Physiological response of green ash (Fraxinus pennsylvanica Marsh.) to drought

Gas exchange measurements were made on leaflets of well-watered and drought-stressed green ash trees (Fraxinus pennsylvanica Marsh.) during the summers of 1986 and 1987 with a view toward understanding drought tolerance in this economically-important Great Plains tree species. On July 19, 1986, net assimilation rate, stomatal conductance and leaf water potential of drought-stressed (DRY) trees were significantly reduced in relation to well-watered (WET) trees, and leaflet temperatures were significantly elevated. However, the temperature differences could not completely account for these reductions in the DRY treatment group. Water deficits caused by leaf xylem cavitation probably limited photosynthesis directly. After irrigation on July 29, 1986, net assimilation rates and stomatal conductances in DRY trees recovered to pre-stress levels in the morning, indicating the photosynthetic machinery was not damaged. However, during the hotter afternoon, water stress was again evident, due to either damage to the root system and/or xylem cavitation. At the beginning of the 1987 growing season (June 4), there were no significant differences in net assimilation rate or stomatal conductance, the values of which were similar to those recorded in May, 1986. Although differences between DRY and WET trees began to develop as in 1986, drier weather in 1987 may have obscured treatment effects. Net assimilation in WET trees may have been depressed by a humidity effect on assimilation in early July as well as by temporary water stress, and a humidity effect on stomata in late July. The results of this study suggest that under drought conditions, green ash is unable to avoid desiccation of its leaf tissues nor can its photosynthetic machinery tolerate desiccation. However, green ash can survive a severe drought, possibly because the stem xylem is not cavitated or the meristematic tissue tolerates desiccation. These results are discussed in the context of selection for drought tolerance