336 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



(e. g., Cu++ on Hg++) may be considered as true poisons. Other ions 

 become active in concentrations of the order of 0.01-0.1 m./l.; while the 

 remainder act only above 0.1 m./l. The latter can be considered as 

 indifferent; they affect photosynthesis only by changing the osmotic 

 pressure (as described in the preceding section). 



Even more than in the case of typical enzyme poisons, the sensitivity of individual 

 plants to inorganic ions is a question of (phylogenetic and sometimes even ontogenetic) 

 adaptation. The behavior of salt-water algae is different from that of soft-water algae, 

 while algae living in mineral springs probably are adapted to the specific composition 

 of their natural media. 



Fromageot (1923) studied the influence of salt concentration on the photosynthesis 

 of marine algae. The highest rate was observed in natural sea water; deviations in 

 salinity in either direction caused a dechne in photosynthesis. The algae retained a 

 certain capacity for photosynthesis even in distilled water, but this was very weak; 

 respiration, on the other hand, proved to be indifferent to changes in salinity. 



Considering the variety of "salt effects" in photosynthesis, it is 

 obviously impossible to give a common explanation for all of them. 

 Some ions affect only photosynthesis in strong light, i. e., they influence 

 the enzymatic mechanism of photosynthesis; while others reduce or 

 stimulate photosynthesis under all conditions. This is true, according 

 to Briggs (1922), of a deficiency in potassium, phosphorus, magnesium, 

 and iron; and according to Greenfield (1941, 1942), of an excess of copper 

 sulfate, cobaltous sulfate, potassium iodide, boric acid, and ammonium 

 sulfate. 



Pirson (1937, 1938, 1940) and Greenfield (1941, 1942) have discussed 

 the mechanisms by which ions may affect photosynthesis, in particular 

 their relation to the colloidal properties of the protoplasm. Ustenko 

 (1941) considered the influence of salts on the disposal of the carbo- 

 hydrates as a possible cause of their effect on the rate of photosynthesis. 



1. Ionic Deficiency Effects 



(a) Potassium 



Potassium is one of the elements whose deficiency often affects the 

 plants; one of the consequences of this deficiency is chlorosis (insufficient 

 development of chlorophyll) and therefore a depressed rate of photo- 

 synthesis. It seems, however, that in addition to this indirect effect, 

 potassium has also a direct effect on photosynthesis, since an addition of 

 this element to the medium may cause an immediate increase in the rate 

 of photosynthesis. 



The importance of potassium for photosynthesis was first stressed by Stoklasa and 

 coworkers (1916, 1917, 1920, 1929), who pointed out that potassium fertihzation is 

 particularly beneficial to sugar-producing plants; they also noticed an accumulation of 

 potassium in chlorophyUous tissues. (However, according to table 14. VI, potassium 

 does not accumulate within the chloroplasts, but is more abundant in the cytoplasm.) 



