With the aid of a sufur-specific flame photometric detector, an emission of volatile sulfur was detected from leaves of cucumber (Cucumis sativus L.), sqush and pumpkin (Cucurbita pepo L.), cantaloupe (Cucumis melo L.), corn (Zea mays L.), soybean (Glycine max [L.] Merr.) and cotton (Gossypium hirsutum L.). Tbe emission was studied in detai in squash and pumpkin. It

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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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1. Nitrate induces the development of NADH-nitrate reductase (EC 1.6.6.1), FMNH(2)-nitrate reductase and NADH-cytochrome c reductase activities in barley shoots. 2. Sucrose-density-gradient analysis shows one band of NADH-nitrate reductase (8S), one band of FMNH(2)-nitrate reductase activity (8S) and three bands of NADH-cytochrome c reductase activity (bottom layer, 8S and 3.7S). Both 8S and 3.7S NADH-cytochrome c reductase activities are inducible by nitrate, but the induction of the 8S band is much more marked. 3. The 8S NADH-cytochrome c reductase band co-sediments with both NADH-nitrate reductase activity and FMNH(2)-nitrate reductase activity. Nitrite reductase activity (4.6S) did not coincide with the activity of either the 8S or the 3.7S NADH-cytochrome c reductase. 4. FMNH(2)-nitrate reductase activity is more stable (t((1/2)) 12.5min) than either NADH-nitrate reductase activity (t((1/2)) 0.5min) or total NADH-cytochrome c reductase activity (t((1/2)) 1.5min) at 45 degrees C. 5. NADH-cytochrome c reductase and NADH-nitrate reductase activities are more sensitive to p-chloromercuribenzoate than is FMNH(2)-nitrate reductase activity. 6. Tungstate prevents the formation of NADH-nitrate reductase and FMNH(2)-nitrate reductase activities, but it causes superinduction of NADH-cytochrome c reductase activity. Molybdate overcomes the effects of tungstate. 7. The same three bands (bottom layer, 8S and 3.7S) of NADH-cytochrome c reductase activity are observed irrespective of whether induction is carried out in the presence or absence of tungstate, but only the activities in the 8S and 3.7S bands are increased. 8. The results support the idea that NADH-nitrate reductase, FMNH(2)-nitrate reductase and NADH-cytochrome c reductase are activities of the same enzyme complex, and that in the presence of tungstate the 8S enzyme complex is formed but is functional only with respect to NADH-cytochrome c reductase activity.

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Nitrite reduction is inhibited by anaerobiosis in barley aleurone layers (5) and wheat roots (18), whereas the reduction of nitrate to nitrite by nitrate reductase is enhanced (4). Nitrite accumulates under such conditions and the anaerobic production of nitrite has been used to assay nitrate reductase in situ in a variety of plant tissues (4, 11-13, 17, 22, 24, 25). The reaction was found to be linear for more than 1 hr and dependent upon the addition of exogenous nitrate (4,13,24

Both mono-and dicotyledonous species catabolize putrescine to -r-aminobutyric acid (GABA), but by two different pathways. GABA is the major labeled product in pea shoots and oat leaves fed with a 2-4h pulse of [1,4-14C]-putrescine (Put) or [1,4-tetramethylene-~*C]-spermidine (Spd), respectively. In the presence of 1-10 ~M gabaculine, a specific inhibitor of GABA:pyruvate-transaminase, the label appearing in GABA increases 2 to 7-fold, which indicates that the transamination reaction is a major fate of GABA formed from Put or Spd in vivo. The conversions to GABA were demonstrated in vitro in coupled assays involving diamine oxidase from pea or polyamine oxidase from oat, and pyrroline dehydrogenase (PYRR-DH). The latter enzyme from either pea or oat is strictly NAD-dependent and is specific for pyrroline. The optimal temperature (40-45 °C) and pH (7.5-8.0) are similar to those of bacterial PYRR-DH. In all cases the enzyme was inhibited by the NAD analogs thionicotinamide and aminopyridine dinucleotide (0.1-1.0 mM). In addition to pea and oat, PYRR-DH was also detected in corn, barley, soybean and broadbean. Di-and polyamine oxidase are released by enzymes which degrade the cell wall, while PYRR-DH remains associated with the protoplast.