MORPHOLOGICAL ADAPTATIONS TO DROUGHT IN SEEDLINGS OF DECIDUOUS ANGIOSPERMS

We conducted comparative studies of putative adaptive morphological attributes in seedlings of two drought tolerant forest tree species: post oak (Quercus stellata Wangenh.) and white oak (Quercus alba L.) and two drought-sensitive species: sugar maple (Acer saccharum Marsh.) and black walnut (Juglans nigra L.). In general, root experiments indicated that seedlings of Quercus species and J. nigra tended to have well-developed taproots and to more rapidly explore deep soil layers than did A. saccharum seedlings. Quercus stellata exhibited a greater capacity than other species for deep root growth. Further, while J. nigra possesses the capacity for vigorous root growth, it also exhibited relatively less root length per unit of leaf area when the soil was moist. When established plants were forced to grow in dry soil, downward root growth was substantially retarded in most species. After several weeks in dry soil, root growth nearly ceased, as seed reserves were depleted and photosynthesis was inhibited by water stress. There was no general redirection of growth to roots when plants were subjected to gradual drying cycles or grown in dry soil. There was no tendency for drought-tolerant species to possess lower rates of cuticular water loss. The species least effective in curtailing water loss after stomatal closure was Q. stellata, the most drought-tolerant species. There were substantial differences among species in leaf abscission responses under drought. Quercus seedlings showed no evidence of water stress induced abscission, even when plants were subjected to severe drought. In contrast, A. saccharum and, particularly, J. nigra seedlings showed substantial leaf abscission under water stress. Production of new leaf area after abscission was observed in non-Quercus species, but was not adequate to compensate for leaf area loss. The results indicated that: (i) very drought-tolerant seedlings of Q. stellata appear to have better relative water supply capacity by roots to leaves than drought-sensitive species in the normal environment presented to newly germinated seedlings; (ii) J. nigra exhibits vigorous root growth after germination that is consistent with its known capacity for stress avoidance, but its allocation of root growth per unit of leaf area is less favorable than the very drought-tolerant Q. stellata; (iii) for this group of species, at least, low cuticular transpiration in water-stressed plants does not appear to be an adaptive trait associated with drought-tolerant species; and (iv) drought-prone sites may present especially detrimental environments for the carbon economy of drought-sensitive species because of the greater probability of recurrent massive leaf abscission.