Growth responses of common ash seedlings (Fraxinus excelsior L.) to total and partial defoliation

Common ash seedlings, grown in controlled conditions, were completely defol iated 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 d after the completion of stem elongation. Complete defoliation up to 80 d after the completion of st em elongation caused renewed growth of terminal buds. The buds had changed from a reversible state to an irreversible state by 80 d after the cessatio n of stem elongation, as shown by the lack of response to defoliation. When leaves were removed before the cessation of stem elongation, rather than a fter, a similar enhancement of stem growth was observed. Partial defoliatio n experiments indicated that the degree and location of defoliation play im portant roles in the plant response. Complete defoliation or complete remov al of leaflets was necessary to obtain 100% budburst. Apical dominance was altered by partial defoliation treatments such that the basal axillary buds began to grow out. Partial defoliation. especially before the cessation of stem elongation, was prejudicial to stem elongation. These results suggest that the induction of compensatory growth mechanisms in ash seedlings requ ire a threshold level of defoliation. (C) 2000 Annals of Botany Company.