Non-stomatal Limitations Research Articles

Dynamic changes in membrane lipid composition of leaves of winter wheat seedlings in response to PEG-induced water stress

BackgroundMembrane lipid composition associates closely with membrane stability and fluidity under water stress. In this study, lipidomic analyses based on electrospray ionization mass spectrometry (ESI-MS/MS) were carried out to explore dynamic changes of membrane lipids in term of molecular species caused by PEG (Polyethylene glycol-6000)-induced water stress in wheat seedlings.ResultsAmong the main phospholipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG) are primary degradation targets, and PC was degraded in the largest degree. Membrane ion leakage dramatically increased later than the significant reduction of these phospholipids, indicating that the loss of membrane integrity lagged behind severe phospholipid degradation. Monogalactosyldiacylglycerol (MGDG) increased firstly and decreased later, while digalactosyldiacylglycerol (DGDG) ratcheted up with stress. DGDG/MGDG increased after stress for 3 days, and unsaturation of DGDG was promoted with stress. Variation trends of galactolipids differed among molecular species. The time when MGDG (34:3), DGDG (34:3) began to decline approached to the time when non-stomatal limitation impaired photosynthesis. While the two predominant molecular species MGDG (36:6) and DGDG (36:6) began to decline later. So we speculated that MGDG (34:3), DGDG (34:3) might be key components in photosynthesis apparatus and participate in photosynthesis directly. While the two predominant molecular species, MGDG (36:6) and DGDG (36:6) might locate in thylakoid lipid bilayer matrix and play roles in stabilizing the membrane. The research provides new insights into the dynamic response of lipid metabolism to PEG-induced water stress.ConclusionIn wheat plants under water stress, the major molecular species of PC, PE and PG were degraded, MGDG and DGDG molecular species had differing degradation time courses.

Leaf carbon and water status control stomatal and nonstomatal limitations of photosynthesis in trees.

Photosynthetic rate is concurrently limited by stomatal limitations and nonstomatal limitations (NSLs). However, the controls on NSLs to photosynthesis and their coordination with stomatal control on different timescales remain poorly understood. According to a recent optimization hypothesis, NSLs depend on leaf osmotic or water status and are coordinated with stomatal control so as to maximize leaf photosynthesis. Drought and notching experiments were conducted on Pinus sylvestris, Picea abies, Betula Pendula and Populus tremula seedlings in glasshouse conditions to study the dependence of NSLs on leaf osmotic and water status, and their coordination with stomatal control, on timescales of minutes and weeks, to test the assumptions and predictions of the optimization hypothesis. Both NSLs and stomatal conductance followed power-law functions of leaf osmotic concentration and leaf water potential. Moreover, stomatal conductance was proportional to the square root of soil-to-leaf hydraulic conductance, as predicted by the optimization hypothesis. Though the detailed mechanisms underlying the dependence of NSLs on leaf osmotic or water status lie outside the scope of this study, our results support the hypothesis that NSLs and stomatal control are coordinated to maximize leaf photosynthesis and allow the effect of NSLs to be included in models of tree gas-exchange.

Responses of leaf gas exchange rate to acute soil drying in Jatropha curcas L

ABSTRACT Development of a water-efficient irrigation system is essential in order to cultivate Jatropha curcas, a bioenergy crop, in semiarid regions. In the present study, to determine the useful parameter for the efficient irrigation system, jatropha plants were grown in pots and root boxes under greenhouse conditions in Okinawa, Japan. At 6 to 9 months after transplanting, we measured the gas exchange rate during ongoing soil water deficits and attempted to identify the threshold of soil matric potential (pF) with changes of both intrinsic water use efficiency ( A/gs ) as a stomatal-based index and carboxylation efficiency ( A/Ci ) as an index of non-stomatal limitation. When irrigation was stopped, A and gs began to decline, with each parameter reaching zero at pF 4.0. However, gs was more sensitive to soil water deficits than A . After re-watering, stomatal openings and carbon uptake recovered to the value observed before water stress treatment. At pF 3.5 and higher values (representing more severe water stress), A/gs intended to increase and A/Ci sharply decreased. These results suggested that approximately pF 3.5 in the soil was the threshold of non-stomatal limitations to photosynthetic rate and should be useful to the re-watering point for the water-efficient irrigation system in jatropha. Abbreviations: A CO2 assimilation rate; Ci : intercellular CO2 concentration; E: transpiration rate; gs : stomatal conductance; pF: soil matrix potential; PFD: photon flux density; TDR: time domain reflectometry; VPD: leaf-to-air vapor pressure difference; VWC: volumetric water content.

