both sites, although aboveground growth parameters and nutrient content in leaves were greater at street level. Both cultivation site and substrate type affected heavy-metal accumulation in plant tissues. Cu, Ni and Fe concentrations in leaves and Pb in roots were higher in street-level-grown plants compared to the roof-grown plants, and concentrations of Cu and Mn in leaves and Fe in both leaves and roots were lower in the soilless substrate compared to the soil-substrate, making the soilless type preferable in the interest of both safer produce for human consumption and lower construction weight in the case of green-roof cultivation.

Signal Transduction in Cereal Plants Struggling with Environmental Stresses: From Perception to Response

Cereal plants under abiotic or biotic stressors to survive unfavourable conditions and continue growth and development, rapidly and precisely identify external stimuli and activate complex molecular, biochemical, and physiological responses. To elicit a response to the stress factors, interactions between reactive oxygen and nitrogen species, calcium ions, mitogen-activated protein kinases, calcium-dependent protein kinases, calcineurin B-like interacting protein kinase, phytohormones and transcription factors occur. The integration of all these elements enables the change of gene expression, and the release of the antioxidant defence and protein repair systems. There are still numerous gaps in knowledge on these subjects in the literature caused by the multitude of signalling cascade components, simultaneous activation of multiple pathways and the intersection of their individual elements in response to both single and multiple stresses. Here, signal transduction pathways in cereal plants under drought, salinity, heavy metal stress, pathogen, and pest attack, as well as the crosstalk between the reactions during double stress responses are discussed. This article is a summary of the latest discoveries on signal transduction pathways and it integrates the available information to better outline the whole research problem for future research challenges as well as for the creative breeding of stress-tolerant cultivars of cereals.

Variation in Root-Related Traits Is Associated With Water Uptake in Lagenaria siceraria Genotypes Under Water-Deficit Conditions

In many agricultural areas, crop production has decreased due to a lack of water availability, which is having a negative impact on sustainability and putting food security at risk. In plants, the plasticity of the root system architecture (RSA) is considered to be a key trait driving the modification of the growth and structure of roots in response to water deficits. The purpose of this study was to examine the plasticity of the RSA traits (mean root diameter, MRD; root volume, RV; root length, RL; and root surface area, SA) associated with drought tolerance in eight Lagenaria siceraria (Mol. Standl) genotypes, representing three different geographical origins: South Africa (BG-58, BG-78, and GC), Asia (Philippines and South Korea), and Chile (Illapel, Chepica, and Osorno). The RSA changes were evaluated at four substrate depths (from 0 to 40 cm). Bottle gourd genotypes were grown in 20 L capacity pots under two contrasting levels of irrigation (well-watered and water-deficit conditions). The results showed that the water productivity (WP) had a significant effect on plasticity values, with the Chilean accessions having the highest values. Furthermore, Illapel and Chepica genotypes presented the highest WP, MRD, and RV values under water-deficit conditions, in which MRD and RV were significant in the deeper layers (20-30 and 30-40 cm). Biplot analysis showed that the Illapel and Chepica genotypes presented a high WP, MRD, and RV, which confirmed that these may be promising drought-tolerant genotypes. Consequently, increased root diameter and volume in bottle gourd may constitute a response to a water deficit. The RSA traits studied here can be used as selection criteria in bottle gourd breeding programs under water-deficit conditions.

Effects of Different Grazing Disturbances on the Plant Diversity and Ecological Functions of Alpine Grassland Ecosystem on the Qinghai-Tibetan Plateau

