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

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.

Key words:

• Geothermal Gases
• Hydrogen Sulfide
• Phytotoxic Gases.