Characterization of the nature of photosynthetic recovery of wheat seedlings from short-term dark heat exposures and analysis of the mode of acclimation to different light intensities

Are tomato plants co-exposed to heat and salinity able to ensure a proper carbon metabolism? - An insight into the photosynthetic hub

Abiotic stress combinations, such as high temperatures and soil/water salinization, severely threaten crop productivity worldwide. In this work, an integrative insight into the photosynthetic metabolism of tomato plants subjected to salt (100 mM NaCl) and/or heat (42 °C; 4 h/day) was performed. After three weeks, the stress combination led to more severe consequences on growth and photosynthetic pigments than the individual stresses. Regarding the photochemical efficiency, transcript accumulation and protein content of major actors (CP47 and D1) were depleted in all stressed plants, although the overall photochemical yield was not negatively affected under the co-exposure. Gas-exchange studies revealed to be mostly affected by salt (single or combined), which harshly compromised carbon assimilation. Additionally, transcript levels of stress-responsive genes (e.g., HsfA1 and NHX2) were differentially modulated by the single and combined treatments, suggesting the activation of stress-signature responses. Overall, by gathering an insightful overview of the main regulatory hub of photosynthesis, we show that the impacts on the carbon metabolism coming from the combination of heat and salinity, two major conditioners of crop yields, were not harsher than those of single stresses, indicating that the growth impairment might be attributed to a proficient distribution of resources towards defense mechanisms.

Exogenous application of moringa leaf extract improves growth, biochemical attributes, and productivity of late-sown quinoa

Quinoa (Chenopodium quinoa Willd.) has gained significant popularity among agricultural scientists and farmers throughout the world due to its high nutritive value. It is cultivated under a range of soil and climatic conditions; however, late sowing adversely affects its productivity and yield due to shorter growth period. Inorganic and organic phyto-stimulants are promising for improving growth, development, and yield of field crops under stressful environments. Field experiments were conducted during crop cultivation seasons of 2016–17 and 2017–18, to explore the role of inorganic (hydrogen peroxide and ascorbic acid) and organic [moringa leaf extract (MLE) and sorghum water extract (sorgaab)] phyto-stimulants in improving growth and productivity of quinoa (cultivar UAF-Q7). Hydrogen peroxide at 100 μM, ascorbic acid at 500 μM, MLE at 3% and sorgaab at 3% were exogenously applied at anthesis stage of quinoa cultivated under normal (November 21^st and 19^th during 2016 and 2017) and late-sown (December 26^th and 25^th during 2016 and 2017) conditions. Application of inorganic and organic phyto-stimulants significantly improved biochemical, physiological, growth and yield attributes of quinoa under late sown conditions. The highest improvement in these traits was recorded for MLE. Application of MLE resulted in higher chlorophyll a and b contents, stomatal conductance, and sub-stomatal concentration of CO₂ under normal and late-sowing. The highest improvement in soluble phenolics, anthocyanins, free amino acids and proline, and mineral elements in roots, shoot and grains were observed for MLE application. Growth attributes, including plant height, plant fresh weight and panicle length were significantly improved with MLE application as compared to the rest of the treatments. The highest 1000-grain weight and grain yield per plant were noted for MLE application under normal and late-sowing. These findings depict that MLE has extensive crop growth promoting potential through improving physiological and biochemical activities. Hence, MLE can be applied to improve growth and productivity of quinoa under normal and late-sown conditions.

Effect of different light intensity on physiology, antioxidant capacity and photosynthetic characteristics on wheat seedlings under high CO2 concentration in a closed artificial ecosystem

