Grain yield and protein concentration relationships in rice

Grain protein concentration (GPC) is an important aspect of rice grain quality, which contributes to nutritional intake requirements; however, high GPC may also reduce eating quality. Both GPC and grain yield (GY) are greatly affected by nitrogen (N) management, and GPC is strongly linked to GY through shared N pathways. This review aims to determine how GPC in rice is affected under different growing conditions and crop management options and how varieties differ in GPC under different conditions and to identify the link between GPC and GY. It highlights the importance of total N uptake by the crop and that GPC gradually increases with the N application rate up to an optimum at which GY reaches a maximum. While GY varies greatly depending on the growing conditions, GPC tends to be maintained within a relatively narrow range. When a number of genotypes are compared, there is often an inverse relationship between GY and GPC, with a mean reduction in GPC of 0.46 percentage point for each 1.0 t ha(-1) increase in GY. However, the balance between GY and GPC is altered based on the genotype's capacity to both take up N from the soil and distribute it to grain, including its ability to translocate N from vegetative organs to growing grain. The balance varies greatly among genotypes, as demonstrated in the case of hybrids, where GY is often higher but GPC is lower compared with inbred varieties. The review concludes with the identification of future research efforts to further understand the GY-GPC relationship.