Effect of partial replacement of NH4+ by NO3- on nitrogen uptake and utilization by different genotypes of rice at the seedling stage

There has been increased evidence of enhanced effect by nitrate (NO3-) on the growth of rice although rice prefers the ammonium (NH4+) to nitrate. Furthermore, rice is being increasingly cultivated in the soil intermittently irrigated or even in aerobic soil in which nitrate nutrition is becoming more important in rice than ever before. Moreover, the rhizosphere of rice is actually exposed to partially oxidized status due to the released oxygen by rice roots. Solution culture experiments were carried out to study the effects of nitrate on the growth and photosynthesis, nitrogen (N) uptake by plant, the glutamine synthetase activity (GSA) and the nitrate reductase activity (NRA) in the leaves of four typical rice (Oryza sativa L.) cultivars (Conventional Indica, Conventional Japonica, Hybrid Indica, Hybrid Japonica) at the seedling stage (28-day-old). The results obtained were as follows: On average, the dry weight of shoots and roots of rice treated with N at the ratio of 50/50 (NH4+-N /NO3--N) was increased by about 20% compared with that at 100/0 (NH4+-N /NO3--N), while chlorophyll content and photosynthesis rates were increased by 4.6% and 16%, respectively, and total N in shoots and roots by 42% and 57%, respectively. It was not the N content but the increased dry matter that contributed to rice production by partial replacement of NH4+ by NO3-. Conventional Indica responsed to nitrate in a greater way than any other cultivars tested. The activities of nitrate reductase (NR) and glutamine synthetase (GS) in the leaves of Indica and hybrid Indica were higher than in Japonica or hybrid Japonica, suggesting that Indica rice could use more NO3- than Japonica rice. The high activity of NR 0.92-4.34 NO2--N, mol /(gh), FW showed that rice was also efficient in absorbing and assimilating nitrate. The increased GSA in the leaves showed that the partial replacement of NH4+ by NO3- could improve the assimilation of NH4+ in the leaves. The increased assimilation of inorganic nitrogen caused by some NO3- was the main contributor to the improved growth of rice. In conclusion, rice, if supplied with partial NO3-, could grow better than without any nitrate and thus nitrification to some degree in paddy soil is of great significance for rice production.