Potential Allelopathic Effects of Two Multipurpose Trees on Germination, Seedling Performance, and Antioxidant Activity of Alfalfa in Mediterranean Arid Lands

The allelopathic effects of some introduced multipurpose trees in arid Mediterranean agricultural ecosystems present a risk to the growth of nearby crops. This study investigates the allelopathic potential of Acacia saligna (Labill.) Wendl. and Cupressus sempervirens L. on the germination and early seedling growth of alfalfa (Medicago sativa L.) in a laboratory bioassay. Aqueous leaf extracts were applied at 0, 5, 10, 15, and 20% concentrations. All aqueous extracts inhibited alfalfa seed germination, with more pronounced inhibitory effects on seedling growth than germination and higher concentrations causing greater inhibition. Total dry mass, root length, and shoot length all declined, more so with Acacia leaf extracts than Cupressus leaf extracts. The deleterious allelopathic effects slightly stimulated the enzymatic antioxidant system in alfalfa seedlings by enhancing the activities of superoxide dismutase, catalase, and glutathione peroxidase. Moreover, these allelopathic effects strongly stimulated the non-enzymatic antioxidant system by accumulating proline and soluble sugars, indicating an adaptive strategy by the alfalfa plants. The extent of these biochemical responses was directly proportional to the concentration of the leaf extracts. These findings underscore the significant role of Acacia saligna and Cupressus sempervirens allelopathy in controlling seed germination and seedling growth of certain economically significant crops.