DIFFUSION AND BUFFER EFFECTS 1407 



C. Interpretation of Induction Phenomena* 

 1. Diffusion and Buffer Effects 



In higher land plants, the gas exchange usually has to take the path 

 through stomata and air channels, the diffusion resistance of which may be- 

 come the main rate-limiting factor in photosynthesis, particularly when the 

 slits are only partially open (c/. chapter 27, page 910). The stomata close 

 regularly during the night, but may do so also during the day, particularly 

 if the plants are temporarily darkened. Sluggish reopening could make 

 photosynthesis in the first moments of illumination "carbon dioxide- 

 limited," even when the outside concentration of carbon dioxide is high. 

 Fortunately, most of the data used in this chapter were obtained either 

 with stomata-free aquatic plants, or with higher plants under conditions 

 minimizing the stomatal effects. 



Lubimenko and Shcheglova (1932) suggested that the low initial value of the photo- 

 synthetic quotient found by Kostychev {cf. sect. A3) may be due to a delay in the out- 

 ward diffusion of oxygen, caused by the necessity to build up its pressure in the leaf until 

 it equals that in the air. However, the building up of a diffusion gradient should delay 

 the movement of carbon dioxide more — and not less — than that of oxygen. In the first 

 place, the diffusion coefficient of carbon dioxide is smaller than that of oxygen; conse- 

 quently, the concentration gradient, required to maintain the same rate of flow, is larger 

 for carbon dioxide than for oxygen. In the second place, the building up of the carbon 

 dioxide pressure can be delayed by the presence of buffers; consequently, the exchange 

 of a certain quantity of carbon dioxide for an equivalent quantity of oxygen, may create 

 a smaller diffusion gradient for carbon dioxide than for oxygen. Lastly, prior to the be- 

 ginning of illumination, the concentration gradient must have been higher for carbon 

 dioxide than for oxygen (since in the dark, too, the flows of the two gases had to be 

 equal). Thus, in the building up of the carbon dioxide gradient required for steady 

 photosynthesis, the cells start further back, progress more slowly and have to go further 

 than in the creation of the oxygen gradient. Consequently, the transition from carbon 

 dioxide liberation into the atmosphere to carbon dioxide absorption from it (or vice 

 versa), must take more time than the change in the direction of the oxygen flow. 



Although diffusion effects should not be overlooked in the quantitative 

 analysis of induction phenomena in higher plants, it will be noted that they 

 can account only for a gradual approach to the steady rate of gas liberation 

 (or consumption), and not for such phenomena as the oxygen "gush," or 

 the carbon dioxide "gulp." Furthermore, all induction losses caused by 

 buffer action or slow diffusion, must be reversible, i. e., at the end of the 

 illumination period, they must be compensated by a continued absorption 

 of carbon dioxide (and evolution of oxygen) in the dark. This is not nor- 

 mally the case in photosjoithesis, where the induction losses are nonrecover- 

 able. 



* BibUography, page 1432 



