340 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



revealed a certain damaging effect of hydroxyl ions. Wilmott (1921) 

 found that the photosynthesis of Elodea is not affected by the substitution 

 of a bicarbonate solution (pH c^ 9) for a carbonic acid solution (pH c^ 6) 

 (assuming that the concentration of the molecular species CO2 remains 

 the same). Emerson and Green (1938) stated that the photosynthesis 

 of Chlorella is independent of pB. in phosphate buffers (pH 4.6-8.9) and 

 probably also in the moderately alkaline carbonate buffers; but carbonate- 

 bicarbonate buffers of higher alkalinity affect the photosynthesis even 

 in this very resistant organism (cf. also Matusima 1939). Some other 

 unicellular algae, e. g., Hormidium (van der Paauw 1932), are injured by 

 all alkaline buffers. 



Treboux (1903) observed a stimulation of photosynthesis in the 

 aquatic higher plants by dilute acids; but this probably was caused by 

 an improved supply of carbon dioxide [cf. page 343) and not by the 

 hydrogen ions as such. 



(6) Alkali Ions 



Some data in the literature suggest a specific inhibiting influence of 

 ammonium ions in photosynthesis. Ewart (1896) found, and Willstatter 

 and Stoll (1918) confirmed the fact, that the rate of photosynthesis in 

 ammonium bicarbonate solutions is lower than in equivalent solutions 

 of other bicarbonates. Benecke (1921) observed that ammonium salts 

 in concentrations as low as 0.01% reduce the formation of starch and 

 the evolution of oxygen by Elodea. This was confirmed by Greenfield 

 (1941), who found that ammonium ions inhibit photosynthesis both in 

 strong light and in weak light {cf. page 336). An unfavorable effect of 

 sodium ions on photosynthesis was noticed by Pirson (1937). According 

 to Pratt (1943) the rate of photosynthesis of Chlorella declines by 60% 

 after 24 hours in a 0.1 molar solution of sodium bicarbonate, as compared 

 with an increase by 25-30% in an equivalent potassium bicarbonate 

 solution. In a mixture of 0.035 m./l. KHCO3 and 0.065 m./l. NaHCOs, 

 the rate remains constant for 15 hours. 



(c) Heavy Metal Ions 



Greenfield (1941, 1942) bathed Chlorella cells in different salt solutions 

 for 20 minutes, washed them out, and studied their photosynthesis by 

 manometric methods in light of five different intensities. Copper sulfate 

 proved to be toxic even in concentrations as low as 10~^ m./l., mercuric 

 chloride in concentrations of 2 X 10~^ m./l., and cobaltous sulfate in 

 concentrations of 10~* m./l., {cf. Fig. 34, Curve 1), whereas 0.1 m./l. of 

 manganous sulfate had no perceptible effect (Fig. 34, Curve 6). Higher 

 concentrations of the "nontoxic" salts also caused inhibition, but this 

 could be attributed to nonspecific osmotic effects, since it occurred in the 



