Temperature responses of photosynthesis and leaf hydraulic conductance in rice and wheat

A novel agrosinters support growth, photosynthetic efficiency and reduce trace metal elements accumulation in oilseed rape growing on metalliferous soil

Soil conditioners to fertilize, improve soil structure and support the phytostabilization of trace metal elements (TMEs) are being used more and more frequently. One of the options are agrosinters - slow-release ceramic fertilizers consisting mainly of SiO2, CaO, P2O5 and K2O, with an alkaline pH and high impact strength. The effect of two different agrosinters, A1 and A2, on the growth and physiological condition of Brassica napus grown in uncontaminated and Pb-, Cd- and Zn-contaminated soil was investigated in a pot experiment. In vivo data were collected using an infrared gas analyzer, a fluorimeter, a pigment content meter and a thermal camera. Elemental composition of the biomass was also investigated. The tested agrosinters promote biomass yield and have an effect on improving leaf chlorophyll content, phenomenological energy fluxes and plant gas exchange. The effect of the agrosinters on the plants was dose- and amendment-specific. An immobilization effect was observed not only in the soil but also in the plants. A reduction in the Cd (22%) and Zn (40%) content in the biomass was measured. All this was related to the effect of increasing the available form of P (50%), K (300%) and Ca (50%) in the soil, which improves soil fertility and reduces the bioavailable forms of the studied TMEs, due to the increase in soil pH and/or the complexation of these with phosphate compounds. The multidimensional analysis of A2 agrosinter shows the most positive effects on plant conditions, indicating that fly ash as a mixed substrate benefits the plants.

The poorly-explored stomatal response to temperature at constant evaporative demand

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapour gradient (Δw) increases with temperature if ambient vapour pressure is held constant, and stomata respond to changes in Δw. However, the direct response of stomata to temperature (DRST; the response when Δw is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meagre available data suggest the response is usually positive, results differ widely and defy broad generalisation. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesised biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.

Variation of mesophyll conductance mediated by nitrogen form is related to changes in cell wall property and chloroplast number

Plants primarily incorporate nitrate (NO3 -) and ammonium (NH4 +) as the primary source of inorganic nitrogen (N); the physiological mechanisms of photosynthesis (A) dropdown under NH4 + nutrition has been investigated in many studies. Leaf anatomy is a major determinant to mesophyll conductance (g m) and photosynthesis; however, it remains unclear whether the photosynthesis variations of plants exposed to different N forms is related to leaf anatomical variation. In this work, a common shrub, Lonicera japonica was hydroponically grown under NH4 +, NO3 - and 50% NH4 +/NO3 -. We found that leaf N significantly accumulated under NH4 +, whereas the photosynthesis was significantly decreased, which was mainly caused by a reduced g m. The reduced g m under NH4 + was related to the decreased intercellular air space, the reduced chloroplast number and especially the thicker cell walls. Among the cell wall components, lignin and hemicellulose contents under NH4 + nutrition were significantly higher than those in the other two N forms and were scaled negatively correlated with g m; while pectin content was independent from N forms. Pathway analysis further revealed that the cell wall components might indirectly regulate g m by influencing the thickness of the cell wall. These results highlight the importance of leaf anatomical variation characterized by modifications of chloroplasts number and cell wall thickness and compositions, in the regulation of photosynthesis in response to varied N sources.

Leaf diffusional capacity largely contributes to the reduced photosynthesis in rice plants under magnesium deficiency

Numerous studies have clarified the impacts of magnesium (Mg) on leaf photosynthesis from the perspectives of protein synthesis, enzymes activation and carbohydrate partitioning. However, it still remains largely unknown how stomatal and mesophyll conductances (gs and gm, respectively) are regulated by Mg. In the present study, leaf gas exchanges, leaf hydraulic parameters, leaf structural traits and cell wall composition were examined in rice plants grown under high and low Mg treatments to elucidate the impacts of Mg on gs and gm. Our results showed that reduction of leaf photosynthesis under Mg deficiency was mainly caused by the decreased gm, followed by reduced leaf biochemical capacity and gs, and leaf outside-xylem hydraulic conductance (Kox) was the major factor restricting gs under Mg deficiency. Moreover, increased leaf hemicellulose, lignin and pectin contents and decreased cell wall effective porosity were observed in low Mg plants relative to high Mg plants. These results suggest that Kox and cell wall composition play important roles in regulating gs and gm, respectively, in rice plants under Mg shortages.

