IONIC INHIBITION EFFECTS 339 



and Lewis (1940) observed that manganese deficiency affects the maxi- 

 mum quantum yield of photosynthesis in weak Hght. The effect of 

 manganese (similarly to that of nitrate) depends on the available supply 

 of potassium — the more potassium, the higher the quantity of manganese 

 required to bring about the full rate of photosynthesis. 



(e) Nitrate and Phosphate 



In addition to the investigations mentioned on page 337 on the 

 interrelation of the effects of nitrogen and potassium, we may quote the 

 observations of Miiller (1932), Hermer (1936), and Pirson (1937) on the 

 importance of an adequate nitrogen supply for photosynthesis. Ac- 

 cording to Miiller, a reduction in nitrogen supply (from 1 g. to 0.05 g. 

 calcium nitrate per liter) leaves photosynthesis in weak light unaffected, 

 but reduces the rate in strong light by almost 50%. Emerson (1929) 

 and Fleischer (1935) found that the capacity for photosynthesis of 

 Chlorella cells made chlorotic by nitrate deficiency was proportional to 

 their content of chlorophyll, and concluded that nitrate has no effect on 

 photosynthesis except through the medium of chlorophyll ; this conclusion 

 is open to the same objections as mentioned in the preceding discussion 

 of the effects of iron deficiency. According to Miiller and Larsen (1935) 

 the rate of photosynthesis of Sinapis alba at 15,000 lux was twice as large 

 when the content of nitrogen in the leaves was 7 mg. per 50 cm.^ than 

 when it was 3.5 mg. per 50 cm.-; this difference was not caused by a 

 different content of chlorophyll. The existence of a nitrate effect on 

 photosynthesis which is not dependent on chlorophyll was confirmed 

 by Pirson (1937), who found that the addition of this anion to a nitrate- 

 deficient medium causes an immediate rise in the rate of photosynthesis. 

 Van Hille (1938) observed that "aging" Chlorella suspensions which have 

 lost much of their eflSciency in photosynthesis despite an unchanged 

 content of chlorophyll, can be "rejuvenated" by a renewed supply of 

 nitrogen. 



The inhibition of photosynthesis by phosphorus deficiency has been 

 observed, for example by Briggs (1922), 



2. Ionic Inhibition Effects 



As mentioned on page 000, many inorganic ions inhibit photosynthesis, 

 some in very low, others only in comparatively high, concentrations. 

 Even the ions whose deficiency retards photosynthesis may become 

 inhibitors if present in excess. 



(a) Hydrogen and Hydroxyl Ions 



The photosynthesis of algae is not particularly sensitive to changes 

 in the acidity of the medium, but experiments in carbonate buffers have 



