The development of hardwood cutting of grapevine comprise 4 succesive phases. Phase I: first period of shoot extension. Phase II: characterized by a slow growth due to a very low activity of the terminal bud. Phase III: reactivation of the growth consecutive to a resumption of the apical activity. Phase IV: when the growth becomes maximal.
The objective of this work has been to identify the cause of the slowdown of the activity of apical bud during the phase II.
In a first experiment, we have compared the effects of three modes of defoliation applied from the beginning of the phase II on the apical activity: total defoliation, defoliation to have two adult leaves at the base of the shoot and defoliation to have two young leaves near the apex. The results of this experiment showed that only treatments depriving the shoot of young leaves induced an activation of the plastochronic activity. So, it appeared that young leaves exercises an inhibitory effect on the apex. The same experiment has been carried out during the phase IV of cuttings development, when the apical activity was maximal. In this condition, no treatments modified the apical activity. This suggests that the inhibition of the apex by young leaves is specifie to the phase II of the develop¬ ment of a grapevine cutting. In a third experiment, we have demonstrated that the foliar inhibition persisted during the all period of the laminart growth. Cytokinins are probably implied in this competition because exogenous applications of these growth regulators during the phase II produced a stimulation of the activity of apical bud. Results of a last experiment showed that, in cuttings prerooted before budburst, the apical activity was not decreased after the phase I. In other words, in this condition, the growth of the shoot was regular without phase II. We explain this resuit as follows. The roots preformed on the cuttings produced cytokinins in sufficient quantity for provide for needs of both foliar growth and apical activity.
In previous studies, MULLINS showed that abortion of the young bunches on cuttings of grapevine was due to an inhibition by deficiency in cytokinins exercised by young leaves.The present work reveales the existence of an exactly similar correlation between the young leaves and the apex. So, in the beginning of the development of a grapevine cutting, young leaves, apical bud and young bunches are implied in a common correlative process which results on the one hand from a small availability of cytokinins due to the lack of roots and, on the other hand from a more efficient sink effect towards the cytokinins for the young leaves than for the apex and young bunches.
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