The growth of plants under high carbon dioxide (CO₂) concentrations (≥ 1000 ppm) is explored for the climate change and the bioregenerative life support system (BLSS) environment of long-duration space missions. Wheat (Triticum aestivum L.) is a grass cultivated for cereal grain-a global staple food including astronauts. Light and CO₂ are both indispensable conditions for wheat seedlings. This study provides insights on the physiology, antioxidant capacity and photosynthetic characteristics of wheat seedlings under a range of photosynthetic photon flux densities in a closed system simulating BLSS with high CO₂ concentration. We found that the F_v/F_m, F_v/F₀, chlorophyll content, intrinsic water use efficiencies (WUE_i), membrane stability index (MSI), and malondialdehyde (MDA) of wheat seedlings grown under an intermediate light intensity of 600 μmol m^-2 s^-1 environment were all largest. Interestingly, the high light intensity of 1200 mol m^-2 s^-1 treatment group exhibits the highest net photosynthetic rate but the lowest MDA content. The stomatal conductance and F₀ of high light intensity of 1000 μmol m^-2 s^-1 treatment group were both significantly higher than that of other groups. Our study provides basic knowledge on the wheat growth in different environments, especially in a closed ecosystem with artificial lights.

Influence of photoluminophore-modified agro textile spunbond on growth and photosynthesis of cabbage and lettuce plants

Light-converting polypropylene spunbond was first used in the study of the key physiological parameters of plants. A comparative study of the functioning of the photosynthetic apparatus and the dynamics of growth in late cabbage plants (Olga variety) and leaf lettuce (Emerald variety) was conducted using the ordinary nonwoven polypropylene fabric (spunbond) (density 30 g·m-2) and the spunbond containing a photoluminophore (PL) (1.6% yttrium oxysulfide doped with europium). The plants were grown in a glass greenhouse without spunbond and under the spunbond containing and not containing the PL that transforms a part of UV-radiation into red light radiation. The use of the spunbond led to a decrease in the rate of photosynthesis, activity of the photosystem 2, and the accumulation of plant biomass and to an increase in the stomatal conductance. By contrast to unmodified spunbond, the application of the spunbond containing the PL led to an increase in the rate of photosynthesis, the water-use efficiency (WUE), and the accumulation of the total biomass of plants by 30-50% but to a decrease in the transpiration rate and the stomatal conductance. It is assumed that the positive effect of the PL is associated with an increase in the fraction of fluorescent red light, which enhances photosynthetic activity and accelerates plant growth.

Role of mineral nutrition in alleviation of heat stress in cotton plants grown in glasshouse and field conditions

Coincidence of high temperature with terminal reproductive pheno-stages of cotton is chief constraint to achieve yield potential. This high temperature interfere plant defensive system, physiological process, water relations and lint yield production. In this study, we modulated the detrimental outcomes of heat stress on cotton through the foliar spray of nutrients. Cotton crop was exposed to sub-optimal and supra-optimal thermal regimes for a period of one week at squaring, flowering and boll formation stages under glass house and field conditions. Foliar spray of potassium (K-1.5%), zinc (Zn-0.2%) and boron (B-0.1%) were applied at three reproductive stages one day prior to expose high temperature regimes. High temperature increased lipid membrane damage through increased malondialdehyde (MDA) contents in cotton leaves. High temperature stress also reduced leaf chlorophyll contents, net photosynthetic rate, stomatal conductance, water potential, averaged boll weight (g) and seed cotton yield per plant. Various nutrients variably influenced growth and physiology of heat-stressed cotton plants. Zinc outclassed all other nutrients in increasing leaf SOD, CAT, POX, AsA, TPC activity, chlorophyll contents, net photosynthetic rate, stomatal conductance, water potential, boll weight and seed cotton yield per plant. For example, zinc improved seed cotton yield under supra-optimal thermal regime by 17% and under sub-optimal thermal regime by 12% of glasshouse study while 19% under high temperature sowing dates of field study than the water treated plants under the same temperatures. Conclusively, increasing intensities of temperature adversely affected the recorded responses of cotton and exogenous application of Zn efficaciously alleviated heat induced perturbations. Moreover, exogenous nutrients mediated upregulations in physiochemical attributes induced heat tolerance at morphological level.

Morpho-physiological evaluation of tomato genotypes under high temperature stress conditions

BACKGROUND

Tomato (Solanum lycopersicum L.) is an important but heat-sensitive vegetable crop. The losses in tomato production associated with heat stress are aggravating further under a global warming scenario. The present study was designed to investigate the comparative performance of tomato genotypes under high temperature stress. Tomato genotypes (191) were exposed to the controlled conditions of high temperature (40/32 °C day/night temperature). Different morphological (shoot length, root length, shoot fresh weight, root fresh weight, shoot dry weight, root dry weight and number of leaves), physiological (photosynthetic rate, transpiration rate, water use efficiency, stomatal conductance to water, sub-stomatal CO2 and leaf temperature) and SPAD value (chlorophyll content) were recorded to check the diversity among genotypes against heat stress.

