EVOLUTION OF ENDODORMANCY IN APPLE ROOTSTOCK M9

During two successive years, from the beginning of October until the end of March and from mid August until March, i.e. before natural bud break, one-year-old cuttings of about 60 cm long were collected in the field every two weeks. These rootstocks were cut in four segments of equal length and brought under growing conditions in a growth chamber. The development stage of the terminal bud of each segment was measured every two days. In these experiments various endodormancy curves and parameters have been used to describe the endodormancy state between buds throughout the endodormant period.

Before maximal endodormancy, the apical buds are more dormant than the lateral ones and a basitonic gradient does exist between the lateral buds, i.e. a faster bud break under forcing conditions downwards along the shoot. The apical buds are longer and more intense endodormant. After maximal endodormancy, the relation between the lateral buds is reversed; there is an acrotonic gradient between the lateral buds. Towards the end of endodormancy, the apical buds burst earlier than the lateral buds. Although at this moment the apical buds reach earlier 50% of bud break than the lateral ones, they both reach 100% of bud break at the same moment. The start, end and depth of endodormancy differ from year to year. We even question if lateral buds become endodormant every year. Our results suggest that the bud development stage, which burst buds reach, increases with increasing chilling. More, the development stage displays a basitonic tendency while the bud break is either basitonic in the beginning or acrotonic at the end of the endodormant period.

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