Foliar influences on the vegetative development of grapevine
Various defoliation treatments were applied to grapevine shoots during the whole duration of the growth period: full defoliation of every shoot of vine, defoliations retaining a various number of adult leaves to the base of every shoot and defoliations retaining a various number of young leaves to the top. The effects of these treatments allow to identify the major foliar influences on the vegetative development. Total defoliation induced a lesser intemodal elongation. This result is probably due, in part, to a carbohydrates deficiency consecutive to this drastic treatment. The defoliations with variation of the number of young leaves showed that the length of internodes increased when this number was greater. This result indicates that young leaves seems exercise a morphogenetical influence on the longitudinal growth of intemodes. For the three defoliation modalities, the rythm of leaves initiation was unchanged. So, it appears that, contrary to many other species, the leaves exercise no control on the plastochronic activity of terminal bud. The results of experiments with variation of the number of leaves maintained on the shoots permit to clarify the relation between foliage and "cane ripening". The already known positive correlation between the length of the suberized part of the shoot and the number of adult leaves has been confirmed. However, the effects of these treatments indicated that this correlation was not linear. Moreover, it seems that every shoot has specific " cane ripening " potentialities determined by other factors that their foliage. The leaves seem act only as revelators of these potentialities. These same experiments retaining adult leaves showed also that such treatments induced a more growth of the not removed leaves. Their laminar extension was increased. The thickness of their lamina and so the one of their palisade parenchyma were more important that in leaves of the control. These observations confirm and explain the compensatory phenomena consecutive to a reduction of the foliar apparatus already described in previous works by other autors. The two mean results of this study consists in the demonstration of the role played by adult leaves both in growth arrest and in apical senescence towards the end of the growing season. A single leaf retained in the upper or lower part of the shoot was sufficient to cause the growth arrest. On the other hand, only old leaves inserted at the base of the shoot can produce the death of the apex. These two last results are analysed and discussed taking in consideration both our experimental results and bibliographical elements. The growth arrest is probably not only determined by the foliar influence. We can expect that a decreasing production of cytokinins by the roots contributes also to this stopping of the apical bud activity. With regard the question of the apical senescence, some results showed that this phenomenom is not caused by a nutrient deficiency. Therefore, it is a question of a correlative process in which the adult leaves inserted to the base ofthe shoot are implicated. This last foliar influence was, so far, completely unknown.
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