Twelve-week-old Ricimis comiminis plants were transplanted to boxes of sand or soil in which the water table was maintained at various levels below the root system. The boxes were placed in a climatological wind tunnel and transpiration varied by altering the humidity of the air stream. Responses in the water potential depression (= suction force or D.P.D.) of the leaves (ΔW1) and stomatal conductance were followed.
With a fine sand it was found that when the water table was high (15 cm below the roots) ΔW1, was 6 atm. irrespective of the rate of transpiration and behaved therefore as if the roots were surrounded by free water as previously shown. When the water table was 25 cm below the roots, ΔW1 was also 6 atm. provided the transpiration rate was low (0.4 g/dm2 leaf/hour) but when the transpiration rate was raised threefold, ΔW1, rose steeply to a value of 12 atm., a steady state value being approached in about 8 hours (see Fig. 3). On reducing the transpiration rate, ΔW1, returned to 6 atm. This rise in ΔW1, is interpreted as due to a rise in the ΔW of the water in the sand at the root surface, this in turn resulting from the low water conductivity of the sand at this height above the water table. Varying the humidity continuously between saturation and about 40% relative humidity through a 24-hour cycle produced a regular 24-hour cycle of transpiration (see Figs.4, 5 and 6). Responses in ΔW1, in plants rooted in fine sand, clay soil and moorland soil were explicable in terms of the above hypothesis of the development of perirhizal zones of soil water tension.
These results suggest that much of the ecologically significant water stress in plants is a manifestation of the dynamic gradients of water potential developing in the perirhizal zones and that these can be considerable even when the water stress in the bulk of the soil is virtually zero.
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