Nitrogen and stress environments on the physiology and yield of grain sorghum

Based on large responses of sorghum grain yield (SGY) to nitrogen application (N) in temperate environments, many specialists and policy makers consider N as an unequivocal solution to the low SGY of the developing world. The objective of this study was to test the null hypothesis that high N, under stress environments (water or weed), results in increased SGY. An N x stress environment x genotype interaction study was conducted in the field in 1990 and 1991 at Mead (Nebraska), and in the greenhouse in 1991 at Lincoln (Nebraska). Stress environment was weeds (no-weeding, one-cultivation and clean-weeding) in the field, and water (8650, 12 350, 18 350 and 32 495 ml-water pot{dollar}sp{lcub}-1{rcub}{dollar}) in the greenhouse. The influence of N on yield and several growth and development parameters was studied.; Nitrogen (40 and 80 kg ha{dollar}sp{lcub}-1{rcub}{dollar}) resulted in large relative increases (40-65%) in SGY with all weed management levels; thus, supporting the null hypothesis. With very severe and severe water stress, N (1.47 and 2.91 g pot{dollar}sp{lcub}-1{rcub}{dollar}) did not affect SGY as the crop was killed by the water stress or failed to develop beyond heading. However, with moderate water stress, N resulted in decreased SGY. This is apparently because of enhanced water stress during later (reproductive) growth due to bigger plants, and/or a smaller water potential gradient between the soil and the plant. Therefore, the greenhouse study partially disputes the null hypothesis and does not agree with the field experiment. However, SGY responses to N in the field may be explained by the absence of a serious environment stress (wet season) and/or amelioration of weed stress during late sorghum reproductive growth. Genotype was important in the responses of SGY to N, however, only under no water stress in the greenhouse study. SGY was highly positively associated with seed number, sorghum dry matter and water use efficiency. SGY was substantially negatively associated with weed drymatter and weed N uptake.; This study indicated that adequate water application is a way to guarantee SGY in water stress environments regardless of N. Nitrogen was shown to be necessary for high SGY only when adequate water or proper weeding is provided. It was also shown that weeding is necessary for high SGY regardless of N. Therefore in developing countries, environmental stresses have to be ameliorated first for SGY to consistently respond to N.