338 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



independent of each other. This conclusion was contradicted by van 

 Hille (1937), but was confirmed by Kennedy (1940), who found that the 

 low yields of photosynthesis, observed in magnesium-deficient media when 

 the chlorophyll content is close to normal, can be improved by the use 

 of flashing light with comparatively long intervals between the flashes. 

 Thus, with intervals of approximately 0.02 sec, the yield per flash was 

 about twice as large in a culture growth with abundant magnesium 

 supply than in one grown with only 1 mg. magnesium per liter; when 

 the dark intervals were increased to 0.4 sec, the yield per flash was 

 only insignificantly smaller in the magnesium-deficient solution. This 

 result is similar to that observed in cyanide-poisoned algae (c/. Vol. II, 

 Chapter 34), and indicates that magnesium deficiency affects the rate 

 of a dark reaction in photosynthesis. 



(c) Iron 



The deprivation of iron is the best known way to produce chloroiic 

 plants (cf. Chapter 15, page 428). Briggs (1922) observed that iron- 

 deficient plants show a depressed photosynthetic activity in the light- 

 limited, hght-saturated, and carbon dioxide-limited states. 



Emerson (1929) and Fleischer (1935) found that the maximum 

 photosynthesis of Chlorella cells which became chlorotic by growth in 

 iron-deficient solutions was proportional to their clilorophyll content, 

 and interpreted this as a proof of the absence of a direct effect of iron on 

 photosynthesis. Kennedy (1940) quoted, in support of this view, the 

 observation that iron-starved, chlorotic leaves show no increase in oxygen 

 yield per light flash with increased dark intervals between the flashes — 

 as was observed in the case of magnesium deficiency. On the other hand, 

 Willstatter and Stoll (1918) found that the photosynthesis of iron- 

 deficient, chlorotic leaves was even lower than one would expect from 

 their content of chlorophyll, and assumed that iron deficiency influences 

 photosynthesis directly, and not merely through its effect on chlorophyll 

 concentration. This discrepancy with the results of Emerson and 

 Fleischer will be discussed in more detail in chapter 32 (Vol. II). It seems 

 probable that, in varying the concentration of chlorophyll by limiting the 

 supply of iron, one varies also the concentration of other enzymatic com- 

 ponents of the photosynthetic mechanism. 



(d) Manganese 



The importance of manganese for photosynthesis has been suspected 

 by McHargue (1922) and Bishop (1928). According to Pirson (1937), 

 the photosynthesis of manganese-starved Chlorella cells is inhibited 

 despite the absence of visible chlorosis; this inhibition can be relieved 

 instantaneously by the addition of manganese to the medium. Emerson 