Grazing is one of the main human disturbance factors in alpine grassland on the Qinghai-Tibet Plateau (QTP), which can directly or indirectly influence the community structures and ecological functions of grassland ecosystems. However, despite extensive field grazing experiments, there is currently no consensus on how different grazing management approaches affect alpine grassland diversity, soil carbon (C), and nitrogen (N). Here, we conducted a meta-analysis of 70 peer-reviewed publications to evaluate the general response of 11 variables related to alpine grassland ecosystems plant diversity and ecological functions to grazing. Overall, the results showed that grazing significantly increased the species richness, Shannon-Wiener index, and Pielou evenness index values by 9.89% (95% CI: 2.75-17.09%), 7.28% (95% CI: 1.68-13.62%), and 3.74% (95% CI: 1.40-6.52%), respectively. Aboveground biomass (AGB) and belowground biomass (BGB) decreased, respectively, by 41.91% (95% CI: -50.91 to -32.88%) and 17.68% (95% CI: -26.94 to -8.52%). Soil organic carbon (SOC), soil total nitrogen (TN), soil C:N ratio, and soil moisture decreased by 13.06% (95% CI: -15.88 to -10.15%), 12.62% (95% CI: -13.35 to -8.61%), 3.27% (95% CI: -4.25 to -2.09%), and 20.75% (95% CI: -27.89 to -13.61%), respectively, whereas, soil bulk density and soil pH increased by 17.46% (95% CI: 11.88-24.53%) and 2.24% (95% CI: 1.01-3.64%), respectively. Specifically, moderate grazing, long-durations (>5 years), and winter grazing contributed to increases in the species richness, Shannon-Wiener index, and Pielou evenness index. However, AGB, BGB, SOC, TN, and soil C:N ratios showed a decrease with enhanced grazing intensity. The response ratio of SOC was positively associated with AGB and BGB but was negatively related to the Shannon-Wiener index and Pielou evenness index. Furthermore, the effects of grazing on plant diversity, AGB, BGB, SOC, and TN in alpine grassland varied with grazing duration, grazing season, livestock type, and grassland type. The findings suggest that grazing should synthesize other appropriate grazing patterns, such as seasonal and rotation grazing, and, furthermore, additional research on grazing management of alpine grassland on the QTP is needed in the future.

Differential Variation in Non-structural Carbohydrates in Root Branch Orders of Fraxinus mandshurica Rupr. Seedlings Across Different Drought Intensities and Soil Substrates

Non-structural carbohydrates (NSCs) facilitate plant adaptation to drought stress, characterize tree growth and survival ability, and buffer against external disturbances. Previous studies have focused on the distribution and dynamics of NSCs among different plant organs under drought conditions. However, discussion about the NSC levels of fine roots in different root branch orders is limited, especially the relationship between fine root trait variation and NSC content. The objective of the study was to shed light on the synergistic variation in fine root traits and NSC content in different root branch orders under different drought and soil substrate conditions. The 2-year-old Fraxinus mandshurica Rupr. potted seedlings were planted in three different soil substrates (humus, loam, and sandy-loam soil) and subjected to four drought intensities (CK, mild drought, moderate drought, and severe drought) for 2 months. With increasing drought intensity, the biomass of fine roots decreased significantly. Under the same drought intensity, seedlings in sandy-loam soil had higher root biomass, and the coefficient of variation of 5th-order roots (37.4, 44.5, and 53% in humus, loam, and sandy-loam soil, respectively) was higher than that of lower-order roots. All branch order roots of seedlings in humus soil had the largest specific root length (SRL) and specific root surface area (SRA), in addition to the lowest diameter. With increasing drought intensity, the SRL and average diameter (AD) of all root branch orders increased and decreased, respectively. The fine roots in humus soil had a higher soluble sugar (SS) content and lower starch (ST) content compared to the loam and sandy-loam soil. Additionally, the SS and ST contents of fine roots showed decreasing and increasing tendencies with increasing drought intensities, respectively. SS and ST explained the highest degree of the total variation in fine root traits, which were 32 and 32.1%, respectively. With increasing root order, the explanation of the variation in root traits by ST decreased (only 6.8% for 5th-order roots). The observed response in terms of morphological traits of different fine root branch orders of F. mandshurica seedlings to resource fluctuations ensures the maintenance of a low cost-benefit ratio in the root system development.

