Fluxes of atmospheric hydrogen sulphide to plant shoots.

Short-term exposure of maize, pumpkin, spinach and spruce to various atmospheric H2 S concentrations resulted in high H2 S fluxes to the shoots. In all species an almost linear relation was observed between flux and H2 S concentration up to around 0.3 μl -1 . At higher H2 S 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 H2 S 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 H2 S flux to the shoots and the sensitivity of the species towards H2 S. A 12 days' exposure of maize, pumpkin and spinach to 0.75 μl 1-1 H2 S resulted in a reduction of shoot yield of 1.36 and 69%, respectively. H2 S 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 H2 S. Even at a concentration of about 0.8 μl 1-1 H2 , which reduced the growth of pumpkin and spinach, the fluxes remained nearly constant. Fluxes of H2 S to the shoots did, however, vary diurnally, being high during the light and low during the dark period. The H2 S 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 H2 S 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 H2 S by the plant.