Evaluation of yield and eating quality of representative indica and japonica rice varieties in response to nitrogen levels.

Nitrogen (N) fertilization is a major contributor to yield, quality and environmental impact in rice production. Conceptually, high level of N application is viewed as associated with high yield, inferior quality and high environmental cost. However, very few studies have investigated how yield, eating quality and protein content respond to N application levels under field conditions, especially with regards to rice varieties of different genotypes for genes specifying nitrogen utilization efficiency (NUE). In this study, we conducted field experiments in Hubei (south) and Heilongjiang (north) Provinces, with four different N fertilizer levels in each site. The south site included 11 indica and 3 japonica varieties, and the north site involved 9 japonica varieties. The results showed that for most varieties, yield increased linearly in response to N application from N0 (0 N fertilizer) to N100 (100 kg·ha-1), with no significant further increase, on average, at higher N levels. Notably, significant varietal differences were observed, with a few high-performing varieties sustaining yield increases even at the highest N rate. While the protein content in grain (GPC) was linearly associated with N application levels up to N200 in indica varieties, the response was not linear in japonica varieties where GPC decreased from N0 to N50, and slightly increased from N50 to N100 and further slightly increased at N150. The taste values of the indica varieties showed significant negative association with the N levels showing lowest taste values at the highest N level. There was a significant increase in taste values of the japonica varieties from N0 to N50 and N100, but decrease at N150. Overall, there was a highly significant negative correlation between taste values and GPC. While a moderate level of N fertilization can simultaneously optimize yield and taste value of japonica in the north, no such optimal level could be found for indica varieties. We genotyped 11 GPC and NUE genes of these varieties, and found that indica varieties carried the high GPC and NUE alleles at almost all the loci while the reverse was the case for the japonica varieties, which was corresponded to differential phenotypic responsiveness. Consequently, indica varieties exhibited higher variation in yield and quality traits in response to nitrogen, while japonica varieties showed more attenuated responses. Furthermore, high N application (N200) reduced the contents of P, K, Mg, Mn, and Zn and increased the contents of Cd, and Hg in the indica rice grains in the south site. Taken together, N fertilization at a rate ~ 100 kg·ha-1 may provide a rational practice balancing yield and quality in rice production under the experimental field conditions.