S16 - Session P3 - The effect of roots and leaves on bud break of apple shoots under forcing conditions during dormancy research

Authors: Esme Louw *, Laura Allderman, Willem Steyn, Nigel Cook

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 the leaves of decideous fruit trees remain 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 roots and leaves 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 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 in the absence of leaves. There was a good correlation between the progressions of dormancy of excised shoots and intact trees. Rest-breaking increased the percentage of bud break and shortened time to bud break independently whether excised shoots or intact trees were used. Although roots and leaves influence dormancy, in the absence of leaves (defoliation) the trends between the assay and the intact tree appeared similar. Furthermore, the shoot assay is a suitable proxy for the evaluation of rest-breaking treatments.