342 



VARIOUS CHEMICAL AND PHYSICAL AGENTS 



CHAP. 13 



synthesis, while an equal concentration of potassium nitrate has no 

 effect at all. 



F. Miscellaneous Chemical Stimulants * 



Numerous data on the stimulation of photosynthesis by chemicals 

 can be found in the literature, but most are based on occasional observa- 

 tions under conditions which are not well defined. In the preceding 

 sections, while dealing with the inhibition phenomena we found that the 

 action of a number of "poisons" can be attributed to their interaction 



350 700 1600 4000 

 Light intansity 



22000 



350 700 1600 4000 

 Light intsnsity 



B 



22000 



Fig. 35. — Effect of ions on photosynthesis in weak and strong light (after Greenfield 

 1942). A, inhibition at all intensities; B, inhibition in strong light only. 



with specific components of the photosynthetic apparatus. So far, no 

 such specific agents have been identified in the reverse case — that of the 

 acceleration of photosynthesis. Here, we still must be content with the 

 vague notion of ''protoplasmic stimulation." Gaffron (1939) suggested 

 that stimulation of photosynthesis may be caused by an inhibition of 

 hack reactions (meaning, not ordinary respiration, but oxidation processes 

 in the chloroplasts which are postulated in certain kinetic theories — cf. 

 Vol. II, Chapter 33 — and represent a direct reversal of photosynthesis). 



From the data presented in chapter 12, it appears as though almost 

 any inhibitor becomes a stimulant if used in suflSciently low concentration. 

 This is true, for example, of hydrocyanic acid, iodoacetic acid, sulfur 

 dioxide, hydrogen sulfide, ether, and urethan. Among alleged stimu- 

 lants of photosynthesis which are not known as inhibitors we may 

 mention acetaldehyde. Sabalitschka and Weidling (1926) found that the 

 rate of oxygen hberation by Elodea canadensis is doubled by 0.001- 

 0.128% of aldehyde. However, Schmucker (1928) failed to confirm this 

 result in experiments with Cahomba caroliniana. 



* BibUography, page 350. 



