No abstract

Strong light decreases the rate of photosynthesis and assimilates production of crop plants. Plants with different carbon reduction cycles respond differently to strong light stress. However, variation in photoinhibition in leaves with different photosynthetic characteristics in maize is not clear. In this experiment, we used the first leaves (with an incomplete C 4 cycle) and fifth leaves (with a complete C 4 cycle) of maize plants as well as the fifth leaves (C 3 cycle) of tobacco plants as a reference to measure the photosynthetic rate (P N) and chlorophyll a parameters under strong light stress. During treatment, P N , the maximal fluorescence (F m), the maximal quantum yield of PSII photochemistry (F v /F m), and the number of active photosystem II (PSII) reaction centers per excited cross-section (RC/CS m) declined dramatically in all three types of leaves but to different degrees. P N , F m , F v /F m , and RC/CS m were less inhibited by strong light in C 4 leaves. The results showed that maize C 4 leaves with higher rates of photosynthesis are more tolerant to strong light stress than incomplete C 4 leaves, and the carbon reduction cycle is more important to photoprotection in C 4 leaves, while state transition is critical in incomplete C 4 leaves.

Malate is the first stable product after CO2 is fixed in NADP-dependent malic enzyme (NADP-ME) type of C4 plants, which transfers CO2 and the reducing equivalent from mesophyll cell (MC) to vascular bundle sheath cell (BSC) chloroplasts and affects the redox state of BSC. The aim of this experiment is to investigate the effect of exogenous malate on the activity of photosystem II (PS II) in C4 and C3 plants. The leaf discs from the 5 th fully expanded leaves of maize (NADP-ME type C4 plants) and the 10 th fully expanded leaves of tobacco (C3 plants) were treated with malate of 50, 100 μM and the chlorophyll fluorescence parameters were measured. Malate treatments decreased the photochemical reaction efficiency (FV/FM) in maize leaves, as a result of rising in initial fluorescence (FO) and decreasing in maximal fluorescence (FM). The number of active PS II reaction center (RC) per excited cross section (RC/CS) declined in malate-treated maize, suggesting that malate inactivated PS II RC. Malate treatments also increased Wk, representing the severity of oxygen-evolving complex (OEC) damage, and decreased the rate of photosynthetic oxygen evolution. We conclude that exogenous malate regulates the activity and structure of PS II in C4 plant maize. No significant changes in the activity of PS II were observed in malate-treated C3 plant tobacco. It is suggested that the short term malate treatment will inhibit PS II of leaves which have C4 anatomy and C4 enzymes.