MORPHOLOGICAL AND PHYSIOLOGICAL RESPONSES TO WATER STRESS FOR SEEDLINGS OF DIFFERENT WALNUT GENOTYPES

Container-grown seedlings of walnuts (Juglans regia L.) were watered either every other day (well watered) or subjected to withholding irrigation for 20 day periods (water-stressed) for a 60 day field experiment. RWC, dry mass of main roots and stem, leaf number, angle between petiole and stem, angle between lateral shoot and stem as well as water relations were measured every 4 days in the stress period. Before and during the experiments, leaf and root samples were collected and analysed for PRO (proline). The levels of PRO increased in parallel with the severity of drought stress in both leaves and roots of tolerant genotypes. Reduction in leaf number as a result of water stress was attributed to reduced leaf initiation, leaf growth rate and leaf size. When subjected to prolonged water stress, 'Lara', and 'Serr' cultivars wilted and lost both leaves and branches more rapidly than 'Chandler' and 'Panegine 20 '. Reduction in most of growth indices resulted from water stress. In contrast, angle between petiole and stem and angle between lateral shoot and stem was increased in response to water stress. These differences between species were attributed to differences in the allocation of carbon between leaves and roots and may relate to osmotic adjustment. Rapid recovery in 'Chandler' and 'Panegin 20 ' following the pro-longed water-stress treatment was attributed to high cell wall elasticity. Previous exposure to water stress contributed to water stress resistance and improved recovery after stress.