High nitrogen input increases the total spikelets but decreases the high-density grain content in hybrid indica rice

Nitrogen (N) application strongly affects both the total spikelets and grain weight in rice (Oryza sativa L.), thereby affecting grain yield. Grain weight is closely related to the grain-filling process after the heading stage, and can be regarded as a combination of grain size and grain density (GD). However, there is a lack of information about the response of GD in rice to N. Here, we examined in a lager panicle hybrid indica rice at two field experimental sites the responses of grain traits (including GD), grain yield, and yield components to various N treatments and their relationships with each other. In this study, the grains with six GD ranges were fractionated into the light grain (1.00 g ml⁻¹ ≤ GD < 1.15 g ml⁻¹) and heavy grain or high-density grain (GD ≥ 1.15 g ml⁻¹). High N application increased the total spikelets m⁻², but did not result in high grain yield due to reduced grain weight (seed-setting rate and 1000-grain weight). Reduction of the 1000-grain weight under high N was related to the decrease in grain volume and high-density grain content (the ratio of the weights of high-density grains to the total grains weight). As N application increased from 0 to 225 kg ha⁻¹, on average, the high-density grain content decreased by 31.93 %, from 78.21 % to 53.24 %. Grain plumpness and GD were the main factors affecting grain weight. GD was more important to the grain weight when grain size was smaller, and plumpness degree and GD were both crucial for grain weight when grain volume was larger. Compared with light grains, heavy grains had higher amylose content and lower crude protein content. These results indicated that the constraint and compensation relationship between spikelet number and grain weight in rice was also reflected in the equilibrium between grain size and GD. Although high N input in rice production increased the total spikelets m⁻², high N input decreased the grain volume, especially the high-density grain content.