Drought induces shifts in soil fungal communities that can be linked to root traits across 24 plant species

Root traits respond to drought in a species-specific manner, but little is known about how soil fungal communities and root traits respond to drought in concert. In a glasshouse experiment, we determined the response of soil pathogens, saprotrophs, and mutualistic and all fungi associated with the roots of 24 plant species subjected to drought. At harvest, soil fungal communities were characterized by sequencing. Data on root traits were extracted from a previously published work. Differences in fungal beta diversity between drought and control were plant species specific. For some species, saprotrophic fungi increased in relative abundance and richness with drought, whereas mutualistic fungi showed the opposite pattern. Community structure of pathogenic fungi was plant species specific but was slightly affected by drought. Pathogen composition was correlated with specific root surface area and root : shoot, saprotroph abundance with root tissue density, whereas mutualist composition was correlated with root : shoot. All these were the fungal attributes that best predicted shoot mass. Fungal response to drought depended highly on the fungal group and was related to root trait adjustments to water scarcity. This provides new insights into the role that root trait adjustments to drought may have in modulating plant-fungus interactions in grasslands ecosystems.

Interactive Effects of Drought and Saline Aerosol Stress on Morphological and Physiological Characteristics of Two Ornamental Shrub Species

Effects of drought and aerosol stresses were studied in a factorial experiment based on a Randomized Complete Design with triplicates on two ornamental shrubs. Treatments consisted of four levels of water container (40%, 30%, 20%, and 10% of water volumetric content of the substrate) and, after 30 days from experiment onset, three aerosol treatments (distilled water and 50% and 100% salt sea water concentrations). The trial was contextually replicated on two species: Callistemon citrinus (Curtis) Skeels and Viburnum tinus L. ‘Lucidum’. In both species, increasing drought stress negatively affected dry biomass, leaf area, net photosynthesis, chlorophyll a fluorescence, and relative water content. The added saline aerosol stress induced a further physiological water deficit in plants of both species, with more emphasis on Callistemon. The interaction between the two stress conditions was found to be additive for almost all the physiological parameters, resulting in enhanced damage on plants under stress combination. Total biomass, for effect of combined stresses, ranged from 120.1 to 86.4 g plant−1 in Callistemon and from 122.3 to 94.6 g plant−1 in Viburnum. The net photosynthesis in Callistemon declined by the 70% after 30 days in WC 10% and by the 45% and 53% in WC 20% and WC 10% respectively after 60 days. In Viburnum plants, since the first measurement (7 days), a decrease of net photosynthesis was observed for the more stressed treatments (WC 20% and WC 10%), by 57%. The overall data suggested that Viburnum was more tolerant compared the Callistemon under the experimental conditions studied.

Evaluation of the relationship between root nutrients and root biomass in lands under different management practices

Mastering ecological dynamics necessitates identifying the substance cycles in biomass. In terms of sustainable soil productivity, the nutrient content of below-ground biomass is just as significant as the above-ground biomass, which fluctuates depending on land use. Yet, there were limited studies on determining the quantity of plant nutrient stocks, particularly in the below-ground biomass, in rangeland, forest, and plantation areas coexisting in the same ecological zone. In this regard, it is expected that the findings of this study will contribute to the literature. For this purpose in mind, distinct samples were taken from three depth levels (0-10 cm, 10-20 cm, 20-30 cm) to determine root biomass and nutrient stocks of roots in neighboring rangeland, forest, and plantation areas, and roots were divided into diameter classes, and below-ground biomass amounts and nutritional contents of below-ground biomass were determined. According to the results obtained, the total root biomass in the rangelands is 8.02 Mg ha^-1, total root biomass was 5.95 Mg ha^-1 in forest areas, and in plantation areas, the total root biomass is 6.94 Mg ha^-1. Root biomass in the 0-10 cm soil layer constituted 78% of the total biomass. Also, for all land uses, the highest below-ground biomass concentrations were observed for Al, Fe, K, Mg, and Ca. The amounts of Al, Fe, K and Mg in the below-ground biomass followed the sequence of rangeland, plantation, and forest from high to low. Nutrient stocks in below-ground biomass and the effects of increases in root biomass on plant growth should be evaluated by future studies.

