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SUMMARY

Previous work has shown that when plants are rooted in unsaturated soil and subjected to high rates of transpiration, the depression of water potential in the leaves (ΔΨ1) may rise to as much as 15 bars, but falls to less than half this value on reducing or stopping transpiration. This fluctuation in water stress in the plant has been interpreted as due to changes in the depression of water potential of the soil at the root surface (ΔΨs) arising dynamically as a result of the hydraulic resistance of the soil. Major gradients of ΔΨs could arise in the soil either around each individual root (perirhizal) or between the root zone and the surrounding root-free water supplying zone (pararhizal).

To decide between these two hypotheses, experiments were made on Helianthus plants subjected to conditions in which pararhizal gradients were unlikely to arise. The above pattern of stress development and recovery were clearly demonstrated, thus favouring the perirhizal hypothesis. The data indicate that under these experimental conditions, the drop in water potential across the perirhizal soil was about the same as that through the plant (approx. 6 bars).