Stomatal responses to humidity in air and helox

Abstract

Abstract. Stomatal responses to humidity were studied in several species using normal air and a helium: oxygen mixture (79:21 v/v, with CO2 and water vapour added), which we termed ‘helox’. Since water vapour diffuses 2.33 times faster in helox than in air, it was possible to vary the water‐vapour concentration difference between the leaf and the air at the leaf surface independently of the transpiration rate and vice versa. The CO2 concentration at the evaporating surfaces (ci), leaf temperature and photon flux density were kept constant throughout the experiments. The results of these experiments were consistent with a mechanism for Stomatal responses to humidity that is based on the rate of water loss from the leaf. Stomata apparently did not directly sense and respond to either the water vapour concentration at the leaf surface or the difference in water vapour concentration between the leaf interior and the leaf surface. In addition, stomatal responses that caused reductions in transpiration rate at low humidities were accompanied by decreases in photosynthesis at constant ci, suggesting heterogeneous (patchy) stomatal closure.

A spatially explicit model of patchy stomatal responses to humidity

“…(2) Water flows frotn the xylem to an evaporating site within the leaf. Most proposed feedback meehanisms for stotnatal responses to transpiration rate incorporate the concept of an evaporating site that experiences a reduetion of water potential proportional to the transpiration rate (Sehulze 1994;Mott & Parkhurst 1991). This idea is consistent with pressure probe studies detnonstrating that mesophyll and epidertnal water potentials decrease as the water vapor mole fraction deficit {Aw) is inereased, while xylem water potential stays relatively constant (Nonami & Schulze 1989).…”

Effects of carbon dioxide concentration on the interactive effects of temperature and water vapour on stomatal conductance in soybean

“…Mott & Parkhurst (1991) have demonstrated that the response to humidity may actually be a response to transpiration rate, which provided motivation for the model of Monteith (1995). However, although Mott & Parkhurst (1991) ascribe the feedforward response to 'patchy closure', their experiment was not capable of differentiating between whole-leaf and peristomatal transpiration. Patchy closure cannot be eliminated in the present study because of the apparent change in the A/Q curve with LAVPD.…”

Stomatal conductance increases with rising temperature

“…Trees often regulate their stomatal conductance to maintain a specific transpiration rate across a wide range of VPD. 23 Loblolly pine adjusted g s in this manner but only at a given temperature. When temperature increased transpiration increased as well.…”