Photosystem damage mechanism in flag leaves of winter wheat under high temperature

High temperatures are one of the main environmental stresses at the filling stage of winter wheat. The reduction in wheat photosynthesis caused by heat stress affects the filling of wheat and reduces grain yield. China is rich in wheat cultivars, and their photosynthetic sensitivity to high temperatures varies. Investigating the mechanism of high-temperature damage to the photosynthetic apparatus of wheat flag leaves can help to rationalize wheat high-temperature resistance resources. In this study, 35 wheat cultivars planted widely in Henan Province during different historical periods were selected. The parameters related to photosynthetic electron transfer of all wheat cultivars were measured and analyzed using fast chlorophyll fluorescence, 820 nm light reflection, and delayed fluorescence synchronization determination. First, according to the maximum photochemical efficiency (F_V/F_M) under high temperature, 35 wheat cultivars were divided into two groups: high-temperature insensitivity and high-temperature sensitivity, and the parameters measured for the two types of wheat cultivars were averaged. The results showed that J and I points of chlorophyll fluorescence induction curves were raised and the maximum quantum yield for primary photochemistry (φP_O), quantum yield for electron transport (φE_o), quantum yield for reduction of end electron acceptors at the PSⅠ acceptor side (φR_o), and performance index (PI_ABS) were significantly reduced in the two types of wheat cultivars under high-temperature stress. Moreover, the rise and decrease extents of high-temperature sensitive type were greater than those of high-temperature insensitive type, indicating that the PSⅡ light energy capture efficiency, the efficiency of the absorbed light energy to drive electrons downstream of the primary electron quinone acceptor Q_A, its efficiency to the PSⅠ end, and the re-reduction ability of the PQ pool decreased more in the leaves of the sensitive wheat cultivars under high temperatures. There was no significant decrease in the maximum decrease slope of the 820 nm light reflection curve (V_PSⅠ) in either types of wheat cultivars, and the maximum increase slope of the 820 nm light reflection curve (V_PSⅡ-PSⅠ) in both types of wheat cultivars significantly decreased. Furthermore, the decline of V_PSⅡ-PSⅠ in the high-temperature sensitive wheat cultivars was greater than in the insensitive ones, indicating that the PSⅠ activity was not affected and the donor side of PSⅠ had a greater extent of damage in the high-temperature sensitive wheat cultivars. Values of both characteristic points of the delayed fluorescence induction curves (I₁ and I₂) decreased. At the same time, I₂/I₁ increased significantly at high temperatures and increased even more in high-temperature sensitive wheat cultivars, indicating that PSⅡ activity decreased and the efficiency of PSⅠ donor-side electron transfer to the end of PSⅠ increased. Based on the complementarity and confirmation of the three research methods, it is concluded that the difference in PSⅡ reaction center activity, PSⅡ light energy capture, and electron transfer from the acceptor side of Q_A to PSⅠ downstream is the main reason for the difference in photosynthetic ability between two types of wheat cultivars under high temperature. The PSⅡ donor side and PSⅠ activity have no direct impact on the high-temperature resistance of wheat. The results of this study help understand the current status of high-temperature resistance of wheat cultivars in the Huanghuai wheat-planting area and provide references for the breeding selection of high-temperature-resistant cultivars and innovation in cultivation techniques.