Exposures of field?grown lettuce to geothermal air pollution — photosynthetic and stomatal responses

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Abstract
Field‐grown lettuce was exposed in‐situ to a facsimile geothermal gas mixture (15 CO2: 1 H2S: 1 CH4: 2 N2 parts by volume added to air). The objective was to determine the effects of geothermal air pollution on photosynthesis and stomatal diffusion resistance during a three‐hour period of exposure. The question was to what degree the extra CO2 might compensate for the phytotoxic effect of H2S during an exposure analogous to fumigation by a typical geothermal power plant plume. The apparent mean values indicate a stimulation of photosynthesis and a relaxation of stomatal resistance at low concentrations. There is no significant depression of photosynthesis until exposure concentrations approach 75 ppm CO2: 5 ppm H2S added to air. The normal variation in photosynthesis is so high (coefficient of variation .5) that it is necessary to take many samples to establish a mean, and this should be considered in designing future experiments.