Effect of low potassium levels on photosynthetic activities in different position leaves of potassium-insensitive rice plants

To investigate the senescence among different position leaves at heading stage in rice plants,gas exchange and chlorophyll fluorescence properties in different leaf positions of the potassium-insensitive rice cultivar Yuanfengzao were examined.The results showed that both the net photosynthetic rate(P_n), stomatal conductance(G_s) declined significantly with lowering of leaf position,while intercellular CO_2 concentration(C_i) in the control plants had little change,but C_i in low potassium-stressed plants greatly increased,which suggesting that the decrease in P_n in both plants were not contributed to the stomatal factors.Further evidence of chlorophyll fluorescence measurement showed that,excitation pressure(1-q_P) and relative constrain of PSⅡ functionality(L_((PSⅡ)) ) increased with lowering of leaf position,and initial fluorescence value(F_o) and non-photochemical quenching coefficient(q_N) maintained rather stable,but PSⅡ maximum photochemical efficiency(F_v/F_m),PSⅡ available photochemical efficiency(F′_v/F′_m), PSⅡ actual photochemical efficiency(Ф_(PSⅡ)) and apparent electron transport rate(ETR) greatly declined.All these results indicated that the damage of PSⅡ reaction center resulted in the decline in CO_2 assimilation.Because no significant differences of all the parameters studied between control and low potassium-stressed rice plants were observed,the photosynthetic functionality maintenance of flag leaf was probably associated with the fact that Yuanfengzao was insensitive to low potassium stress.