100 MINERAL SALTS ABSORPTION IN PLANTS 



Donnan-free space of maize roots is about 10 m eq/1. In this 

 calculation, the possibility that a fraction of the immobile anions 

 were already associated with cations (e.g. calcium ions) when the 

 tissue was placed in salt was neglected, and this may have led to an 

 underestimation of the concentration of immobile anions in the 

 Donnan-free space. More recent experiments with salt-saturated 

 red beet tissue in which this source of error was minimised have 

 yielded values of about 560 m eq/1. (Briggs et al. 1958). 



Although the reality of a Donnan-free space has been established, 

 its precise location remains a subject of debate. Levitt (1957) has 

 argued convincingly that it can be adequately accommodated in cell 

 wall spaces. Dainty and Hope (1959) have compared the reversible 

 physical component of uptake in intact Chara plants with that 

 in separated cellulose walls of the same organism, and have con- 

 cluded likewise that the bulk of the Donnan-free space is located in 

 the wall. According to this view the plasmalemma is an effective 

 barrier to passive penetration and active mechanisms must be 

 involved in the movement of ions across it. Arisz (1953) has supplied 

 evidence that chloride absorption into the cytoplasm of Vallisneria 

 leaf cells is more sensitive to inhibition by cyanide, and less sensitive 

 to dinitrophenol poisoning than is the transfer of the ions into 

 vacuoles. Both processes apparently depend on respiration. 



Others (e.g. Robertson, 1951; Hope and Stevens, 1952, Butler, 

 1953) have suggested that the apparent free space may include at 

 least a part of the cytoplasm. Possibly the truth lies between these 

 two views, for the plasmalemma sometimes behaves as a cation- 

 permeable/anion-impermeable membrane, across which anions are 

 actively transported but cations exchange freely. Two facts make it 

 exceedingly difficult to reach a definite conclusion regarding 

 location of the DFS exclusively in the cell wall or partly in the 

 cytoplasm. 



(i) There may be no sharp line of demarcation between the two, 

 especially in young cells with actively growing walls which 

 seem to originate from the bulk of the cytoplasm, and 

 (ii) the outer surface of the cytoplasm may be extensively 

 convoluted, enclosing within the cell an appreciable space 

 which is in direct communication with the external medium. 

 Electrical charges on the membrane would cause fine 

 invaginations to behave as Donnan-free spaces. 



