Root and shoot traits for rice varieties with higher grain yield and higher nitrogen use efficiency at lower nitrogen rates application

Understanding plant traits that are associated with high grain yield and high nitrogen use efficiency (NUE) is very important in breeding program to develop N-efficient varieties. However, such traits are yet to be identified in rice. We investigated this issue using rice varieties differing in response to N rates. Four japonica rice varieties, Huaidao 5 (HD-5), Lianjing 7 (LJ-7), Ninjing 1 (NJ-1) and Yangjing 4038 (YJ-4) were grown in the field, and four N rates, 0, 100, 200 and 300kgha^-1, were applied during the growing season. Results show that both HD-5 and LJ-7 produced higher grain yield, took up higher amount of N from the soil, and exhibited higher NUE than NJ-7 or YJ-4 at lower N rates (0, 100 or 200kgha^-1). Grain yield and NUE were comparable among the four varieties at the N rate of 300kgha^-1. When compared with NJ-1 or YJ-4, both HD-5 and LJ-7 had greater root and shoot biomass, deeper root distribution, longer root length, greater root length density, root oxidation activity and crop growth rate, higher photosynthetic NUE, and more remobilization of nonstructural carbohydrate from stems during grain filling at lower N rates. Our results suggest that HD-5 and LJ-7 can maintain grain yield at lower N rates as N-efficient varieties. The shoot and root traits, especially the deeper roots, greater root oxidation activity and higher photosynthetic NUE at lower N rates, could be used in selection for N-efficient rice varieties.