Vertical Gradients of Water Potential and Tissue Water Relations in Sitka Spruce Trees Measured with the Pressure Chamber

Abstract

Under static conditions of no water flow, a change in water potential of about 0.1 bar m-' is expected over the distance between the soil and the leaves in the canopy of a tree. Under conditions where there is a flow of water through the soil-plant-atmosphere continuum, a larger difference in water potential is to be expected as the result of the resistances to flow encountered by the moving water. The magnitude of the depression of water potential in the leaves depends on both the resistances to flow through the soil-plant-atmosphere continuum and on the rate of flow which is determined by the atmospheric onditions at the leaves. In small herbaceous plants, the resistance of the plant itself to water movement is small (Tinklin & Weatherley 1966, 1968), and the major part of the plant resistance lies in the leaves and the roots, the contribution of the stem to the total being small (Jensen, Taylor & Wiebe 1961; Boyer 1971; Stoker & Weatherley 1971). Nonetheless, gradients in water potential of 5 bar m-' between roots and leaves have been reported under conditions of high evaporation (Begg & Turner 1970; Cary & Fisher 1971). (It should be noted that much of the literature showing low leaf water potentials in plants in the field, even in the leaves of plants growing in moist soil, is not relevant in this connection because of the likely occurrence of an appreciable resistance in the soil surrounding the roots, or at the root-soil interface (Tinklin & Weatherley 1968).) In larger plants, such as trees, it might be expected that the stem resistance would be sufficiently arge to contribute appreciably to the reduction in leaf water potential even under quite moderate conditions of evaporation and that differences in stem anatomy amongst species might be partly responsible for different reductions in leaf water potential. For example, Farmer (1918) showed that the resistance of coniferous hoots to the passage of water was three to six times that of broad-leaved trees, and Peel (1965) found a lower resistance to flow of water in the xylem of ring porous Fraxinus than in the xylem of diffuse porous Acer and Salix. The significance of this resistance in the stem to the development of low water potentials in the leaves depends on its size in relation to the total resistance in the soil-plant-atmosphere continuum. If it is relatively large, the implication is that low leaf water potentials may occur in tall trees largely as a result of the distance of the leaves from the source of water. Recently there has been some dispute as to whether the gradient of water potential in the stems of tall trees does in fact exceed that required to maintain the hydrostatic head, viz. 0 1 bar m-'. Gradients no steeper than this have been reported in tall trees of Sequoia sempervirens, Pseudotsuga menziesii and Sequoiadendron giganteum even at * Permanent address: Institute of Physiological Botany, University of Uppsala, Uppsala, Sweden.