Behaviour of Sugar‐beet Plants ( Beta vulgaris L. ssp. vulgaris var. altissima [Doell]) under Conditions of Changing Water Supply: Abscisic Acid as Indicator

Moderate drought stress increases the root‐to‐shoot ratio in sugar‐beet plants. The question arises whether abscisic acid occurs in these plants and whether it plays a role in their adaptation to limited water supply. Thus, after identification by GC/MS, the content of free ABA and its alkaline hydrolysable conjugates was determined by capillary GLC in sugar‐beet plants of pot and field experiments with differentiated water supply. It was shown that leaves had a higher content of free ABA than tap roots. Within the leaf apparatus the young organs contained more ABA than the fully developed leaves. Moderate drought stress increased both free and alkaline hydrolysable ABA contents in leaves of intact plants, whereas the ABA content of tap roots was not demonstrably influenced. In contrast, severe and rapid osmotic stress treatment of detached leaves by dipping the petioles in mannitol solutions increased the free ABA content at the expense of the hydrolysable conjugates. This suggests an ABA release by the latter. Therefore, sugar‐beet plants seem to have two mechanisms for increasing the free ABA content under drought stress conditions: the gradual synthesis of ABA including its conjugates and the release of free ABA from ABA conjugates.