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Environmental and Stress Physiology

Internal CO₂ Measured Directly in Leaves ¹

Abscisic Acid and Low Leaf Water Potential Cause Opposing Effects

College of Marine Studies, University of Delaware, Lewes, Delaware 19958

Observations of nonuniform photosynthesis across leaves cast doubt on internal CO₂ partial pressures (p_i) calculated on the assumption of uniformity and can lead to incorrect conclusions about the stomatal control of photosynthesis. The problem can be avoided by measuring p_i directly because the assumptions of uniformity are not necessary. We therefore developed a method that allowed p_i to be measured continuously in situ for days at a time under growth conditions and used it to investigate intact leaves of sunflower (Helianthus annuus L.), soybean (Glycine max L. Merr.), and bush bean (Phaseolus vulgaris L.) subjected to high or low leaf water potentials (_w) or high concentrations of abscisic acid (ABA). The leaves maintained a relatively constant differential (p) between ambient CO₂ and measured p_i throughout the light period when water was supplied. When water was withheld, _w decreased and the stomata began to close, but measured p_i increased until the leaf reached a _w of –1.76 (bush bean), –2.12 (sunflower) or –3.10 (soybean) megapascals, at which point p = 0. The increasing p_i indicated that stomata did not inhibit CO₂ uptake and a p of zero indicated that CO₂ uptake became zero despite the high availability of CO₂ inside the leaf. In contrast, when sunflower leaves at high _w were treated with ABA, p_i did not increase and instead decreased rapidly and steadily for up to 8 hours even as _w increased, as expected if ABA treatment primarily affected stomatal conductance. The accumulating CO₂ at low _w and contrasting response to ABA indicates that photosynthetic biochemistry limited photosynthesis at low _w but not at high ABA.

¹ Supported by grants from the DuPont Company and Department of Energy (DE-FG02-87ER13776).

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