ANALYSIS OF THE RESPONSE OF GRAPEVINE BUDS TO CONTROLLED FREEZING

The response of latent grapevine buds (Vitis vinifera L cv Chardonnay) from a Champagne vineyard was studied by means of differential thermal analysis from November 1987 to May 1988. During the rest period between November 1987 and March 15th, freezing thermograms (1-degrees-C/min) displayed 3 crystallization peaks (E1, E2 and E3) as previously described (fig 1). On and after the 15th of March, before any apparent morphological change in the bud, the freezing point (first exotherm) and starting temperatures of the 2 last exotherms (E2 and E3) increased progressively. At stage 2, corresponding to the swelling of the bud according to Eichhorn and Lorenz terminology, the starting temperature of the third exotherm increased, leading to the merging of the 2 peaks E1 and E2: the thermograms displayed only 2 crystallization peaks. From stage 3 to stage 9 which corresponded to rapid elongation of the shoot and to growth of the leaf and inflorescence, the starting temperatures of the 2 remaining peaks continued to increase. This evolution, which was related to phenological stages, affected the second crystallization peak and led to the fusion of the 2 peaks: freezing thermograms at the end of the growth recovery period only displayed 1 exotherm. At stage 2, the first exotherm induced irreversible histological alterations in the main bud, whereas histological organization of the axillary bud remained apparently unaltered. At stage 5, which was particularly studied in this work, destruction affected every tissue: bark (fig 2), xylem and liber (fig 3), meristematic tissues of the apical dome (fig 4), and the leaf and inflorescence (fig 5).