Chapter IX — 163 — Uptake and Movement 



a known pressure difference was observed by attaching a suction pump to 

 the stem. On the assumption that the resistance in the stump remained 

 constant and that the rate of water movement into the stem was proportional 

 to the pressure drop across the resistance, Renner cakulated that tensions 

 of 1.0 to 1.5 atmospheres were customary in herbaceous plants. He found 

 an extreme tension of 5 atmospheres in one experiment with ForsytJiia. The 

 soil around all of the experimental plants was well watered, which probably 

 explains the low tensions in the plants. 



Earlier (1911a) Renner had used potometer methods to determine 

 tensions in rooted bean plants as well as in leafy shoots. He recorded pres- 

 sure deficits of wilted leaves of 10 to 20 atmospheres, while tensions of 

 culture solution plants were found to be about 1 atmosphere. Renner be- 

 lieved that the maximum tension was limited by the osmotic pressure of the 

 leaves. 



Nordhausen (1916, 1917, 1921) was skeptical of Renner's results, 

 feeling that important changes occurred in the resistance of the stem caus- 

 ing a reduced flow under the force of the pump. This would result in the 

 tension values being too high. He modified Renner's method by intro- 

 ducing a porous clay disk of known resistance at the base of the cut stem. 

 By comparison of the rates of flow through this resistance under the action 

 of the stem, with the rates obtained with a known pressure drop, he cal- 

 culated tensions of 3 to 4 atmospheres in trees and 7 to 8 atmospheres in 

 cut shoots. The higher values in the cut shoots he explained as arising 

 from increased water deficits from restricted supply caused by cutting and 

 applying the clay resistance. 



Dixon (1924) believed that Nordhausen's values represented mini- 

 mum tensions as the clay resistance would cause some plugging of the stem 

 and air would be introduced during the preparation of the experiment. 



HoLLE (1915) pointed out that the vessels in wilted leaves of Alliaria 

 officinalis are full of water as shown by direct observation. Under this con- 

 dition the diffusion pressure deficit of water in the vessels should equal the 

 osmotic pressure of the mesophyll cells (since turgor equals zero). The 

 osmotic pressure of the xylem sap due to solutes would reduce this value 

 slightly. 



A modification of Renner's method was used by Eaton (1941) in cal- 

 culating the overall tension in the xylem. A water culture plant was placed 

 with half of its roots growing in a high salt solution and the other half grow- 

 ing in a low salt solution. The roots were alternated between the solutions 

 so that for a given time period there was an equal area of absorbing sur- 

 face in each solution. Measurement was made of the amount of water ab- 

 sorbed from each solution for a given length of time. It was assumed that 

 the flow of solution into the roots was proportional to the pressure head 

 and that the resistance was the same in both cases. An average value for 

 thjs tension at the base of the shoot which satisfied both sets of water uptake 

 and osmotic values was found to be 3.55 atmospheres. 



No consideration was given to the effect of the different solutions on the 

 permeability of the roots, and the influence of time and temperature on the 

 osmotic pressure of the solutions was neglected. As Eaton points out, 

 such a calculation represents at best only an overall figure, and a fluctua- 

 tion from perhaps positive pressures at night to relatively high tensions dur- 

 ing the periods of rapid transpiration is probably undergone by water in the 

 xylem. 



