THE INTERNAL RESISTANCE IN SPINACH LEAVES TO ATMOSPHERIC H2S DEPOSITION IS DETERMINED BY METABOLIC PROCESSES

Spinach (Spinacia oleracea) leaves formed an active sink for atmospheric H2S. Upon short-term exposure, H2S flux to the leaves showed saturation kinetics with respect to the H2S concentrations and could be described by the Michaelis-Menten equation. The kinetics of H2S flux to spinach leaves were similar for plants grown in a nutrient solution and in soil, and plants exposed in light and in darkness. The apparent J(H2S)max was about 1.7 pmol cm-2 s-1 and K[H2S] (concentration at which 1/2 J(H2S)max was reached) ranged from 5 to 11 pmol cm-3 H2S (0.12 to 0.26-mu-11(-1) H2S). The H2S flux to spinach leaves depended on the exposure temperature, with low fluxes at low and high fluxes at high temperatures. The apparent J(H2S)max increased from 0.4 pmol cm-2 s-1 at 1-degrees-C to 2.4 pmol cm-2 s-1 at 32.5-degrees-C. H2S exposure did not affect diffusive conductance of leaves to aqueous vapor efflux at the various conditions. At exposure temperatures of 12-degrees-C and higher and at low atmospheric H2S concentrations (4 pmol cm-3 and lower), H2S deposition velocities were close to that of diffusive conductance of leaves to aqueous vapor efflux, illustrating a very low internal resistance of the leaves to H2S. The internal resistance to H2S increased with increasing atmospheric H2S concentrations. At exposure temperatures of 8-degrees-C and lower, the internal resistance to H2S was high, even at low H2S concentrations. It is proposed that the internal resistance of plant leaves to H2S is determined by the rate of metabolism of the deposited H2S.