Nitrogen Dynamics and the Physiological Basis of Stay‐Green in Sorghum

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( r=0.751**, n=27 )and rate( r=−0.783**, n=27 )of leaf senescence under terminal water deficit.