Crop load (CL) is a critical regulator of production and quality on apple. It affects leaf photosynthetic rate and usually an increase is detected in leaves close to developing sinks. The objective of this work was to test if 13C discrimination during photosynthesis could be an indicator of carbon sink limitation. The natural plant carbon isotope composition (13C/12C ratio: d13C) is an indicator of water use efficiency and it is an effective tool to study environmental stresses in plants. Seven-year-old trees of Imperial Gala/Bud 9 (n=30), field-grown at the Clarksville Horticultural Research Station in Michigan, were hand-thinned to five levels of CL manipulating the leaf to fruit ratio (LFR: 4, 8, 16, 32, 64) after June drop. Net photosynthetic rate (A) of leaves was monitored daily during the season and elevated rates were observed in low LFR. The A was inhibited in low CL trees (LFR 32 and 64) more in the afternoon (from 20% to 42% in relation to normal CL: LFR 16) than in the morning (from 5% to 20%), and stomatal conductance declined over the afternoon. Shoot and fruit growth were affected (fruit size –11/+11%, shoot length –13/+18% from normal CL; LFR 16). Variations of the stable carbon isotope composition of leaves show a significant reduction of 13C discrimination in low CL trees (–3.2%: d13C –25.82) and an increase of 13C discrimination (+2.7%: d13C –27.38) in relation to normal CL trees (LFR 15.63). The results were similar to those reported in 2004, which imply isotopic discrimination in relation to source limitation. This is the opposite of what you would expect under water stress conditions. Although trees were well-watered during the season, the effect of water stress on apple trees and its interaction with source limitation will be discussed.