Transpiration and leaf water potentials of wheat in relation to changing soil water potential

Abstract

Changes in the transpiration rate of wheat in drying soils were followed in experiments in which plants were grown in two small weighable lysimeters in a glasshouse. Hourly measurements of soil water potential (Ψ_s) were made at three depths in each lysimeter. The water potential of flag leaves was measured with a pressure chamber, and stomatal resistance with a pressure drop porometer. Data on root densities and distribution were also obtained. Transpiration rates fell below estimated potential levels when the average value of Ψ_s in the root zone was reduced to –1 to –5 bars, depending on soil storage, root distribution and potential transpiration rate. From this point Ψ_s fell rapidly in the surface layers, more slowly at depth. It was found that accurate calculations of daily water uptake could be made from changes in soil water content. The minimum value of leaf water potential (°1 )attained each day declined progressively through the drying cycle, but there was evidence that stomatal resistance (r_s) is not uniquely related to Ψ₁; initial stomatal closure occurred at Ψ₁, values which decreased from –11 to –25 bars as drying progressed. This adaptive mechanism is related to changes in osmotic potential of the leaves.

Whole plant resistances (R_p), derived from leaf water potentials and fluxes through individual stems, increased as stem populations increased. In the high population lysimeter R_p decreased from 300 to 100 bar sec mm^-3 as canopy transpiration rates increased from 1.5 to 4.5 x 10^-4 mm sec^-1. In the low population lysimeter R_p decreased from 70 to 30 bar sec mm^-3 as transpiration increased from about 2.2 to 4.5 x 10^-4 mm sec^-1. The higher resistances appear to confer significant advantages in terms of water conservation and adaptation to drought.