| Growth and photosyhtnesis of maize seedlings, such as seedling height, dry mass accumulation, net photosynthetic rates (Pn),. quantum efficiency of CO2 assimilation (φCO2), carboxylation efficiency (CE) and actual photochemical efficiency (φ PSII), negatively responded to level of exogenous ABA in a 7-day treatment, but the tolerance to photoinhibition was enhanced in maize seedlings with ABA prior treatment at 10 and 25 μmol L-1, while photoinhibition was aggravated in maize seedlings with ABA prior treatment at 50μmol L-1. Then the 25μmol L-1 ABA-pretreated maize seedlings were used to investigate the protective effect of exogenous ABA on light-induced photoinhibition. Results showed that decreases of Pn, CE and φ PS II were obviously alleviated in ABA-treated maize seedlings, and which were associated with relatively higher photochemical quenching (qP) and non-photochemical quenching (NPQ) as compared with control. In addition, data of middle relaxing component of non-photochemical fluorescence quenching (qm) and time required PSII maximal photochemical efficiency (Fv/Fm) recovery revealed that state transition-dependent and PSII cyclic reparation-dependent energy dissipation were stronger in ABA-treated maize seedlings than control under strong light. Besides these, higher levels of xanthophyll cyclic pigment components of violaxanthin (V), antheraxanthin (A) and zeaxanthin (Z), and a bigger size of xanthophyll cyclic pool (V+A+Z) presented in ABA-treated maize seedlings, while relatively lower levels of V, A and Z, and a smaller size of xanthophyll cyclic pool (V+A+Z) exhibited in control seedlings. These results demonstrated that stronger xanthophyll cycle-dependent energy dissipation occurred in ABA-treated maize seedlings and which was based on a bigger size of ABA-induced xanthophyll cyclic pool. In ABA-treated maize, the higher ratio of φ PSII/Pn, higher activities of antioxidative enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR), and higher contents of nonenzymatic antioxidants such as ascorbic acid (AsA), dehydroascorbate acid (DHAsA), reduced and oxidative glutathione (GSH and GSSH), suggested that Mehler-peroxidase (MP) reaction also acted effectively in protection ABA-treated maize seedlings against photoinhibition. The ABA-induced increase, in MP reaction was further supported by the alleviation of MDA increase and of Chla+b and protein decrease in ABA-treated maize seedlings in the course of photoinhibition.When the excitation of photosystem I and II (PSI and PSII) of maize leaf was balanced from the illumination of PSI and PSII light both at the same photon flux density, leaf photosynthetic status was induced in state 1 and the PSII maximal fluorescence (Fm1) was relatively high. When the PSI light was turned off and leaf was illuminated under PSII light only, the PSII maximal fluorescence (Fm2) decreased significantly, especially at the beginning 20 min, and this decrease was not affected bv dithiothreitol (DTT). an inhibitor of de-epoxidase in xanthophvll cycle. In thecourse of PSII light illumination, PSII photochemical efficiency (Fv/Fm) stayed constant, while the PSII initial fluorescence (Fo') decreased fast in the initial 20 min. In addition, energy distribution to PSII ( β ) decreased and to PSI (α ) increased remarkably. Though the tendency of these changes was similar in maize between the control and abscisic acid (ABA) treatment at 25 μ mol L-1, higher Fm|/Fm2-1 associated with lowerβ/α -1 presented in the ABA-treated maize. Moreover, ABA-treated maize exhibited a high level of middle-relaxing non-photochemical fluorescence quenching (qm) during PSII light illumination, but the qm difference between the control and the ABA-treated maize disappeared in the presence of N-ethylmaleimide (NEM), an inhibitor of PSII light harvesting complex (LHCII) phosphorylation. Furthermore, the high ability to redistribute light energy between PSI and PSII in the ABA-treated maize also displayed in the great...
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