EFFECT OF PREVIOUS-SEASON AND CURRENT LIGHT ENVIRONMENTS ON EARLY-SEASON SPUR DEVELOPMENT AND ASSIMILATE TRANSLOCATION IN GOLDEN DELICIOUS APPLE

Flowering spurs of 'Golden Delicious' apple showed variation in patterns of leaf area development during the period between petal fall and June drop. Differences were the result of changes to bourse leaf area in response to shading of the spur occurring in the previous season or during current-season development. Bourse leaf area was lowest on spurs which were shaded in the previous season. These spurs developed bourse shoots with fewer leaves and which ceased extension growth earlier than bourse shoots on well illuminated spurs or those temporarily shaded in the current season. Dry weight per unit leaf area of spur leaves was markedly greater for well exposed spurs than for spurs shaded in either the previous or the current season. Similar responses were observed with bourse-shoot leaves in the period five to eight weeks after full bloom. Patterns of allocation of C-14-labelled photoassimilate derived from spur or bourse leaves, two weeks after full bloom, were not affected by shade treatments. In contrast, the temporary shading of well-exposed spurs, five weeks after full bloom, significantly reduced the proportion of spur leaf assimilate partitioned to the fruitlets, compared with unshaded spurs. A similar but non-significant trend was also evident for assimilate originating from the bourse leaves. The early termination of bourse shoot growth on spurs which were shaded in the previous season resulted in more bourse-derived assimilate being partitioned to fruitlets, in comparison with spurs from well-exposed sites, the differences increasing with time after full bloom. Overall, shade-induced effects on spur 'vigour' appeared to be related to changes in total spur photosynthetic potential rather than to qualitative differences in the partitioning of assimilates.