Scaling up experimental stress responses of grass invasion to predictions of continental-level range suitability

Understanding how the biological invasion is driven by environmental factors will improve model prediction and advance early detection, especially in the context of accelerating anthropogenic ecological changes. Although a large body of studies has examined how favorable environments promote biological invasions, a more comprehensive and mechanistic understanding of invasive species response to unfavorable/stressful conditions is still developing. Grass invasion has been problematic across the globe; in particular, C₄ grass invaders, with high drought tolerance, adaptations to high temperatures, and high water use efficiency, could become more severe. Here, we conducted a rigorous microcosm experiment, with one of the most damaging invasive C₄ grass, cogongrass (Imperata cylindrica), to explore how cogongrass responds to soil water and nutrient stress. We further integrated the results of the microcosm study with a species distribution model to (1) corroborate greenhouse results with field observations and (2) validate the robustness of our findings at subcontinental scales. Both the microcosm experiments and species distribution model agreed that soil water stress had a stronger impact on cogongrass than the nutrient one. New vegetative growth of cogongrass continued to be inhibited by the prior water stress. The significant water effect on cogongrass total biomass was supported by the finding that both allometric and biochemical traits of cogongrass did not show significant responses to the changes in water treatment. Different to the conventional wisdom that nutrient enrichment plays a bigger role in facilitating biological invasions, this study highlighted the possibility that water conditions may have a more substantial effect on some aggressive invaders. Therefore, an important implication of this study on biological conservation is that field managers might take advantage of the negative effect of global drought on some invasive species to increase the efficiency of their controlling efforts because invasive species may become more vulnerable under drought effect.

Fine-root functional trait responses to experimental warming: a global meta-analysis

Whether and how warming alters functional traits of absorptive plant roots remains to be answered across the globe. Tackling this question is crucial to better understanding terrestrial responses to climate change as fine-root traits drive many ecosystem processes. We carried out a detailed synthesis of fine-root trait responses to experimental warming by performing a meta-analysis of 964 paired observations from 177 publications. Warming increased fine-root biomass, production, respiration and nitrogen concentration as well as decreased root carbon : nitrogen ratio and nonstructural carbohydrates. Warming effects on fine-root biomass decreased with greater warming magnitude, especially in short-term experiments. Furthermore, the positive effect of warming on fine-root biomass was strongest in deeper soil horizons and in colder and drier regions. Total fine-root length, morphology, mortality, life span and turnover were unresponsive to warming. Our results highlight the significant changes in fine-root traits in response to warming as well as the importance of warming magnitude and duration in understanding fine-root responses. These changes have strong implications for global soil carbon stocks in a warmer world associated with increased root-derived carbon inputs into deeper soil horizons and increases in fine-root respiration.

Responses of native and invasive woody seedlings to combined competition and drought are species-specific

Woody species invasions are a major threat to native communities with intensified consequences during increased periods of summer drought as predicted for the future. Competition for growth-limiting nitrogen (N) between native and invasive tree species might represent a key mechanism underlying the invasion process, because soil water availability and N acquisition of plants are closely linked. To study whether the traits of invasive species provide an advantage over natives in Central Europe in the competition for N under drought, we conducted a greenhouse experiment. We analyzed the responses of three native (i.e., Fagus sylvatica L., Quercus robur L. and Pinus sylvestris L.) and two invasive woody species (i.e., Prunus serotina Ehrh. and Robinia pseudoacacia L.) to competition in terms of their organic and inorganic N acquisition, as well as allocation of N to N pools in the leaves and fine roots. In our study, competition resulted in reduced growth and changes in internal N pools in both native and invasive species mediated by the physiological characteristics of the target species, the competitor, as well as soil water supply. Nitrogen acquisition, however, was not affected by competition indicating that changes in growth and N pools were rather linked to the remobilization of stored N. Drought led to reduced N acquisition, growth and total soluble protein-N levels, while total soluble amino acid-N levels increased, most likely as osmoprotectants as an adaptation to the reduced water supply. Generally, the consequences of drought were enhanced with competition across all species. Comparing the invasive competitors, P. serotina was a greater threat to the native species than R. pseudoacacia. Furthermore, deciduous and coniferous native species affected the invasives differently, with the species-specific responses being mediated by soil water supply.

