Are the effects of winter temperatures on spring budburst mediated by the bud water status or related to a whole-shoot effect? Insights in the apple tree

UMR 1334 AGAP : Equipe AFEF ‘Architecture et Fonctionnement des Espèces fruitières’The effects of winter temperatures, i.e., during dormancy, on shoot architecture are well known with budburst preferentially in the distal or the proximal part of the parent shoot in cold and mild winter conditions, respectively. However, the link with the overwintering bud water status is still scarcely documented. Our study was developed on four apple (Malus domestica Borkh.) cultivars covering a range of chilling requirements from low (‘Condessa’) to medium (‘Granny Smith’) and high (‘Royal Gala’, ‘Starkrimson’), and maintained in either cold (1,428 h below 7.2 °C) or mild (99 h below 7.2 °C) fluctuating winter temperatures. Our aim was to analyze xylem conductance at the stem-to-bud junction, and relative water content and water potential of the bud itself, for buds situated in the distal third of one-year-old shoots. From dormancy to the pre-budburst stage, xylem conductance at the stem-to-bud junction increased or decreased or did not show consistent changes depending on the cultivar and the winter temperature treatment. Whatever the cultivar, there were no significant trends across dates for the effects of winter temperatures on bud water potential and relative water content. Water potential had negative values, between −4.35 and −2.24 MPa, across cultivars and winter temperature treatments without a consistent relationship with actual spring budburst frequency. These results suggested that lateral buds were hydraulically isolated from the parent stem during winter until a few days before budburst. We discussed that the temperature-related spring budburst was likely more related to a whole-shoot effect mediated by hormonal, hydraulics and/or sugar signaling, than to the individual bud water status during dormancy