DROUGHT AND FLOOD STRESS EFFECTS ON PLANT DEVELOPMENT AND LEAF WATER RELATIONS OF 5 TAXA OF TREES NATIVE TO BOTTOMLAND HABITATS

Responses of five bottomland tree taxa to drought and flooding were studied to identify those adapted to urban environments. During one experiment, containerized 'Franksred' red maple [Acer rubrum L. 'Franksred' (trademark = Red Sunset)], sweetbay magnolia (Magnolia virginiana L.), black tupelo (Nyssa sylvatica Marsh.), bald cypress [Taxodium distichum (L.) Rich.], and pawpaw [Asimina triloba (L.) Dunal.] mere treated with various irrigation regimes for up to 118 days. Net assimilation rate (NAR) and relative growth rate (RGR) were reduced more by flooding than by drought for plants of ail tare, except pawpaw, which showed similar NAR and RGR during flooding and drought. Only sweetbay magnolia and bald cypress maintained positive NAR and RGR during flooding, and sweetbay magnolia was the only taxon that did not produce significantly less leaf surface area, shoot dry mass, and root dry mass during flooding and drought. Apparent morphological mechanisms of stress resistance included an increase in specific mass of leaves (mg-cm(-2)) during drought for red maple and bald cypress and a 385% increase in the root : shoot mass ratio for droughted plants of pawpaw. Leaf water relations of drought- and flood-stressed 'Franksred' red maple and sweetbay magnolia were determined in a second experiment. Predawn and mid-day leaf water potential (psi) decreased with decreasing root-zone matric potential for both taxa, and transpiration rate was reduced by drought and flooding. Pressure-volume analysis showed that leaves of 'Franksred' red maple responded to drought by shifting symplastic water to the apoplast. Leaves of drought-stressed sweetbay magnolia adjusted osmotically by reducing osmotic potential (psi(pi)) at full turgor by 0.26 MPa. Our results suggest that sweetbay magnolia and bald cypress will perform well at urban planting sites where episodes of drought and flooding regularly occur.