Developmental changes in the potential for h(2)s emission in cucurbit plants

Based on results obtained with leaf discs exposed to sulfate, leaves on cucurbit plants (Cucurbita pepo L. cv Small Sugar Pumpkin and Cucumis sativus cv Chipper) 1 to 2.5 weeks old have a low potential for H(2)S emission (less than 10 picomoles per min per cm(2) leaf area) in response to sulfate, whereas discs from most of the leaves on plants 3 to 4 weeks old emit H(2)S at a higher rate (50 to 150 picomoles per min per cm(2) leaf area). This difference is determined by the age of the plant, and is independent of the leaves' age or developmental stage. In response to l-cysteine, however, discs from leaves on cucurbit plants 1 to 2.5 weeks old emit H(2)S at higher rates (15 to 50 picomoles per min per cm(2) leaf area) than in response to sulfate. Furthermore, the potential for H(2)S emission in response to l-cysteine decreases with increasing age of the individual leaf. Thus, most of the potential for H(2)S emission in response to l-cysteine is developed during germination and the early growth of cucurbit plants, but most of the potential for H(2)S emission in response to sulfate arises later in the development of the plants.The developmental changes in the potential for H(2)S emission in response to l-cysteine in vivo are paralleled by changes in the cysteine desulfhydrase activity extractable from the leaves. This extractable activity, which is thought to be responsible for the generation of H(2)S by leaf tissue in response to l-cysteine, can be increased by preincubation of leaf discs in l-cysteine. Overt cysteine desulfhydrase activity is up to 2-fold higher, and latent cysteine desulfhydrase activity is up to 4-fold higher, in leaves on cucurbit plants 1 to 2.5 weeks old than in leaves on plants 3 to 4 weeks old. Thus, most of the cysteine desulfhydrase activity develops during the early period of growth of a cucurbit plant. Overt cysteine desulfhydrase activity passes through a maximum value during the development of each leaf; total as well as latent cysteine desulfhydrase activities, however, decline with increasing age of the individual leaf in much the same way as H(2)S emission in response to l-cysteine declines.