An Innovative Approach to Alleviate Zinc Oxide Nanoparticle Stress on Wheat through Nanobubble Irrigation

The extensive utilization of zinc oxide nanoparticles in consumer products and the industry has led to their substantial entry into the soil through air and surface runoff transportation, which causes ecotoxicity in agro-ecosystems and detrimental effects on crop production. Nanobubbles (diameter size < 1 µm) have many advantages, such as a high surface area, rapid mass transfer, and long retention time. In this study, wheat seedlings were irrigated with a 500 mg L-1 zinc oxide nanoparticle solution delivered in the form of nanobubble watering (nanobubble-ZnO-NPs). We found that nanobubble watering improved the growth and nutrient status of wheat exposed to zinc oxide nanoparticles, as evidenced by increased total foliar nitrogen and phosphorus, along with enhanced leaf dry mass per area. This effect can be attributed to nanobubbles disassembling zinc oxide aggregates formed due to soil organic carbon, thereby mitigating nutrient absorption limitations in plants. Furthermore, nanobubbles improved the capability of soil oxygen input, leading to increased root activity and glycolysis efficiency in wheat roots. This work provides valuable insights into the influence of nanobubble watering on soil quality and crop production and offers an innovative approach for agricultural irrigation that enhances the effectiveness and efficiency of water application.

When drought meets heat - a plant omics perspective

Changes in weather patterns with emerging drought risks and rising global temperature are widespread and negatively affect crop growth and productivity. In nature, plants are simultaneously exposed to multiple biotic and abiotic stresses, but most studies focus on individual stress conditions. However, the simultaneous occurrence of different stresses impacts plant growth and development differently than a single stress. Plants sense the different stress combinations in the same or in different tissues, which could induce specific systemic signalling and acclimation responses; impacting different stress-responsive transcripts, protein abundance and modifications, and metabolites. This mini-review focuses on the combination of drought and heat, two abiotic stress conditions that often occur together. Recent omics studies indicate common or independent regulators involved in heat or drought stress responses. Here, we summarize the current research results, highlight gaps in our knowledge, and flag potential future focus areas.

Two Congeneric Shrubs from the Atacama Desert Show Different Physiological Strategies That Improve Water Use Efficiency under a Simulated Heat Wave

Desert shrubs are keystone species for plant diversity and ecosystem function. Atriplex clivicola and Atriplex deserticola (Amaranthaceae) are native shrubs from the Atacama Desert that show contrasting altitudinal distribution (A. clivicola: 0-700 m.a.s.l.; A. deserticola: 1500-3000 m.a.s.l.). Both species possess a C4 photosynthetic pathway and Kranz anatomy, traits adaptive to high temperatures. Historical records and projections for the near future show trends in increasing air temperature and frequency of heat wave events in these species' habitats. Besides sharing a C4 pathway, it is not clear how their leaf-level physiological traits associated with photosynthesis and water relations respond to heat stress. We studied their physiological traits (gas exchange, chlorophyll fluorescence, water status) before and after a simulated heat wave (HW). Both species enhanced their intrinsic water use efficiency after HW but via different mechanisms. A. clivicola, which has a higher LMA than A. deserticola, enhances water saving by closing stomata and maintaining RWC (%) and leaf Ψ_md potential at similar values to those measured before HW. After HW, A. deserticola showed an increase of A_max without concurrent changes in g_s and a significant reduction of RWC and Ψ_md. A. deserticola showed higher values of Chla fluorescence after HW. Thus, under heat stress, A. clivicola maximizes water saving, whilst A. deserticola enhances its photosynthetic performance. These contrasting (eco)physiological strategies are consistent with the adaptation of each species to their local environmental conditions at different altitudes.

Reduction of photosynthesis under P deficiency is mainly caused by the decreased CO2 diffusional capacities in wheat (Triticum aestivum L.)

