EFFECTS OF CROP LOAD ON APPLE PHOTOSYNTHETIC RESPONSES AND YIELD

Different crop loads (100, 66, 33 and 0%) were established at full bloom by flower cluster removal on 5-year-old field grown apple trees. Crop load tightly controlled vegetative development and, after full canopy was attained, a negative relation was found between final canopy surface area and corresponding fruit number. No differences were found among the fruiting treatments relative to the leaf and fruit biomass accumulation per tree. The non-fruiting trees, however, showed a reduced deciduous biomass increase, despite the larger leaf area produced. Dry matter partitioning to fruits, expressed per unit leaf area, was positively affected by crop load and a rapid decrease in mean fruit weight with high crop densities (up to 16 fruits/m² leaf surface area) could be ascribed to a source-limiting condition due to fruit number. Leaf net photosynthetic rates, estimated during the growing season, did not show differences among the fruiting treatments, but the values were greater than for the non-fruiting trees. Whole canopy net photosynthetic rates, estimated in late summer on a tree with the highest crop load and a non-fruiting one, showed a lower specific daily activity in the non-fruiting condition. On the contrary, the total carbon gain was higher for the non-fruiting canopy than the fruiting one. After harvest, whole canopy photosynthetic activity was still high, comparable to corresponding pre-harvest rates. This could be ascribed to reserve accumulation, wood maturation, and root growth, which is particularly active late in the season. The presence of alternative sinks, able to offset the effect of fruit load alone on assimilate demand, could also explain the weak relationship found between crop load and photosynthetic response during the growing season.