Genotypic variations in leaf and whole-plant water use efficiencies are closely related in bread wheat genotypes under well-watered and water-limited conditions during grain filling

Wheat plants growing under Mediterranean rain-fed conditions are exposed to water deficit, particularly during the grain filling period, and this can lead to a strong reduction in grain yield (GY). This study examines the effects of water deficit after during the grain filling period on photosynthetic and water-use efficiencies at the leaf and whole-plant level for 14 bread wheat genotypes grown in pots under glasshouse conditions. Two glasshouse experiments were conducted, one in a conventional glasshouse at the Universidad de Talca, Chile (Experiment 1), and another at the National Plant Phenomics Centre (NPPC), Aberystwyth, UK (Experiment 2), in 2015. Plants were grown under well-watered (WW) and water-limited (WL) conditions during grain filling. The reductions in leaf water potential (Ψ), net CO2 assimilation (An) and stomatal conductance (gs) due to water deficit were 79, 35 and 55%, respectively, during grain filling but no significant differences were found among genotypes. However, chlorophyll fluorescence parameters (as determined on dark-adapted and illuminated leaves) and chlorophyll content (Chl) were significantly different among genotypes, but not between water conditions. Under both water conditions, An presented a positive and linear relationship with the effective photochemical quantum yield of Photosystem II (Y(II)) and the maximum rate of electron transport (ETRmax), and negative with the quantum yield of non-photochemical energy conversion in Photosystem II (Y(NPQ)). The relationship between An and Chl was positive and linear for both water conditions, but under WL conditions An tended to be lower at any Chl value. Both, instantaneous (An/E) and intrinsic (An/gs) water-use efficiencies at the leaf level exhibited a positive and linear relationship with plant water-use efficiency (WUEp = plant dry weight/water use). Carbon discrimination (Δ13C) in kernels presented a negative relationship with WUEp, at both WW and WL conditions, and a positive relationship with GY. Our results indicate that during grain filling wheat plants face limitations to the assimilation process due to natural senesce and water stress. The reduction in An and gs after anthesis in both water conditions was mainly due a decline in the chlorophyll content (non-stomatal limitation), whereas the observed differences between water conditions were mainly due to a stomatal limitation.

Energy conversion processes and related gene expression in a sunflower mutant with altered salicylic acid metabolism

Salicylic acid (SA) is involved in several responses associated with plant development and defence against biotic and abiotic stress, but its role on photosynthetic regulation is still under debate. This work investigated energy conversion processes and related gene expression in the brachytic mutant of sunflower lingering hope (linho). This mutant was characterized by a higher ratio between the free SA form and its conjugate form SA O-β-D-glucoside (SAG) compared to wild type (WT), without significant changes in the endogenous level of abscisic acid and hydrogen peroxide. The mutant showed an inhibition of photosynthesis due to a combination of both stomatal and non-stomatal limitations, although the latter seemed to play a major role. The reduced carboxylation efficiency was associated with a down-regulation of the gene expression for both the large and small subunits of Rubisco and the Rubisco activase enzyme. Moreover, linho showed an alteration of photosystem II (PSII) functionality, with reduced PSII photochemistry, increased PSII excitation pressure and decreased thermal energy dissipation of excessive light energy. These responses were associated with a lower photosynthetic pigments concentration and a reduced expression of genes encoding for light-harvesting chlorophyll a/b binding proteins (i.e. HaLhcA), chlorophyll binding subunits of PSII proteins (i.e. HaPsbS and HaPsbX), phytoene synthase enzyme and a different expression level for genes related to PSII repair cycle, such as HaPsbA and HaPsbD. The concomitant stimulation of respiratory metabolism, suggests that linho activated a coordinate modulation of chloroplast and mitochondria activities to compensate the energy imbalance and regulate energy conversion processes.

