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 (Ψs) 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 Ψs in the root zone was reduced to –1 to –5 bars, depending on soil storage, root distribution and potential transpiration rate. From this point Ψs 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 (rs) 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 (Rp), derived from leaf water potentials and fluxes through individual stems, increased as stem populations increased. In the high population lysimeter Rp 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 Rp 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.

The effects of drought on barley: soil and plant water relations

“…The relationship between \jr t and E was markedly non-linear. Such non-linearity has been observed by many workers (Jones, 1978;Seaton, Landsberg & Sedgley, 1977;Hailey et al 1973). Landsberg & Fowkes (1978) have shown that non-linearity can occur because of variation in i/r, through the profile.…”

Use of the root contact concept, an empirical leaf conductance model and pressure-volume curves in simulating crop water relations

“…The root radius was 0.072 (_+0.014) mm as determined from microscopy of 8 field grown barley root samples in 0.1 m deep layers from 0-0.4 m depth. In order to avoid interference from few thick main roots (Seaton et al, 1977) only the radii of laterals were determined. Within each sample 20 measurements of root radius were undertaken.…”

A two-layer model of soil hydrology

“…Thick lines represent cases where transpiration was thought to be of minimal importance. Individual curves are: (M), 5 cm layer of 50-60% sand covered by little vegetation (Marsh et al, 1981); (DA), 5 cm layer of sandy loam covered by ryegrass (Davies and Allen, 1973); (R), 1.5 m layer of sandy loam, vegetation recently burned (Rouse et al, 1977); (D), 1 or 2 cm layer of soil, vegetation recently burned (Barton, 1979); (W), 5 cm layer of soil covered by dry rangeland grass (Williams et al, 1978); (MN), lo-20 cm layer of heavy clay covered by shallow rooted grass (Mukammal and Neuman, 1977); (BD), 25 cm layer of sandy loam with soybean crop (Bailey and Davies, 1981); (SC), 37 cm layer of sandy clay loam with wheat crop (Seaton er al., 1977); (S), 37 cm layer of sandy loam with wheat crop (Seaton et al, 1977).…”