Nitrogen dynamics and the physiological basis of stay-green in sorghum

Sorghum [Sorghum bicolor (L,) Moench] hybrids containing the stay-green tra it retain more photosynthetically active leaves under drought than do hybri ds that do not contain this trait. Since the Longevity and photosynthetic c apacity of a leaf are related to its N status, it is important to clarify t he role of N in extending leaf greenness in stay-green hybrids. Field studi es were conducted in northeastern Australia to examine the effect of three water regimes and nine hybrids on N uptake and partitioning among organs. N ine hybrids varying in the B35 and KS19 sources of stay-green were grown un der a fully irrigated control, post-flowering water deficit, and terminal w ater deficit. For hybrids grown under terminal water deficit, stay-green wa s viewed as a consequence of the balance between N demand by the grain and N supply during gain filling. On the demand side, grain numbers were 16% hi gher in the four stay-green than in the five senescent hybrids. On the supp ly 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 senes cent hybrids. Matching the N supply from these two sources with grain N dem and found that the shortfall in N supply for grain filling in the stay-gree n and senescent hybrids averaged 32 and 41 kg N ha(-1) resulting in more ac celerated leaf senescence in the senescent hybrids. Genotypic differences i n delayed onset and reduced rate of leaf senescence were explained by diffe rences in specific leaf nitrogen and N uptake during grain filling. Leaf ni trogen concentration at anthesis was correlated with onset (r = 0.751**, n = 27) and rate (r = -0.783**, n = 27) of leaf senescence ender terminal wat er deficit.