Developmental Changes in the Potential for H 2 S Emission in

Based on results obtained with leaf discs exposed to sulfate, leaves on cucurbit plants (Cucurbitapepo L. cv Small Sugar Punpkn and Cucuzs sativws cv Chipper) 1 to 2.5 weeks old have a low potential for H2S emission (less than 10 pkomoles 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 mn per cm2 leaf area). This difference is determined by the age of the plat, and is indepndent of the leaves' age or developmental stage. In response to L-cysteine, however, discs from leaves on cucurbit plants I to 2.5 weeks old emit H2S at higher rates (15 to 50 picomoles per mn per cm2 leaf area) thaa in response to sulfate. Furthermore, the potential for H2S emission in response to Lcysteine 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 .-cysteine. Overt cysteine desulfhydrase activity is up to 2-fold higher, and latent cysteine desulHhydrase 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 oldL Thus, most of the cysteine desulfhydrase activity develops duing the early penod of growth of a cucurbit plant. Overt cysteine desulfhydrase activity passes through a n value during the development ofeach leaf, total as well as latent cystelne desuibhydrase activities, however, declbe with increasing age of the individual leaf in much the same way as H2S emissio in response to t-cysteine declines. In contrast to mammals which develop all their organs at the same time, plants develop new organs, e.g. new leaves, during almost all stages in their life. Because the whole plant itself is continually developing, each leaf undergoes a life cycle which may be different from that of every other leaf on the same plant, the differences being determined by the developmental stages of the whole plant. 1This work was supported by United States Department of Energy Contract DE-ACO2-ERO-1338. 2Reipient of a Deutsche Forschungsgemeinschaft Fellowship. Present address: Botanisches Institut der Universitat Koln, Gyrhofstrasse 15, D5000 Koln41, FRG. 3 Present address: ARCO Plant Cell Research Institute, 6560 Trinity Court, Dublin, CA 94566. Little is known about how metabolic changes in leaves are related to, or determined by, the developmental stage of the leaf or of the plant. Most investigations which bear on this matter deal with development of photosynthetic capacity during greening, or with the final part of plant or leaf development, senescence (15). An interesting exception is CAM, which has been shown to be controlled by the leaves' development (6). In most succulentleaved plants, the capacity for CAM increases with leaf age (7); the low capacity for malic acid synthesis in the dark observed in young leaves of Bryophyllum is accompanied by a lack of net CO2 fixation in the dark (5). Sulfur metabolism in leaves also changes during leaf ontogenesis. The activity of ATP sulfurylase, which catalyzes the initial activation step in light-dependent sulfate assimilation, is low in leaves of young seedlings of soybean plants; high levels of activity are not observed in the leaves until seedlings are about 3 weeks old (1). Furthermore, ATP sulfurylase activity increases with increasing leaf position number, independent of the age of the plant. During development of the individual leaf, ATP sulfurylase passes through a maximum level or decreases with increasing leaf age (1). Therefore, ATP sulfurylase activity in soybean leaves is controlled by both the development of the whole plant and the development of the individual leaf. Resistance of cucurbit leaves to acute exposure to S02 changes with the leaf position on the plant axis, decreasing gradually from the apex downward. This gradient in resistance to SO2 is not due to avoidance, since resistant leaves actually have an appreciably higher rate of S02 absorption than do sensitive leaves (2). Resistance is, however, positively correlated with the rate of conversion of the absorbed SO2 to H2S: expanding leaves emit H2S in higher amounts and at higher rates than do mature leaves (12). It has recently been demonstrated that cucurbit leaves exposed to high concentrations of either sulfate or L-cysteine emit H2S into the atmosphere (9-11, 13, 14, 16). As observed with S02 as the sulfur source (2), the rates of H2S emission in response to Lcysteine were found to be high in expanding leaves and to decrease gradually from the apex downward (14). However, these experiments on H2S emission were performed with plants 30 to 40 d old; therefore, it was unclear whether the observed differences between the emission of H2S from leaves at different positions on the axis of cucurbit plants reflected developmental changes of the entire plant during its growth, or developmental changes of individual leaves independent of plant age. The present investigation was undertaken to characterize more thoroughly the relationship between development of cucurbit plants and their potential to emit H2S in response to sulfate and L-cysteine. MATERIALS AND METHODS Plant Material. The present investigation was performed with Cucurbita pepo L. cv Small Sugar Pumpkin and Cucumis sativus 269 www.plantphysiol.org on December 30, 2017 Published by Downloaded from Copyright © 1983 American Society of Plant Biologists. All rights reserved. RENNENBERG AND FILNER