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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.

Atmospheric and soil drought reduce nocturnal conductance in live oaks