Energy costs of salinity tolerance in crop plants: night-time transpiration and growth

Relationships between Wheat Development, Soil Properties, and Rhizosphere Mycobiota

Wheat is a vital global food crop, yet it faces challenges in saline-alkali soils where Fusarium crown rot significantly impacts growth. Variations in wheat growth across regions are often attributed to uneven terrain. To explore these disparities, we examined well-growing and poorly growing wheat samples and their rhizosphere soils. Measurements included wheat height, root length, fresh weight, and Fusarium crown rot severity. Well-growing wheat exhibited greater height, root length, and fresh weight, with a lower Fusarium crown rot disease index compared to poorly growing wheat. Analysis of rhizosphere soil revealed higher alkalinity; lower nutrient levels; and elevated Na, K, and Ca levels in poorly growing wheat compared to well-growing wheat. High-throughput sequencing identified a higher proportion of unique operational taxonomic units (OTUs) in poorly growing wheat, suggesting selection for distinct fungal species under stress. FUNGuild analysis indicated a higher prevalence of pathogenic microbial communities in poorly growing wheat rhizosphere soil. This study underscores how uneven terrains in saline-alkali soils affect pH, nutrient dynamics, mineral content, wheat health, and rhizosphere fungal community structure.

Integrative multi-omics analyses of date palm (Phoenix dactylifera) roots and leaves reveal how the halophyte land plant copes with sea water

Date palm (Phoenix dactylifera L.) is able to grow and complete its life cycle while being rooted in highly saline soils. Which of the many well-known salt-tolerance strategies are combined to fine-tune this remarkable resilience is unknown. The precise location, whether in the shoot or the root, where these strategies are employed remains uncertain, leaving us unaware of how the various known salt-tolerance mechanisms are integrated to fine-tune this remarkable resilience. To address this shortcoming, we exposed date palm to a salt stress dose equivalent to seawater for up to 4 weeks and applied integrative multi-omics analyses followed by targeted metabolomics, hormone, and ion analyses. Integration of proteomic into transcriptomic data allowed a view beyond simple correlation, revealing a remarkably high degree of convergence between gene expression and protein abundance. This sheds a clear light on the acclimatization mechanisms employed, which depend on reprogramming of protein biosynthesis. For growth in highly saline habitats, date palm effectively combines various salt-tolerance mechanisms found in both halophytes and glycophytes: "avoidance" by efficient sodium and chloride exclusion at the roots, and "acclimation" by osmotic adjustment, reactive oxygen species scavenging in leaves, and remodeling of the ribosome-associated proteome in salt-exposed root cells. Combined efficiently as in P. dactylifera L., these sets of mechanisms seem to explain the palm's excellent salt stress tolerance.

Abiotic Stress-Induced Leaf Senescence: Regulatory Mechanisms and Application

Leaf senescence is a natural phenomenon that occurs during the aging process of plants and is influenced by various internal and external factors. These factors encompass plant hormones, as well as environmental pressures such as inadequate nutrients, drought, darkness, high salinity, and extreme temperatures. Abiotic stresses accelerate leaf senescence, resulting in reduced photosynthetic efficiency, yield, and quality. Gaining a comprehensive understanding of the molecular mechanisms underlying leaf senescence in response to abiotic stresses is imperative to enhance the resilience and productivity of crops in unfavorable environments. In recent years, substantial advancements have been made in the study of leaf senescence, particularly regarding the identification of pivotal genes and transcription factors involved in this process. Nevertheless, challenges remain, including the necessity for further exploration of the intricate regulatory network governing leaf senescence and the development of effective strategies for manipulating genes in crops. This manuscript provides an overview of the molecular mechanisms that trigger leaf senescence under abiotic stresses, along with strategies to enhance stress tolerance and improve crop yield and quality by delaying leaf senescence. Furthermore, this review also highlighted the challenges associated with leaf senescence research and proposes potential solutions.

