Nitrate supply affects root growth differentially in two rice cultivars differing in nitrogen use efficiency

Partial nitrate nutrition was testified to improve rice (Oryza sativa L.) growth. However, how partial nitrate nutrition is related to root growth in rice cultivars with different N-use efficiency is still unclear. Two rice cultivars, Nanguang (high N-use efficiency) and Elio (low N-use efficiency), were grown on six ratios of NH4+/NO3- in the solution. The response of root growth to partial nitrate nutrition was investigated and N status and auxin concentration were recorded in order to elucidate the mechanisms by which the optimal ratio of NH4+/NO3- that controls the architecture of the root system. The length of adventitious and lateral roots was stimulated only in cv. Nanguang by partial nitrate nutrition. Nitrate-stimulated root length in Nanguang resulted mainly from root initiation rather than root elongation. Root biomass was similar between two rice cultivars under NH4+ supplied, while higher root biomass was observed in cv. Nanguang than in cv. Elio under 25% nitrate supplied for 10 weeks. Significant IAA increase was recorded in cv. Nanguang under 25% nitrate presence in the nutrition compared to under sole NH4+ solution. The presence of nitrate increased root initiation in the rice with high N-use efficiency, which led to faster biomass accumulation and higher N-use efficiency at later growth stages.