15.1.2 Hydraulic architecture

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Figure 15.3 Each morning, transpirational water flux increases and leaf water potential declines. Leaf water potential declines because of the time lag between loss of water from the leaves and increased uptake by roots. The slope of leaf transpiration as a function of leaf water potential is a measure of hydraulic conductivity (Lp) of the soil-plant system. This relationship is not always a single-phase straight line. Change in slope reflects changes in soil-plant Lp over time (based on Eamus et al. 1996a)

Transpiration from canopies lowers leaf water potential (Ψleaf), which is immediately translated to a tension within the soil–plant–atmosphere continuum. Water moves from soil to canopy according to overall differences between Ψsoil and Ψleaf and resistances to flow in between. Whole-plant hydraulic conductivity can be estimated by plotting whole-plant transpiration rates against leaf water potential from dawn to midday (Figure 15.3). The slope of the resultant line is a measure of the hydraulic conductivity (Lp). This outcome can be deduced from restating the basic Ohm’s Law analogue that flux of water between two points (Jv) is equal to the hydraulic conductivity (Lp) × the gradient in water potential (ΔΨw). Thus:


This model assumes that water potential in the root–soil interface remains constant and is usually defined, in experimental conditions, as being zero or equal to the pre-dawn water potential of plant foliage. The test plant is assumed to have equilibrated to soil water potential overnight and this method further assumes that there is little change in this value during the following day. These qualifications are discussed below.