Role of nitrogen utilization in facilitating photosynthetic compensation of soybean under vertically heterogeneous light

Photosynthetic compensation enables high-density planted crops to use the available light efficiently. However, the underlying mechanism remains unclear. Herein, soybeans (Glycine max L. Merr.) were treated with vertically heterogeneous light (HL) to simulate the light conditions in high-density planting. The net photosynthetic rate (PN) increased in upper unshaded leaves (UL) while it decreased in lower shaded leaves (LL) under HL. This evident photosynthetic compensation was accompanied by a preferential distribution of N to UL. Correspondingly, the amounts of chlorophyll (Chl) and Rubisco increased in UL. At the same time, the ratio of Rubisco to Chl (Rubisco/Chl) and the photosynthetic N-use efficiency (PNUE) of UL decreased under HL. In contrast, there was little change in the PNUE of LL. This study suggests that the within-plant distribution and within-leaf allocation of N play significant roles in photosynthetic compensation. The decline in PNUE at the whole-plant level should be viewed as a cost associated with facilitating this process.