The Effect of Transpiration Rate on the Leaf Water Potential of Sand and Soil Rooted Plants

Abstract

SUMMARYTwelve-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 (AH^i) and stomatal conductance were followed.With a fine sand it was found that when the water table was high (15 cm below the roots) AIF, was 6 atm. irrespective of the rate of transpiration and behaved therefore as if the roots were surrounded by f^ree water as previously shown. \\'hen the water table was 25 cm below the roots, A IF, was also 6 atm. provided the transpiration rate was low (0.4 g/dm^ leaf/hour) but when the transpiration rate was raised threefold, Afr, 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, A IF, returned to 6 atm. This rise in A IF, is interpreted as due to a rise in the AfF 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 ^0°/^ relative humidity through a 24-hour cycle produced a regular 24-hour cycle of transpiration (see Figs. 4, 5 and 6). Responses in A IF, 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.

The Location of the Resistance to Water Movement in the Soil Supplying the Roots of Transpiring Plants

“…This constancy in A;//i was also reflected in the smallness of the changes in R.W.C., stomatal aperture and (5 gauge reading. These results confirm the findings of Tinklin and Weatherley (1966) using Rieinus eommunis.…”

“…It will be noticed that Ai//i values obtained psychrometrically and with the pressure bomb agree closely under conditions of water stress, however, when the rate of transpiration approached zero (periods 3 and 5) Ai/zj as measured by the psychrometer was distinctly higher than that obtained with the pressure bomb. It is widely recognized that as tissues approach full turgidity, psychrometric values of A\JJI are overestimates (Tinklin, 1967). It is likely that no such error occurs with the pressure bomb and it is interesting to note therefore that such pressure bomb values approach those of A\pg in periods 3 and 5.…”

“…The results obtained have differed widely with different species and with different experimenters (see Weatherley, 1976). Using Ricinus, Gossypium and Helianthus it has invariably been found in this laboratory that 338 S. M. A. FAIZ and P. E. WEATHERLEY remains constant (plateau value) over a wide range of transpiration rates (Macklon and Weatherley, 1965;Tinklin and Weatherley, 1966;Stoker and Weatherley, 1971). This means that the resistance of the plant (/},) is not a constant but declines with increasing rate of transpiration.…”

Vertical Gradients of Water Potential and Tissue Water Relations in Sitka Spruce Trees Measured with the Pressure Chamber

“…(It should be noted that much of the literature showing low leaf water potentials in plants in the field, even in the leaves of plants growing in moist soil, is not relevant in this connection because of the likely occurrence of an appreciable resistance in the soil surrounding the roots, or at the root-soil interface (Tinklin & Weatherley 1968). )…”

Response of Plant Water Potential to the Irrigated Environment of Southern Idaho1

“…While basic laboratory and controlled environment studies are increasing the knowledge of water-potential effects on plant growth, little is known about the actual water-energy levels of plants under field conditions. It is known that plant water potential is influenced by available soil water and the evaporative demand of the atmosphere (6,9,12,20). Some data are available showing plant water potential during short periods of time under field conditions (1, 3, 7, 10, 11, 13, 14, and 18); however, there is almost no information on the daily changes of water potential in field crops throughout the growing season.…”