Stay-green: A consequence of the balance between supply and demand for nitrogen during grain filling?

Retention of green leaf area in grain sorghum under post-anthesis drought, known as stay-green, is associated with greater biomass production, lodging resistance and yield. The stay-green phenomenon can be examined at a cell, leaf, or whole plant level. At a cell level, the retention of chloroplast proteins such as LHCP2, OEC33 and Rubisco until late in senescence has been reported in sorghum containing the KS19 source of stay-green, indicating t hat photosynthesis may be maintained for longer during senescence in these genotypes. At a leaf level, longevity of photosynthetic apparatus is intima tely related to nitrogen (N) status. At a whole plant level, stay-green can be viewed as a consequence of the balance between N demand by the grain an d N supply during grain filling. To examine some of these concepts, nine hy brids varying in the B35 and KS19 sources of stay-green were grown under a postanthesis water deficit. Genotypic variation in delayed onset and reduce d rate of leaf senescence were explained by differences in specific leaf ni trogen (SLN) and N uptake during grain filling. Matching N supply from age- related senescence and N uptake during grain tilling with grain N demand fo und that the shortfall in N supply for grain filling was greater in the sen escent than stay-green hybrids, resulting in more accelerated leaf senescen ce in the former. We hypothesise that increased N uptake by stay-green hybr ids is a result of greater biomass accumulation during grain filling in res ponse to increased sink demand (higher grain numbers) which, in turn, is th e result of increased radiation use efficiency and transpiration efficiency due to higher SLN. Delayed leaf senescence resulting from higher SLN shoul d, in turn, allow snore carbon and nitrogen to be allocated to the roots of stay-green hybrids during grain filling, thereby maintaining a greater cap acity to extract N from the soil compared with senescent hybrids.