Diurnal variations in hydraulic conductivity and root pressure can be correlated with the expression of putative aquaporins in the roots of Lotus japonicus

The hydraulic conductivity of excised roots (Lp(r)) of the legume Lotus jap onicus (Regel) K. Larsen grown in mist (aeroponic) and sand cultures, was f ound to vary over a 5-fold range during a day/night cycle. This behaviour w as seen when Lp(r) was measured in roots exuding, either under root pressur e (osmotic driving force), or under an applied hydrostatic pressure of 0.4 MPa which produced a rate of water flow similar to that in a transpiring pl ant. A similar daily pattern of variation was seen in plants grown in natur al daylight or in controlled-environment rooms, in plants transpiring at am bient rates or at greatly reduced rates, and in plants grown in either aero ponic or sand culture. When detached root systems were connected to a root pressure probe, a marked diurnal variation was seen in the root pressure ge nerated. 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. Measurement s were made on the first four cell layers of the cortex, but no evidence of any diurnal fluctuation could be found. It was concluded that the conducta nce 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 wi th cDNA from the Arabidopsis aquaporin AthPIP1a gene, an abundant transcrip t was found to vary in abundance diurnally under high-stringency conditions . The pattern of fluctuations resembled closely the diurnal pattern of vari ation in root Lp(r). The plasma membranes of root cells were found to conta in an abundant hydrophobic protein with a molecular weight of about 31 kDa which cross-reacted strongly to an antibody raised against the evolutionari ly conserved N-terminal amino acid sequence of AthPIP1a.