Concentrations of Abscisic Acid and Indoleacetic Acid in Cotton Fruits and Their Abscission Zones in Relation to Fruit Retention

An experiment was conducted with field-grown cotton (Gossypium hirsutum L.) to determine the effects of drought and an increase in available photosynthate on the abscisic acid (ABA) and indoleacetic acid (IAA) contents of 3-day-old bolls and their abscission zones. Photosynthate availability was manipulated by removing about two-thirds of the plants to permit increased irradiance, and thus photosynthesis, in the plant canopy. The demand for photosynthate was decreased by removing all bolls from the remaining plants. The thinning and defruiting operations were performed about 3 weeks after first flower. Control plants were neither thinned nor defruited. Effects of water deficit were observed by making three harvests at different times during a 2-week irrigation cycle. Increasing the availability of photosynthate increased boll retention, but had relatively little effect on the concentrations of ABA and IAA in bolls. However, it did increase the concentration of IAA in abscission zones. Water deficit increased the ABA content of bolls and abscission zones and decreased the IAA content of bolls and abscission zones. Across all treatments, the IAA content of abscission zones was positively correlated, and the ABA content of bolls was negatively correlated, with boll retention. The results indicate that stresses change the hormonal balance in ways that are consistent with observed increases in fruit abscission.