Peatland warming strongly increases fine-root growth

Belowground climate change responses remain a key unknown in the Earth system. Plant fine-root response is especially important to understand because fine roots respond quickly to environmental change, are responsible for nutrient and water uptake, and influence carbon cycling. However, fine-root responses to climate change are poorly constrained, especially in northern peatlands, which contain up to two-thirds of the world's soil carbon. We present fine-root responses to warming between +2 °C and 9 °C above ambient conditions in a whole-ecosystem peatland experiment. Warming strongly increased fine-root growth by over an order of magnitude in the warmest treatment, with stronger responses in shrubs than in trees or graminoids. In the first year of treatment, the control (+0 °C) shrub fine-root growth of 0.9 km m-2 y-1 increased linearly by 1.2 km m-2 y-1 (130%) for every degree increase in soil temperature. An extended belowground growing season accounted for 20% of this dramatic increase. In the second growing season of treatment, the shrub warming response rate increased to 2.54 km m-2 °C-1 Soil moisture was negatively correlated with fine-root growth, highlighting that drying of these typically water-saturated ecosystems can fuel a surprising burst in shrub belowground productivity, one possible mechanism explaining the "shrubification" of northern peatlands in response to global change. This previously unrecognized mechanism sheds light on how peatland fine-root response to warming and drying could be strong and rapid, with consequences for the belowground growing season duration, microtopography, vegetation composition, and ultimately, carbon function of these globally relevant carbon sinks.

Effects of Microplastic Fibers and Drought on Plant Communities

Microplastics in soils can affect plant performance, as shown in studies using individual plants. However, we currently have no information about potential effects on plant community productivity and structure. In a plant community consisting of seven plant species that co-occur in temperate grassland ecosystems, we thus investigated the effect of microplastics (i.e., microfibers) and drought, a factor with which microfibers might interact, on plant productivity and community structure. Our results showed that at the community level, shoot and root mass decreased with drought but increased with microfibers, an effect likely linked to reduced soil bulk density, improved aeration, and better penetration of roots in the soil. Additionally, we observed that microfibers affected plant community structure. Species such as Calamagrostis, invasive in Europe, and the allelophatic Hieracium, became more dominant with microfibers, while species that potentially have the ability to facilitate the establishment of other plant species (e.g., Holcus), decreased in biomass. As microfibers affect plant species dominance, the examination of cascade effects on ecosystem functions should be a high priority for future research.

Fine Root Length of Maize Decreases in Response to Elevated CO2 Levels in Soil

To assess the environmental risks of carbon capture and storage (CCS) due to underground CO2 leakage, many studies have examined the impact on plant growth; however, the effect of leaked CO2 on root morphology remains poorly understood. This study simulated the effects of CO2 leakage from CCS on maize (Zea mays L.) root systems through pot experiments—one control treatment (no added CO2) and two elevated soil CO2 treatments (1000 g m−2 d−1 and 2000 g m−2 d−1). Compared with the control, root length, root surface area, and root volume were reduced by 44.73%, 34.14%, and 19.16%, respectively, in response to CO2 treatments with a flux of 2000 g m−2 d−1. Meanwhile, the fine root length in CO2 treatments with a flux of 1000 g m−2 d−1 and 2000 g m−2 d−1 were reduced by 29.44% and 45.88%, respectively, whereas no obvious difference in regard to coarse roots was found. Understanding changes in plant root morphology in this experiment, especially the decrease in the fine root length, are essential for explaining plant responses to CO2 leakage from CCS.

Distinct fine-root responses to precipitation changes in herbaceous and woody plants: a meta-analysis

Precipitation is one of the most important factors that determine productivity of terrestrial ecosystems. Precipitation across the globe is predicted to change more intensively under future climate change scenarios, but the resulting impact on plant roots remains unclear. Based on 154 observations from experiments in which precipitation was manipulated in the field and root biomass was measured, we investigated responses in fine-root biomass of herbaceous and woody plants to alterations in precipitation. We found that root biomass of herbaceous and woody plants responded differently to precipitation change. In particular, precipitation increase consistently enhanced fine-root biomass of woody plants but had variable effects on herb roots in arid and semi-arid ecosystems. In contrast, precipitation decrease reduced root biomass of herbaceous plants but not woody plants. In addition, with precipitation alteration, the magnitude of root responses was greater in dry areas than in wet areas. Together, these results indicate that herbaceous and woody plants have different rooting strategies to cope with altered precipitation regimes, particularly in water-limited ecosystems. These findings suggest that root responses to precipitation change may critically influence root productivity and soil carbon dynamics under future climate change scenarios.

