Validation of the shoot assay as a proxy to determine progression of dormancy in intact apple trees

Forcing defoliated, excised, one-year-old shoots to bud break is the norm in dormancy research and this method has been used successfully to describe dormancy progression patterns in deciduous trees, especially during the release from bud dormancy, and in the evaluation of rest-breaking agents. In warm fruit growing regions, leaves are present well into winter and shoots must be defoliated prior to forcing. The entrance into bud dormancy under these conditions is protracted compared to colder regions. The possible effects of the leaves and roots on the bud dormancy could influence findings.

To test the validity of the shoot assay and the effects of the roots and leaves on the bud dormancy, we compared the dormancy curves and bud break results from: a) the traditional shoot assay (excised, defoliated shoots) against, b) the forcing of single shoot, intact, potted trees, as well as c) single shoot, defoliated potted trees, and d) excised shoots with leaves, during dormancy over two consecutive seasons. To assess the shoot assay as a proxy during the evaluation of rest-breaking agents we tested a rest-breaking treatment before spring using a) excised shoots and b) single shoot, potted trees.

Leaves influenced dormancy during the entrance into dormancy in both excised shoots and intact trees. While roots influenced the depth of bud dormancy when leaves were not present. A significant, strong correlation between the progressions of dormancy of excised, defoliated shoots and defoliated trees were found in both trial seasons.

The shoot assay is therefore a good proxy for dormancy progression caused by factors within the bud itself. Using shoot assay to screen rest breaking agents showed the same significant time advancement of bud burst but gave conflicting results in terms of maximum bud burst between the two trial seasons. More research is suggested to understand this phenomenon.

This research was crucial in providing credibility to all past and future dormancy research done using the “shoot assay”. It provides us with the necessary scientific proof that the methodology through which dormancy research has been conducted for studying progression of dormancy is sound. However, in rest-breaking research, a potted plant is a more reliable proxy for an orchard tree until year-on-year discrepancies in bud burst have been explained.