22 MINERAL SALTS ABSORPTION IN PLANTS 



of salt absorption by these organelles has not been investigated. 

 Further studies of ion uptake in biochemically-active preparations 

 of sub-cellular particles will undoubtedly lead to a greater under- 

 standing of the process of absorption, and the spatial location of 

 ion-transport mechanism in cells. 



B. Experimental Methods 



1. Qualitative Determinations 



The intake of salts by plant cells can sometimes be detected by 

 microscopic observations, and various ingenious techniques were 

 devised by early investigators for this purpose. One of the most 

 successful of these methods involves measurement of the rate of 

 recovery of a vacuolated cell after it has been plasmolysed in a 

 hypertonic solution. If solute is absorbed, the osmotic pressure of 

 the vacuolar sap gradually increases and its volume expands through 

 the uptake of water. This procedure was used by De Vries (1871) 

 in an attempt to demonstrate the penetration of sodium chloride 

 into red beet cells. 



Entry of coloured ions into cells can sometimes be followed by 

 direct microscopic observation. If, however, the cell remains 

 colourless upon immersion in a solution of dye, it is not certain, 

 that the coloured ions have failed to enter because some dyes are 

 decolourized at the redox potential of the cell. Penetration of 

 colourless ions can occasionally be detected as a result of formation 

 of coloured complexes with cell constituents; iron, for example, gives 

 a blue coloration in the presence of tannins. Alternatively, micro- 

 chemical tests can be applied to show the presence of certain ions 

 in cells following immersion of a tissue in salt solutions. Dipheny- 

 lamine has been used in this way to demonstrate uptake of nitrate. 

 If a cell contains soluble carbonates or oxalates, absorption of 

 calcium ions is indicated by the appearance of crystals of insoluble 

 calcium salts. 



More recently, the technique of autoradiography has been 

 used to show the uptake by root tips of such ions as phos- 

 phate and sulphate, labelled with radioactive isotopes and even 

 to demonstrate their intracellular location (Howard and Pelc, 

 1951). 



