Developmental Changes in the Potential for H₂S Emission in Cucurbit Plants 1

Abstract

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 cm² leaf area) in response to sulfate, whereas discs from most of the leaves on plants 3 to 4 weeks old emit H₂S at a higher rate (50 to 150 picomoles per min per cm² 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 H₂S at higher rates (15 to 50 picomoles per min per cm² 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 the 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 developmental 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. 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 H₂S emission in response to l-cysteine declines.

2

Recipient of a Deutsche Forschungsgemeinschaft Fellowship. Present address: Botanisches Institut der Universität Köln, Gyrhofstrasse 15, D-5000 Köln 41, FRG.

3

Present address: ARCO Plant Cell Research Institute, 6560 Trinity Court, Dublin, CA 94566.

1

This work was supported by United States Department of Energy Contract DE-ACO2-ERO-1338.