Characteristics of Gas Exchange and Chlorophyll Fluorescence in Different Position Leaves at Booting Stage in Rice Plants

To investigate the senescence of rice leaves at different positions at booting stage, gas exchange and chlorophyll fluorescence properties in leaves at different positions were examined by using six rice materials. The net rates of photosynthesis (Pn), stomatal conductance (gs), transpiration rate (E) and water use efficiency (WUE) decreased significantly with lowering of leaf positions, while intercellular CO2 concentration (Ci) had a little change, indicating that the decrease of photosynthetic rate was not resulted from the decrease of gs. The decrease of SPAD reading which had a close correlation with chlorophyll content was one of apparent reasons resulting in the decease of Pn. Further evidence by chlorophyll fluorescence showed that the photochemical efficiency of PSⅡ and its electron transport rate (ETR) decreased substantially with lowering of the leaf positions but the variable-to-maximum fluorescence ratio (Fv/Fm) and efficiency of energy conversion of open PSⅡ (Fv'/Fm') remained rather stable, suggesting that primary limitation of Pn was suppression of electron transport of PSⅠ, and probably associated with carbon assimilation.