176 PHOTOSYNTHESIS 



work is necessary in order to employ these observations for the formula- 

 tion of a theory of the rate of atmospheric ionization in photosynthesis. 



3. The Compensation Point 



The light intensity at which the respiratory and photosynthetic activities 

 compensate each other, i.e., where there is an equilibrium, or steady state, 

 CnHonOn + uO. ^ uCOo + uHoO, and the gaseous exchange is conse- 

 quently zero, has been designated by Plaetzer ~'^^ as the compensation 

 point. As this point is dependent upon the rate of respiration it varies 

 greatly in dififerent plants. Plaetzer studied only acj[uatic plants and em- 

 ployed the bubble counting method with the precautions prescribed by 

 Kniep for plants with an intercellular system. (See Chapter 4.) In 

 using plants without an intercellular system the titration method of 

 Winkler was employed for determining the oxygen in the water. 



The efifect of temperature on the compensation point as determined by 

 Plaetzer is of considerable interest. It appears that with decreasing tem- 

 perature the compensation point is lowered. Thus, the compensation 

 point, expressed in Hefner candles, changes with temperature as follows: 



Spirogxra. 174 at 20° ; 26.7 at 5°. Fontinalis, 150 at 20° ; 40 at 5°. 



Cladofhora, 253.3 " " ; 62.9 " " Cinclidotus. 400 " " ; 75 " " 



It would appear from the above that with a given light intensity photo- 

 synthesis increases with decreasing temperature. In fact, Plaetzer was 

 able to demonstrate that a light intensity which at 20° represented the 

 compensation point of Cinclidotus, at 5^ produced a decided evolution of 

 oxygen due to photosynthesis. That is, with the same light intensity 

 which at 20' just balances the energy transfers, at 5° there is a gain 

 for the plant. It should be noted, however, that the actual total rate of 

 photosynthesis at 5° is lower than at 20", but in relation to the respira- 

 tion it is higher. Paradoxical as this appears at first glance, it must be 

 clearly borne in mind that we are dealing here with light of low intensity, 

 i.e. at these temperatures the light is the limiting factor. It must be re- 

 called again that in photosynthesis the three chief external factors, any 

 one of which may, according to circumstances, be "limiting"' are carbon 

 dioxide-concentration, light intensity and temperature. Now then, given 

 an ample supply of carbon dioxide, if a plant at a definite temperature, 

 is exposed to a light intensity just sufficient to surpass the compensa- 

 tion point, oxygen will be emitted. If the light intensity is increased, 

 oxygen emission (photosynthesis) will also increase; if the temperature 

 is increased photosynthesis will not increase. That indicates that the 

 intensity of illumination — not the temperature — determines the rate of 

 photosynthesis, i.e. light is the limiting factor. If the light intensity is 

 further increased, a point will be finally reached at which photosynthesis, 

 even with the most intense light, will not increase. Here light is no 



'""Plaetzer, Vcrhandlungen Physik-Mcd. Gcs. IVurzhurg N. F., 45, 31 (1917). 



