One-year-old apple trees ( Malus domestica Borkh., MM. 106) were subjected to all combinations of 10° and 20°C root and shoot temperatures after their chilling requirement had been satisfied at 4°. Tree root temperature of 20° significantly increased percent budbreak over trees with roots at 10°. Trees with a shoot temperature of 20° caused accelerated budbreak over trees with 10° shoots, but the final (day 18) total budbreak was not different due to shoot temperatures. Shoot temperatures strongly influenced extension of new shoots, starch breakdown in fibrous roots, and soluble sugar increase in fibrous roots and root shank. Shoot temperature of 20° increased all these parameters over the 10° shoot temperature.
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Belding et al. (1987) studied this question.