Arrested Leaf Abscission in the Non-Abscising Variety of Pubescent Birch: Developmental,Morphological and Hormonal Aspects

A novel type of abscission development, arrested abscission,is described for a rare variety of pubescent birch (Betula pubescensEhrh. f. hibernifolia Ulvinen), the leaves of which dehydrateand remain attached to the plant. Anatomical examination ofthe abscission zone revealed that its development is temporallyand spatially similar to that of normal leaves of Betula pubescens.Floodingand exogenous ethylene were effective in inducing shedding ofthe leaves at the beginning of the growing season, butthis effectwas lost in senescing leaves. Leaf hydration and abscissionwere retained in the presence of external abscisic acid (ABA)inthe non-abscising variety, which also had a lower level of endogenousABA. The observed responses together with the alteredhormonallevel in this variety suggest that apart from ethylene, ABAis involved in autumnal abscission indirectly by regulatingtheleaf water status. Key words: ABA deficiency, abscisic acid, abscission zone, ethylene, senescenc