1900 



KINETICS OF PHOTOSYNTHESIS 



CHAP. 37c 



partially to compensate respiration long before the alleged "threshold" 

 of 0.009% CO2 is reached. Fig. 37D.6 shows clearly the dependence of the 

 CO2 compensation point on light intensity (not found by Audus). The 

 compensation point moves from about 0.004% at 13 klux, to 0.016% at 

 2.1 klux, and > 0.04% at 1 klux. Thus, compensation results from a dy- 

 namic equilibrium between a dark process of constant velocity, and a photo- 

 chemical process that increases in rate (within certain limits) with in- 

 creasing light intensity and carbon dioxide concentration, and not from 

 "switching photosynthesis on and off." 



Vol-% CO2 

 0.05 



OM- 



0.03- 



0.02- 



0.01- 



7S0mm 180 



Fig. 37D.6. Carbon dioxide compensation as function of light 

 intensity for same plant as in fig. 37D.5 (after Egle 1951). Solid 

 curve at left is respiration in dark. 



Egle used stomata-free plants. In leaves, which were used by Garreau, Audus, and 

 Gabrielsen and Schou, stomatal movements, induced by low CO2 concentration, compli- 

 cate the results. However, the observations of Heath and co-workers indicate that these 

 movements should make the slowing-down of photosynthesis at low CO2 concentrations 

 more gradual rather than more sudden. Heath (1948) and Heath and Milthorpe (1950) 

 reported that stomatal openings grow wider as the CO2 concentration declines from 

 0.03 to 0.01%; the response ceases below 0.009%. Heath also found (1949, 1950, 1951) 

 that a concentration of about 0.01% is established in air pumped through illuminated 

 leaves (25 klux), at pumping speeds Ijotween 0.4 and 2.5 l./hr. (per 9 cm.^), whether the 

 entering air carried 0.03% CO2 or 0.14-0.18% CO2. It can thus be concluded that 

 compensation occurs when the CO? concentration in the suhslomaial spaces is down to 

 about 0.01%, and that the stomata respond to changes in the CO2 pressure in these 

 spaces, rather than in the outside air. We expect that systematic study will prove this 



