Ethylene Biosynthesis and Cadmium Toxicity in Leaf Tissue of Beans (Phaseolus vulgaris L.)

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Abstract
Stress ethylene production in bean (P. vulgaris L., cv. Taylor''s Horticultural) leaf tissue was stimulated by Cd2+ at concentrations > 1 .mu.M. Cd2+-induced ethylene biosynthesis was dependent upon synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase. Activity of ACC synthase and ethylene production rate peaked at 8 h of treatment. The subsequent decline in enzyme activity was most likely due to inactivation of the enzyme by Cd2+, which inhibited ACC synthase activity in vitro at concentrations as low as 0.1 .mu.M. Decrease in ethylene production rate was accompanied by leakage of solutes and increasing inhibition of ACC-dependent ethylene production. Ca2+, present during a 2-h preincubation, reduced the effect of Cd2+ on leakage and ACC conversion. Cd2+ may exert its toxicity through membrane damage and inactivation of enzymes. The possibility of an indirect stimulation of ethylene biosynthesis through a wound signal from injured cells is discussed.