Cyclic electron flow protects photosystem I donor side under low night temperature in tomato

Cyclic electron flow (CEF) around photosystem I (PSI) is an important photoprotective mechanism, mediated mainly by PGR5/PGRL1 proteins and the chloroplast NAD(P)H dehydrogenase (NDH) complex. Here, chemical inhibition of PGR5/PGRL1- or NDH-dependent CEF was performed to determine their roles in preventing photoinhibition in tomato leaves under low night temperature (LNT) stress. Experimental treatments were with either antimycin A (A) or rotenone (R) to inhibit PGR5/PGRL1- or NDH-dependent CEF, respectively. The results showed that net photosynthetic rate (Pn), antioxidant enzyme activity, maximal photochemistry efficiency (Fv/Fm), effective photochemical quantum yield of PSII [Y(II)], non-photochemical quenching (NPQ), coefficient of photochemical fluorescence quenching (qP), and effective photochemical quantum yield of PSI [Y(I)] decreased more under LNT following A treatment than following R treatment. In A-treated plants, analysis of the normalized relative variable fluorescence at the K step (Vk) and the relative variable fluorescence at the J step (VJ) showed that both the donor and acceptor sides of PSII were severely damaged under LNT. Moreover, the donor side limitation of PSI [Y(ND)] increased whereas the acceptor side limitation of PSI [Y(NA)] decreased and the expression of genes encoding photosystem core subunits was significantly reduced. A treatment also resulted in increased ROS content and inhibition of SOD and APX activities under LNT. These data suggest that PGR5/PGRL1-dependent CEF plays a more important role than NDH-dependent CEF regarding resistance to LNT stress in tomato.