The Carbon and Nitrogen Balance in Developing Grains Affects Grain Protein Concentrations in Two Wheat Varieties with Similar Yields

The grain protein concentration (GPC) and yield are intimately and negatively correlated in wheat. However, how carbon (C) and nitrogen (N) supplies influence the wheat growth that underlies yield and quality formation has not been fully explored. Two wheat varieties with high GPCs (ZM578) and low GPCs (LY502) were grown under low and optimum N supplies in the field, and their growth, yields, and GPCs were closely investigated. The grain yield and total N uptake of the wheat variety ZM578 were comparable to those of LY502, but the GPCs increased by 12.4–16.1% on average. Compared with LY502, ZM578 had lower maximum stem numbers but similar spike numbers at maturity due to a lower rate of tiller mortality, which partly contributed to its grain yield. The grain N and C assimilation rates strongly and linearly correlated with each other, while steeper slopes were observed for the optimum- vs. low-N treatments, as well as for the high- vs. low-GPC treatments. Similarly, the C to N ratios in the developing grains of ZM578 were lower than those in the developing grains of LY502, regardless of N availability, which indicated that more N was transported to the grains per unit C in high-GPC ZM578. The random forest model revealed that the grain N uptake and leaf senescence (SPAD during grain filling) were the major controlling factors of yields and GPCs in cultivar ZM578. Overall, this study demonstrated that high yields and high quality can be simultaneously achieved by optimizing N management and selecting superior varieties.