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 (<i>Glycine max</i> L. Merr.) were treated with vertically heterogeneous light (HL) to simulate the light condition<b>s</b> in high-density planting. The net photosynthetic rate (<i>P</i> _N) 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.