RESULTS

All the genotypes showed a significantly variable response in almost all the attributes under high-temperature conditions. Correlation among the variables provided a clear understanding of the phenomena involved. Based on all the attributes studied, genotypes L00090 and L00091 were found to be the most heat tolerant compared to other genotypes, whereas CLN1462A and CLN 1466E were found to be comparatively sensitive.

CONCLUSION

It was concluded that the studied attributes were genotype dependent, and significant diverse performance was noted. The findings of this study pave the way towards the selection of tolerant genotypes, not only for use under high-temperature conditions but also to employ them in breeding programs to produce heat-tolerant hybrids. © 2015 Society of Chemical Industry.

Drought-induced modifications of photosynthetic electron transport in intact leaves: analysis and use of neural networks as a tool for a rapid non-invasive estimation

Water deficit is one of the most important environmental factors limiting sustainable crop yields and it requires a reliable tool for fast and precise quantification. In this work we use simultaneously recorded signals of photoinduced prompt fluorescence (PF) and delayed fluorescence (DF) as well as modulated reflection (MR) of light at 820nm for analysis of the changes in the photosynthetic activity in detached bean leaves during drying. Depending on the severity of the water deficit we identify different changes in the primary photosynthetic processes. When the relative water content (RWC) is decreased to 60% there is a parallel decrease in the ratio between the rate of excitation trapping in the Photosystem (PS) II reaction center and the rate of reoxidation of reduced PSII acceptors. A further decrease of RWC to 20% suppresses the electron transfer from the reduced plastoquinone pool to the PSI reaction center. At RWC below values 15%, the reoxidation of the photoreduced primary quinone acceptor of PSII, Q(A)(-), is inhibited and at less than 5%, the primary photochemical reactions in PSI and II are inactivated. Using the collected sets of PF, DF and MR signals, we construct and train an artificial neural network, capable of recognizing the RWC in a series of "unknown" samples with a correlation between calculated and gravimetrically determined RWC values of about R(2)≈0.98. Our results demonstrate that this is a reliable method for determination of RWC in detached leaves and after further development it could be used for quantifying of drought stress of crop plants in situ. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Functional aspects of the photosynthetic light reactions in heat stressed Arabidopsis deficient in digalactosyl-diacylglycerol

Plants are often submitted, in their natural environment, to various abiotic stresses such as heat stress. However, elevated temperature has a detrimental impact on overall plant growth and development. We have examined the physiological response of the dgd1-2 and dgd1-3 Arabidopsis mutants lacking 30-40% of digalactosyl-diacylglycerol (DGDG) exposed to heat constraint. These mutants, which grow similarly to wild type under normal conditions, were previously reported to be defective in basal thermotolerance as measured by cotyledon development. However their functional properties were not described. Chlorophyll fluorescence measurements and absorbance changes at 820nm were used to monitor photosystem II (PSII) and PSI activity, respectively. It was observed that both mutants have similar photosystem activities with some differences. The mutants were less able to use near saturation light energy and elicited higher rates of cyclic PSI electron flow compare to wild type. Arabidopsis leaves exposed to short-term (5min) mild (40°C) or strong (44°C) heat treatment have shown a decline in the operating effective quantum yield of PSII and in the proportion of active PSI reaction centers. However, cyclic PSI electron flow was enhanced. The establishment of the energy-dependent non-photochemical quenching of chlorophyll fluorescence was accelerated but its decline under illumination was inhibited. Furthermore, heat stress affected the process implicated in the redistribution of light excitation energy between the photosystems known as the light state transitions. All the effects of heat stress mentioned above were more intense in the mutant leaves with dgd1-3 being even more susceptible. The decreased DGDG content of the thylakoid membranes together with other lipid changes are proposed to influence the thermo-sensitivity of the light reactions of photosynthesis towards heat stress.

