EFFECTS OF FRUITING ON SEASONAL APPLE LEAF CHLOROPHYLL FLUORESCENCE

The effect of fruiting (non-, low-, medium-, and high-crop loads) on leaf chlorophyll fluorescence of ‘Braeburn’/M.26 was measured in situ at monthly intervals throughout the 1995/96 growing season, using a pulse-modulated fluorescence system. Differences in net photosynthesis indicated that various crop loads may use differently the absorbed light energy available to the leaves through the growing season. Therefore, this study investigated the differences in energy dissipation, through both the photochemical and thermal pathways. It was hypothesized that the non-cropping compared to the high-cropping trees would have a higher proportion of energy dissipated thermally rather than by the photochemical process, due to lower leaf net photosynthesis which would reduce the utilization of absorbed light energy. Leaf fluorescence was measured initially in dark adapted leaf disks to establish the intrinsic efficiency of photosystem II with all reaction centers open (F_v/F_m) and, subsequently, the photochemical yield (ΔF/F_m') of leaves exposed to natural irradiances was measured in each treatment through the day. The light-dependency of the photochemical yield of leaves within each treatment was then determined by analysis of these data. Results showed that only at harvest time were there crop load effects on photochemical yield, with the low- to non-cropping trees having a significantly lower ΔF/F_m' than trees with medium - or high-crop load. Underscoring this decline in yield was a marked increase in thermal dissipation as crop load declined. Diverting excess energy to heat was therefore an effective method for maintaining high photochemical efficiency in response to changes in sink demand induced by variation in crop load.