High temperatures affect practically all physiological and metabolic processes in plants. Therefore, plants are forced to change their metabolism in order to minimize damage caused by heat because they are stationary organisms that cannot escape the heat in true sense of word. This process is known as acclimation.

The main goal of the study was to assess how long-term plant acclimation to high temperatures affected the ability of photosynthetic apparatus to withstand heat stress. In experiment focusing on the effects of high temperature, three genotypes of wheat differing in leaf and photosynthetic traits were analyzed: Thesee (Triticum aestivum L.), Roter Samtiger Kolbenweizen (Triticum compactum) and ANK 32A (Triticum aestivum L.). The pot experiment was carried out in outdoor conditions (non-acclimated variant). Acclimation to high temperature was induced by transferring half of the plants to a foil tunnel with elevated temperature for 12 days as soon as all plants had fully developed flag leaves (acclimated variant).

Subsequently, exposition of the both groups of plants (acclimated and non-acclimated) in a growth chamber with artificial light and air temperature up to 45 °C for approximately 12 hours prior to the measurements induced a severe heat stress. The realized measurements of gas-exchange and rapid kinetics of chlorophyll a fluorescence demonstrated that the ability of the genotypes to respond to acute heat stress varies. The effects of high temperature were manifested by non-stomatal inhibition of the photosynthetic process and the rapid kinetics of chlorophyll fluorescence confirmed the mostly reversible effects due to a moderate decrease of parameters associated with PSII activity.

All genotypes exhibited a decrease in the total activity of PSII (PIabs) ,the number of active PS II reaction centers (RC/ABS) and maximum quantum yield of PSII (Fv/Fm) due to a effect of acute heat stress. On the other hand, severe heat stress adversely affected the PS I (parameter ψREo) in the non-acclimated variants only, while a significant increase was observed in the acclimated variants, which was the most pronounced in heat sensitive and chlorophyll b-deficient genotype ANK-32.

This finding supports the hypothesis that increased PSI activity may play a key role in protection of plants against adverse effects of acute temperature stress. Research activities were realized under the projects: APVV-22-0392, VEGA 1-0664-22, VEGA 1-0425-23.