50 MINERAL SALTS ABSORPTION IN PLANTS 



(1939) observed that Nitzschia closterium (a marine diatom) will 

 normally absorb phosphate only in the light, but it can be induced 

 to do so to a limited extent in the dark if the cells have previously 

 been grown in light under conditions of phosphate deficiency. 

 Dark-induced absorption may depend on chemical combination 

 between the ions and binding substances which are synthesized in 

 the light. Uha cells gradually lose potassium ions when kept in the 

 dark and reabsorb them upon reillumination (Fig. 15b). The 

 reason for the temporary increase in potassium content to a supra- 

 optimal level soon after transfer of the plants from darkness to 

 light, followed by a rapid decrease to the normal level, is not clear. 



An interesting light-dependent ion transport mechanism was 

 observed in the leaves of certain water plants by Arens (1936) and 

 has been further studied by Steemann Nielsen (1951) and Lowen- 

 haupt (1956). Briefly, when a solution of calcium bicarbonate is 

 applied to the abaxial surface of a Potomogeton leaf in the light, salt 

 is absorbed and calcium ions are subsequently excreted, together 

 with hydroxyl ions, on the other side. The cation movement ceases 

 in the dark or when salts other than bicarbonate are supplied, and is 

 presumably hnked to utilization of bicarbonate in photosyn- 

 thesis. 



Ultra-violet light inhibits salt absorption and may cause leakage 

 of ions from cells. Destruction of lipo-protein complexes which 

 participate in the maintenance of the semi-permeability of proto- 

 plasm, and decomposition of ribonucleic acid, have been implicated 

 in this effect (Lepeschkin, 1930; Tanada, 1955). 



3. Oxygen Pressure 



In the absence of oxygen, the metabolic component of salt 

 absorption is inhibited in aerobic organisms. Vlamis and Davis 

 (1944) found that excised roots of various plants showed maximum 

 absorption at oxygen concentrations of 2-3 per cent, and higher 

 concentrations up to 100 per cent did not increase the rate of uptake 

 further (Fig. 16a). Hopkins (1956) found that phosphate absorption 

 by excised barley roots is independent of the partial pressure of 

 oxygen over the range 3-100 per cent when the total gas pressure is 

 kept at 1 atm. Over the range 3-0 per cent a hyperbolic relationship 

 was observed with 0-3 per cent giving half of the maximum rate 

 (Fig. 16b). 



