© 2007 Heron Publishing—Victoria, Canada
Nighttime transpiration in woody plants from contrasting ecosystems
Todd E. Dawson (1, 2, 3), Stephen S. O. Burgess (4), Kevin P. Tu (1), Rafael S. Oliveira (1, 5), Louis S. Santiago (1, 6), Joshua B. Fisher (3, 7), Kevin A. Simonin (1) and Anthony R. Ambrose (1)
1. Department of Integrative Biology, Valley Life Sciences Building, University of California, Berkeley, CA 94720, USA / 2. Corresponding author (tdawson@berkeley.edu) / 3. Department of Environmental Science, Policy and Management, Mulford Hall, University of California, Berkeley, CA 94720, USA / 4. School of Plant Biology, University of Western Australia, 35 Stirling Highway, Nedlands, WA 6009, Australia / 5. Lab. Ecologia Isotópica - CENA, Universidade de São Paulo, Av. Centenário, 303, 13.416-000, Piracicaba, SP, Brazil / 6. Present address: Department of Botany and Plant Sciences, 2150 Batchelor Hall, University of California, Riverside, CA 92521,USA / 7. Present address: Oxford University Centre for the Environment, South Parks Road, Oxford, OX1 0EZ, U.K. / Received June 27, 2006; accepted October 19, 2006; published online January 2, 2007
Summary
It is commonly assumed that transpiration does not occur at night because leaf stomata are closed in the dark. We tested this
assumption across a diversity of ecosystems and woody plant species by various methods to explore the circumstances when this
assumption is false. Our primary goals were: (1) to evaluate the nature and magnitude of nighttime transpiration, En, or stomatal conductance, gn; and (2) to seek potential generalizations about where and when it occurs. Sap-flow, porometry and stable isotope tracer
measurements were made on 18 tree and eight shrub species from seven ecosystem types. Coupled with environmental data, our
findings revealed that most of these species transpired at night. For some species and circumstances, nighttime leaf water
loss constituted a significant fraction of total daily water use. Our evidence shows that En or gn can occur in all but one shrub species across the systems we investigated. However, under conditions of high nighttime evaporative
demand or low soil water availability, stomata were closed and En or gn approached zero in eleven tree and seven shrub species. When soil water was available, En or gn was measurable in these same species demonstrating plasticity for En or gn. We detected En or gn in both trees and shrubs, and values were highest in plants from sites with higher soil water contents and in plants from
ecosystems that were less prone to atmospheric or soil water deficits. Irrespective of plant or ecosystem type, many species
showed En or gn when soil water deficits were slight or non-existent, or immediately after rainfall events that followed a period of soil
water deficit. The strongest relationship was between En or gn and warm, low humidity and (or) windy (> 0.8 m s–1) nights when the vapor pressure deficit remained high (> 0.2 kPa in wet sites, > 0.7 kPa in dry sites). Why En or gn occurs likely varies with species and ecosystem type; however, our data support four plausible explanations: (1) it may facilitate
carbon fixation earlier in the day because stomata are already open; (2) it may enhance nutrient supply to distal parts of
the crown when these nutrients are most available (in wet soils) and transport is rapid; (3) it may allow for the delivery
of dissolved O2 via the parenchyma to woody tissue sinks; or (4) it may occur simply because of leaky cuticles in older leaves or when stomata
cannot close fully because of obstructions from stomatal (waxy) plugs, leaf endophytes or asymmetrical guard cells (all non-adaptive
reasons). We discuss the methodological, ecophysiological, and theoretical implications of the occurrence of En or gn for investigations at a variety of scales.
Keywords:
nighttime stomatal conductance, porometry, sap flow, water balance, water relations, woody plants.
