Developmental changes in the potential for H2S 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 H2S emission (less than 10 picomoles per min per square cm leaf area) in response to sulfate, whereas discs from most of the leaves on plants 3 to 4 weeks old emit Hs2S at a higher rate (50 to 150 picomoles per min per square cm 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 and old emit H2S at higher rates (15 to 50 picomoles per min per square cm leaf area) than in response to sulfate. Furthermore, the potential for H2S emission in response to L-cysteine decreases with increasing age of the individual leaf. Thus, most of the potential for H2S emission in response to L-cysteine is developed during germination and th e early growth of cucurbit plants, but most of the potential for H2S emission in response to sulfate arises later in the development of the plants. The development changes in the potential for H2S 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 H2S by leaf tissue in response to L-cysteine, can be increased by preincubation of leaf discs in L-cysteine. (IFU