Heat stress and recovery of photosystem II efficiency in wheat (Triticum aestivum L.) cultivars acclimated to different growth temperatures

- Heat stress effect on photosystem II efficiency in four wheat cultivars were studied.
- The cultivars C518 and KK3 showed heat tolerance performing better PSII efficiency.
- The cultivars PWS7 and KKM1353 belonged to the heat susceptible group.
- The cultivars varied for their rapid recovery capability after heat stress.
- Heat stress significantly decreased relative chlorophyll content in all cultivars.

The effect of heat stress on photosystem II (PS II) efficiency and post-stress recovery was studied in four wheat cultivars using chlorophyll fluorescence. The main aim was to examine the cultivar differences in relation to inhibition and recovery of PSII functionality after heat stress at different growth stages. The secondary aim was to investigate whether a pre-acclimation of plants to elevated temperature during the growth period induces a better tolerance to heat stress than for plants grown in ambient temperature or not. The plants were grown in two growth temperature conditions (15 °C and 25 °C) and subjected to heat stress (40 °C) for two days at early tillering and three days at anthesis and early grain development stages. The plants were returned to their original growth conditions after heat stress and recovery was observed for three days. The maximum photochemical efficiency (Fv/Fm) and the quantum yield of PSII (F′q/F′m) were measured before, during and after the heat stress. The heat stress significantly inhibited the Fv/Fm and F′q/F′m in all wheat cultivars at all growth stages. There were significant differences in Fv/Fm among the cultivars at anthesis and at early grain development but not at early tillering stage. However, the cultivars differed significantly in F′q/F′m at all growth stages. At anthesis and early grain development, the cultivar C518 had the lowest reduction in Fv/Fm and F′q/F′m after heat stress and recovered fully after 72 h in both growth conditions illustrating higher heat tolerance characteristics as compared to the other three cultivars. The largest decrease in Fv/Fm and F′q/F′m after heat stress occurred in the cultivar PWS7, which did not recover completely after 72 h. All cultivars grown at 25 °C had a slightly increased heat tolerance and better recovery compared to plants grown at 15 °C. The relative leaf chlorophyll content decreased significantly after heat stress in all cultivars at all growth stages. The elevated growth temperature (25 °C) accelerated plant growth resulting in early heading and reduced grain yield in comparison to ambient temperature (15 °C).