Fluxes of atmospheric hydrogen sulphide to plant shoots

summary Short‐term exposure of maize, pumpkin, spinach and spruce to various atmospheric H2S concentrations resulted in high H2S fluxes to the shoots. In all species an almost linear relation was observed between flux and H2S concentration up to around 0.3 μl −1. At higher H2S concentrations the flux reached a maximum, which was about 0.08, 0.04, 0.1 and 0.03 μmol g f. wt−1 h−1 for maize, pumpkin, spinach and spruce, respectively. The transpiration rate was not affected during H2S exposure. Up to a level of 0.3 μl 1−1, the shoot conductance for a influx of the various species varied between 61 and 92% of that predicted from shoot conductance for aqueous vapour efflux. There was no relation between the H2S flux to the shoots and the sensitivity of the species towards H2S. A 12 days' exposure of maize, pumpkin and spinach to 0.75 μl 1−1 H2S resulted in a reduction of shoot yield of 1.36 and 69%, respectively. H2S flux to the shoots of pumpkin, spinach and spruce remained rather constant during exposure for three or four days to about 0.22μl l−l H2S. Even at a concentration of about 0.8 μl 1−1H2, which reduced the growth of pumpkin and spinach, the fluxes remained nearly constant. Fluxes of H2S to the shoots did, however, vary diurnally, being high during the light and low during the dark period. The H2S flux to transpiration ratio was constant during both light and dark periods, indicating that uptake was predominantly via the stomata. Only part of the total flux of H2S to the shoots of pumpkin and spinach could be recovered in the water‐soluble non‐protein sulphydryl fraction of the shoots (maximum 34%). It is proposed that cysteine synthase is directly involved in the fixation and metabolism of atmospheric H2S by the plant.