Effects of Drought Stress on Photosynthetic and Physiological Parameters of Tomato

Drought has become an important factor limiting crop yields in China. As an important greenhouse horticultural crop in China, the research of tomato (Solanum lycopersicum L. cv. Jinpeng No.10) is of great theoretical and practical significance. In the study, four different relative soil moisture contents (74% to 80%, 55% to 61%, 47% to 52%, and 25% to 30%) were used to induce drought stress. We investigated changes in photosynthetic gas exchange, chlorophyll fluorescence, and other related physiological parameters in response to different relative soil moisture contents. Drought inhibited the photosynthesis of tomato significantly, as shown by a clear decline in the net photosynthetic rate. Our results indicated stomatal limitation and nonstomatal limitation were responsible for the photosynthesis reduction.

Allelopathic effects of Cinnamomum septentrionale leaf litter on Eucalyptus grandis saplings

Allelopathy plays an important role in plant-plant interactions, particularly through compounds released from litter exudates and decomposition. We conducted a greenhouse experiment to examine how additions of Cinnamomum septentrionale Hand. Mazz leaf litter (A) versus leaf litter aqueous extracts (B) can impact Eucalyptus grandis Hill ex Maid saplings, focusing on growth, physiology and photosynthesis. We show that both A and B significantly inhibited the growth and photosynthesis of E. grandis saplings, and this inhibition strengthened with increasing soil A content (range from 0 to 120 g pot−1) and concentrations of B (range from 0 to 80 g L−1). Additions of leaf litter or its aqueous extracts decreased chlorophyll content and intercellular CO2 concentration (Ci) while stomatal conductance (Gs) increased, reflecting that non-stomatal limitation might be the reason for the reduction of the photosynthetic rate. After treatment with A, the peroxidase (POD) and superoxide dismutase (SOD) activity were reduced, while there was a reduction in POD and non-changed SOD activity after treatment with B. Furthermore, sugar and proline levels declined under both A and B treatments. This study demonstrates that both A and B of C. septentrionale influenced the growth, chlorophyll synthesis and photosynthesis of E. grandis saplings, and caused oxidative damage in E. grandis. The Synthesis Effect (SE) indicates that B has stronger allelopathic effect than A under the same treatment. This stronger allelopathic effect of C. septentrionale leaf litter aqueous extracts than its decomposing leaf litter can be reflected by greater damage to membrane systems, and greater reductions of both chlorophyll content and photosynthesis on treated plants.

Do plants respond and recover from a combination of drought and heatwave in the same manner under adequate and deprived soil nutrient conditions?

Extreme climatic conditions with extended drought periods and heatwaves are predicted to increase in frequency and severity in many regions of the world. Aside from this, other abiotic stress factors such as nutrient deficiency could pose a serious problem to plants when combined with other stressors resulting in more complex underpinning mechanisms. In the present study, we evaluated the response of Brassica napus to single and combined impacts of drought and heatwave (HW) under adequate or deprived (N-A and N-D) soil nutrient conditions. In addition, to get better insights in the plant response to combined stress, a post-stress period, pointing out a degree of the recovery after the cessation of stress, was also included. The results showed a different manner of single drought and heatwave action. The adverse effect of drought on leaf gas exchange was lagged on the growth and became more apparent only after recovery period with no obvious difference between different nutrient levels. Contrary, the growth response of nutrient-deprived plants to single HW was weak and in most cases, insignificant. Heatwave applied simultaneously with drought highly exacerbated the adverse effect of drought both under N-A and N-D conditions. Combined drought and heatwave stress resulted in the sharper decline of Asat and it was attributed to both stomatal and non-stomatal limitations. Interestingly, plants underwent combined drought and HW treatment under N-D conditions showed better aboveground growth recovery, compared to those grown under N-A conditions, while displayed far more diminished photochemistry of photosystem II and badly disturbed the C/N balance. This discrepancy came from the fact that soil nutrient deficiency, by itself, evoked strong stress under control climate conditions resulting in a dramatically slower aboveground growth of nutrient-deprived plant. In turn, although combined drought and HW stress had similar effect on the aboveground growth either under N-A or N-D conditions, the recovery of later one was better. These results highlight the necessity to look at plants’ performance under unfavorable environmental conditions beyond the actual event, since it can be depended not only on the duration of exposure but also on the legacy effect after treatment.

