Hydraulic Architecture of Whole Plants and Plant Performance

Abstract

Most of the early work on the hydraulic architecture has focused on the stems of the shoots. The tacit assumption has been that because trees are tall, most of the resistance ought to be in the stems. Based on an incomplete data set, I once even suggested that the shoots of large trees might have more hydraulic resistance than the roots in the ratio of 3:1 (Tyree 1988) but this is unlikely to be true generally and may have been wrong in the specific case cited. Roberts (1977) measured the midday xylem pressure, P _x , of large trees and then cut off the trees near ground level and placed them in a bucket of water. Removing the resistance of the roots raised P _x to a less negative value, which we will designate P′ _x .