Analysis of Water Potential Isotherms in Two Ornamental Shade Tree Species Entering Winter Dormancy

Abstract

Abstract Osmotic adjustment in response to onset of winter dormancy was characterized in well-watered, potted sweetgum (Liquidambar styraciflua L.) and southern magnolia (Magnolia grandiflora L.) growing outdoors in Knoxville, Tenn. Analyses of water potential isotherms indicated that adjustment occurred in both species, with osmotic potential (ψπ) at full turgor decreasing 0.8 MPa in sweetgum (by the time of first color, 27 Oct.) and 1.0 MPa in magnolia (by 1 Dec.). Osmotic adjustment occurred despite the fact that plants did not suffer osmotic stress; morning and afternoon leaf relative water content (RWC) and leaf water potential (ψ) remained high throughout the fall. Leaf conductance was halved in sweetgum and doubled in magnolia as the autumn progressed. A correlation was found in magnolia between turgid : dry weight ratio and ψπ at full turgor. Tissue elasticity decreased somewhat, as the elastic modulus increased ≈2 to 3 MPa in each species through the autumn. Water potential isotherms changed most dramatically through the autumn in magnolia. Initially, ψ was −1 MPa at 82% RWC and, by December, leaves were able to withstand ψs of −3 MPa before RWC dropped to 82%. These changes are similar to those commonly reported as responses to drought or salinity.