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

Nitrate uptake characteristics and ammonium effects on nitrate uptake 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 depending on membrane potential of root cells. And upland rice and paddy rice presented obviously different responses. For all cultivars, the nitrate treatments induced rapid depolarization and then slow repolarization of membrane potential in root epidermal cells, and even hyperpolarization was observed when nitrate concentration was low. The membrane potential of epidermal cells in Brazilian upland rice roots was larger and its response to NO3− was bigger than those of two paddy rice cultivars. 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 NH4+ differed obviously between upland rice and paddy rice.