The Physiological And Molecular Mechanism Of Nitrate Transport In Rice (Oryza Sativa L.)

Ammonium is the main source of nitrogen (N) for rice in paddy soil. However, nitrate is also available to rice roots in paddy soil and previously published results have shown that the amount of N taken up by rice is increased by adding more nitrate in hydroponic experiments. But what is the reason behind the difference of rice nitrate nutrition? How does rice transport and utilize the nitrate? All of these questions need to be answered.This thesis is focused on these questions and I try to find an insight of the physiological and molecular mechanisms underpinning these questions. Hydroponic experiments were conducted to measure cellular nitrate; the effect of nitrate on rice growth and N assimilation; the physiological and molecular differences in nitrate transport of rice cultivars; the structure and function of high affinity nitrate transporter genes in rice, and vacuolar nitrate remobilization and regulation by external nitrate. Together this information provides a relatively complete picture for understanding nitrate transport in rice.Firstly, intracellular measurement of nitrate activity in rice cells in vivo was studied using double-barreled nitrate selective microelectrodes. These microelectrodes showed a typical log linear response to nitrate concentration from 100 mol m^-3 to 0.01mol m^-3 with a slope of 48 -58 mV and the nitrate activity detectable was around 0.0001 M. The microelectrode was characterized by ion high selectivity and sensitivity and good economy when compared with other intracellular measuring methods. In addition, the in vivo method is better than the chemical analysis of the samples because it provides a more detailed picture of the intracellular pools of nitrate. The results showed that nitrate activity in root cells of rice grown at the3 to 4 leaf stage fell into two different populations, i.e.2mM ⁵mM in cytosol and 15 mM²0mM in vacuole growing at low nitrate supply and 35 mM⁶5mM in vacuole with high nitrate supply. Since vacuolar volume accounts for 90% of a whole cell, most of the nitrate in plants is...