Incorporating non-stomatal limitation improves the performance of leaf and canopy models at high vapour pressure deficit.

Vapour pressure deficit (D) is projected to increase in the future as temperature rises. In response to increased D, stomatal conductance (gs) and photosynthesis (A) are reduced, which may result in significant reductions in terrestrial carbon, water and energy fluxes. It is thus important for gas exchange models to capture the observed responses of gs and A with increasing D. We tested a series of coupled A-gs models against leaf gas exchange measurements from the Cumberland Plain Woodland (Australia), where D regularly exceeds 2kPa and can reach 8kPa in summer. Two commonly used A-gs models were not able to capture the observed decrease in A and gs with increasing D at the leaf scale. To explain this decrease in A and gs, two alternative hypotheses were tested: hydraulic limitation (i.e., plants reduce gs and/or A due to insufficient water supply) and non-stomatal limitation (i.e., downregulation of photosynthetic capacity). We found that the model that incorporated a non-stomatal limitation captured the observations with high fidelity and required the fewest number of parameters. Whilst the model incorporating hydraulic limitation captured the observed A and gs, it did so via a physical mechanism that is incorrect. We then incorporated a non-stomatal limitation into the stand model, MAESPA, to examine its impact on canopy transpiration and gross primary production. Accounting for a non-stomatal limitation reduced the predicted transpiration by ~19%, improving the correspondence with sap flow measurements, and gross primary production by ~14%. Given the projected global increases in D associated with future warming, these findings suggest that models may need to incorporate non-stomatal limitation to accurately simulate A and gs in the future with high D. Further data on non-stomatal limitation at high D should be a priority, in order to determine the generality of our results and develop a widely applicable model.

Physiological and biochemical responses and fruit production of noni (Morinda citrifolia L.) plants irrigated with brackish water

Searching for plants that are more tolerant to salinity for commercial purposes is necessary for cultivation in soils that are saline and/or irrigated with brackish water. Thus, this study aimed to evaluate the effects of irrigation water salinity on soil salinization, and some physiological and biochemical parameters in noni plants, aiming at a better understanding of the mechanisms of salt tolerance in this species. The experiment was conducted in a completely randomized design with 5 replicates. Noni plants (Morinda citrifolia L.) were grown in pots containing 20 kg of soil and irrigated using different levels of electrical conductivity of the water (ECw): 0.3; 2; 4; 6 or 8 dS m−1, adopting a leaching fraction of 0.10. Electrical conductivities of soil saturation extract and drainage water, plant growth, production, gas exchange, relative water content, absolute integrity percentage of cell membrane, chlorophyll a fluorescence, pigments, and accumulation of solutes were evaluated in two different periods (60 and 210 days after treatment (DAT)). The leaching fraction used (0.10) was not enough to prevent soil salinization with increase of ECw. The increase of ECw reduces CO2 assimilation due to stomatal (60 and 210 DAT) and non-stomatal (210 DAT) limitations compromising the growth of noni plants. The increase of Na+ and Cl− contents induced by the increase of ECw causes disturbances in the ion homeostasis and reduces the membrane integrity. However, together with the accumulation of solutes contribute to the maintenance of the water status of noni plants under saline conditions.

