Effect of pH on nitrate uptake by rice seedlings

The uptake of nitrate(NO3-) by roots of rice seedlings(Yangdao 6) at different pHs was investigated using single barreled microelectrodes.The results obtained are listed as follows: 1) NO3- uptake of non-nitrate-induced rice provoked instantaneous depolarization or hyperpolarization of membrane potential of rice root cells.Membrane potential was raised with the increasing of NO3- concentration.The biggest change of membrane potentials occurred when NO3- was 0.5 mmol/L and thereafter maintain almost steady.2) The response sensitivity of membrane potentials of rice root cortical cells to NO3- was decreased as the increase of surrounding pH from 4.4 to 7.0,but neglected inhibition on the membrane potential change caused by NO3- uptake was observed at pH 8.0.3) Intracellular pH could be decreased by 0.16 unit using sodium butyrate(5 mmol/L)while it could be increased by 0.2 unit using procaine(3 mmol/L),and their impact on membrane potential change during NO3- uptake was almost neglected.4) There was a significant variation of above ground fresh weight of rice after 2 weeks cultivation under different pHs.The highest fresh weight and the highest NO3- and NH4+ content were observed at pH 5.5.As a conclusion,the intracellular pH was highly regulated and buffered by a series of physiological and biochemical reactions,and the uptake of NO3- was more influenced by external pH than by(intracellular) pH.The influx of NO3-was stimulated at acidic pH while the efflux of NO3- of rice roots was stimulated at(basic) pH.This is why the uptake of NO3- by plants was always increased at acidic pH.