© 2007 Heron Publishing—Victoria, Canada
Atmospheric and soil drought reduce nocturnal conductance in live oaks
Jeannine Cavender-Bares (1, 2), Lawren Sack (3) and Jessica Savage (1)
1. Department of Ecology, Evolution and Behavior, 1987 Upper Buford Circle, University of Minnesota, St. Paul, MN 55108, USA / 2. Corresponding author (cavender@umn.edu) / 3. Department of Botany, University of Hawai’i at Manoa, 3190 Maile Way, Honolulu, HI 96822, USA / Received March 27, 2006; accepted September 17, 2006; published online January 2, 2007
Summary
Nocturnal and daytime whole-canopy transpiration rate (E) and conductance (g = E/VPD, where VPD is leaf to air vapor pressure difference) were assessed gravimetrically in drought-treated and well-watered
3-year-old saplings of live oak species (Quercus series Virentes Nixon) from the southeastern USA (Quercus virginiana Mill.) and Central America (Q. oleoides Cham. and Schlecter). Our objectives were to: (1) quantify nocturnal and daytime E and g in a controlled environment; (2) determine the impact of severe drought on nocturnal E and g; and (3) examine whether unavoidable water loss through the epidermis could account for nocturnal water loss. We calculated
daytime E during peak daylight hours (between 0930 and 1330 h) and nocturnal E during complete darkness (between 2200 and 0500 h). In addition to reducing E and g during the daytime, drought-treated plants reduced nocturnal E and g on a whole-canopy basis by 62–64% and 59–61%, respectively, and on a leaf-level basis by 27–28% and 19–26%, respectively.
In well-watered plants, nocturnal g declined with increasing VPD, providing evidence for stomatal regulation of nocturnal transpiration. In drought-treated plants,
g was low and there was no relationship between nocturnal g and VPD, indicating that water loss could not be reduced further through stomatal regulation. Both daytime and nocturnal
g declined curvilinearly with predawn water potential for all plants, but nocturnal g was unrelated to predawn water potentials below –1 MPa. The reductions in daytime and nocturnal E and g during drought were associated with decreases in whole-plant and leaf hydraulic conductances. Observed nocturnal g was within the same range as epidermal conductance for oak species determined in previous studies under a range of conditions.
Nocturnal E rose from 6–8% of daytime E for well watered plants to 19–20% of daytime E for drought-treated plants. These results indicate that, during drought, saplings of live oak species reduce g to a minimum through stomatal closure, and experience unavoidable water loss through the epidermis.
Keywords:
epidermal conductance, hydraulic conductance, leaf hydraulic properties, nocturnal conductance, Quercus oleoides, Quercus virginiana, stomatal pore index.