Photosystem II photochemistry, photoinhibition, and the xanthophyll cycle in heat-stressed rice leaves

To investigate how high light affects the responses of photosynthesis to heat stress, the effects of high temperature (25-42.5 degrees C) either in the dark or in the light (1000 micromol m(-2) s(-1)) on photosystem II (PSII) photochemistry and the xanthophyll cycle were investigated in rice plants. At temperatures higher than 35 degrees C, there was a decrease in the CO(2) assimilation rate, and this decrease was greater in the light than in the dark. The maximal efficiency of PSII photochemistry (F(v)/F(m)) showed no significant change in the dark, but did show a significant decrease in the light. In addition, there was an increase in non-photochemical quenching (NPQ) and this increase was greater in the light than in the dark. Furthermore, the de-epoxidation status of the xanthophyll cycle increased significantly with increasing temperature in the light. Compared to the control leaves, the dithiothreitol-fed leaves showed a greater decrease in F(v)/F(m) but a very small increase in NPQ and de-epoxidation status of the xanthophyll cycle at temperatures higher than 35 degrees C. On the other hand, the ascorbate-fed leaves showed less of a decrease in F(v)/F(m) but a greater increase in NPQ and the de-epoxidation status of the xanthophyll cycle. Ascorbate peroxidase and glutathione reductase activities in leaves and chloroplasts were enhanced and this enhancement was greater in the light than in the dark. Heat stress had no significant effect on the contents of ascorbate and glutathione in leaves and chloroplasts in the dark, but led to an increase in the contents of reduced ascorbate and glutathione in leaves and chloroplasts in the light at the temperatures higher than 35 degrees C. Our results suggest that the xanthophyll cycle plays an important role in protecting PSII against heat-induced photoinhibition by an increase in the ascorbate pool in the chloroplast.

Heterogeneous behavior of PSII in soybean (Glycine max) leaves with identical PSII photochemistry efficiency under different high temperature treatments

The purpose of this study is to demonstrate the heterogeneous behavior of PSII in soybean (Glycine max) leaves and identical maximum PSII photochemistry efficiency (F(V)/F(M)) under different high temperature treatments. We observed that, with an identical decrease in F(V)/F(M) in soybean leaves caused by different high temperature treatments, chlorophyll a fluorescence differed significantly, indicating different behaviors in the photosynthetic apparatus. The quantitative analysis showed that, with an identical F(V)/F(M), leaves treated at 48 degrees C showed a higher W(K), an indicator of damage to the oxygen-evolving complex along with a lower O(2) evolution rate compared with leaves treated at 45 degrees C. This demonstrated that the donor side of PSII was damaged more severely at 48 degrees C than at 45 degrees C despite the same decrease in F(V)/F(M) in the two heat-treated leaves. The ratios of Q(A)- and Q(B)-reducing PSII reaction centers to total PSII reaction centers were both lower in leaves treated at 48 degrees C than in leaves treated at 45 degrees C with an identical F(V)/F(M), indicating that the acceptor side of PSII was also more damaged by heat treatment at 48 degrees C than at 45 degrees C. However, when damage to the donor side of PSII was similar in leaves treated at two different temperatures, the acceptor side of PSII was damaged less severely at 48 degrees C, which accounted for higher electron transport rate at the acceptor side of PSII in leaves treated at 48 degrees C than in leaves treated at 45 degrees C.

Heat-induced impairments and recovery of photosynthetic machinery in wheat seedlings. Role of light and prooxidant-antioxidant balance

The extent of damage caused to the photosynthetic machinery of 10-d-old wheat seedlings by short-term exposure to mild heat, their capacity to recover from it and the possible roles of H2O2, SOD, catalase and ascorbate peroxidase on the recovery process were investigated. Seedlings were subjected to heat treatments at 40/42/44 °C for 20 min in the dark and allowed to grow for 72 h in light of different irradiances (40-800 μE m(-2) s(-1)) at 20 °C for recovery from heat induced damage. Complete or partial recovery of photosynthetic activities was observed in the seedlings treated at 40 °C and 42 °C, but not at 44 °C. Our data suggest that the balance between (pro)oxidant and antioxidant levels poised by heat stress subsequent light is the crucial factor for the extent of recovery from heat induced damage.