Salt Stress-Regulation of Root Water Uptake in a Whole-Plant and Diurnal Context

This review focuses on the regulation of root water uptake in plants which are exposed to salt stress. Root water uptake is not considered in isolation but is viewed in the context of other potential tolerance mechanisms of plants-tolerance mechanisms which relate to water relations and gas exchange. Plants spend between one third and half of their lives in the dark, and salt stress does not stop with sunset, nor does it start with sunrise. Surprisingly, how plants deal with salt stress during the dark has received hardly any attention, yet any growth response to salt stress over days, weeks, months and years is the integrative result of how plants perform during numerous, consecutive day/night cycles. As we will show, dealing with salt stress during the night is a prerequisite to coping with salt stress during the day. We hope to highlight with this review not so much what we know, but what we do not know; and this relates often to some rather basic questions.

Drought stress mitigation by foliar application of L-carnitine and its effect on radish morphophysiology

Drought is the major abiotic stress limiting crop production worldwide, with drought events being expected to be harsher and more frequent due to the global warming. In this context, the development of strategies to mitigate the deleterious effects of drought, such as the use of biostimulants, is imperative. Radish is a globally cultivated root vegetable, with high nutritional and phytochemical value. Thus, this study aimed to evaluate the potential of exogenous carnitine application in the mitigation of drought stress on radish morphophysiology. For this, radish plants were grown for 30 days, being irrigated with 80% (well-watered) or 15% (drought stress) of water holding capacity and sprayed with carnitine (5, 50, and 500 µM) or water (0 µM-no carnitine). The experimental design was completely randomized, in a 4 × 2 factorial scheme (carnitine concentrations × water conditions) with six replicates, and each experimental unit consisted of one plant. The gas exchanges, chlorophyll a fluorescence, photosynthetic pigments, electrolyte leakage, relative water content, and biomass production and allocation were evaluated. Drought reduced the photosynthetic capacity of plants by impairing water balance and membrane integrity, decreasing biomass accumulation, mainly in globular roots. The application of low carnitine (5 µM) mitigated these negative effects caused by drought, increasing membrane integrity and water balance of plants, while higher carnitine concentration (50 and 500 µM) aggravated drought stress. This study highlights the potential of carnitine in the mitigation of drought stress on radish plants, supporting its role as a biostimulant.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01308-6.

Changes in root hydraulic conductivity in wheat (Triticum aestivum L.) in response to salt stress and day/night can best be explained through altered activity of aquaporins

Salt stress reduces plant water flow during day and night. It is not known to which extent root hydraulic properties change in parallel. To test this idea, hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150 and 200 mM NaCl) for 5-8d before harvest (d14-18) and subjected to a range of analyses to determine diurnal changes in hydraulic conductivity (Lp) at cell, root and plant level. Cell pressure probe analyses showed that the Lp of cortex cells was differentially affected by salt stress during day and night, and that the response to salt stress differed between the main axis of roots and lateral roots. The Aquaporin (AQP) inhibitor H₂ O₂ reduced Lp to a common, across treatments, level as observed in salt-stressed plants during the night. Analyses of transpiring plants and exuding root systems provided values of root Lp which were in the same range as values modeled based on cell-Lp. The results can best be explained through a change in root Lp in response to salt stress and day/night, which results from an altered activity of AQPs. qPCR gene expression analyses point to possible candidate AQP isoforms.

Does night-time transpiration provide any benefit to wheat (Triticum aestivum L.) plants which are exposed to salt stress?

The study aimed to test whether night-time transpiration provides any potential benefit to wheat plants which are subjected to salt stress. Hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150, and 200 mM NaCl) for 5-8 days prior to harvest (day 14-18). Salt stress caused large decreases in transpiration and leaf elongation rates during day and night. The quantitative relation between the diurnal use of water for transpiration and leaf growth was comparatively little affected by salt. Night-time transpirational water loss occurred predominantly through stomata in support of respiration. Diurnal gas exchange and leaf growth were functionally linked to each other through the provision of resources (carbon, energy) and an increase in leaf surface area. Diurnal rates of water use associated with leaf cell expansive growth were highly correlated with the water potential of the xylem, which was dominated by the tension component. The tissue-specific expression level of nine candidate aquaporin genes in elongating and mature leaf tissue was little affected by salt stress or day/night changes. Growing plants under conditions of reduced night-time transpirational water loss by increasing the relative humidity (RH) during the night to 95% had little effect on the growth response to salt stress, nor was the accumulation of Na⁺ and Cl^- in shoot tissue altered. We conclude that night-time gas exchange supports the growth in leaf area over a 24 h day/night period. Night-time transpirational water loss neither decreases nor increases the tolerance to salt stress in wheat.

