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In parts of peninsular India, sorghum (Sorghum bicolor L.) is grown during the dry season using water stored in the root zone. The optimum application of nitrogen is difficult to assess because no comprehensive model exists for the interaction of water and N. To explore this system as a basis for modelling in the first instance and ultimately for better management, sorghum (cv. SPH-280) was grown in the post-rainy season at ICRISAT (Andhra Pradesh, India) with and without irrigation and at six rates of nitrogen from zero to 150 kg/ha applied before sowing. The biomass of top components was measured weekly and of roots every 2 weeks. Interception of solar radiation was monitored continuously in all treatments.

Leaf expansion was strongly influenced both by water and by N, whereas specific leaf area was almost independent of treatment. In the irrigated treatment, the Biomass Radiation Coefficient (e) for the main growth period was almost independent of N application at 1.3-1.4 g/MJ and was also independent of leaf N. In consequence, the main source of differences in yield was a decrease in radiation interception with decreasing N. In contrast, without irrigation, biomass, yield, e and leaf N were all maximal at 60 kg/ha N.

At 33 days after emergence (DAE), root mass was almost independent of N whether water had been applied or not, but was somewhat smaller with irrigation. Later, root, leaf, and panicle mass all responded to N and to water, but stem mass was unresponsive to N with irrigation. There was evidence of translocation from stem to grain in most treatments. With irrigation, a maximum grain yield of 4.8 t/ha was obtained at 150 kg/ha N and without irrigation the maximum was 3.2 t/ha at 90 kg/ha.