EFFECTS OF DEFRUITING ON SOURCE-SINK RELATIONSHIP, CARBON BUDGET, LEAF CARBOHYDRATE CONTENT AND WATER-USE EFFICIENCY OF APPLE-TREES

Three-year-old apple (Malus domestica Borkh. cv. Golden Delicious) tre es with ca 5 m(2) leaf area each were grown in situ in lysimeters and defruited either in August, September or October. Net photosynthesis, dark respiration and water use efficiency were measured for 5 days pri or to and after fruit removal, using tree canopy chambers and infrared gas analysis with a dedicated interface and software. Net photosynthe sis of the tree canopy declined after defruiting, irrespective of the month of defruiting. The decline after fruit removal was 30-40% in Aug ust or September and 57% in October. After defruiting in August or Sep tember, trees respired 50-60% slower in the dark, but 39% faster after defruiting in October. The excessive respiratory losses after fruit r emoval in October, when the tree lost more carbon than it assimilated, may have been induced by translocation of carbohydrates from the leav es to the perennial woody parts of the tree and by the onset of leaf s enescence. Fruit removal in August or September increased water use ef ficiency by 31-41%, but decreased water use efficiency in October by 7 5%.