The long-term response of photosynthesis in walnut (Juglans regia L.) leaf to a leaf-to-fruit ratio

For clarifying the relationship between a leaf-to-fruit ratio (LFR) and photosynthesis, LFR manipulation was performed with Juglans regia cv. Xinxin2 in order to test the photosynthesis response to LFR in source leaves. Results showed that LFR with one and two leaves was positively correlated with net photosynthetic rate (PN), chlorophyll content, and specific leaf mass, implying extremely low LFR inhibited the leaf development. However, LFR with five leaves was negatively correlated with PN, positively correlated with starch, but not related to intercellular CO2 concentration, indicating the high LFR caused the nonstomatal limitation and feedback inhibition of photosynthetic production. No significant differences in PN between LFRs (with three and four leaves) probably indicated a balanced state of coordinated supply and demand between the source leaf and sink fruit. The above results indicated that the response of photosynthesis in the source leaves to LFR depends on the variation range of LFR.

Melatonin confers drought stress tolerance in soybean (Glycine max L.) by modulating photosynthesis, osmolytes, and reactive oxygen metabolism

In order to investigate the role of melatonin in the drought tolerance, we examined pigments, gas exchange, osmolytes, and reactive oxygen radical metabolism in soybean plants. Drought declined photosynthetic pigments and caused irreversible reduction in net photosynthesis, which was followed by stomatal limitation for 5 and 10 d and nonstomatal limitation for 15 d. Soluble sugar, soluble proteins, and proline concentrations were higher in drought-stressed seedlings compared with the control. The contents of superoxide anion, hydrogen peroxide, and malondialdehyde increased during drought stress indicating oxidative stress. Drought stress also increased superoxide dismutase, peroxidase, and catalase activities. Melatonin treatment improved the tolerance of drought-treated plants, which was possibly due to the enhanced content of osmolytes and higher antioxidant enzyme activities that retard dehydration and lipid peroxidation.

Effect of Water Stress on Photosynthesis, Chlorophyll Fluorescence Parameters and Water Use Efficiency of Common Reed in the Hexi Corridor

Water stress is the major environmental stress that affect agricultural production worldwide, especially in arid and semi-arid regions. This research investigated the effect of water stress on common reed (Phragmites australis (Cav.) Trin. ex Steud.) grown in the West Lake Wetland of Zhangye City, China. We designed with four water treatments (100, 95, 85 and 75% of water field capacity, i.e. CK (100%), mild (95%), moderate (85%) and severe (75%) water deficit regimes). The effect of water stress on photosynthesis and chlorophyll fluorescence parameters was investigated. There was a midday depression in net photosynthetic rate (Pn) for CK, mild and moderate drought, but not for severe drought. Stomatal limitation was dominant for mild treatment. But under severe drought stomatal limitation to photosynthesis was dominating in the morning and nonstomatal limitation was dominating in the afternoon. Compared with CK treatment, Pn, stomatal conductance (gs), intercellular CO2 concentration (Ci), transpiration rate (E) and water use efficiency (WUE) were always lower at drought stress. This result suggests that water stress caused the photosynthesis of common reed to be completely blocked and the common reed was a drought-sensitive plant. We found 75% is the moisture threshold for common reed. The decrease in Fm, Fv, Fv/Fm and increase F0 suggested that Rubisco activity reduced and PSII partially inactivated during the day under drought. However, part of this inactivation of PSII might be alleviated under mild or moderate drought, but severe drought cannot. Drought stress also affected the photosynthesis Pn-PAR response curve. Drought stress increased LCP, RD and decreased LSP, Pmax and AQY. This indicates that when common reed suffers from drought, the utilization range of light intensity become narrowed and photosynthetic ability or adaptability to light reduced.

Detection of Cadmium Risk to the Photosynthetic Performance of Hybrid Pennisetum.

