EFFECT OF FRUITING AND DROUGHT OR FLOODING ON CARBON BALANCE OF APPLE-TREES

The response of fruiting or deblossomed trees to water stress such as drought or flooding was investigated in six semi open-top cuvettes eac h containing one apple (Malus domestica Borkh, cv. Golden Delicious) t ree. Xylem water potentials of leaves dropped from -1.2 to -4.1 MPa wi thin 7 d of drought, the effect being enhanced by fruiting. Apple tree s without fruits showed smaller reductions in net photosynthetic rate (P-N) and dark respiration rate (R-D) after 2 d of drought and hence m ore positive carbon balances relative to fruiting trees. Flooding for 4 d had a more pronounced effect on P-N than on transpiration, resulti ng in a reduced water use efficiency (WUE). This reduction in WUE was greater in the non-fruiting trees. Flooding reduced P-N of the whole a pple canopies irrespective of fruiting; apple trees without fruits inc reased R-D resulting in a less positive carbon balance relative to fru iting trees. Fruiting increased the sensitivity to drought of apple tr ees (R-D and P-N), but decreased their sensitivity to flooding (R-D an d WUE), suggesting different adaptation mechanisms for the two forms o f water stress.