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Sorghum Sorghum bicolor (L.) Moench hybrids containing the stay‐green trait retain more photosynthetically active leaves under drought than do hybrids that do not contain this trait. Since the longevity and photosynthetic capacity of a leaf are related to its N status, it is important to clarify the role of N in extending leaf greenness in stay‐green hybrids. Field studies were conducted in northeastern Australia to examine the effect of three water regimes and nine hybrids on N uptake and partitioning among organs. Nine hybrids varying in the B35 and KS19 sources of stay‐green were grown under a fully irrigated control, post‐flowering water deficit, and terminal water deficit. For hybrids grown under terminal water deficit, stay‐green was viewed as a consequence of the balance between N demand by the grain and N supply during grain filling. On the demand side, grain numbers were 16% higher in the four stay‐green than in the five senescent hybrids. On the supply side, age‐related senescence provided an average of 34 and 42 kg N ha −1 for stay‐green and senescent hybrids, respectively. In addition, N uptake during grain filling averaged 116 and 82 kg ha −1 in stay‐green and senescent hybrids. Matching the N supply from these two sources with grain N demand found that the shortfall in N supply for grain filling in the stay‐green and senescent hybrids averaged 32 and 41 kg N ha −1 , resulting in more accelerated leaf senescence in the senescent hybrids. Genotypic differences in delayed onset and reduced rate of leaf senescence were explained by differences in specific leaf nitrogen and N uptake during grain filling. Leaf nitrogen concentration at anthesis was correlated with onset and rate of leaf senescence under terminal water deficit.
Borrell et al. (Fri,) studied this question.