Physiological adaptation of Alternanthera philoxeroides under water stress condition induced by polyethylene glycol

Water stress may induce active oxygen species generation and cause injuries to plant. The objective of this study was to examine the mechanism of physiological adaptation of Alternanthera philoxeroides (Mart.) Griseb, an alien invasive species, under three water stress conditions. A. philoxeroides plants grown in 1/2 Hogland nutrient solution were treated with polyethylene glycol 6000 corresponding to water potential of -0.15 MPa, -0.30 MPa and -0.50 MPa. Chlorophyll content, membrane electrolyte leakage, soluble protein, MDA and activities of several antioxidant enzymes were measured. Chlorophyll content, membrane electrolyte leakage and MDA content of plant tissue did not change markedly under -0.15 MPa water potential, but increased slightly when the water stress was induced by -0.30 MPa PEG, whereas the -0.50 MPa PEG stress led to decrease in chlorophyll content and appearance of the peak values of membrane electrolyte leakage and MDA content on the 5th day. However, soluble protein content initially increased and then decreased under all of the three different water stress conditions, so that a level higher than that of the control resulted. Activities of antioxidant enzymes (SOD, POD and CAT) remained unchanged or increased slightly under condition of -0.15 MPa PEG stress, but showed an initial increase followed by decrease under both -0.30 MPa and -0.50 MPa PEG stress, although the peak values occurred at different time periods under different degrees of water stress. The enzyme activities remained at normal level on the 5th day. Furthermore, as water stress was aggravated, antioxidant enzyme activities decreased while lipid peroxidation increased. The experiment suggested that A. philoxeroides were capable of surviving low water stress, and its ability of adaptation to water stress was related to the maintenance of or increased capacity to the elimination superoxided radicals by the action of antioxidant enzymes.