Effects of Nitrogen on Leaf Area Index, Leaf Area Duration, Net Assimilation Rate, and Yield of Wheat 1

The low returns from nitrogen fertilization in central and eastern Sudan stimulated an investigation into the response of grain yields of irrigated wheat ( Triticum aestivum L. em. Thell.) to two sources of nitrogen (ammonium sulfate and urea), two methods (broadcasting and sidedressing), and several times of application. Grain yields were unaffected by either source of nitrogen or by method of application. Application of the fertilizer at different stages of growth showed that the addition of fertilizer in the early phases of growth of the crop (at sowing or tillering) gave the greatest response. The response to application at ear emergence was much less. Splitting the nitrogen amount with half given at sowing and half at tillering or ear emergence had no extra merit, and a second application given early in the growing season had more effect on yields than a later one at ear emergence. Growth analysis indicated that the variation in grain yield was mainly a reflection of the effect of the treatments on the leaf area duration (LAD) after ear emergence. Early nitrogen application effectively increased LAD by increasing the leaf area index (LAI) toward the time of ear emergence, while late nitrogen had very little effect on LAI and consequently on LAD after ear emergence.