GROWTH OF HONEY LOCUST SEEDLINGS DURING HIGH ROOT-ZONE TEMPERATURE AND OSMOTIC-STRESS

Growth of honey locust (Gleditsia triacanthos var. inermis Willd.) seedlings was studied during exposure to reduced osmotic potential (psi-pi) and high temperature in the root zone. Half-sib plants were cultured in solution. Root-zone temperature was increased from ambient (23C) to 35C for 0, 6, 12, or 24 hours.day-1. Within each temperature treatment, solution psi-pi of -0.05, -0.10, and -0.20 MPa were maintained by additions of polyethylene glycol (PEG) 8000. Root and shoot dry weights decreased with increasing exposure to 35C among seedlings in -0.05-MPa solution and decreased for seedlings in -0.10- and -0.20-MPa solutions in all temperature regimes. Growth of epicotyls displayed similar trends, but epicotyls of plants in -0.20-MPa solution were longest with 6 hours.day-1 at 35C. Significant interactions between effects of temperature and osmotic regimes indicated that water-stressed honey locust seedlings are relatively insensitive to elevated root-zone temperatures. However, related studies showed that PEG caused reductions in growth that could not be explained by decreases in psi-pi and suggested that responses of honey locust to PEG differed from those when drought was imposed by withholding irrigation in an aggregate medium.