Phosphorus is one of the most important essential mineral elements for plant growth and development. It has been widely recognized that phosphorus deficiency can lead to the significant declines in leaf photosynthetic rate and leaf area. However, the internal mechanism associated with the leaf anatomical traits has not been well understood. In present study, a hydroponic experiment was conducted to study the effect of phosphorus deficiency on leaf growth and photosynthesis in Jimai 22 (JM22, Triticum aestivum L.) and Suk Landarace 26 (SL26, Triticum aestivum L.). With the decrease in phosphorus concentration, leaf photosynthetic rate and leaf area in SL26 and JM22 all decreased significantly, but the decrease in leaf area occurred earlier than that in leaf photosynthetic rate. The thresholds of phosphorus concentration to maintain a high photosynthesis were 145.5 and 138.7 mg m^-2, respectively, in SL26 and JM22; and they were 197.5 and 212.0 mg m^-2, respectively, for leaf growth. The decrease in leaf photosynthetic rate under low P conditions was mainly caused by the lowered stomatal conductance and mesophyll conductance, and to a less extent by the decrease in biochemical capacities. The decrease in stomatal conductance was attributed to the smaller vascular bundle area, xylem conduits area and the lower leaf hydraulic conductance. However, the reduction in mesophyll conductance was not related to either the cell wall thickness or the development of chloroplast.

Sustainable Soil Management: Effects of Clinoptilolite and Organic Compost Soil Application on Eco-Physiology, Quercitin, and Hydroxylated, Methoxylated Anthocyanins on Vitis vinifera

Climate change and compostinS1g methods have an important junction on the phenological and ripening grapevine phases. Moreover, the optimization of these composting methods in closed-loop corporate chains can skillfully address the waste problem (pomace, stalks, and pruning residues) in viticultural areas. Owing to the ongoing global warming, in many wine-growing regions, there has been unbalanced ripening, with tricky harvests. Excessive temperatures in fact impoverish the anthocyanin amount of the must while the serious water deficits do not allow a correct development of the berry, stopping its growth processes. This experiment was created to improve the soil management and the quality of the grapes, through the application of a new land conditioner (Zeowine) to the soil, derived from the compost processes of industrial wine, waste, and zeolite. Three treatments on a Sangiovese vineyard were conducted: Zeowine (ZW) (30 tons per ha), Zeolite (Z) (10 tons per ha), and Compost (C) (20 tons per ha). During the two seasons (2021-2022), measurements were made of single-leaf gas exchange and leaf midday water potential, as well as chlorophyll fluorescence. In addition, the parameters of plant yield, yeast assimilable nitrogen, technological maturity, fractionation of anthocyanins (Cyanidin-3-glucoside, Delphinidin-3-glucoside, Malvidin-3-acetylglucoside, Malvidin-3-cumarylglucoside, Malvidin-3-glucoside, Peonidin-3-acetylglucoside, Peonidin-3-cumarylglucoside, Peonidin-3-glucoside, and Petunidin-3-glucoside), Caffeic Acid, Coumaric Acid, Gallic Acid, Ferulic Acid, Kaempferol-3-O-glucoside, Quercetin-3-O-rutinoside, Quercetin-3-O-glucoside, Quercetin-3-O-galactoside, and Quercetin-3-O-glucuronide were analyzed. The Zeowine and zeolite showed less negative water potential, higher photosynthesis, and lower leaf temperature. Furthermore, they showed higher levels of anthocyanin accumulation and a lower level of quercetin. Finally, the interaction of the beneficial results of Zeowine (soil and grapevines) was evidenced by the embellishment of the nutritional and water efficiency, the minimizing of the need for fertilizers, the closure of the production cycle of waste material from the supply chain, and the improvement of the quality of the wines.