Photosynthesis plays an essential role in plant growth and crop yield, and the mechanisms of the effects of cadmium (Cd) on photosynthetic performance require more attention. The acute toxicity of Cd in soil to the photosynthetic capacity of Hybrid Pennisetum was evaluated using gas exchange parameters, A/Ci curves, light response curves, and chlorophyll a fluorescence transients after exposure to elevated Cd concentrations (0, 10, 20, 50, 70, and 100 mg kg−1) for a 3-month period. The results indicated that leaf Cd concentration in Hybrid Pennisetum increased with the strength of soil Cd stress and ranged from 4.9 to 15.8 μg g−1 DW. The accumulation of leaf Cd severely restricted photosynthesis and its non-stomatal limitation in regulating the photosynthetic performance of Hybrid Pennisetum. The leaf chloroplasts at 10 and 20 mg kg−1 Cd concentrations showed no noticeable change, but the chlorophyll content significantly decreased by 9.0–20.4% at 50–100 mg kg−1 Cd concentrations. The Cd treatments also decreased plant ribulose-1,5-bisphosphate (RuBP) activity (Vcmax) and regeneration capacity (Jmax), triose phosphate utilization (TPU), light-saturated photosynthesis (Amax), apparent quantum yield (AQY), light saturation point (LSP), and dark respiration (Rday), but Cd treatment increased the light compensation point (LCP). The shape of chlorophyll a fluorescence transients in leaves was altered under different Cd treatments. The increased OJ phase and the decreased IP phase in fluorescence induction curves suggested that Cd toxicity inhibited both light use efficiency and photodamage avoidance ability. These results suggested that the decrease in photosynthesis through exposure to Cd may be a result of the decrease in leaf chlorophyll content, Rubisco activity, and RuBP regeneration, inhibition of triose phosphate utilization, reduction of the ability to use light and provide energy, and restrictions on electron transport in PSII.

Stomatal and non-stomatal limitations at different leaf temperatures to the photosynthetic process during the post-harvest period for Vitis vinifera cv. Chardonnay vines.

ABSTRACTPhotosynthesis in Vitis vinifera cv. Chardonnay leaves was measured during the post-harvest period in relation to leaf temperatures and chloroplast CO2 concentrations to evaluate the limitations to photosynthesis once the fruit sink was removed. Photosynthesis was optimal at 30°C and rates were higher at high compared to low temperatures, consistent with the current climate. Stomatal limitation of photosynthesis ranged from 20% at low temperatures to 45% at high temperatures. RuBP carboxylation (Vcmax) and regeneration (Jmax) were optimal at 39 and 36°C, but their capacities were low below 25°C accounting for low assimilation rates. Chloroplast CO2 concentration transitioning between RuBP regeneration and RuBP carboxylation-limited assimilation, increased steeply with increasing temperature. Assimilation of Chardonnay leaves was limited by RuBP regeneration below 35°C and by RuBP carboxylation above 35°C, consistent with increased photorespiration at high temperatures. Results provide a quantitative explanation for the photosynthetic performance of Chardonnay vines in the post-harvest period.

Photosynthetic characteristics and chlorophyll a fluorescence transient in Lonicera japonica under drought stress

Chlorophyll a fluorescence transient can reveal the degree of thermal damage to plant photosynthetic organs and provide much physiological information about PSII fleetly. However, it is not clear how it reveals photosynthetic mechanisms of plants under decreasing soil moisture. In this paper, the photosynthesis characteristics and photosystem II (PSII) activities in leaves of Lonicera japonica under 11-level soil moisture were explored by gas-exchange analysis and JIP test. Both parameters of net photosynthetic rate (Pn) and water use efficiency (WUE) exhibited an increasing–decreasing trend with the decreasing relative soil water content (RSWC). At 29.7% ≤ RSWC≤ 79.6%, the main reason for Pn decrease was stomatal limitation; at 36.4% ≤ RSWC≤ 55.1%, both Pn and WUE were higher than the average. When RSWC was below 29.7%, Pn, Tr, and WUE significantly decreased. Meanwhile, a JIP-test analysis revealed a distinct K peak with decreases in the quantum yield and energy distribution ratio (ψo or φEo) and increases in the K phase (Wk) and J phase (Vj) at RSWC≤ 29.7%, indicating that severe drought (RSWC≤ 29.7%) damaged the leaf oxygen-evolving complex, caused the accumulation of Q A − in PSII electron acceptors, and, therefore, hindered the progression of the PSII electron transport chain. In addition, at RSWC≤ 29.7%, the absorption flux per CS (ABS/CSo) increased, while the phenomenological energy fluxes for TRo/CSo, ETo/CSo, RC/CSo decreased, indicating that the PSII reaction center was damaged and excessive light energy was accumulated. Therefore, the main reason for the decrease of photosynthetic efficiency was non-stomatal limitation at RSWC≤ 29.7%, and the maximum moisture deficit for the growth of Lonicera japonica is RSWC= 29.7%.

