Effects of root exudates and aqueous root extracts of cucumber (Cucumis sativus) and allelochemicals, on photosynthesis and antioxidant enzymes in cucumber

The effects of root exudates and aqueous root extracts of cucumber (Cucumis sativus) and allelochemicals on root antioxidant enzymes and leaf photosynthesis, transpiration and stomatal conductance were investigated in cucumber. Cucumber seedlings were incubated in solutions containing root exudates at 50 or 100 mg/l, root extracts at 1:100, 1:50, 1:25 and 1:10 (root dry weight: distilled water), and derivatives of benzoic and cinnamic acids at 0.25 mM, respectively. Root peroxidase and superoxide dismutase activities increased significantly after exposure to allelopathic agents. Membrane peroxidation was also enhanced by root exudates, root extracts and some of the tested acids. Allelopathic agents reduced leaf stomatal conductance from 1.28 mol m(-2) s(-1) to 0.02-0.33 Mol m(-2) s(-1). Leaf transpiration was reduced by 67% by: root exudates at 100 mg/l, by 85-95% by root extracts, and by 51-87% by tested acids, respectively. Net assimilate rate was also inhibited by 29% by root exudates at 100 mg/l, by 49-83% by root extracts, and by 7-43% by tested acids, respectively. The intercellular CO2 concentrations for the treated seedlings were only 47-89% of the control. However, leaf temperature was increased by 2-4degreesC by the tested agents. (C) 2002 Elsevier, Science Ltd. All rights reserved.