PARENT SESSION
Contributed Oral Session 16: Photosynthesis and Water Relations: Conductivity; Stress
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 524 A, Level 5, Palais des congrès de Montréal

Does leaf water potential limit stomatal conductance at high water vapor pressure deficits?

Bunce, James^*,1, ¹ USDA-ARS, Beltsville, MD, USA

ABSTRACT- A reduction in leaf stomatal conductance with increasing leaf to air difference in water vapor pressure (D) is nearly ubiquitous. Ecological comparisons of sensitivity have led to the concept that the reduction in stomatal conductance with increasing D serves to limit transpiration and maintains leaf water potentials above those which would cause cavitation of xylem and loss of hydraulic conductivity. Further, reduction in leaf water potential is commonly suggested as the proximate signal which limits stomatal conductance at high D. Both hypotheses were tested by exposing velvetleaf and soybean to D high enough to reduce stomatal conductance and then decreasing the ambient carbon dioxide concentration and observing the resulting changes in conductance, transpiration rate and leaf water potential, and their reversibility. Regardless of whether the treatments were applied to small parts of leaves, whole leaves, or whole shoots, reducing the carbon dioxide concentration at high D increased stomatal conductance and transpiration rate and lowered leaf water potential. Maintaining the low carbon dioxide treatment and high transpiration rate for as long as an hour did not result in reductions in stomatal conductance or hydraulic conductance. These results indicate that low leaf water potential did not limit stomatal conductance at high D and that even lower leaf water potentials did not cause collapse of the hydraulic system.

Key words: stomatal conductance, hydraulic conductance, vapor pressure deficit, water potential