The second heat wave leads to changes in the efficiency of alfalfa’s photosynthetic energy use

Extreme climatic events, which have become more common, more intense, and more frequent, threatencrop productivity and food security. The aim of this study was to investigate the response of chlorophyll a fluorescence parameters of Medicago sativa L. under regulated environment and recurrent heat waves stress effect. Plants were grown in pots. Two four-day heat waves (35/28°C day/night temperature) were simulated simultaneously with drought (10% soil moisture). Each four-day heat wave was followed by a five-day recovery period. Measurements of chlorophyll a fluorescence parameters were taken on the last, fourth, day of the exposure of each heat wave and after each recovery period. Results of this study showed that both the first and second heat waves significantly reduced the quantum yield of PSII photochemistry. During the second heat wave, the negative effects persisted, but were already weaker. The same pattern of change wasfound for the performance index (PIABS). PIABS decreased by 54.0% and 46.8% during the first and second heat waves, respectively, compared to controls (p < 0.05). Despite the increase in absorbed and trapped energy due to the effect of the heat waves, the electron transport rate from QA to QB in the plants exposed to heat waves was not followed by an increase. The above-mentioned changes in the viability of the photosystem may have been caused by a decrease in the density of the active reaction centers (RC/CSo) and an increase in the amount of dissociated energy (DIo/CSo). After the second recovery period, the RC/CSo in heat and drought stressed plants was equal to that of control plants, which resulted in lower energy waste in the form of heat (DIo/CSo).