DISCRIMINATION AGAINST (CO2)-C-13 IN THE LEAVES OF WATER-STRESSED [BRAEBURN] APPLE

During CO2 fixation, C3 plants discriminate against the naturally occu rring stable isotope (CO2)-C-13. The extent of discrimination may be a useful screening tool to evaluate the extent of stress, as compared t o gas exchange measurements, as it is an integrated measure of interna l physiological and environmental parameters influencing the performan ce of the plant throughout the season. Twelve four-year-old <Braeburn> apple trees (Malus domestica Borkh.) were used in this study The thre e treatments, each randomly assigned to four trees, were: control (C) which received sufficient irrigation to initiate drainage, early defic it (ED) which was deficit irrigated from 55 days after full bloom (DAF B) to 10000 DAFB and then irrigated as for C, and late deficit (LD) wh ich was deficit irrigated from 105 DAFB until final Harvest at 177 DAF B. Leaf water potential was reduced in ED and LD treatments during the stress period as was photosynthesis and stomatal conductance. A signi ficant reduction in the discrimination against (CO2)-C-13 occurred in the ED and LD trees when evaluated at the end of the stress period. Th is manuscript highlights the usefulness of (CO2)-C-13 discrimination i n the evaluation of the impact of water deficit on the physiological p erformance of apple trees.