Increasing temperature and vapour pressure deficit lead to hydraulic damages in the absence of soil drought

Temperature (T) and vapour pressure deficit (VPD) are important drivers of plant hydraulic conductivity, growth, mortality, and ecosystem productivity, independently of soil water availability. Our goal was to disentangle the effects of T and VPD on plant hydraulic responses. Young trees of Fagus sylvatica L., Quercus pubescens Willd. and Quercus ilex L. were exposed to a cross-combination of a T and VPD manipulation under unlimited soil water availability. Stem hydraulic conductivity and leaf-level hydraulic traits (e.g., gas exchange and osmotic adjustment) were tracked over a full growing season. Significant loss of xylem conductive area (PLA) was found in F. sylvatica and Q. pubescens due to rising VPD and T, but not in Q. ilex. Increasing T aggravated the effects of high VPD in F. sylvatica only. PLA was driven by maximum hydraulic conductivity and minimum leaf conductance, suggesting that high transpiration and water loss after stomatal closure contributed to plant hydraulic stress. This study shows for the first time that rising VPD and T lead to losses of stem conductivity even when soil water is not limiting, highlighting their rising importance in plant mortality mechanisms in the future.

Interspecific variation in the temperature response of mesophyll conductance is related to leaf anatomy

Although mesophyll conductance (g_m ) is known to be sensitive to temperature (T), the mechanisms underlying the temperature response of g_m are not fully understood. In particular, it has yet to be established whether interspecific variation in g_m -T relationships is associated with mesophyll anatomy and vein traits. In the present study, we measured the short-term response of g_m in eight crop species, and leaf water potential (Ψ_leaf ) in five crop species over a temperature range of 15-35°C. The considered structural parameters are surface areas of mesophyll cells and chloroplasts facing intercellular airspaces per unit leaf area (S_m and S_c ), cell wall thickness (T_cw ), and vein length per area (VLA). We detected large interspecific variations in the temperature responses of g_m and Ψ_leaf . The activation energy for g_m (E_a,gm ) was found to be positively correlated with S_c , although it showed no correlation with T_cw . In contrast, VLA was positively correlated with the slope of the linear model of Ψ_leaf -T (a), whereas E_a,gm was marginally correlated with VLA and a. A two-component model was subsequently used to model g_m -T relationships, and the mechanisms underlying the temperature response of g_m are discussed. The data presented here indicate that leaf anatomy is a major determinant of the interspecific variation in g_m -T relationships.

Evaporative flux method of leaf hydraulic conductance estimation: sources of uncertainty and reporting format recommendation

BACKGROUND

The accurate estimation of leaf hydraulic conductance (K_leaf) is important for revealing leaf physiological characteristics and function. However, the K_leaf values are largely incomparable in previous studies for a given species indicating some uncertain influencing factors in K_leaf measurement.

RESULT

We investigated the potential impacts of plant sampling method, measurement setup, environmental factors, and transpiration steady state identification on K_leaf estimation in Oryza sativa and Cinnamomum camphora using evaporation flux method (EFM). The effects of sampling and rehydration time, the small gravity pressure gradients between water sources and leaves, and water degassing on K_leaf estimation were negligible. As expected, the estimated steady flow rate (E) was significantly affected by multiple environmental factors including airflow around leaf, photosynthetically active radiation (PARa) on leaf surfaces and air temperature. K_leaf decreased by 40% when PARa declined from 1000 to 500 µmol m^-2 s^-1 and decreased by 15.1% when air temperature increased from 27 to 37 °C. In addition, accurate steady-state flow rate identification and leaf water potential measurement were important for K_leaf estimation.

CONCLUSIONS

Based on the analysis of influencing factors, we provided a format for reporting the metadata of EFM-based K_leaf to achieve greater comparability among studies and interpretation of differences.

Leaf Photosynthesis and Its Temperature Response Are Different between Growth Stages and N Supplies in Rice Plants

Leaf photosynthesis is highly correlated with CO₂-diffusion capacities, which are determined by both leaf anatomical traits and environmental stimuli. In the present study, leaf photosynthetic rate (A), stomatal conductance (g_s), mesophyll conductance (g_m) and the related leaf anatomical traits were studied on rice plants at two growth stages and with two different N supplies, and the response of photosynthesis to temperature (T) was also studied. We found that g_m was significantly higher at mid-tillering stage and at high N treatment. The larger g_m was related to a larger chloroplast surface area facing intercellular air spaces and a thinner cell wall in comparison with booting stage and zero N treatment. At mid-tillering stage and at high N treatment, g_m showed a stronger temperature response. The modelling of the g_m-T relationships suggested that, in comparison with booting stage and zero N treatment, the stronger temperature response of g_m was related to the higher activation energy of the membrane at mid-tillering stage and at high N treatment. The findings in the present study can enhance our knowledge on the physiological and environmental determinants of photosynthesis.

