428 THE PIGMENT SYSTEM CHAP. 15 



and thus indirectly also the stationary concentration of the pigment in 

 the mature plant — depends on a number of chemical and physical factors. 



(a) Nutrient Elemevts 



The absence of certain nutrient elements makes plants chlorotic, 

 that is, deficient in chlorophyll. Among these are potassium, nitrogen, 

 and magnesium, as well as the heavy metals, iron and manganese. These 

 effects were mentioned in chapter 13, in the discussion of inhibition and 

 stimulation of photosynthesis by inorganic ions. It was stated there 

 that deficiencies in mineral nutrients may produce both a direct and an 

 indirect inhibition of photosynthesis. The first is removed immediately 

 by the supply of the deficient element, while the second one, being 

 associated with chlorosis, can be remedied only more slowly by the 

 increased formation of chlorophyll (and probably also of other catalytic 

 components which are deficient in the photosynthetic apparatus of chlo- 

 rotic plants). 



Potassium. — An inadequate supply of potassium causes chlorosis, but 

 an excessive quantity of this element may have the same effect if the 

 supply of nitrogen is not large enough. These interrelations between 

 potassium and nitrogen fertilization, investigated by Gassner and Goeze 

 (1934) and Eckstein (1939), among others, were mentioned in chapter 13. 

 According to Pirson (1937, 1938, 1940), rubidium is only an imperfect sub- 

 stitute for potassium in relieving chlorosis, while sodium or cesium cannot 

 be used as substitutes at all (c/. page 337). 



Nitrogen. — Nitrogen deficiency also causes chlorosis (of., for example, 

 Pirson 1937). Fleischer (1935) used varying degrees of nitrogen de- 

 ficiency to produce Chlorella cells with different contents of chlorophyll. 



Magnesium. — This element is present in chlorophyll, and its complete 

 absence in the diet must inevitably lead to chlorosis. Emerson and Ar- 

 nold (1929) used magnesium-deficient nutrient solutions to produce Chlo- 

 rella cells with a low chlorophyll content. Fleischer (1935) and Kennedy 

 (1940) confirmed these results, and also found that much more magnesium 

 is required to bring about the full rate of photosynthesis of Chlorella {cf. 

 page 337) than to prevent chlorosis. The influence of magnesium on 

 chlorophyll has also been investigated by Mameli (1918) and Zaitseva 

 (1929). 



Iron. — The chlorosis of iron-deficient plants is a well-known fact, but 

 no satisfactory explanation of the necessity of iron for the formation of 

 chlorophyll has as yet been given. Pollacci and Oddo (1915), Oddo and 

 Polacci (1920), Polacci (1935), and Lodoletti (1938) asserted that both 

 phanerogams and algae can produce chlorophyll in iron-free nutrient 

 solutions if they are supplied with a-pyrrole carbonate, and concluded 

 that iron is only necessary for the synthesis of the pyrrole group; but 

 Deuber (1926), Godnev (1927), and Aronoff and Mackinney (1943) were 



