Effect of Vapor Pressure Deficit on Gas Exchange in Wild-Type and Abscisic Acid-Insensitive Plants

Abstract

Stomata control the gas exchange of terrestrial plant leaves, and are therefore essential to plant growth and survival. We investigated gas exchange responses to vapor pressure deficit (VPD) in two gray poplar (Populus × canescens) lines: wild type and abscisic acid-insensitive (abi1) with functionally impaired stomata. Transpiration rate in abi1 increased linearly with VPD, up to about 2 kPa. Above this, sharply declining transpiration was followed by leaf death. In contrast, wild type showed a steady or slightly declining transpiration rate up to VPD of nearly 7 kPa, and fully recovered photosynthetic function afterward. There were marked differences in discrimination against ¹³CO₂ (Δ¹³C) and C¹⁸OO (Δ¹⁸O) between abi1 and wild-type plants. The Δ¹³C indicated that intercellular CO₂ concentrations decreased with VPD in wild-type plants, but not in abi1 plants. The Δ¹⁸O reflected progressive stomatal closure in wild type in response to increasing VPD; however, in abi1, stomata remained open and oxygen atoms of CO₂ continued to exchange with ¹⁸O enriched leaf water. Coupled measurements of Δ¹⁸O and gas exchange were used to estimate intercellular vapor pressure, e _i In wild-type leaves, there was no evidence of unsaturation of e _i, even at VPD above 6 kPa. In abi1 leaves, e _i approached 0.6 times saturation vapor pressure before the precipitous decline in transpiration rate. For wild type, a sensitive stomatal response to increasing VPD was pivotal in preventing unsaturation of e _i In abi1, after taking unsaturation into account, stomatal conductance increased with increasing VPD, consistent with a disabled active response of guard cell osmotic pressure.