Comparisons of photosynthetic and anatomical traits between wild and domesticated cotton

Mesophyll conductance (gm) is a crucial leaf trait contributing to the photosynthetic rate (AN). Plant domestication typically leads to an enhancement of AN that is often associated with profound anatomical modifications, but it is unclear which of these structural alterations influence gm. We analyzed the implication of domestication on leaf anatomy and its effect on gm in 26 wild and 31 domesticated cotton genotypes (Gossypium sp.) grown under field conditions. We found that domesticated genotypes had higher AN but similar gm to wild genotypes. Consistent with this, domestication did not translate into significant differences in the fraction of mesophyll occupied by intercellular air spaces (fias) or mesophyll and chloroplast surface area exposed to intercellular air space (Sm/S and Sc/S, respectively). However, leaves of domesticated genotypes were significantly thicker, with larger but fewer mesophyll cells with thinner cell walls. Moreover, domesticated genotypes had higher cell wall conductance (gcw) but smaller cytoplasmic conductance (gcyt) than wild genotypes. It appears that domestication in cotton has not generally led to significant improvement in gm, in part because their thinner mesophyll cell walls (increasing gcw) compensate for their lower gcyt, itself due to larger distance between plasmalemma and chloroplast envelopes.

Limiting factors for panicle photosynthesis at the anthesis and grain filling stages in rice (Oryza sativa L.)

Panicle photosynthesis is crucial for grain yield in cereal crops; however, the limiting factors for panicle photosynthesis are poorly understood, greatly impeding improvement in this trait. In the present study, pot experiments were conducted to investigate the limiting factors for panicle photosynthesis at the anthesis stage in seven rice genotypes and to examine the temporal variations in photosynthesis during the grain filling stage in the Liangyou 287 genotype. At the anthesis stage, leaf and panicle photosynthesis was positively correlated with stomatal conductance and maximum carboxylation rate, which were in turn associated with hydraulic conductance and nitrogen content, respectively. Panicle hydraulic conductance was positively correlated with the area of bundle sheaths in the panicle neck. During grain filling, leaf and panicle photosynthesis remained constant at the early stage but dramatically decreased from 8 to 9 days after anthesis. The trends of variations in panicle photosynthesis were consistent with those in stomatal conductance but not with those in maximum carboxylation rate. At first, the maximum carboxylation rate and respiration rate in the panicle increased, through elevated panicle nitrogen content, but then drastically decreased, as a result of dehydration. The present study systematically investigated the limiting factors for panicle photosynthesis, which are vital for improving photosynthesis and crop yield.

Transpiration increases under high-temperature stress: Potential mechanisms, trade-offs and prospects for crop resilience in a warming world

The frequency and intensity of high-temperature stress events are expected to increase as climate change intensifies. Concomitantly, an increase in evaporative demand, driven in part by global warming, is also taking place worldwide. Despite this, studies examining high-temperature stress impacts on plant productivity seldom consider this interaction to identify traits enhancing yield resilience towards climate change. Further, new evidence documents substantial increases in plant transpiration rate in response to high-temperature stress even under arid environments, which raise a trade-off between the need for latent cooling dictated by excessive temperatures and the need for water conservation dictated by increasing evaporative demand. However, the mechanisms behind those responses, and the potential to design the next generation of crops successfully navigating this trade-off, remain poorly investigated. Here, we review potential mechanisms underlying reported increases in transpiration rate under high-temperature stress, within the broader context of their impact on water conservation needed for crop drought tolerance. We outline three main contributors to this phenomenon, namely stomatal, cuticular and water viscosity-based mechanisms, and we outline research directions aiming at designing new varieties optimized for specific temperature and evaporative demand regimes to enhance crop productivity under a warmer and dryer climate.

