Research article Daily dynamics of leaf and soil-to-branch hydraulic conductance in silver birch (Betula pendula) measured in situ Eele Õunapuu * , Arne Sellin Institute of Ecology and Earth Sciences, University of Tartu, Vanemuise 21, 51014 Tartu, Estonia article info Article history: Received 19 October 2012 Accepted 11 April 2013 Available online 27 April 2013 Keywords: Canopy position Circadian regulation Diurnal variation Environmental factors Evaporative flux method Leaf water status Plant water relations abstract Daily dynamics of leaf (K L ) and soil-to-branch hydraulic conductance (K SeB ) was investigated in silver birch (Betula pendula Roth.) using evaporative flux method in situ: water potential drop was measured with a pressure chamber and evaporative flux was estimated as sap flux density measured with sap flow gauges. Canopy position had a significant (P < 0.001) effect on both K L and K SeB . Upper-canopy leaves exhibited 1.7 and soil-to-branch pathway 2.3 times higher hydraulic efficiency than those for lower- canopy. K L varied significantly with time of day: K L for both upper- and lower-canopy leaves was lowest in the morning and rose gradually achieving maximal values in late afternoon (4.75 and 3.38 mmol m 2 s 1 MPa 1 , respectively). Relevant environmental factors affecting K L were photosyn- thetic photon flux density (Q P ), air relative humidity (RH) and air temperature (T A ). K SeB started rising in the morning and reached maximum in the lower canopy (1.44 mmol m 2 s 1 MPa 1 ) at 1300 h and in the upper canopy (2.52 mmol m 2 s 1 MPa 1 ) at 1500 h, decreasing afterwards. Environmental factors controlling K SeB were J S and Q P . The diurnal patterns of K L reflect a combination of environmental factors and endogenous rhythms. The temporal pattern of K SeB refers to daily up- and down-regulation of hydraulic conductance of water transport pathway from soileroot interface to leaves with respect to changing irradiance. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction Water is a ubiquitous medium in which biochemical processes take place. Because the aerial parts of terrestrial plants continually lose water by evaporation, plants need a constant water supply from the soil. For plants growing in temporally and spatially changing environments, the ability to regulate water use efficiently and maintain a positive water balance becomes the primary requirement for survival [1]. Ample evidence suggests that a functional relationship exists between plant hydraulic properties and stomatal conductance and photosynthetic capacity [2e5]. Stomatal regulation provides a means to balance water loss by transpiration with the efficiency of water-conducting tissues to supply water to leaves, while simul- taneously avoiding leaf desiccation and maximizing carbon dioxide uptake under given environmental conditions [6,7]. Hydraulic conductance represents a measure of how efficiently liquid water is transferred through conducting tissues. Because leaves account for most (47e94%) of the hydraulic resistance to water flow through the shoots of woody plants [8,9] and contribute more than 30% to the total hydraulic resistance of plants [10,11], the hydraulic conductance of leaves (K L ) is an important physiological parameter of plant water relations. Being tightly coupled to stomatal conductance and photosynthetic capacity, K L is ultimately associated with plant productivity, growth rate, competitiveness and survival. Although leaf hydraulics has received increasingly more atten- tion over the last decade, our knowledge of the factors determining leaf hydraulic capacity remains deficient. Leaf hydraulic conduc- tance has been shown to be highly variable both among and within those species studied to date [10]. K L varies over a range of time scales, from minutes to months, demonstrating circadian [12e15], seasonal and developmental trends [16e18]. K L depends on growing conditions and is sensitive to changes in microclimate, being influenced by multiple environmental factors including light [9,14,19,20], temperature [18,21] and water availability [22e24]. Most studies of leaf hydraulics have been carried out on her- baceous plants or tree seedlings grown in controlled environments [15,25], or on excised shoots and leaves of mature forest trees in the laboratory [9,12,14,19,26]. Few studies have focused on measuring K L in forest trees in situ [13,18,27], therefore data on the diurnal patterns of leaf hydraulic conductance measured on forest trees in their natural environment are scarce. * Corresponding author. Tel.: þ372 7376167; fax: þ372 7376222. E-mail address: eele.ounapuu@ut.ee (E. Õunapuu). Contents lists available at SciVerse ScienceDirect Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy 0981-9428/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.plaphy.2013.04.011 Plant Physiology and Biochemistry 68 (2013) 104e110