Studies on the Movement of Water Through Apple Trees

Abstract

Resistances to the flow of water through young potted apple trees were estimated by measuring the transpiration rate of trees with and without root systems. Root system resistances were obtained by difference. Whole-plant resistances were of the order 10 x 1013 Pa s m-3 and there was some evidence that root resistances (Rr) varied with transpiration rate; the ratio liT■ Rx (where Rx is resistance to water flow in the stem system) altered from 2:1 at relatively high transpiration rates to 1:1 at lower rates. The trunk of a 9-year-old orchard tree (trunk diameter ~7 cm, height ~2-5 m) was cut under water and estimates of the flow resistances in this tree were obtained. These were much lower than the resistances to flow in the potted trees. Capacitance (defined as the change in stored water content per unit change in plant water potential) values were calculated for the small trees and the large tree from measurements of weight and water potential changes after the trees were removed from water. They were very similar on a weight basis (approx. 2-0 x 10~8 kg kg-1 Pa-1). Leaf capacitance values (~1 x 10~8 kg Pa-1 m-2) were also obtained. Stomatal conductances decreased with water potential and increased with short-wave radiation, but the relationships were not definitive. Estimates of boundary layer conductance in a greenhouse (very low wind speeds) were of the same order (~ 5 mm s-1) as values obtained previously.