24-epibrassinolide improved chilled tomato photosynthetic performance by stabilizing electron transport chain and function of photosystem II

To explore the protective mechanisms of brassinosteroids in the chill-induced photoinhibition in tomato (Solanum lycopersicum), we studied the effect of foliar sprayed 24-epibrassinolide (EBR, 0.1µM) on the gas exchange, chlorophyll fluorescence characteristics, and chlorophyll a fluorescence transient in tomato seedlings under chilling stress (a temperature of 8 ℃ and an irradiance of 200 µmol m-2 s-1) for 4 d. Results showed that chilling significantly inhibited CO2 assimilation and induced photoinhibition of photosystem II (PS II). However, photosystem I (PS I) was relatively tolerant to chilling stress, which was due to the downregulation of PS II activity and increase of cyclic electron transport around PS I (CEF). Chilling led to the inactivation of PS II reaction centers (RCs) and blocked the electron transport at the PS II acceptor side, but did not affect the oxygen-evolving complex (OEC) on the donor side of PS II. Exogenous EBR could alleviate chill-induced PS II photoinhibition mainly by the increase of CO2 assimilation and thermal dissipation of excitation energy in the PS II antennae, while the protective effect of CEF was relatively smaller. This study demonstrated that EBR maintained the stability of the electron transport chain and the function of PS II in chilled tomatoes. EBR promoted the absorption (ABS/CS), trapping (TRo/CS), and electron transport (ETo/CS) per leaf area in tomatoes under chilling stress, which was due to increasing the density of active reaction centers (RC/CS), rather than the activity of active RCs.