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CROP PHYSIOLOGY & METABOLISM

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

^a Hermitage Research Station, Dep. of Primary Industries, Warwick Queensland 4370, Australia
^b QDPI/CSIRO Agricultural Production Systems Research Unit, Toowoomba Queensland 4350, Australia

borrela{at}dpi.qld.gov.au

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.

Abbreviations: DAE, days after emergence • GLAM, green leaf area at maturity (cm²/m²) • LAI, leaf area index • LNC, leaf nitrogen concentration (mg N g^-1) • ND, No water deficit • NUE, Nitrogen use efficiency (kg kg^-1) • PFD, Post-flowering water deficit • SLN, Specific leaf nitrogen (g N m^-2) • SLW, Specific leaf weight (g m^-2) • TD, Terminal water deficit

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