Drought-induced leaf abscission and whole-plant drought tolerance of seedlings of seven black walnut families

Whole-plant drought tolerance and leaf abscission in response to drought of 5-month-old, half-sib black walnut (Juglansnigra L.) seedlings representing seven geographic origins were examined. Seedlings were subjected to six different levels of drought stress and then reirrigated. Mortality, leaf abscission, and refoliation responses were measured. Only one of 404 seedlings actually died. Survival of stem cambial tissues at predawn leaf xylem pressure potential values as low as −4.0 MPa was associated with the capacity for drought-induced leaf abscission. The percentage of seedlings exhibiting at least 80% leaf abscission increased linearly as predawn leaf xylem pressure potential decreased from −1.5 to −3.5 MPa. Although substantial differences in leaf abscission among families were observed, these differences were not statistically significant (P ≤ 0.05). Eighteen percent of all seedlings exhibited some degree of refoliation upon recovery of plant water status. However, the cumulative area of regrowth foliage was insignificant in terms of its adaptive value for resumption of photosynthetic activity following stress relief.