Artificial spur extinction alters light interception by 'Royal Gala' apple trees

Total dry matter production by apple orchards is positively related to light interception. Consequently, maximising light interception is important in commercial apple orchards, as it directly affects tree growth and yield. Artificial spur extinction (ASE) is a method of crop load control that reduces the density and alters distribution of floral buds in whole trees. Because ASE reduces total bud numbers on the tree, total light interception may be affected. The objective of this study was to investigate the impact of ASE treatments on canopy light interception. In a mature 'Royal Gala'/M9 orchard in Hawke's Bay, New Zealand, we compared unmodified trees with trees managed using ASE. Bud densities in ASE trees were set to 2, 4 and 6 buds cm^-2 branch cross-sectional area (BCA) in late winter, while unmodified trees were not altered. On both ASE and unmodified trees, crop loads were set after final fruit drop by hand thinning to 2 fruit bud^-1 on 2 buds cm^-2 BCA or single fruit on 4 and 6 buds cm^-2 BCA. Over one season, fractional light interception by the canopies was calculated from the difference between the mean of irradiance readings above the canopy and the mean of irradiance readings below the canopy. Light interception of unmodified trees increased from ∼30% at 2.5 weeks after budbreak (WABB) to ∼60% at 8 WABB and thereafter did not change until leaf-fall. Prior to 8 WABB, light interception by trees set at ASE 6 did not differ from that by the unmodified trees. In trees set at ASE 4 and 2, light interception was initially lower (25.7 and 22.7% respectively) than other treatments and this effect lasted until 5 WABB in ASE 4 and 8 WABB in ASE 2. At full canopy, trees managed with ASE intercepted ∼4% more light than unmodified trees. Higher light interception of ASE trees is probably because ASE stimulates a higher proportion of fruiting spurs to produce short- to medium-length annual bourse shoots with greater leaf area than spur bourse buds.