The Molecular And Physiological Responses Of Nitrate Reductase And Glutamine Synthetase To Nitrate In Rice (Oryza Sativa L.)

Nitrate and ammonium are the predominant inorganic N forms in soil thatutilized by plants. Since nitrification is strongly inhibited and ammonium becomes the major form of N in paddy soil, previousstudies concern more ammonium than nitrate nutrition of rice plants. Although NH₄⁺ is usually considered to be the preferred nitrogen source of rice (Oryza sativa L.), the roots are actually exposed to a mixed nutrition of NH₄⁺ and NO₃^-due to the excretion of O₂ by the roots and subsequent nitrification in rhizosphere. But what are the reasons behind the difference in their capability of utilizing nitrate among rice genotypes? How is nitrate be metabolised once absorbed by rice roots? To answer these questions,this thesis studied the effects of NO₃^-on N utilization by rice plants at molecular physiological level. Hydroponic experiments were conducted to study the effect of nitrate on the growth, the changes in enzyme activities and transcript levels of nitrate reductase and glutamine synthetase by different rice cultivars.Firstly, four rice cultivars, Indica, Japonica and two hybrid cultivars were grown in solution culture containing 1 mmol.L^-1 NO₃^- and their growth and utilization of nitrate were compared. Indica produced the greatest biomass while Japonica the lowest growth among the four cultivars in 1 mmol.L^-1 NO₃^-. Indica rice had higher nitrate reductase activity (NRA) and glutamine synthetase activity (GSA) than Japonica rice in 1 mmol.L^-1NO₃^- , suggesting that Indica rice had a greater ability to utilize nitrate than Japonica.The difference in nitrogen recovery is not only found between Indica and Japonica rice, but also within different Japonica rice cultivars. Therefore, we selected two rice cultivars of different NUE based on the results of field experiments. Solution culture experiments were conducted to study the influence of nitrate (NO₃^-) on the activities of nitrate reductase (NR) and glutamine synthetase (GS) and transcript levels of two NR genes, i.e. OsNial, OsNia2, two cytosolic GS1 genes, i.e. OsGln1;1...