The relative sensitivity of the cotton plant to water stress throughout the growing season was determined to identify when irrigation will have the greatest beneficial effect. Daily plant water stress for 72 different data sets of water applications was correlated to corresponding yield criteria. The magnitude of significant correlation was interpreted as the degree of sensitivity. Plant water stress during square formation and early flowering resulted in fewer bolls to reach maturity, but this detrimental effect was cancelled by the development of bigger bolls due to greater lint growth. This resulted in better seedcotton and lint yields. Flower and boll senescence which resulted from water stress during flowering peak, however, inhibited seedcotton and lint yields. The most pronounced inhibiting effect stress had on yields, was during boll development well after the end of effective flowering, when it inhibited boll development. Stress during ripening of the bolls increased lint and boll development and consequently enhanced yields. When limited quantities of water is available, preference should be given to irrigation during boll development, then by irrigation when the first flowers appear, followed by irrigation during peak flowering. Water should be withheld from opening of the first bolls.

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