Sap Flow Velocity in Fraxinus pennsylvanica in Response to Water Stress and Microclimatic Variables

In arid and semiarid regions with water shortage, forestry development is limited by water availability. Understanding how tree sap flow responds to water stress and microclimatic variables is essential for the management of trees and the understanding of the eco-physiological properties of trees in arid areas. In the city of Tianjin in northern China, we measured the sap flow of Fraxinus pennsylvanica, a widely distributed urban greening tree species in semiarid regions of China. We measured the sap flow in four F. pennsylvanica trees over 6 months (April-September 2021), using a thermal diffusion probe method, and simultaneously monitored microclimatic variables and soil moisture. Results indicated that high nighttime sap flow velocity might be produced under the water stress condition. In addition, the nighttime sap flow velocity under the water stress condition was more susceptible to the combined effects of meteorological factors at night. The daytime sap flow velocity exerted a highly significant positive effect on the nighttime sap flow velocity during the whole research period, and the model fit was higher in the early growing season than that in the late growing season (early growing season: R ² = 0.51, P < 0.01; late growing season: R ² = 0.36, P < 0.01). Vapor pressure deficit had a positive effect on daytime sap flow. However, net vapor pressure deficit restrained daytime sap flow velocity when the intercorrelation between the microclimatic variables was removed. Our study highlights that drought areas perhaps have higher nighttime sap flow and that more emphasis should be placed on nighttime sap flow and the response of nighttime sap flow to microclimatic variables. In addition, the influence of other microclimatic variables on vapor pressure deficit needs to be considered when analyzing the relationship between daytime sap flow and vapor pressure deficit. An increase in net VPD can suppress the daytime sap flow.

Fullerenol regulates oxidative stress and tissue ionic homeostasis in spring wheat to improve net-primary productivity under salt-stress

The effects of fullerenol nanopriming (0, 10, 40, 80 and 120 nM concentration) on salt stressed-wheat (0 and 150 mM NaCl) were investigated under natural conditions. Salinity resulted in a shift in wheat growth pattern in the form of LAR (+ 40.9% increase) and RGR (+ 13.4% increase) while decreased NAR (- 31.7%). It also disturbed shoot and root biomass, ion uptake and reduced chlorophyll contents. Despite increase in enzyme activities, higher ROS generation (+ 48.1% O₂^- anion; and + 62.2% H₂O₂) and lipid peroxidation (+ 40.8% MDA) were detected in salt-stressed wheat plants. Possibly, the increases in enzyme activities were not up to the level to completely counteract the salinity induced oxidative stress. Nanopriming with fullerenol improved NAR (+ 8.77% to 23.2%), ROS metabolism and decreased indicators of oxidative stress. Hydropriming treatment also promoted NAR recovery by 21.9% than control plants. Compared to Na⁺ ions, improvements in shoot relative concentrations of K⁺, Ca²⁺ and P also recorded along with soluble sugars and amino acids, which improved osmotic balance. These biochemical modifications contributed to improvements in grain yield attributes (+11.8% to 18.3% in 100 grain-weight) than salinity stressed control. Hydropriming also contributed to a recovery in grain yield attributes by 12.6%. Above all, the harvested seeds from fullerenol treated plants also showed better germination and seedlings growth traits. Conclusively, we report non-toxic, growth-promoting effects of fullerenol nanoparticles on wheat crop and as a way forward; we suggest its exogenous application to recover crop productivity under saline environments.

