In cotton (Gossypium hirsutum L.), the pattern of assimilation (leaf development) and sink development (fruit development) are out of phase. The phase lag arises because at each fruiting site the leaf expands before the flower bud is fertilized, and senesces before the boll is mature. As a result, assimilation declines as the fruit sink expands. It has been suggested that delaying leaf senescence may result in higher lint yields. This study examined apparent photosynthesis (AP) of aging leaves of cotton cultivars with differing maturities. The objectives of the study were to detect differences in AP during the leaf life span and to study its correlation with lint yield.

In the 1980 study, maximum leaf AP (P(max)) occurred at leaf age of 20-26 days, while in the 1981 study P(max) occurred at a leaf age of 17-23 days. In both studies P(max) rates were maintained until approximately 30 days of age, declining linearly thereafter. Considerable variation among genotypes in AP rates at 45 days of leaf age (p(45)) was observed in both studies. In 1980, the F, hybrid of STV:825 and DPL-7146 showed the highest value, while the lowest was measured in STV-825. Similar genotypic variation was observed during the 1981 study. DES-56 displayed the highest P(45) rate while Coker-SM exhibited the lowest rate.

Simple correlation coefficients of the different AP parameters measured with lint yield were positive but not significant. The data suggest that morphological development and growth habits are more important in determining lint yield than leaf photosynthetic rate and that an optimum balance between vegetative and reproductive growth must be obtained in order to maximize yields. Further yield improvements can then be made by increasing the photosynthetic efficiency of the selected genotypes.