Water Movement from Soil to Root Investigated through Simultaneous Measurement of Soil and Stem Water Potential in Potted Trees

Abstract

Legge, N. J. 1985. Water movement from soil to root investigated through simultaneous measure ment of soil and stem water potential in potted trees.—J. exp. Bot. 36: 1583-1589. Osmotic tensiometers implanted in the stems of three mountain ash (Eucalyptus regnans F. Muell.) saplings growing in large plastic bins recorded stem water potential, Wsv while soil water potential, f „ was simultaneously recorded by instruments near the trees' roots and in the surrounding root-free soil. Early in a drying cycle, with the soil still wet, the diurnal variation in W, was often slight, despite diurnal variations in Vst approaching 2 0 MPa. Late in a drying cycle the diurnal fluctuations in fs and fst were very similar although changes in fs still lagged up to 1-5 h behind changes in fst. f5 values at this time occasionally reached —3-0 MPa with no apparent damage to the trees. Watering the bins in daytime led to a response in fs values within about 5 min, whereas ¥n values did not respond for a further 20 min. fst values then rose rapidly but after only 1 h began to decline again, while fs values remained at or near saturation for the rest of the day. Water uptake hypotheses which attribute an important role to a soil-root interface resistance are not supported by these data. Key words—Soil water potential, perirhizal gradients. Correspondence to: School of Agriculture, La Trobe University, Bundoora, Victoria 3083, Australia.