A redundant hydraulic function of root hairs in barley plants grown in hydroponics

The root hair-less brb of Hordeum vulgare L. (bald root barley) mutant was used to assess the significance that root hairs have for the hydraulic properties of roots and response to a limited supply of mineral nutrients in plants grown on hydroponics. The barley brb mutant and its parent wild-type (H. vulgare cv. Pallas) were grown under nutrient sufficient control conditions, and under conditions of low supply of P and N. Plants were analysed when they were 14-18 days old. Root hydraulic conductivity (Lp) was determined for excised root systems and intact transpiring plants, and cell Lp was determined through cell pressure probe measurements. The formation of Casparian bands and suberin lamellae was followed through staining of cross-sections. The presence or absence of root hairs had no effect on the overall hydraulic response of plants to nutritional treatments. Root and cell Lp did not differ between the two genotypes. The most apparent difference between brb and wild-type plants was the consistently reduced formation of apoplastic barriers in brb plants. Any hydraulic function of root hairs can be redundant in barley, at least under the hydroponic conditions tested.

Salt stress reduces root water uptake in barley (Hordeum vulgare L.) through modification of the transcellular transport path

The aim of the study was to understand the hydraulic response to salt stress of the root system of the comparatively salt-tolerant crop barley (Hordeum vulgare L.). We focused on the transcellular path of water movement across the root cylinder that involves the crossing of membranes. This path allows for selective water uptake, while excluding salt ions. Hydroponically grown plants were exposed to 100 mM NaCl for 5-7 days and analysed when 15-17 days old. A range of complementary and novel approaches was used to determine hydraulic conductivity (Lp). This included analyses at cell, root and plant level and modelling of water flow. Apoplastic barrier formation and gene expression level of aquaporins (AQPs) was analysed. Salt stress reduced the Lp of root system through reducing water flow along the transcellular path. This involved changes in the activity and gene expression level of AQPs. Modelling of root-Lp showed that the reduction in root-Lp did not require added hydraulic resistances through apoplastic barriers at the endodermis. The bulk of data points to a near-perfect semi-permeability of roots of control plants (solute reflection coefficient σ ~1.0). Roots of salt-stressed plants are almost as semi-permeable (σ > 0.8).

Photosynthetically active radiation impacts significantly on root and cell hydraulics in barley (Hordeum vulgare L.)

Photosynthetically active radiation (PAR) affects transpirational water loss, yet we do not know through which mechanisms root water uptake is adjusted in parallel. Here, we exposed hydroponically grown barley plants to three levels of PAR [Normal (control), Low, High] and focused on the role which aquaporins (AQPs), apoplastic barriers (Casparian bands, suberin lamellae) and root morphology play in the adjustment of root hydraulic conductivity (Lp). Plants were analyzed when they were 14-18 days (d) old. Root and cell Lp, which involves AQP activity, was determined through exudation and cell pressure probe measurements, respectively. Gene expression of AQPs was analyzed through qPCR. The formation of apoplastic barriers was studied through staining of cross-sections. The rate of transpirational water loss per plant and unit leaf area increased in response to high-PAR and decreased in response to low-PAR treatments, both during day and night. Hydraulic conductivity in roots decreased significantly at organ and cell level in response to Low-PAR, and increased (organ) or did not change (cell level) in response to High-PAR. The formation of apoplastic barriers was little affected by PAR. Gene expression of AQPs tended to be highest in the Low-PAR treatment. Lateral roots, showing few apoplastic barriers, contributed the least in Low- and the most to root surface area in High-PAR plants. It is concluded that barley plants which experience changes in shoot transpirational water loss in response to PAR adjust root water uptake through changes in root Lp, and that these changes are mediated through altered AQP activity and root morphology.

Halotropism: Phytohormonal Aspects and Potential Applications

Halotropism is a sodium specific tropic movement of roots in order to obtain the optimal salt concentration for proper growth and development. Numerous results suggest that halotropic events are under the control and regulation of complex plant hormone pathway. This minireview collects some recent evidences about sodium sensing during halotropism and the hormonal regulation of halotropic responses in glycophytes. The precise hormonal mechanisms by which halophytes plant roots perceive salt stress and translate this perception into adaptive, directional growth forward increased salt concentrations are not well understood. This minireview aims to gather recently deciphered information about halotropism focusing potential hormonal aspects both in glycophytes and halophytes. Advances in our understanding of halotropic responses in different plant species could help these plants to be used for sustainable agriculture and other future applications.

