Effect of partial replacement of NH4+ by NO3- on nitrate reductase activity and their genetic expression patterns in rice

Nitrogen is one of the most important nutrients limiting the production of cereal crops.There has been increased evidence that nitrate(NO3-) had an enhanced effect on rice production although it prefers ammonium(NH4+) nutrition.Furthermore,rice is being increasingly cultivated in the soil intermittently irrigated or even in aerobic soil condition in China.Thus,nitrate plays a more remarkable role than ever before.Hydroponic experiments were carried out to study the effects of partial replacement of NH4+ by NO3- on the growth,nitrogen(N) accumulation,nitrate concentration,nitrate reductase(NR) activity and expression patterns of two rice(Oryza sativa L.) NR genes,i.e.OsNia1,OsNia2 in two cultivars(Nanguang and Yunjing) at the seedling stage(35-day-old) under control conditions.The results obtained were as follows.Both rice cultivars growing in mixed N supply achieved a maximum biomass.NO3- supply could increase the biomass of rice shoots and roots on average by about 23% and 18%,respectively.Total N accumulation in Nanguang and Yunjing applying mixed N sources(NH4+ and NO3-) was 18% and 8% higher than those single applying NH4+.RT-PCR analysis of different samples revealed that transcript levels of OsNia1,OsNia2 increased in response to mixed N supply.The transcript level of OsNia2 was significantly higher than OsNia1 in both rice cultivars tested.Nanguang responses to NO3- in a greater way that the transcript levels of OsNia2 both in leaves and roots and OsNia1 transcripts in leaves were higher than Yunjing.NR activities of both in leaves and roots of Nanguang were more significantly promoted than Yunjing fed with NH4NO3.The high activity of NR NO2 67 mol/(gh) showed that rice was also efficient in absorbing and assimilating NO3-.The more efficient inorganic N assimilation resulted in an improved growth of rice.In conclusion,rice,like many plants,supplied with mixed N have a beneficial effect on crop growth NO3-.Therefore,promoting nitrification to a certain extent in paddy soil is of great significance for rice production.