OPTIMUM AND UPPER TEMPERATURE LIMITS; HEAT INJURY 1221 



Thomas (1950) found for the purple bacterium, Rhodos-pir ilium ruhrum, 

 a peak of efficiency at 40° C, followed by a sharp drop. 



As previously mentioned, P becomes time-dependent when the limit 

 of the "biokinetic" range is approached, usually even before the optimum 

 has been reached. The position of the optimum thus becomes a function of 

 the duration of the experiment. Matthaei (1904) found slow heat injury 

 to cherry laurel leaves at temperatures above 25° C; figure 31.3 was 



10 20 



TEMPERATURE, "C. 



Fig. 31.2. Temperature curves of respiration (R) and photosynthesis 

 (P) in the lichen Ramalina farinacea (after St§,lfelt 1939). 



constructed from her data by Jost (1906). By extrapolating them to 

 zero time, Blackman (1905) extended the exponentially ascending tem- 

 perature curve of the photosynthesis of cherry laurel leaves up to 37.5° C. 

 Decker (1944) observed a decline of 45% in the (apparent) photosynthesis 

 of two species of pine between 30° and 40° C, in light of about 40,000 lux. 

 Green algae behave similarly: Wurmser and Jacquot (1923) found com- 

 plete stoppage of photosynthesis in Ulva after 2 minutes exposure to 45° C. 



Stalfelt (1939) found in Usnm dasypoga (a lichen) time dependence 

 above 21° C, independently of light intensity (2-32 klux), but subject to 

 adaptation to the temperature of the ambient medium. 



Noddack and Kopp (1940) found that the photosynthesis of Chlorella 

 became time-dependent above 22° C. By using experiments of not 

 over 30 miimtes duration, they could extend the exponentially ascend- 

 ing curve up to 30° C. ; but a 150 minute exposure to the latter temperature 

 gave an average rate 25% less than that found in 30 minute experiments 