Seminal roots of wild and cultivated barley differentially respond to osmotic stress in gene expression, suberization, and hydraulic conductivity

Wild barley, Hordeum vulgare spp. spontaneum, has a wider genetic diversity than its cultivated progeny, Hordeum vulgare spp. vulgare. Osmotic stress leads to a series of different responses in wild barley seminal roots, ranging from no changes in suberization to enhanced endodermal suberization of certain zones and the formation of a suberized exodermis, which was not observed in the modern cultivars studied so far. Further, as a response to osmotic stress, the hydraulic conductivity of roots was not affected in wild barley, but it was 2.5-fold reduced in cultivated barley. In both subspecies, osmotic adjustment by increasing proline concentration and decreasing osmotic potential in roots was observed. RNA-sequencing indicated that the regulation of suberin biosynthesis and water transport via aquaporins were different between wild and cultivated barley. These results indicate that wild barley uses different strategies to cope with osmotic stress compared with cultivated barley. Thus, it seems that wild barley is better adapted to cope with osmotic stress by maintaining a significantly higher hydraulic conductivity of roots during water deficit.

High-throughput three-dimensional visualization of root system architecture of rice using X-ray computed tomography

BACKGROUND

X-ray computed tomography (CT) allows us to visualize root system architecture (RSA) beneath the soil, non-destructively and in a three-dimensional (3-D) form. However, CT scanning, reconstruction processes, and root isolation from X-ray CT volumes, take considerable time. For genetic analyses, such as quantitative trait locus mapping, which require a large population size, a high-throughput RSA visualization method is required.

RESULTS

We have developed a high-throughput process flow for the 3-D visualization of rice (Oryza sativa) RSA (consisting of radicle and crown roots), using X-ray CT. The process flow includes use of a uniform particle size, calcined clay to reduce the possibility of visualizing non-root segments, use of a higher tube voltage and current in the X-ray CT scanning to increase root-to-soil contrast, and use of a 3-D median filter and edge detection algorithm to isolate root segments. Using high-performance computing technology, this analysis flow requires only 10 min (33 s, if a rough image is acceptable) for CT scanning and reconstruction, and 2 min for image processing, to visualize rice RSA. This reduced time allowed us to conduct the genetic analysis associated with 3-D RSA phenotyping. In 2-week-old seedlings, 85% and 100% of radicle and crown roots were detected, when 16 cm and 20 cm diameter pots were used, respectively. The X-ray dose per scan was estimated at < 0.09 Gy, which did not impede rice growth. Using the developed process flow, we were able to follow daily RSA development, i.e., 4-D RSA development, of an upland rice variety, over 3 weeks.

CONCLUSIONS

We developed a high-throughput process flow for 3-D rice RSA visualization by X-ray CT. The X-ray dose assay on plant growth has shown that this methodology could be applicable for 4-D RSA phenotyping. We named the RSA visualization method 'RSAvis3D' and are confident that it represents a potentially efficient application for 3-D RSA phenotyping of various plant species.

Combined Effects of Drought and Shading on Growth and Non-Structural Carbohydrates in Pinus massoniana Lamb. Seedlings

