Sonal Mathur

Chlorophyll a fluorescence study revealing effects of high salt stress on Photosystem II in wheat leaves.

In order to study the effects of high salt stress on PS II in detached wheat (Triticum aestivum) leaves, the seedlings were grown in Knop solution and temperature was 20 +/- 2 degrees C. Detached leaves were exposed to high salt stress (0.1-0.5 M NaCl) for 1 h in dark and Chl a fluorescence induction kinetics was measured. Various parameters like Fv/Fm, ABS/RC, ETo/TRo, performance index and area over the florescence curve were measured and the energy pipeline model was deduced in response to salt stress. Our results show that the damage caused due to high salt stress is more prominent at the donor side rather than the acceptor side of PS II. Moreover the effects of high salt stress are largely reversible, as the acceptor side damage is completely recovered (approximately 100%) while the recovery of the donor side is less than 85%. Based on our results we suggest that in response to high salt stress, the donor side of PS II is affected more as compared to the acceptor side of PS II.

PHOTOSYNTHESIS: RESPONSE TO HIGH TEMPERATURE STRESS

Global warming has led to increased temperature of the earth which is a major abiotic stress posing a serious threat to the plants. Photosynthesis is amongst the plant cell functions that is highly sensitive to high temperature stress and is often inhibited before other cell functions are impaired. The primary sites of targets of high temperature stress are Photosystem II (PSII), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) while Cytochrome b559 (Cytb559) and plastoquinone (PQ) are also affected. As compared to PSII, PSI is stable at higher temperatures. ROS production, generation of heat shock proteins, production of secondary metabolites are some of the consequences of high temperature stress. In this review we have summarized the physiological, biochemical and molecular aspects of high temperature stress on the process of photosynthesis, as well as the tolerance and adaptive mechanisms involved.

Analysis of high temperature stress on the dynamics of antenna size and reducing side heterogeneity of Photosystem II in wheat leaves (Triticum aestivum)

This study demonstrates the effect of high temperature stress on the heterogeneous behavior of PSII in Wheat (Triticum aestivum) leaves. Photosystem II in green plant chloroplasts displays heterogeneity both in the composition of its light harvesting antenna i.e. on the basis of antenna size (α, β and γ centers) and in the ability to reduce the plastoquinone pool i.e. the reducing side of the reaction centers (Q(B)-reducing centers and Q(B)-non-reducing centers). Detached wheat leaves were subjected to high temperature stress of 35°C, 40°C and 45°C. The chlorophyll a (Chl a) fluorescence transient were recorded in vivo with high time resolution and analyzed according to JIP test which can quantify PS II behavior using Plant efficiency analyzer (PEA). Other than PEA, Biolyzer HP-3 software was used to evaluate different types of heterogeneity in wheat leaves. The results revealed that at high temperature, there was a change in the relative amounts of PSII α, β and γ centers. As judged from the complementary area growth curve, it seemed that with increasing temperature the PSII(β) and PSII(γ) centers increased at the expense of PSII(α) centers. The reducing side heterogeneity was also affected as shown by an increase in the number of Q(B)-non-reducing centers at high temperatures. The reversibility of high temperature induced damage on PSII heterogeneity was also studied. Antenna size heterogeneity was recovered fully up to 40°C while reducing side heterogeneity showed partial recovery at 40°C. An irreversible damage to both the types of heterogeneity was observed at 45°C. The work is a significant contribution to understand the basic mechanism involved in the adaptation of crop plants to stress conditions.

Changes in PS II heterogeneity in response to osmotic and ionic stress in wheat leaves (Triticum aestivum)

High salt stress involves ionic stress as well as osmotic stress. In this work we have tried to differentiate between the ionic and osmotic effects of salt stress on the basis of their ability to cause changes in PS II heterogeneity. PS II heterogeneity is found to vary with environmental conditions. Osmotic stress caused no change in the QB reducing side heterogeneity and a reversible change in antenna heterogeneity. The number of QB non-reducing centers increased under ionic stress but were unaffected by osmotic stress. On the other hand ionic stress led to a partially irreversible change in QB reducing side heterogeneity and a reversible change in antenna heterogeneity. In response to both ionic and osmotic effect, there is conversion of active PS IIα centres to inactive PSIIβ and γ centres.

Computational analysis of fluorescence induction curves in intact spinach leaves treated at different pH.

Effects of change in pH have been investigated on spinach leaf discs by measuring fluorescence induction kinetics using plant efficiency analyzer (PEA). On the basis of computational analysis of the results, we have reported that acidic pH causes a significant inhibition of the donor and the acceptor side of PS II. Energy flux models have been presented using the software Biolyzer HP 3. Effects of pH were investigated on the antenna size heterogeneity of PS II and a relative change in the proportions of α, β, and γ centers was observed.

