Osmotic Adjustment, Induced by Drought, in Seedlings of Three Eucalyptus Species

Abstract

Osmotic adjustment was observed in pot-grown seedlings of Eucalyptus behriana, E. microcarpa and E. polyanthemos that had been subjected to one and two periods of drought. The osmotic potential of sap expressed from rehydrated leaves was significantly lower in seedlings which had wilted twice (-2.02 ± 0.05 MPa) compared with those which had wilted once (-1.86 ± 0.05 MPa) and those which had been watered daily (-1.66 ± 0.05 MPa). After two drought cycles, seedlings began to wilt at lower mean values of plant water potential (- 3.51 ± 0.22 MPa) than those which had not wilted previously (-3.14 ± 0.22 MPa). Thus drought-induced osmotic adjustment apparently enhanced turgor maintenance. The ratio of turgid weight to dry weight was slightly, but significantly, smaller in the seedlings subjected to two drought cycles (3.83 ± 0.04 MPa) compared with those subjected to one drought cycle (4.05 ± 0.04). The osmotic adjustment that was induced by two drought cycles in these seedlings was about one third of the observed seasonal osmotic adjustment in mature trees of E. behriana and E. microcarpa in the field.