The physiological and biochemical photosynthetic properties of Lycium ruthenicum Murr in response to salinity and drought

To estimate the salt tolerance and drought tolerance of Lycium ruthenicum Murr, we designed a pot experiment to measure the physiological and biochemical photosynthetic mechanisms of this species by establishing a range of NaCl concentrations (0, 50, 100, 200 and 400 mM) and degrees of drought stress (30%, 60%, 75% and 100% of field capacity). The results showed that the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), chlorophyll (Chl), carotenoids (Cars), maximal fluorescence (Fm), optimal quantum yield of photosystem II (PSII) (Fv/Fm), performance index (PIABS), activity of the water-splitting complex (Fv/Fo), actual photochemical efficiency of PSII (ΦPSII) and photochemical quenching (qP) were stimulated by moderate salinity (0–100 mM) and low water deficit (75–100%) and inhibited by high salinity (200–400 mM) and severe water deficit (30–60%). The changes in minimal fluorescence (Fo) and nonphotochemical quenching (qN) were opposite. Furthermore, the contents of malondialdehyde (MDA), proline, superoxide dismutase (SOD), and peroxidase (POD) significantly increased with increasing stress levels. In summary, salt and drought stress can separately enhance the photosynthetic capability to some extent because of increases in Gs and PSII activity. With an increase in stress, nonstomatal limitations, inhibition of electron transport and the activity of PSII can lead to reductions in photosynthetic capability. In addition, increases in osmotic substances and antioxidant enzymes can increase photosynthesis to some extent, but the physiological photosynthetic mechanisms are not protected from damage as these substances and enzymes continually increase. In summary, the resulting information can provide a theoretical basis and reference for construction of the ecological environment of the Qaidam area and the popularization and cultivation of Lycium ruthenicum Murr, thereby increasing ecological, social and economic benefits.

Stomatal and non-stomatal limitations are responsible in down-regulation of photosynthesis in melon plants grown under the saline condition: Application of carbon isotope discrimination as a reliable proxy

Salinity is one of the most severe environmental stresses limiting agricultural crop production worldwide. Photosynthesis is one of the main biochemical processes getting affected by such stress conditions. Here we investigated the stomatal and non-stomatal factors during photosynthesis in two Iranian melon genotypes “Ghobadlu” and “Suski-e-Sabz”, as well as the “Galia” F1 cultivar, with an insight into better understanding the physiological mechanisms involved in the response of melon plants to increasing salinity. After plants were established in the greenhouse, they were supplied with nutrient solutions containing three salinity levels (0, 50, or 100 mM NaCl) for 15 and 30 days. With increasing salinity, almost all of the measured traits (e.g. stomatal conductance, transpiration rate, internal to ambient CO2 concentration ratio (Ci/Ca), Rubisco and nitrate reductase activity, carbon isotope discrimination (Δ13C), chlorophyll content, relative water content (RWC), etc.) significantly decreased after 15 and 30 days of treatments. In contrast, the overall mean of water use efficiency (intrinsic and instantaneous WUE), leaf abscisic acid (ABA) and flavonol contents, as well as osmotic potential (ΨS), all increased remarkably with increasing stress, across all genotypes. In addition, notable correlations were found between Δ13C and leaf gas exchange parameters as well as most of the measured traits (e.g. leaf area, biomass, RWC, ΨS, etc.), encouraging the possibility of using Δ13C as an important proxy for indirect selection of melon genotypes with higher photosynthetic capacity and higher salinity tolerance. The overall results suggest that both stomatal and non-stomatal limitations play an important role in reduced photosynthesis rate in melon genotypes studied under NaCl stress. This conclusion is supported by the concurrently increased resistance to CO2 diffusion, and lower Rubisco activity under NaCl treatments at the two sampling dates, and this was revealed by the appearance of lower Ci/Ca ratios and lower Δ13C in the leaves of salt-treated plants.

