This study aimed to characterise the specific phenotypic responses and the sensitivity of photosynthetic parameters to progressive drought in modern wheat genotypes. In pot experiments, we tested eight wheat genotypes (Triticum sp.) that differed in ploidy level and country of origin. Water stress was simulated by the restriction of irrigation, which led to a decreased leaf relative water content of up to 70%. During gradual dehydration, changes in the structure and function of photosystem II (PSII) were analysed using the fluorescence parameters derived from fast fluorescence kinetics (OJIP transient). The results indicated that a group of JIP test-based parameters demonstrated sensitivity to drought, including genotype-specific responses. Severe drought stress led to a decrease in the photochemical efficiency of PSII (Fv/Fm), a reduction in the number of active PSII reaction centers (RC/ABS) and a decrease in parameters, indicating overall photochemical performance at the PSII level (performance indices PIabs and PItot). These findings demonstrate that the approaches used in our experiments were useful and reliable in monitoring the physiological responses of individual varieties of wheat exposed to stress conditions, and they have application potential as selection criteria in crop breeding. The contribution of the high-temperature effects on the photochemical responses under water deficit conditions is also discussed.
The importance of high temperature as an environmental factor is growing in proportion to deepening global climate change. The study aims to evaluate the effects of long-term acclimation of plants to elevated temperature on the tolerance of their photosynthetic apparatus to heat stress. Three wheat (Triticum sp. L.) genotypes differing in leaf and photosynthetic traits were analyzed: Thesee, Roter Samtiger Kolbenweizen, and ANK 32A. The pot experiment was established in natural conditions outdoors (non-acclimated variant), from which a part of the plants was placed in foil tunnel with elevated temperature for 14 days (high temperature-acclimated variant). A severe heat stress screening experiment was induced by an exposition of the plans in a growth chamber with artificial light and air temperature up to 45 °C for ~12 h before the measurements. The measurements of leaf photosynthetic CO2 assimilation, stomatal conductance, and rapid kinetics of chlorophyll a fluorescence was performed. The results confirmed that a high temperature drastically reduced the photosynthetic assimilation rate caused by the non-stomatal (biochemical) limitation of photosynthetic processes. On the other hand, the chlorophyll fluorescence indicated only a moderate level of decrease of quantum efficiency of photosystem (PS) II (Fv/Fm parameter), indicating mostly reversible heat stress effects. The heat stress led to a decrease in the number of active PS II reaction centers (RC/ABS) and overall activity o PSII (PIabs) in all genotypes, whereas the PS I (parameter ψREo) was negatively influenced by heat stress in the non-acclimated variant only. Our results showed that the genotypes differ in acclimation capacity to heat stress, and rapid noninvasive techniques may help screen the stress effects and identify more tolerant crop genotypes. The acclimation was demonstrated more at the PS I level, which may be associated with the upregulation of alternative photosynthetic electron transport pathways with clearly protective functions.
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