Response And Physiological Mechanisms Of Root Growth Of Rice Genotypes With Different Nitrogen-use-efficiency To Enhanced Nitrate Nutrition

Rice roots can excrete O₂ to surrounding soil and this can convert ammonium (NH₄⁺) to nitrification (NO₃^-) in the process of nitrification at root surface or in rhizosphere and, thus, rice roots are actually always exposed to a mixture of NH₄⁺ and NO₃^-although the predominant species of mineral nitrogen (N) in the soil collected from the lowland rice field is NH₄⁺. Therefore NO₃^- nutrition on the surface of rice roots is an important part of N nutrition of rice plants. Two hydroponic experiments, including whole root culture and root-split culture in NH₄⁺ and NH₄⁺/ NO₃^-(75/25) conditions, were conducted to study the effect of enhanced NO₃^- nutrition on root growth of rice cultivars with different N use efficiency (NUE). High NUE rice cultivar (Nanguang) and low NUE rice cultivar (ELIO) selected from field experiments in 2003 and 2004 were used in experiments. The results were listed as follows.1.Compared to NH₄⁺ solution, enhanced NO₃^- nutrition increased biomass accumulation and N accumulation of rice plants in Nanguang cultivar by 33% and 64%. Enhanced NO₃^- nutrition also increased the leaf area and photosynthesis rate in Nanguang cultivar by 32% and 22%. Significent difference between enhanced NO₃^-nutrition and no NO₃^- nutrition existed in high NUE cultivar (Nanguang) but not in low NUE cultivar (ELIO).2. Results of root-split culture showed that localized NO₃^- nutrition increased biomass accumulation of lateral roots in Nanguang cultivar by 27%, the main reason of which was that localized NO₃^- nutrition increased formation of the lateral roots in high NUE cultivar (Nanguang). Compared to lateral roots in sole NH₄⁺ solution, the number of lateral roots of Nanguang in localized NO₃^- solution were increased by 20%. Moreover, localized NO₃^- nutrition increased the surface area and volume of localized roots of Nanguang. Rate of root exudation significantly was significantly increased in Nanguang cultivar in localized NO₃^- nutrition. These results indicated that lateral root growth of high NUE rice cultivar had higher responses to enhancd NO₃^- nutrition than that of low NUE rice cultivar.3.Compared to NH₄⁺ solution, there was no differences in sugar concentration, and no significant correlation was observed between the growth of lateral roots and sugar concentration in enhanced NO₃^- nutrition. This might imply that the sugar concentration is not a main factor to determine lateral root growth of rice. compared to sole NH₄⁺ solution, the content of nitrate and the activity of NR in Nanguang significantly increased when rice roots were exposed to a mixture of NH₄⁺ and NO₃^- , while no significant differences in nitrate concentration and the activity of NR were observed in ELIO.