OPTIMUM AND UPPER TEMPERATURE LIMITS; HEAT INJURY 1223 



If a plant is subjected to superoptimal temperatures for more than a 

 few minutes, reversible thermal inhibition may be replaced by an irrevers- 

 ible (or only slowly reversible) thermal injury. Van Amstel (1916) observed 

 thermal injury to the photosynthetic apparatus of Elodea at 40° C. Wurm- 

 ser and Jacquot (1923) observed that the inhibition of photosynthesis in 



200 



10 14 18 22 26 30 10 14 18 22 26 30 

 TEMPERATURE, "C. 



Fig. 31.6. Temperature curves of photosynthesis and res- 

 piration of four algae (after van der Paauw 1934). 



green, red or brown algae, caused by a 2 minute exposure to temperatures of 

 36-45° C, was not entirely reversible, but left a permanent decrease in 

 photosynthetic efficiency after return to 16°. 



Kennedy (1940) studied quantitatively the effects of preheating to 

 40 and 45° C. on the photosynthesis of Chlorella at 25°. One hour ex- 

 posure to 40° had no effect on chlorophyll concentration and respiration, 

 but caused a decline in photosynthesis by about 30%. Exposure to 45° 

 caused a very rapid decline in the capacity for photosynthesis at 25° (cf. 



