*e-mail: marek.zivcak@uniag.sk

Heat waves are projected to become more frequent and severe in the near future, significantly impacting crop plants’ physiological functions and yield. The development of technical tools for non-invasive monitoring of photosynthetic traits enables the uncovering of valuable responses and mechanisms associated with crop tolerance to stress applicable in crop plant phenotyping and screening.

In our study, we applied the non-invasive techniques to investigate the residual post-heat stress effects influencing photosynthetic responses in six diverse winter wheat genotypes (Triticum sp.), which vary in origin, taxonomy, and ploidy (tetraploids vs. hexaploids). After subjecting the plants to elevated temperatures (up to 38 °C) for five days, we compared their photosynthetic parameters: gas exchange analyses, simultaneous measurements of photosystem I (PSI) and photosystem II (PSII) photochemistry, electrochromic bandshift (ECS), at the beginning of recovery and after five additional recovery days compared to control plants grown under moderate temperatures. Notably, the plants belonging to hexaploid and tetraploid species exhibited distinct responses to heat stress based on CO2 assimilation rate (A) and maximum carboxylation rates (VCmax).

Further analyses revealed that tetraploid genotypes fully recovered their photosynthetic and photoprotective functions in the later stage of recovery, while the hexaploid group showed limited recovery. The latter was associated with reduced photosystem I activity and elevated electric membrane potential in chloroplasts. A poor recovery was associated with an overly reduced acceptor side of photosystem I, as well as high values of the electric membrane potential (Δψ) derived from ECS measurements in the chloroplast.

Our results suggest that a high Δψ can be associated with an excessive proton flux via the thylakoid membrane due to membrane leakiness as a symptom of membrane impairment. On the other hand, a good recovery of photosynthetic capacity and photoprotective functions were clearly associated with an enhanced proton gradient (ΔpH) obtained by ECS records, thus demonstrating a key role of efficient regulation of proton transport to produce sufficient transthylakoid proton gradient needed for photosynthesis restoration after high-temperature episodes.

In a broader sense, our results also demonstrate a diversity of photosynthetic responses to heat stress and the crucial role of photoprotective responses at the photosystem I (PSI) level that are associated with overall recovering capacity after heat stress, which deserves more attention in phenotyping and screening of crops in stress conditions. The work was supported by the research projects APVV-22-0392, APVV-20-0071, VEGA 1-0664-22 and VEGA 1-0425-23.