Abscisic acid as a potent regulator of the transition from juvenile to mature stage inXanthium strumarium

The cotyledons ofXanthium strumarium plants are of low sensitivity to photoperiodic treatment and contain only trace amounts of ABA under long-day conditions. The first pair of leaves, very sensitive to photoperiodic treatment, contains a higher level ofABA, decreasing with age of the plant. Prolonged short-day photoperiodic treatment increases the ABA level in the cotyledons but this is still 10times lower than in the first two leaves. Exogenous 10^?4M ABA increases the ABA level in the cotyledons to the level corresponding to that in the first leaves, and enhances the photoperiodic sensitivity of cotyledons. In contrast to cotyledons, the photoperiodic treatment affects the ABA level in the first pair of leaves only slightly. The authors propose that a high ABA level supports the transition of plants to flowering, while a low ABA level may be responsible for a low photoperiodic sensitivity of cotyledons inXanthium plants.