Transcriptional regulation-mediating ROS homeostasis and physio-biochemical changes in wild tomato (Solanum chilense) and cultivated tomato (Solanum lycopersicum) under high salinity

Salinity intrusion is one of the biggest problems in the context of sustainable agricultural practices. The major concern and challenge in developing salt-resistance in cultivated crops is the genetic complexity of the trait and lack of natural variability for stress-responsive traits. In this context, tomato wild relatives are important and have provided novel alleles for breeding abiotic stress tolerance including salt tolerance. We provide here a case study, involving tomato wild relative Solanum chilense and cultivated variety Solanum lycopersicum, carried out under high salt stress to investigate comparative transcriptional regulation mediating ROS homeostasis and other physiological attributes. Salt dependent oxidative stress in S. lycopersicum was characterized by a relatively higher H₂O₂ content, generation of O₂^•−, electrolytic leakage and lipid peroxidation whereas reduced content of both ascorbate and glutathione. On the contrary, the robust anti-oxidative system in the S. chilense particularly counteracted the salt-induced oxidative damages by a higher fold change in expression profile of defense-related salt-responsive genes along with the increased activities of anti-oxidative enzymes. We conclude that S. chilense harbours novel genes or alleles for salt stress-related traits that could be identified, characterized, and mapped for its possible introgression into cultivated tomato lines.

Determining Ion Toxicity in Cucumber under Salinity Stress

Cucumber (Cucumis sativus L.), an important vegetable crop, is sensitive to NaCl. Its salinity tolerance can be improved by grafting onto pumpkin rootstocks, which restricts the uptake of Na+, but not of Cl−. Although Na+ seems to be more toxic than Cl− in cucumber, tissue tolerance to Na+ and Cl− is still unclear. In this study, a mixed-salt experiment, designed for equal osmolarity and equimolar concentrations of ions between treatments, was conducted using cucumber genotypes “Aramon” and “Line-759,” which are different in Na+ and Cl− exclusion. This combination of treatments generated various patterns of ion concentrations in leaves for deriving the response curves of photosynthesis and stomatal conductance to ion concentrations. In both cultivars, photosynthesis and stomatal conductance were sensitive to leaf Na+ concentration but insensitive to Cl− concentration. In these genotypes, tissue tolerance to Na+ varied independently of Na+ exclusion. Grafting “Aramon” onto pumpkin rootstock modified the Na+/Cl− ratio in leaves, reduced Na+ uptake, enhanced K+ transport towards the young leaves, and induced Cl− recirculation to the old leaves. These results suggest that (1) cucumber cannot restrict the Na+ accumulation in leaves but is able to avoid overaccumulation of Cl−, and (2) pumpkin rootstock regulates the recirculation of K+ and Cl−, but not Na+.

Osmotic adjustment and energy limitations to plant growth in saline soil

Plant roots must exclude almost all of the Na⁺ and Cl^- in saline soil while taking up water, otherwise these ions would build up to high concentrations in leaves. Plants evaporate c. 50 times more water than they retain, so 98% exclusion would result in shoot NaCl concentrations equal to that of the external medium. Taking up just 2% of the NaCl allows a plant to osmotically adjust the Na⁺ and Cl^- in vacuoles, while organic solutes provide the balancing osmotic pressure in the cytoplasm. We quantify the costs of this exclusion by roots, the regulation of Na⁺ and Cl^- transport through the plant, and the costs of osmotic adjustment with organic solutes in roots.

The salinity challenge

This article is a Commentary on https://doi.org/10.1111/nph.15955; https://doi.org/10.1111/nph.15713; https://doi.org/10.1111/nph.15773; https://doi.org/10.1111/nph.15864; https://doi.org/10.1111/nph.15862; https://doi.org/10.1111/nph.15852; https://doi.org/10.1111/nph.15758.

Energy costs of salt tolerance in crop plants

Agriculture is expanding into regions that are affected by salinity. This review considers the energetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion uptake are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H⁺ -ATPase also is a critical component. One proposed leak, that of Na⁺ influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na⁺ and Cl^- concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will inform experimentation and allow a quantitative assessment of the energy costs of NaCl tolerance to guide breeding and engineering of molecular components.