Ammonium Effects on Nitrate Uptake by Roots of Upland and Paddy Rice Seedlings Related to Transmembrane Potential Differences

Nitrate uptake characteristics and ammonium effects were compared between upland rice (Brazilian upland rice) and paddy rice (Wuyujing 3 and Yangdao 6) through the glass microelectrode technique and the concentration gradient method of uptake kinetics. Results indicated that nitrate uptake by rice seedlings and ammonium effects were related to transmembrane potential difference, and upland rice and paddy rice presented obviously different responses. Nitrate induced a rapid depolarization and then a slow repolarization of transmembrane potential in root epidermal cells, and even hyperpolarization was observed when nitrate concentration was low. Resting transmembrane potential of epidermal cells in Brazilian upland rice roots was lower and its response to NO3- was bigger than that of two paddy rice species. Depolarization of membrane potential was amplified when ammonium was simultaneously added with nitrate into the measure medium, but repolarization was reduced, even disappeared. Brazilian upland rice seedlings had high Vmax of nitrate uptake and low Km, furthermore, Vmax and Km were little affected by ammonium, but Vmax of Wuyujing 3 was reduced significantly. Therefore, inhibition of ammonium to nitrate uptake possibly due to decrease of membrane potential and influence of nitrate transporters. Moreover, inhibition of NH4+ differed obviously between upland rice and paddy rice