Plant Physiol, July 2000, Vol. 123, pp. 1021-1028

Increases in Water Potential Gradient Reduce Xylem Conductivity in Whole Plants. Evidence from a Low-Pressure Conductivity Method¹

Department of Plant Science, University of Tasmania, G.P.O. Box 252-55, Hobart 7001, Tasmania, Australia (T.J.B.); and Department of Environmental Biology, University of Adelaide, South Australia 5005, Australia (R.S.H.)

A new method using hydrostatic suctions (less than 0.02 MPa) was used to measure whole-root conductivity (K_r) in saplings of two angiosperm pioneer trees (Eucalyptus regnans and Toona australis) and two rainforest conifers (Dacrycarpus dacrydioides and Nageia fleurii). The resultant K_r was combined with measurements of stem and leaf hydraulic conductivity to calculate whole-plant conductivity and to predict leaf water potential (_l) during transpiration. At normal soil temperatures there was good agreement between measured and predicted _l during transpiration in all species. Changes in the soil-to-leaf water potential gradient were produced by root chilling, and in three of the four species, changes in _l corresponded to those expected by the effect of increased water viscosity on K_r. In one species, however, root chilling produced severe plant wilting and a decline in _l significantly below the predicted value. In this species _l decreased to a value close to, or below, the _l at 50% xylem cavitation. It is concluded that decreased whole-plant conductivity in T. australis resulted from a decrease in xylem conductivity due to stress-induced cavitation.