Response of linear and cyclic electron flux to moderate high temperature and high light stress in tomato

Objective: To evaluate the possible photoprotection mechanisms of cyclic and linear electron flux (CEF and LEF) under specific high temperature and high light (HH) stress. Methods: Six-leaf-stage tomato seedlings (Liaoyuanduoli, n=160) were divided into four parts: Part 1, served as control under 25 degrees C, 500 mu mol/(m(2).s); Part 2, spayed with distilled water (H2O) under 35 degrees C, 1000 mu mol/(m(2).s) (HH); Part 3, spayed with 100 mu mol/L diuron (DCMU, CEF inhibitor) under HH; Part 4, spayed with 60 mu mol/L methyl viologen (MV, LEF inhibitor) under HH. Energy conversion, photosystem I (PSI), and PSII activity, and trans-thylakoid membrane proton motive force were monitored during the treatment of 5 d and of the recovering 10 d. Results: HH decreased photochemical reaction dissipation (P) and the maximal photochemical efficiency of PSII (F-v/F-m), and increased the excitation energy distribution coefficient of PSII (beta); DCMU and MV aggravated the partition imbalance of the excitation energy (gamma) and the photoinhibition degree. With prolonged DCMU treatment time, electron transport rate and quantum efficiency of PSI (ETRI and Y-I) significantly decreased whereas acceptor and donor side limitation of PSI (Y-NA and Y-ND) increased. MV led to a significant decline and accession of yield of regulated and non-regulated energy Y-NPQ and Y-NO, respectively. Membrane integrity and ATPase activity were reduced by HH stress, and DCMU and MV enhanced inhibitory actions. Conclusions: The protective effects of CEF and LEF were mediated to a certain degree by meliorations in energy absorption and distribution as well as by maintenance of thylakoid membrane integrity and ATPase activity.