GAS EXCHANGE OF IMPATIENS PALLIDA NUTT. (BALSAMINACEAE) IN RELATION TO WILTING UNDER HIGH LIGHT

Impatiens pallida, a succulent annual herb of moist temperate forests, typically wilts on summer days after several minutes of direct sunlight. Time courses of gas exchange and leaf water potential were measured to determine if wilting resulted in substantially reduced photosynthesis, stomatal conductance, or leaf internal CO₂ concentrations. Leaves quickly wilted with the onset of high-light, but photosynthesis and stomatal conductance increased markedly. Photosynthetic rates and stomatal conductance declined slightly after several hours of high-light, and from morning to late afternoon shade conditions. Leaf internal CO₂ declined with increased photosynthesis, but there was no evidence that stomatal conductance limited photosynthesis through the day. We propose that rapid wilting is an adaptation that facultatively limits heat loading and extreme water loss under high-light. Further whole plant studies in natural settings are needed to fully evaluate the quantitative significance of wilting in relation to water use and photosynthesis.