Question

On the relationship between transpiration rate and leaf water potential

Answer 1

Answer:

The aim was to vary the transpiration rate of water culture plants and to measure corresponding values of ΔW, the leaf water potential depression (= DPD = suction force), as well as relative water content (relative turgidity) and stomatal aperture. On the Ohm‐law analogy ΔW and transpiration rate should be linearly related.

Ricinus communis plants were placed in a wind tunnel and the transpiration rate was varied by changing the level of the relative humidity of the air stream. With increasing rate of transpiration, ΔW rose steeply to a value of about 6 atmospheres at which it remained virtually constant (Fig. 1). Thus from very low transpiration rates to the highest obtainable, far from ΔW rising linearly with transpiration, there was no response in ΔW and similarly no response in relative water content and stomatal resistance. When transpiration was reduced to zero the leaves still maintained a ΔW of about 3 atmospheres instead of zero, but reasons are put forward for considering this value to present water saturation.

The main resistance to water movennent in the plant was shown to reside in the roots. If a leaf was detached from a transpiring plant and allowed to continue transpiration at a similar rate, water being freely supplied to the cut end of the petiole, its ΔW fell from 6 to 3.5 atmospheres (Fig. 3). Thus the resistance must lie‘below’the petiole. Similarly it was shown to lie below the stem.

The constancy of leaf ΔW in the face of large changes in flux of water through the roots, implies that the root resistance declines in step with the rise in flux. A simple hydraulic model is presented which would behave in this way.