Zinc priming and foliar application enhances photoprotection mechanisms in drought-stressed wheat plants during anthesis

Drought is one of most important limiting factors in wheat productivity worldwide. The need to increase drought tolerance during anthesis is of the utmost importance for high yield potentials and yield stability. Photosynthesis is one of the major physiological processes affected by drought. Damages in the photosynthetic apparatus may also arise due to non-regulated dissipation of excessive energy. Zinc (Zn) is an indispensable micronutrient for plants and is required for a wide range of physiological and biochemical processes. In this work we evaluated the stress mitigation effects of Zn seed priming alone and coupled with Zn foliar application in wheat plants submitted to severe drought during anthesis, followed by a recovery period. Under such severe drought stress, photosynthesis was constrained by both stomatal and non-stomatal limitation. Severe drought also induced an increase in non-regulated energy dissipation and hindered a full recovery of the plant's photosynthetic processes after rewatering. We also report possible activation of transposable elements due to drought stress and Zn application. Yield was severely decreased by drought and Zn treatments were unable to counteract this effect. Although unable to oppose the reduction of net photosynthesis, Zn treatments positively enhance photoprotection. At the end of drought period, Zn priming alone and coupled with Zn foliar application increased, respectively, over 2- and 3- fold the regulated dissipation of excess energy. Zn treatments lessened the non-regulated energy dissipation caused by drought, protected the plants against irreversible damages to the photosynthetic apparatus and enabled a better recovery of wheat plants after stress relief.

Hypernodulating soybean mutant line nod4 lacking 'Autoregulation of Nodulation' (AON) has limited root-to-shoot water transport capacity.

Although hypernodulating phenotype mutants of legumes, such as soybean, possess a high leaf N content, the large number of root nodules decreases carbohydrate availability for plant growth and seed yield. In addition, under conditions of high air vapour pressure deficit (VPD), hypernodulating plants show a limited capacity to replace water losses through transpiration, resulting in stomatal closure, and therefore decreased net photosynthetic rates. Here, we used hypernodulating (nod4) (282.33 ± 28.56 nodules per plant) and non-nodulating (nod139) (0 nodules per plant) soybean mutant lines to determine explicitly whether a large number of nodules reduces root hydraulic capacity, resulting in decreased stomatal conductance and net photosynthetic rates under high air VPD conditions. Plants were either inoculated or not inoculated with Bradyrhizobium diazoefficiens (strain BR 85, SEMIA 5080) to induce nitrogen-fixing root nodules (where possible). Absolute root conductance and root conductivity, plant growth, leaf water potential, gas exchange, chlorophyll a fluorescence, leaf 'greenness' [Soil Plant Analysis Development (SPAD) reading] and nitrogen content were measured 37 days after sowing. Besides the reduced growth of hypernodulating soybean mutant nod4, such plants showed decreased root capacity to supply leaf water demand as a consequence of their reduced root dry mass and root volume, which resulted in limited absolute root conductance and root conductivity normalized by leaf area. Thereby, reduced leaf water potential at 1300 h was observed, which contributed to depression of photosynthesis at midday associated with both stomatal and non-stomatal limitations. Hypernodulated plants were more vulnerable to VPD increases due to their limited root-to-shoot water transport capacity. However, greater CO2 uptake caused by the high N content can be partly compensated by the stomatal limitation imposed by increased VPD conditions.