Effects of iron deficiency on photosynthesis and photosystem II function in soybean leaf.

Gas exchange and chlorophyll a fluorescence in soybean plants were investigated to explore the effects of iron deficiency on photosynthesis and photosystem II function in vivo. Iron deficiency induced a drastic decrease in net photosynthesis (Pn). Compared with normal plants, the maximal quantum yield of PSII photochemistry (psipo) in iron-deficient plants was only slightly lower; whereas, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than QA-(Psio) and quantum yield of electron transport beyond QA (psiEo) were significantly depressed. Iron deficiency also caused a clear enhancement of the relative variable fluorescence at K step (VK). When exposed to light, iron-deficient plants had considerably lower efficiency of excitation energy capture by open PSII reaction centers (Fv'/Fm'), quantum yield of PSII electron transport (PhiPSII), and photochemical quenching coefficient (qP), but markedly higher non-photochemical quenching (NPQ). In addition, post-illumination transient increase in chlorophyll fluorescence was clearly enhanced in iron-deficient plants. Basing on these data, we suggest that both the donor and the acceptor sides of PSII complex were damaged by iron deficiency; cyclic electron transport around PSI in iron-deficient soybean plants might play an important role in inducing the excitation energy dissipation and meeting the demand for extra ATP as a compensation for the loss of phosphorylation capability.