Carbon assimilation is reduced by stress. Under such conditions, the trade-off between growth and non-structural carbohydrate (NSC) storage becomes crucial for plant survival and continued growth. However, growth and NSC responses to drought and shading in Pinus massoniana Lamb. remain unclear. Here, we investigated the effects of drought, shading, and combined drought and shading on leaf gas exchange parameters, stem basal diameter, plant height, biomass accumulation, and NSC concentration in 2-year old seedlings after a 2 month treatment. The results showed that (1) both drought and shading significantly reduced photosynthetic rate, increment of stem basal diameter and plant height, and biomass accumulation, while NSC concentration increased under drought but decreased under shading; (2) the combined drought-shading treatment had a stronger effect on photosynthetic rate and growth than either stress factor individually, whereas the concentration of NSC did not change significantly; and (3) drought, shading, and their combination had a lower effect on biomass than on NSC partitioning, in which case clear effects were observed. Drought increased NSC proportion in roots by 5.4%; conversely, shading increased NSC proportion in leaves by 3.7%, while the combined treatment increased NSC proportion in roots by 5.1% but decreased it in the leaves by 5.4%. These results suggest that the mechanism inhibiting P. massoniana growth is different under drought and shading conditions according to carbon partitioning. Furthermore, complex environmental stress may lead to different mechanisms of carbon partitioning compared with either dry or shaded environments. Our findings will be helpful in predicting the impact of climate change on P. massoniana growth.

Cigarette butts have adverse effects on initial growth of perennial ryegrass (gramineae: Lolium perenne L.) and white clover (leguminosae: Trifolium repens L.)

Cigarette filters (butts) are currently the most abundant form of anthropogenic litter on the planet, yet we know very little about their environmental impacts on terrestrial ecosystems, including plant germination and primary production. When discarded, filters contain a myriad of chemicals resulting from smoking tobacco and some still contain unsmoked remnants. A greenhouse experiment was used to assess the impacts of discarded filters of regular or menthol cigarette, either from unsmoked, smoked, or smoked cigarettes with remnant tobacco, on the growth and development of Lolium perenne (perennial ryegrass) and Trifolium repens (white clover). After 21 days, shoot length and germination success were significantly reduced by exposure to any type of cigarette filter for the grass and clover. Although total grass biomass was not measurably affected, the root biomass and root:shoot ratio were less in the clover when exposed to filters from smoked regular cigarettes and those with remnant tobacco. Cigarette filters caused an increase in chlorophyll-a in clover shoots and an increase in chlorophyll-b in grass shoots. Accordingly, whilst the chlorophyll a:b ratio was increased in the clover exposed to cigarette filters, it was decreased in grass. This study indicates the potential for littered cigarette filters to reduce growth and alter short-term primary productivity of terrestrial plants.

The Dynamics of Living and Dead Fine Roots of Forest Biomes Across the Northern Hemisphere

Research Highlights: A detailed picture of the seasonality in fine root biomass (FRB), necromass (FRN), and the biomass/necromass ratio (FRBN) throughout the whole year is crucial to uncover profound effects of long-term environmental changes on fine root dynamics. Materials and Methods: We used meta-analysis to characterize the variability of FRB, FRN and FRBN, and determined their relations with climatic (monthly versus annual), edaphic and geomorphic factors for tropical, temperate and boreal forest biomes across the Northern Hemisphere. Results: Boreal forests exhibited the highest FRB and FRN, while tropical forests yielded the lowest FRN, and thus the greatest FRBN. FRB and FRN significantly decreased with sampling depth, but increased with soil organic carbon content and elevation, while an opposite pattern was found for FRBN. Temperature and precipitation at different time scales (monthly versus annual) and latitude had varying influences on fine roots. High FRB and FRN were observed during dry season for tropical forests, but in the late growing season for temperate forests. The three forest biomes exhibited the high root activity (measured as FRBN) in June or July. Conclusions: It is crucial to realize the universal and specific responses of fine roots to multiple environmental factors when attempting to incorporate these parameters into fine root monthly dynamic models in forest ecosystems. The biome-specific fluctuation of fine roots contributes to identify the influence factors on fine root seasonal patterns throughout the whole year. Our analysis is expected to improve the understanding of the key role of fine roots at monthly level in modeling and predicting carbon budget of various forest biomes under future climate change.

