SummaryThe influence of defruiting on vegetative development and its implications for the time response of canopy net CO2 exchange rate (NCERcanopy), transpiration (Tcanopy) and midday stem water potential ( stem) were analysed during the last 4 months of apple fruit development. Two crop-load treatments were applied 2 months after full bloom: defruited (DF) and fruited (F). The latter did not receive any fruit thinning. Four trees in two groups (with two replications of two trees per crop-load treatment), growing on a dwarfing rootstock (M-9), were monitored diurnally for NCERcanopy and Tcanopy, using a whole canopy gas exchange technique. The effects of defruiting were observed shortly after fruit removal. All parameters measured remained constant or increased over the 4 months until harvest. Canopy gas exchange decreased for all DF trees. Leaf NCER was also lower in DF trees until harvest. In contrast the leaf area of DF trees increased by 20% compared to F trees. Differences in NCERcanopy between treatments were reversed after harvest, when DF trees registered higher NCERcanopy values than F trees because of their larger leaf area. Leaf conductance and transpiration decreased, and stem increased for DF trees compared to F trees during the 4 month period of differential cropping. Daily NCERcanopy values decreased from late May until September, as did incident solar radiation, and temperatures were cooler in September. Inhibition of NCER at the leaf level increased linearly over time, from 10% immediately after thinning, up to 35% before harvest, and exceeded the 20% increase in vegetative growth in DF trees. This down-regulation, which increased proportionally with time, was interpreted as an inadequate replacement of fruit by vegetative sinks in DF trees over the 4 month research period.