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.of IAA in young bolls. Rodgers (26) used a bioassay to estimate the IAA content of bolls and reported that abscising bolls contained much less IAA than retained bolls. Differences were not evident, however, until the boils were more than 5 d old. Bolls frequently abscise within 5 d of anthesis (13). Changes in hormone concentrations must precede abscission ifthey are to cause abscission.The concentration of IAA in the abscission zone may be more important than the IAA content of bolls in regulating boll abscission. Auxin transport inhibitors stimulated leaf abscission (3, 21, 22) presumably because they decreased the concentration of IAA in the abscission zone. It is possible that water deficit affects the IAA content of abscission zones by decreasing auxin transport (5). Water deficit and auxin transport inhibitors had similar effects in promoting leaf abscission induced by ethephon (23). No direct data are available, however, on the effects of water deficit, or a shortage of photosynthate, on the concentration of IAA in abscission zones of either leaves or fruits.The purpose of the work reported here was to determine the effects of a nutritional stress (shortage of photosynthate) on the ABA and IAA contents ofyoung cotton bolls and their abscission zones in relation to boll retention rates. Because harvests were made at different times during an irrigation cycle, information was also obtained on the effects of water deficit on ABA, IAA, and boil retention. The results support the hypothesis that stresses decrease boll retention because they affect the hormonal balance in bolls and their abscission zon...