Chapter VIII _ 131 _ Active Relations 



Measurements of Diffusion Pressure Deficits : — Another manifesta- 

 tion of an "active" force has been observed in potato tuber tissue by Lyon 

 (1942), who used the simplified method (cf. Chapter VII). Comparison 

 was made between the observed DPD value (OP of sucrose solution effect- 

 ing no change in the volume of the tissue), and the calculated DPD value, 

 derived as follows : 



DPNn = On — TPn (i) 



where On = Og • — ^ (2) 



Vn 



^ Vs (Vs — Vg) ^^ 



The resuhs indicated that calculated DPDn values exceeded observed values 

 by as much as 14 atm. for tubers in warm (70° F.) storage, or over half 

 again as much as the observed DPDn. The average discrepancy was about 

 3 atm. for tubers which had been in storage at 42° F. and about 6 atm. for 

 those at 70° F. 



Certain trends are apparent in the data — the significantly higher dis- 

 crepancy at the warmer storage temperatures ; its absence at harvest time ; 

 apparent increase in turgor simultaneous with water loss. These observa- 

 tions are difficult to explain by means of the classical theory of osmotic pres- 

 sure. The author suggests that while the water secretion hypothesis is a 

 possibility, its proof is lacking, and that "the force could be one of pure 

 chemistry, such as an effect of colloids on the diffusion pressure of water 

 within the cells or a difference in electrical potentials, either of which could 

 vary during storage through physiological changes." 



Lyon points out that the discrepancy could reasonably be due to: (a) 

 underestimating the calculated normal wall pressure or (b) overestimation 

 of the osmotic pressure at normal cell volume. As one limitation of the 

 simplified method, there is the possibility of excessive Og values, for rea- 

 sons stated on page 97 (Chapter VII). Such an error, if sufficiently great, 

 would account for the discrepancy. It may be a matter of plasticity of 

 cell walls, the degree varying under differing conditions. One might also 

 question the validity of assuming a direct proportionality between volume 

 and osmotic pressure at the three measurement states. The non-solvent 

 fraction in potato tuber cells would seem to be significant. Regardless of 

 these suggestions with respect to the method, there are indications here of 

 some kind of active regulation. The wide variation in the calculated DPD 

 value and the consistent trends are otherwise difficult to explain. 



The water secretion theory of Bennet-Clark finds support in some 

 investigations of Brauner and Hasman (1946), who have proposed that 

 an electrokinetic component is partly responsible for the DPD of cells. 

 Using potato and carrot tissue in isotonic solutions of K2SO4, sucrose and 

 CaCl2, the DPD (gravimetric method) varied in a manner suggesting that 

 the charge on cell membranes may exert a control on the movement of 

 water. The differences were not attributable to permeability factors. Small 

 amounts of CaCl2 had a marked eft'ect in lowering the DPD. In potato, 

 about 10 per cent of the total DPD was ascribed to the electroosmotic fac- 

 tor. Somewhat higher values were subsequently (1947) demonstrated in 

 beetroot and in carrot. 



