Nitrogen stress alters trade-off strategies between reproduction and vegetative growth in soybean

Soybean is the most abundant and cheapest source of protein with a relatively high economic value. Excess nitrogen negatively affects the growth of soybean plants; however, few studies have investigated whether the vegetative and reproductive organs of soybean respond similarly to nitrogen deposition. The effect of nitrogen deposition on the growth of soybean plants was determined herein at six different levels (0, 25, 50, 100, 150, and 200 kg N ha(-1) yr(-1)). The results indicated that the vegetative organ biomass (roots, stems, and leaves) and the ratio of vegetative organ biomass to total biomass decreased at increasing levels of nitrogen deposition. The vegetative biomass decreased by 24.02% at the highest level of nitrogen deposition. However, the reproductive biomass (seeds and pods) and the ratio of reproductive biomass to total biomass exhibited an opposite trend. The seed biomass increased by 13.83% at the highest level of nitrogen deposition. The chlorophyll fluorescence parameters indicated that photosynthetic carbon fixation was slightly suppressed at high levels of nitrogen deposition. The slight inhibition in photosynthetic capacity following nitrogen deposition led us to speculate that the reduction in total and vegetative biomass was attributed to the reduction in total photosynthetic area due to a decrease in leaf number. However, the reduction in total photosynthetic productivity did not limit the accumulation of reproductive biomass (seeds and pods). The findings suggested that nitrogen deposition stress alters the energy allocation strategy of soybean, and the energy is allocated for maintaining reproductive growth while reducing vegetative growth.