DROUGHT RESPONSE OF YOUNG APPLE-TREES ON 3 ROOTSTOCKS .2. GAS-EXCHANGE, CHLOROPHYLL FLUORESCENCE, WATER RELATIONS, AND LEAF ABSCISIC-ACID

'Imperial Gala' apple trees (Malus xdomestica Borkh.) on M.9 EMLA, MM. 111, and Mark rootstocks were subjected to two drought-stress and reco very periods in a rainshelter. Water relations, gas-exchange parameter s per unit leaf area and per tree, chlorophyll fluorescence, and leaf abscisic acid content were determined during each stress and recovery period. Whole-plant calculated gas exchange best indicated plant respo nse to drought stress, with consistent reductions in CO2 assimilation, transpiration, and leaf conductance. Variable and maximal chlorophyll fluorescence and fluorescence quenching were not as sensitive to stre ss. Other fluorescence parameters showed little difference. The most c onsistent decreases due to stress for gas exchange per square meter we re in transpiration and leaf conductance, with few differences in CO2 assimilation and fewer for mesophyll conductance, internal CO2 concent ration, and water-use efficiency. Leaf water potential was consistentl y lower during drought stress and returned to control values upon irri gation. Leaf abscisic acid content was higher for drought-stressed tre es on M.9 EMLA than control trees during the stress periods but incons istently different for the other rootstock treatments. Trees on M.9 EM LA were least affected by drought stress, MM.111 was intermediate, and Mark was the most sensitive; these results are consistent with the gr owth data.