Water deficit under continuous light enhances the thermal stability of photosystem II in Homalothecium lutescens moss

The thermal stability of photosystem II was examined under different (light and water deficit) treatments in the moss H. lutescens. The decrease in water content under continuous light effected a heat-tolerance increase, further intensified by increasing excitation energy levels. The breakpoints (T, T, F1/2) of the F, vs. T curves significantly shifted towards higher temperatures even under a 30-minute moderate (-1.3 MPa) osmotic treatment, and this was partially inhibited by DTT. Both moderate and higher water deficit (-2.5 MPa) resulted in an increase in thermal stability, independent of the excitation energy level. This effect of water deficit remained observable over a fairly long period. Since in a dark-adapted state the critical values of the F0 vs. T curves did not shift towards significantly higher temperatures with an increase in water deficit, it seems likely that rapid thermal stability increase of PS II induced by water deficit occurs only in energized photosynthetic membranes

Effects of drought on thermal stability of photosynthetic apparatus in bread wheat and in Aegilops species originating from various habitats

The responses of thermal stability of photosystem II to drought induced by dessication in soil pots were examined in two wheat (Triticum aestivum L.) cultivars and in Aegilops genotypes originating from habitats with different annual rainfalls and daily temperatures. The three days drought treatment did not effect a considerable water loss in leaves and parallel with this a significant heat-tolerance increase of PS II determined on the basis of breakpoints (Tc, Tp) of the temperature dependence of steady-state fluorescence level (Fs vs. T curves) at 1000 μE m-2 s-1 actinic light (AL) intensity was not observable. Higher water deficit (RWC<75%,10-14 day drought treatment) resulted in a significant increase in thermal stability, for wheat and for several Aegilops genotypes. In a dark-adapted state the critical values of the F0 vs. T curves did not shift towards significantly higher temperatures with an increase in water deficit. The results indicate that some Aegilops genotypes originating from arid habitats have better heat tolerance than wheat during drought, making them appropriate for improving the heat tolerance of wheat enabling it to survive the dry and hot periods in the field

Effects of drought on photosynthetic parameters and heat stability of PSII in wheat and in Aegilops species originating from dry habitats

The effects of water deficit induced by withholding water in soil pots were examined on processes related to photosynthesis and heat stability of PSII in wheat cultivars and in Aegilops species. Decrease in relative water content (RWC) of leaves resulted in fast and considerable stomatal closure and decrease in net photosynthetic CO2 fixation (A) in Ae. bicornis and in wheat cultivars, while in Ae. tauschii and Ae. speltoides stomatal conductance (gs) and A remained relatively high between 90 and 70% RWC. Parallel with this, A was limited by the CO2 diffusion to the intercellular spaces (stomatal limitation, Ls) even at a lower RWC in Ae. taushii and in Ae. speltoides, while a significant mesophyll limitation (Lm) was observed for Ae. bicornis and for wheat. On the other hand, drought stress resulted in a significant increase in the thermal stability of PSII in wheat and Aegilops genotypes. The results indicate that some genotypes of Ae. taushii and Ae. speltoides have better drought tolerance with satisfactory heat stability than wheat, making them appropriate for improving the heat tolerance of wheat to survive dry and hot periods in the field

Does increased photorespiration protect the leaves of common reed living in fragmented patches from excess light?

The effects of non-photorespiratory conditions on some photosynthetic parameters were studied in the leaves of common reed from fragmented patches and closed stands at 1500 umol m2 AL intensity. At the steady-state photosynthesis level there was no significant difference in the effective quantum yield of PS II (DFIFm') between untreated leaves from closed stands and fragmented patches. The application of phosphinothricin (PPT) and air containing 2% 02 resulted in a substantial decrease in DFIFm', which was more significant in leaves from fragmented patches. Both PPT and low oxygen tension produced a similar effect on DFIFm'. In addition the photorespiration (R) was twice as high in leaves from fragmented patches than in closed stands and in parallel with this a linear regression ratio was found between RP and in vitro GS activity. The results suggest that the leaves of fragmented patches and closed stands might have divergent defensive strategies against excess light

Changes of photosynthetic parameters in wheat/barley introgression lines during salt stress

Salt stress induced photosynthetic responses were investigated in the wheat/barley introgression lines developed in the Agricultural Research Institute, Martonvásár, Hungary, and in different parental wheat and barley Hordeum vulgare L. genotypes. An increase in NaCl concentration of the nutrient solution to 200 mmol L-1 resulted in a considerable stomatal closure and a decreased net CO2 assimilation rate (A) for wheat genotypes, barley cv. Betzes and 4H Asakaze komugi-Manas addition line while changes of these parameters were less significant for barley cv. Manas and the 7H Asakaze komugi-Manas and 6H Mv9 kr1-Betzes-Seneca addition lines. These preliminary results suggest that the 7H Akom-Manas addition line may be a good candidate for improving the salt tolerance of wheat in the future

Can the drought tolerance traits of Ae. biuncialis manifest even in the wheat genetic background?

The physiological responses to water stress induced by PEG were investigated in Aegilops biuncialis (Vis.) genotypes which differ in the annual rainfall of their habitat (1050, 550 and 225 mm/year) and in Triticum aestivum (L.) wheat genotypes differing in drought tolerance, in order to find Ae. biuncialis accessions suitable for improving wheat drought tolerance through intergeneric crossing. A decrease in the osmotic pressure of the nutrient solution from –0.027 MPa to –1.8 MPa resulted in intense water loss, a low extent of stomatal closure and a decrease in the intercellular CO2 concentration (Ci) in Aegilops genotypes originating from dry habitats, while in wheat genotypes high osmotic stress induced increased stomatal closure, resulting in a low level of water loss and high Ci. Nevertheless, under saturating light at normal atmospheric CO2 level, the rate of CO2 assimilation was higher for the Aegilops accessions under strong osmotic stress than for the wheats. Moreover, in the wheat genotypes, CO2 assimilation exhibited less or no O2 sensitivity. These physiological responses were manifested in changes in the growth rate and biomass production, since Aegilops (Ae550, Ae225) genotypes retain a higher growth rate (especially in the roots), biomass production and yield formation after drought stress than wheat. On the basis of the results it seems that Aegilops genotypes originating from a dry habitat have better drought tolerance than wheat, making them good candidates for improving the drought tolerance of wheat through intergeneric crossing

ENVIRONMENTAL FACTORS AFFECTING THE HEAT STABILITY OF THE PHOTOSYNTHETIC APPARATUS

Owing to greenhouse effect and severe dry periods in the agricultural fields, cultivated plants are increasingly exposed to the adverse impact of several abiotic stresses. Therefore, an increasing emphasis should be placed on how multiple stresses affect the physiological processes in plants and how plants respond to the coexistence of combined stress factors. Simultaneous environmental factors may elicit a response different from that given to a single factor, resulting in intensification, overlapping or antagonistic effects. Although the rate of photosynthesis is significantly reduced by salinity and decreased water availability, the thermotolerance of the photosynthetic apparatus may be altered by salt and drought preconditioning. In this short review, we focused on the individual effect of heat stress and the influence of dehydration and NaCl treatment on the heat tolerance of plants. According to our present knowledge, the thermostability of the photosynthetic apparatus may usually be improved by pretreatment of drought or NaCl. At the same time, several different mechanisms in the background of the higher thermostability are hypothesized. These possible drought- and salt-induced processes are also summarized by this review article