Effects of Heat Stress on Growth and Crop Yield of Wheat (Triticum aestivum)

Wheat is an important staple crop of tropical countries, and its productivity largely regulates the economy of the country. At the same time, high temperature is one of the major abiotic stresses in tropical countries like India that has adverse impact on development, growth and overall yield of wheat. At some particular stage of life cycle of wheat, even a little increase or rise in temperature can lead to a complete loss of crop yield. High temperature differentially affects various metabolic processes including the stability of various proteins and membranes and the effectiveness of enzymatic reactions in the cell via denaturation, resulting in metabolic imbalance. Heat stress-induced membrane and protein damage can result in increased reactive oxygen species and thus oxidative stress. High-temperature stress leads to decrease in photosynthetic rates by affecting photosystem II and Rubisco activity and hence influencing the yield. Effects of high temperature on various metabolic reactions with particular emphasis on photosynthetic processes have been discussed.

High salt stress in coupled and uncoupled thylakoid membranes: a comparative study.

The effect of high salt concentration on photosystem II (PS II) electron transport rates and chlorophyll a fluorescence induction kinetics was investigated in coupled and uncoupled spinach thylakoid membranes. With increase in salt concentration, the rates of electron transport mediated by PS II and the Fv/Fm ratio were affected more in uncoupled thylakoids as compared to coupled thylakoid membranes. The uncoupled thylakoid membranes seemed to behave like coupled thylakoid membranes at high NaCl concentration (∼1 M). On increasing the salt concentration, the uncoupler was found to be less effective and Na+ probably worked as a coupling enhancer or uncoupling suppressor. We suggest that positive charge of Na+ mimics the function of positive charge of H+ in the thylakoid lumen in causing coupled state. The function of NaCl (monovalent cation) could be carried out by even lower concentration of Ca2+ (divalent cation) or Al3+ (trivalent cation). We conclude that this function of NaCl as coupling enhancer is not specific, and in general a positive charge is required for causing coupling in uncoupled thylakoid membranes.

Improved photosynthetic efficacy of maize (Zea mays) plants with arbuscular mycorrhizal fungi (AMF) under high temperature stress

In this study, pot experiments were performed to investigate the effects of high temperature stress (44 °C) in maize plants colonized with and without arbuscular mycorrhizal fungi (AMF). Various parameters characterizing photosynthetic activity were measured in order to estimate the photosynthetic efficiency in maize plants. It was observed that density of active reaction centers of PSII, quantum efficiency of photosystem II (PSII), linear electron transport, excitation energy trapping, performance index, net photosynthesis rate increased in AMF (+) plants at 44 °C ± 0.2 °C. Efficiency of primary photochemical reaction (represented as Fv/Fo) increased in AMF (+) plants as compared to AMF (-) plants. AMF seems to have protected water splitting complex followed by enhanced primary photochemistry of PSII under high temperature. Basic morphological parameters like leaf width, plant height and cob number increased in AMF (+) plants as compared to AMF (-) plants. AMF (+) plants grew faster than AMF (-) plants due to larger root systems. Chl content increased in AMF (+) plants as compared to AMF (-) maize plants. AMF hyphae likely increased Mg uptake which in turn increased the total chlorophyll content in AMF (+) maize plants. This subsequently led to a higher production in photosynthate and biomass. Thus AMF (+) plants have shown better photosynthesis performance as compared to AMF (-) maize plants under high temperature stress.

Elucidating the site of action of oxalate in photosynthetic electron transport chain in spinach thylakoid membranes.

The effects of oxalate on PS II and PS I photochemistry were studied. The results suggested that in chloride-deficient thylakoid membranes, oxalate inhibited activity of PS II as well as PS I. To our knowledge, this is the only anion so far known which inhibits both the photosystems. Measurements of fluorescence induction kinetics, YZ• decay, and S2 state multiline EPR signal suggested that oxalate inhibited PS II at the donor side most likely on the oxygen evolving complex. Measurements of re-reduction of P700+ signal in isolated PS I particles in oxalate-treated samples suggested a binding site of oxalate on the donor, as well as the acceptor side of PS I.

Photosynthetic efficiency in sun and shade plants

Photosynthesis is amongst the plant cell functions that are highly sensitive to any type of changes. Sun and shade conditions are prevalent in fields as well as dense forests. Dense forests face extreme sun and shade conditions, and plants adapt themselves accordingly. Sun flecks cause changes in plant metabolic processes. In the field, plants have to face high light intensity and survive under such conditions. Sun and shade type of plants develops a respective type of chloroplasts which help plants to survive and perform photosynthesis under adverse conditions. PSII and Rubisco behave differently under different sun and shade conditions. In this review, morphological, physiological, and biochemical changes under conditions of sun (high light) and shade (low light) on the process of photosynthesis, as well as the tolerance and adaptive mechanisms involved for the same, were summarized.