Differences in leaf anatomy determines temperature response of leaf hydraulic and mesophyll CO2 conductance in phylogenetically related C4 and C3 grass species

Leaf hydraulic and mesophyll CO₂ conductance are both influenced by leaf anatomical traits, however it is poorly understood how the temperature response of these conductances differs between C₄ and C₃ species with distinct leaf anatomy. This study investigated the temperature response of leaf hydraulic conductance (K_leaf ), stomatal (g_s ) and mesophyll (g_m ) conductance to CO₂ , and leaf anatomical traits in phylogenetically related Panicum antidotale (C₄ ) and P. bisulcatum (C₃ ) grasses. The C₄ species had lower hydraulic conductance outside xylem (K_ox ) and K_leaf compared with the C₃ species. However, the C₄ species had higher g_m compared with the C₃ species. Traits associated with leaf water movement, K_leaf and K_ox , increased with temperature more in the C₃ than in the C₄ species, whereas traits related to carbon uptake, A_net and g_m , increased more with temperature in the C₄ than the C₃ species. Our findings demonstrate that, in addition to a CO₂ concentrating mechanism, outside-xylem leaf anatomy in the C₄ species P. antidotale favours lower water movement through the leaf and stomata that provides an additional advantage for greater leaf carbon uptake relative to water loss with increasing leaf temperature than in the C₃ species P. bisulcatum.

Mesophyll conductance: walls, membranes and spatial complexity

A significant resistance to CO₂ diffusion is imposed by mesophyll tissue inside leaves. Mesophyll resistance, r_m (or its reciprocal, mesophyll conductance, g_m ), reduces the rate at which Rubisco can fix CO₂ , increasing the water and nitrogen costs of carbon acquisition. g_m varies in proportion to the surface area of chloroplasts exposed to intercellular airspace per unit leaf area. It also depends on the thickness and effective porosity of the cell wall and the CO₂ permeabilities of membranes. As no measurements exist for the effective porosity of mesophyll cell walls, and CO₂ permeability values are too low to account for observed rates of CO₂ assimilation, conclusions from modelling must be treated with caution. There is great variation in the mesophyll resistance per unit chloroplast area for a given cell wall thickness, which may reflect differences in effective porosity. While apparent g_m can vary with CO₂ and irradiance, the underlying conductance at the cellular level may remain unchanged. Dynamic changes in apparent g_m arise for spatial reasons and because chloroplasts differ in their photosynthetic composition and operate in different light environments. Measurements of the temperature sensitivity of membrane CO₂ permeability are urgently needed to explain variation in temperature responses of g_m .

Heat-induced changes in the abundance of wheat Rubisco activase isoforms

The Triticum aestivum (wheat) genome encodes three isoforms of Rubisco activase (Rca) differing in thermostability, which could be exploited to improve the resilience of this crop to global warming. We hypothesized that elevated temperatures would cause an increase in the relative abundance of heat-stable Rca1β. Wheat plants were grown at 25° C : 18°C (day : night) and exposed to heat stress (38° C : 22°C) for up to 5 d at pre-anthesis. Carbon (C) assimilation, Rubisco activity, CA1Pase activity, transcripts of Rca1β, Rca2β, and Rca2α, and the quantities of the corresponding protein products were measured during and after heat stress. The transcript of Rca1β increased 40-fold in 4 h at elevated temperatures and returned to the original level after 4 h upon return of plants to control temperatures. Rca1β comprised up to 2% of the total Rca protein in unstressed leaves but increased three-fold in leaves exposed to elevated temperatures for 5 d and remained high at 4 h after heat stress. These results show that elevated temperatures cause rapid changes in Rca gene expression and adaptive changes in Rca isoform abundance. The improved understanding of the regulation of C assimilation under heat stress will inform efforts to improve wheat productivity and climate resilience.

Mechanisms of the Morphological Plasticity Induced by Phytohormones and the Environment in Plants

Plants adapt to environmental changes by regulating their development and growth. As an important interface between plants and their environment, leaf morphogenesis varies between species, populations, or even shows plasticity within individuals. Leaf growth is dependent on many environmental factors, such as light, temperature, and submergence. Phytohormones play key functions in leaf development and can act as molecular regulatory elements in response to environmental signals. In this review, we discuss the current knowledge on the effects of different environmental factors and phytohormone pathways on morphological plasticity and intend to summarize the advances in leaf development. In addition, we detail the molecular mechanisms of heterophylly, the representative of leaf plasticity, providing novel insights into phytohormones and the environmental adaptation in plants.