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Leaf discs and detached leaves exposed to L-cysteine emitted a volatile sulfur compound which was proven by gas chromatography to be H2S. This phenomenon was demonstrated in all nine species tested (Cucumis sativus, Cucurbita pepo, Nicotiana tabacum, Coleus bbumei, Beta vulgaris, Phaseolus vulgaris, Medicago sativa, Hordeum vulgare, and Gossypium hirsutum). The emission of volatile sulfur by cucumber leaves occurred in the dark at a similar rate to that in the light. The emission of leaf discs reached the maximal rate, more than 40 picomoles per minute per square centimeter, 2 to 4 hours after starting exposure to L-cysteine; then it decreased. In the case of detached leaves, the maximum occurred 5 to 10 h after starting exposure. The average emission rate of H2S during the first 4 hours from leaf discs of cucurbits in response to 10 milimlar L-cysteine, was usually more than 40 picomoles per minute per square centimeter, i.e. 0.24 micromoles per hour per square decimeter. Leaf discs exposed to 1 milHimolar Lcysteine emitted only 2% as much as did the discs exposed to 10 millmolar L-cysteine. The emission from leaf discs and from detached leaves lasted for at least 5 and 15 hours, respectively. However, several hours after the maximal emission, injury of the leaves, manifested as chlorosis, was evdent.H2S emission was a specific consequence of exposure to L-cysteine; neither D-cysteine nor L-cystine elicited H2S emission. Aminooxyacetic acid, an inhibitor ofpyridoxal phosphate dependent enzymes, inhibited the emission.In a celi free system from cucumber leaves, H2S formation and its release occurred in response to L-cysteine. Feeding experiments with 135SIL_CyS_ teine showed that most of the sulfur in H2S was derived from sulfur in the L-cysteine supplied and that the H2S emitted for 9 hours accounted for 7 to 10% of L-cysteine taken up. 35S-labeled S032-and S042-were found in the tissue extract in addition to internal soluble S2-. These findigs suggest the existence of a sulfur cycle which converts L-cysteine to S042-through cysteine desulfhydration.Illuminated green leaves emit H2S when plants are exposed to s042- (32,35) or SO2 (6,27). Plants have the potential for reduction of s042-to a bound form of sulfide, which is incorporated into L-cysteine, by a light-driven assimilation pathway (1, 26). Therefore, the conversion of bound sulfide to free sulfide and its release as H2S is one possible origin of the H2S emitted in response to s042- (35 reductase (1,26). Still another possibility is that L-cysteine could be a precursor of H2S. L-Cysteine is a precursor ofmost organic sulfur compounds (9), and it regulates s042-uptake (13,16,29,30) Salmonella, L-cysteine is degraded to pyruvate, NH4' and sulfide by L-cysteine desulfhydrase, which is induced by L-cysteine (4,5,14,15). L-cysteine desulfhydrase activity has also been reported to exist in the XD strain of cultured tobacco cells and to be induced by L-cysteine in these cells (12). The H2S could also arise by cyanide-dependent desulfhydration of L-cyste...

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In Cucurbitaceae young leaves are resistant to injury from acute exposure to S02, whereas mature leaves are sensitive. After exposure of cucumber (Cucumis sativus L.) plants to S02 at injurious concentrations, illuminated leaves emit volatile sulfur, which is solely H2S. Young leaves emit H2S many times more rapidly than do mature leaves. Young leaves convert approximately 10% of absorbed 135SIS02 to emitted 135SIH2S, but mature leaves convert less than 2%. These results suggest that a high capability for the reduction of S02 to H2S and emission of the H2S is a part of the biochemical basis of the resistance of young leaves to S02.Plants are injured by far lower doses of SO2 than are animals (18), in spite of the fact that SO2 is closely related to one or more intermediates in the path of sulfate assimilation in plants (1,19), and plants possess sulfite reductase, an enzyme specific for sulfite (13,20,23 When plants are exposed to SO2, they absorb it rapidly, probably through stomata (22,29). The SO2 dissolves in tissue water, whereupon it ionizes to HS03-or So32-. These may be normal intermediates, albeit at lower concentrations, because plants can synthesize s032-from s042- (9,26). It has been shown in several plant species that most of SO2 absorbed is oxidized to So42-rapidly and to a lesser extent the sulfur is incorporated into organic sulfur compounds such as cysteine and glutathione (6,10,11,24,25). On the other hand, light-dependent reduction of SO2 to sulfide has been suggested as a possible metabolism of SO2 (6,17,21 Fumigation with SO2. Plants in plastic pots with the top sealed around the stem with Parafilm were fumigated individually with air containing SO2 at 24.5 ± 1°C in a closed 40-L Plexiglas chamber. The chamber had an air stirrer built in, and was illuminated with cool-white fluorescent lamps (0.8 mw cm-2). When the plant was placed in the chamber, a beaker containing a mixture which would generate SO2 upon acidification was also placed in the chamber. After sealing the chamber, lactic acid was added to the beaker contents through a port connected to the beaker by Teflon tubing. The mixture after acidification contained in 30 ml: KHSO3 (60 ,tmol), Na2CO3 (40 imol), 6.7% (v/v) ethanol, and 12% (v/v) lactic acid.Measurement of Volatile Sulfur Emission. Immediately after a whole plant was fumigated with SO2, a leaf to be used for measurement of the sulfur emission rate was detached. The cut end of the petiole of the detached leaf was placed in H20 in a small sealed vial in a sealed Plexiglas leaf chamber (0.4 L) with two ports, one of which was the air inlet and the other was the air outlet. The vial was sealed around the petiole with Parafilm in order to prevent the possible absorption or release of volatile sulfur compounds via the petiole. The outlet was connected 437 www.plantphysiol.org on May 9, 2018 -Published by Downloaded from

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