Abstract. The hydraulic conductivity of excised roots (Lp r ) of the legume Lotus japonicus (Regel) K. Larsen grown in mist (aeroponic) and sand cultures, was found to vary over a 5-fold range during a day/night cycle. This behaviour was seen when Lp r was measured in roots exuding, either under root pressure (osmotic driving force), or under an applied hydrostatic pressure of 0.4 MPa which produced a rate of water ¯ow similar to that in a transpiring plant. A similar daily pattern of variation was seen in plants grown in natural daylight or in controlled-environment rooms, in plants transpiring at ambient rates or at greatly reduced rates, and in plants grown in either aeroponic or sand culture. When detached root systems were connected to a root pressure probe, a marked diurnal variation was seen in the root pressure generated. After excision, this circadian rhythm continued for some days. The hydraulic conductivity of the plasma membrane of individual root cells was measured during the diurnal cycle using a cell pressure probe. Measurements were made on the ®rst four cell layers of the cortex, but no evidence of any diurnal ¯uctuation could be found. It was concluded that the conductance of membranes of endodermal and stelar cells may be responsible for the observed diurnal rhythm in root Lp r . When mRNAs from roots were probed with cDNA from the Arabidopsis aquaporin AthPIP1a gene, an abundant transcript was found to vary in abundance diurnally under high-stringency conditions. The pattern of ¯uctuations resembled closely the diurnal pattern of variation in root Lp r . The plasma membranes of root cells were found to contain an abundant hydrophobic protein with a molecular weight of about 31 kDa which cross-reacted strongly to an antibody raised against the evolutionarily conserved N-terminal amino acid se- quence of AthPIP1a. Key words: Aquaporin ± Diurnal cycle ± Hydraulic conductivity ± Lotus (aquaporin) ± Plasma membrane ± Pressure probe Introduction Water and mineral nutrients are vital resources acquired through the roots of land plants. In their passage through the plant, these resources share a number of common pathways and there can be linkages between their ¯uxes. In the plasma membrane, there are at least two pathways along which water is believed to move. One is diusion through the lipid bilayer, a process which depends on the thermal motion of the membrane lipids, and the other is conduction through water channels which are established by polypeptides of the aquaporin type. Flow through these two pathways may be independently regulated (Henzler and Steudle 1995; Steudle and Henzler 1995). When considering water ¯ow across a structure as complex as a root, the principal resistance in the pathway is often envisaged as being due to plasma membranes of cells in the pathway. A common view is that the plasma membrane of the endodermis may contribute the major part of this resistance (Weatherley 1982), but there are dierent opinions about this matter (Newman 1973; Kramer and Boyer 1995; Steudle and Peterson 1998). In classic experiments, it was observed that the root hydraulic conductivity (Lp r ) declined towards the end of the light period and rose again at the end of the dark period (Parsons and Kramer 1974). Such eects have been taken as an indication that root resistance to water Abbreviations and symbols: A (root) = cell (root) surface area; (Ath)PIP = plasma intrinsic protein (from A. thaliana); C (x/o) = concentration (in xylem/medium); d cell/layer = cell diameter/ distance of cell or layer to root surface; e = cell elastic modulus; J vr = rate of water ¯ow through root system; l cell = cell length; Lp (r) = cell (root) hydraulic conductivity; PIP = plasma-mem- brane intrinsic protein; P (r/bomb) = cell (root/bomb) pressure; r sr = root re¯ection coecient; T w 1=2 = half-time of water ex- change; t 1/2 = half-time of exponential kinetic; V (x/xylem/root) = cell (exuded/xylem/root) volume Correspondence to: T. Henzler; E-mail: tobias.henzler@uni-bayreuth.de; Fax: 49 (921) 55 2564 Planta (1999) 210: 50±60 Diurnal variations in hydraulic conductivity and root pressure can be correlated with the expression of putative aquaporins in the roots of Lotus japonicus Tobias Henzler 1 , Rosemary N. Waterhouse 2 , Audra J. Smyth 2 , Micaela Carvajal 3 , David T. Cooke 2 , Anton R. SchaÈner 4 , Ernst Steudle 1 , David T. Clarkson 2 1 Lehrstuhl P¯anzenoÈkologie, UniversitaÈt Bayreuth, 95400 Bayreuth, Germany 2 Department of Plant Sciences, IACR-Long Ashton, University of Bristol, BS41 9AF, UK 3 Departamento Nutricion y Fisiologia Vegetal, CEBAS-CSIC, 30080 Murcia, Spain 4 Institute of Biochemical Plant Pathology, GSF National Research Center, 85764 Neuherberg, Germany Received: 3 February 1999 / Accepted: 25 June 1999