Increases in water potential gradient reduce xylem conductivity in whole plants. Evidence from a low-pressure conductivity method

A new method using hydrostatic suctions (less than 0.02 MPa) was used to me asure whole-root conductivity (K-r) in saplings of two angiosperm pioneer t rees (Eucalyptus vegnans and Toona australis) and two rainforest conifers ( Dacrycarpus dacrydioides and Nageia fleurii). The resultant K-r was combine d with measurements of stem and leaf hydraulic conductivity to calculate wh ole-plant conductivity and to predict leaf water potential (psi(1)) during transpiration. At normal soil temperatures there was good agreement between measured and predicted psi(1) during transpiration in all species. Changes in the soil-to-leaf water potential gradient were produced by root chillin g, and in three of the four species, changes in psi(1), corresponded to tho se expected by the effect of increased water viscosity on K-r. In one speci es, however, root chilling produced severe plant wilting and a decline in p si(1) significantly below the predicted value. In this species psi(1) decre ased to a value close to, or below, the psi(1) at 50% xylem cavitation. It is concluded that decreased whole-plant conductivity in T. australis result ed from a decrease in xylem conductivity due to stress-induced cavitation.