Nitrogen manipulation affects leaf senescence during late seed filling in soybean

It is known that soybean plants store nitrogen during vegetative growth stage, and then remobilize it to the seeds during seed filling. This nitrogen remobilization is assumed to induce leaf senescence. The objective of this study was to investigate whether the manipulation of nitrogen availability could affect leaf senescence during seed filling. Soybean variety Fukuyutaka was grown in the side-opened vinyl house in 2014 and 2015 in Saga University, Japan. The plants were sufficiently watered with the solution containing 100 mg L⁻¹ nitrogen and other nutrients before starting the treatments. The nutrient solution with different nitrogen concentrations (5–800 mg L⁻¹) was applied to the plants from R6 (full-seed) to R7 (physiological maturity) stages. Parallel to the progress of plant senescence after R6 stage, the leaf-SPAD value, leaf nitrogen and soluble protein contents decreased gradually in control plants (100 mg L⁻¹ nitrogen); however, these parameters did not change in higher nitrogen concentration treatments, whereas decreased quickly in lower nitrogen treatments. Therefore, the leaves and stems in higher nitrogen treatments remained green even at harvest time when the whole plant senesced in control and lower nitrogen treatments. The relative expression of an autophagy gene, GmATG8c, which is associated with nitrogen remobilization, showed the lower the nitrogen availability the earlier the up-regulation. The present results revealed that the shortage of nitrogen could stimulate the leaf senescence, while increasing soil nitrogen availability could delay even stop the leaf senescence, indicating that the nitrogen availability could be a key regulating factor of monocarpic senescence in soybean.