110 MINERAL SALTS ABSORPTION IN PLANTS 



tend to accumulate salt in vacuoles, and therefore little is available 

 for transport elsewhere when the absorption zone is exposed to light. 

 Illumination of cells in the receptor zones, likewise stimulates 

 vacuolar accumulation, and promotes transport of salts towards 

 them from the absorption zone. 



Evidence that movement from zone to zone occurs via cytoplasm 

 rather than by diffusion through cell walls was obtained by experi- \ 

 ments with metabolic inhibitors. Accumulation of chloride in all \ 

 parts of the leaf was prevented when potassium cyanide (3 X lO^'^M) 

 was applied to the absorption zone (Fig. 37g, h). This would not ; 

 be expected if salt moved passively through cell walls, unless, 

 indeed, the inhibitor also travelled along a similar path in sufficient i 

 amounts to prevent absorption in the recipient zones. That the i 

 latter is not the case was shown by the failure of potassium cyanide 

 to inhibit uptake by the absorption zone when it was supplied i 

 elsewhere (Fig. 37j). Cyanide at this concentration evidently 

 inhibits uptake of chloride into the protoplasm of Vallisneria cells, 

 but does not interfere with accumulation in vacuoles or with trans- 

 port through the symplast. 



Dinitrophenol (10~'*M) was found to inhibit accumulation in the j 

 zone to which it was applied, presumably through an effect on 

 transport into vacuoles (Fig. 37 e, f, i, k). When supplied to the 

 absorption zone (Fig. 39 e, f, i) it did not apparently interfere with | 

 transport and accumulation of salt elsewhere, but in the case (Fig. ; 

 37 k) where it was applied to a recipient zone, there is some : 

 indication that it prevented transport across that region into a j 

 terminal zone. In more recent work (Arisz, 1958) it has been 

 demonstrated that a number of other metabolic inhibitors, which 

 affect either vacuolar accumulation (e.g. azide) or uptake into j 

 cytoplasm (arsenate and uranyl ions) do not influence transport i 

 from zone to zone, and this seems to support the somewhat surprising j 

 conclusion that movement of salts in cytoplasm is mainly a passive 

 process. More experiments with Vallisneria leaves and other 

 materials are required before such a conclusion can be fully justified. 



For further reading 



Arisz, W. H. (1953). Active uptake, vacuole-secretion and plasmatic transport ] 



of chloride ions in leaves of Vallisneria spiralis. Acta Bot. Neerl. 1, 506-15. | 



Briggs, G. E. and Robertson, R. N. (1957). Apparent free space. Ann. Rev. ' 

 Plant Physiol. 8, 11-29. 



