IONIC DEFICIENCY EFFECTS 337 



Stoklasa made the fantastic suggestion that the radioactivity of potassium may con- 

 tribute its energy to photosynthesis; even stranger was the idea of Jacob (1928) that the 

 primary effect of light in photosynthesis may be a photoelectric hberation of electrons 

 from potassium! 



Briggs (1922) found that a deficiency in potassium affects photo- 

 synthesis in the carbon dioxide-Hmited, as well as in the Hght-Hmited and 

 in the hght-saturated, state. That the effect of potassium on photo- 

 synthesis goes beyond the improvement attributable to an increase in 

 chlorophyll concentration was shown by Gassner and Goeze (1934), and 

 confirmed by Eckstein (1939) and Tiedjens and Wall (1939). According 

 to Miiller and Larsen (1935), the photosynthesis of Sinapis alba is doubled 

 concurrent with an increase of potassium concentration in the leaves from 

 1.1 to 3.8 mg. per 50 cm.^; this change cannot be attributed to an increase 

 of chlorophyll content. 



Pirson (1937, 1938, 1940) found that supplying potassium to Chlorella 

 cells grown in a potassium-deficient medium causes an instantaneous 

 increase in the rate of photosynthesis (both in weak and in strong light). 

 In this direct action — which Pirson attributed to changes in the colloidal 

 structure of the protoplasm, potassium can be replaced by rubidium, and 

 (less efficiently) by cesium. In the delayed, secondary effect of potassium 

 on photosynthesis, which is associated with an increase in chlorophyll 

 concentration, no replacement by cesium is possible, and even rubidium 

 proves to be a poor substitute. Improvement in photosynthesis with 

 increased supply of potassium continues only up to a certain concentration 

 (Gassner and Goeze 1934, Brilliant 1936, and Alten and Goeze 1937). 

 This limit is higher if the supply of nitrogen is abundant; when the 

 nitrogen supply is low, an increase in potassium concentration may cause 

 a decline instead of a rise in photosynthesis. Inversely, an abundant 

 supply of nitrogen can become detrimental to photosynthesis if the 

 supply of potassium is low (cf. Gassner and Goeze 1934; Rohde 1936'-^; 

 Maiwald and Frank 1935; and Alten and Goeze 1937). 



(6) Magnesium 



Magnesium is a component of chlorophyll; it is therefore natural 

 that plants grown without magnesium are chlorotic and incapable of 

 photosynthesis (c/. page 428). Briggs (1922) observed that magnesium 

 deficiency depresses photosynthesis in the light-limited and in the light- 

 saturated state, as well as in the carbon dioxide-limited state. Fleischer 

 (1935) found that changes in magnesium concentration affect the chloro- 

 phyll concentration of Chlorella in a range in which they have only a 

 slight effect on the rate of photosynthesis, and alter the rate of photo- 

 synthesis in the concentration range in which the quantity of chlorophyll 

 remains practically constant, thus indicating that the two effects are 