Plants alter their vertical root distribution rather than biomass allocation in response to changing precipitation

Elucidating the variation of allocation pattern of ecosystem net primary productivity (NPP) and its underlying mechanisms is critically important for understanding the changes of aboveground and belowground ecosystem functions. Under optimal partitioning theory, plants should allocate more NPP to the organ that acquires the most limiting resource, and this expectation has been widely used to explain and predict NPP allocation under changing precipitation. However, confirmatory evidence for this theory has mostly come from observed spatial variation in the relationship between precipitation and NPP allocation across ecosystems, rather than directly from the influences of changing precipitation on NPP allocation within systems. We performed a 6-yr five-level precipitation manipulation experiment in a semiarid steppe to test whether changes in NPP allocation can be explained by the optimal partitioning theory, and how water requirement of plant community is maintained if NPP allocation is unaltered. The 30 precipitation levels (5 levels × 6 yr) were divided into dry, nominal, and wet precipitation ranges, relative to historical precipitation variation over the past six decades. We found that NPP in both aboveground (ANPP) and belowground (BNPP) increased nonlinearly as precipitation increased, while the allocation of NPP to BNPP (f_BNPP ) showed a concave quadratic relationship with precipitation. The declined f_BNPP as precipitation increased in the dry range supported the optimal partitioning theory. However, in the nominal range, NPP allocation was not influenced by the changed precipitation; instead, BNPP was distributed more in the surface soil horizon (0-10 cm) as precipitation increased, and conversely more in the deeper soil layers (10-30 cm) as precipitation decreased. This response in root foraging appears to be a strategy to satisfy plant water requirements and partially explains the stable NPP allocation patterns. Overall, our results suggest that plants can adjust their vertical BNPP distribution in response to drought stress, and that only under extreme drought does the optimal partitioning theory strictly apply, highlighting the context dependency of the adaption and growth of plants under changing precipitation.

Root Plasticity in the Pursuit of Water

One of the greatest challenges of terrestrial vegetation is to acquire water through soil-grown roots. Owing to the scarcity of high-quality water in the soil and the environment's spatial heterogeneity and temporal variability, ranging from extreme flooding to drought, roots have evolutionarily acquired tremendous plasticity regarding their geometric arrangement of individual roots and their three-dimensional organization within the soil. Water deficiency has also become an increasing threat to agriculture and dryland ecosystems due to climate change. As a result, roots have become important targets for genetic selection and modification in an effort to improve crop resilience under water-limiting conditions. This review addresses root plasticity from different angles: Their structures and geometry in response to the environment, potential genetic control of root traits suitable for water-limiting conditions, and contemporary and future studies of the principles underlying root plasticity post-Darwin's 'root-brain' hypothesis. Our increasing knowledge of different disciplines of plant sciences and agriculture should contribute to a sustainable management of natural and agricultural ecosystems for the future of mankind.

Effects of livestock grazing on grassland carbon storage and release override impacts associated with global climate change

Predicting future carbon (C) dynamics in grassland ecosystems requires knowledge of how grazing and global climate change (e.g., warming, elevated CO₂ , increased precipitation, drought, and N fertilization) interact to influence C storage and release. Here, we synthesized data from 223 grassland studies to quantify the individual and interactive effects of herbivores and climate change on ecosystem C pools and soil respiration (Rs). Our results showed that grazing overrode global climate change factors in regulating grassland C storage and release (i.e., Rs). Specifically, grazing significantly decreased aboveground plant C pool (APCP), belowground plant C pool (BPCP), soil C pool (SCP), and Rs by 19.1%, 6.4%, 3.1%, and 4.6%, respectively, while overall effects of all global climate change factors increased APCP, BPCP, and Rs by 6.5%, 15.3%, and 3.4% but had no significant effect on SCP. However, the combined effects of grazing with global climate change factors also significantly decreased APCP, SCP, and Rs by 4.0%, 4.7%, and 2.7%, respectively but had no effect on BPCP. Most of the interactions between grazing and global climate change factors on APCP, BPCP, SCP, and Rs were additive instead of synergistic or antagonistic. Our findings highlight the dominant effects of grazing on C storage and Rs when compared with the suite of global climate change factors. Therefore, incorporating the dominant effect of herbivore grazing into Earth System Models is necessary to accurately predict climate-grassland feedbacks in the Anthropocene.