THERMAL ADAPTATION 1225 



associated with the denaturation of proteins. The destruction of enzymes, 

 which also was suggested as a possible cause of the decline of photosynthe- 

 sis at high temperatures (Blackman 1905) may be but a consequence of 

 changes in structure of their proteinaceous components. Lipides (the 

 role of which in the composition of the chloroplasts was discussed in chapter 

 14) also may be responsible for heat sensitivity. (It was mentioned in 

 chapter 14, page 361, and in chapter 24, page 817, that the "melting" of 

 grana in chloroplasts, observed under the fluorescence microscope, was at- 

 tributed by Metzner to a liquefaction of lipides.) 



It may be asked whether the rapidly reversible thermal inhibition (by 

 comparatively low temperatures and short exposures) is fundamentally dif- 

 ferent from the irreversible, or only slowly reversible, injury caused by 

 higher temperatures and longer exposures. In addition to reversible shifts 

 of chemical equilibria, such as the one responsible for the formation of the 

 ACO2 complex, various other reversible changes could affect reversibly the 

 rate of photosynthesis at high temperatures. One of them is the increased 

 viscosity of the protoplasm. It wa smentioned on page 1220 that, at low 

 temperatures, the viscosity of the protoplasm, hke that of most other mate- 

 rials, decreases upon heating. Characteristically, however, it passes 

 through a minimum, usually in the neighborhood of 15° C, and then in- 

 creases again. This increase can slow down the diffusion of carbon di- 

 oxide and thus bring photosynthesis from the carbon dioxide-saturated 

 into the carbon dioxide-limited state. Like the effect of the dissociation 

 of the ACO2 complex, this kind of thermal inhibition should disappear 

 upon an increase in the concentration of carbon dioxide. Thus, in order to 

 clarify the role of viscosity in the decline of photosynthesis at low and high 

 temperatures, it would be useful to measure these effects at different con- 

 centrations of carbon dioxide, and to compare the results with the change 

 in viscosity. Much quantitative work remains to be done in this field. 



Thermal deactivation of enzymes by the denaturation of proteins also 

 may be more or less rapidly reversible, depending on how far the denatura- 

 tion has been allowed to proceed. Reversible denaturation as an explana- 

 tion of the temperature effect on enzymatic processes has been discussed, 

 e. g., by Johnson, Brown and Marsland (1942) on the basis of experiments 

 with lucif erase. 



3. Thermal Adaptation 



Ewart's figures, given on page 1219, indicated already that the lower 

 limit of photosynthesis differs in plants from different climatic zones, and 

 thus reveals a considerable degree of "thermal adaptation." This adapta- 

 tion also affects the positions of the optimum and of the upper limit of photo- 



