TEMPERATURE EFFECTS ON PHOTOSYNTHESIS 355 



able to accomplish sufficient photosynthesis in the course of the winter to 

 compensate for winter consumption of carbohydrates in respiration. The 

 occurrence of photosynthesis in the leaves of the cherry laurel {Prunus 

 laurocerasus) has been demonstrated at —6° C. Tropical plants cannot 

 carry on photosynthesis at temperatures as low as those at which many tem- 

 perate zone plants can synthesize simple carbohydrates. In most tropical 

 species photosynthesis apparently will not occur at temperatures below about 

 5° C. At the other end of the temperature range for photosynthesis stand 

 the species of algae indigenous to hot springs which can survive 75° C. and 

 probably carry on photosynthesis at temperatures close to this value. Many 

 semi-desert and tropical species can withstand air temperatures of 55° C. and 

 probably photosynthesize at temperatures not far below this. In most plants 

 of temperate regions the range of temperatures within which photosynthesis 

 occurs at a relatively rapid rate is about 10-35° C. 



2. The Effect of Temperatures on the Rate of Photosynthesis. — The gen- 

 eral relation betAveen temperature and the rate of photosynthesis is rather 

 complex and can best be discussed in terms of a specific example. If the 

 rate of photosynthesis of an Elodea canadensis plant be measured by the 

 bubble-counting method for a number of intervals of time at each of several 

 successively higher temperatures under such conditions that neither light nor 

 carbon dioxide are limiting factors, the results can be plotted as a family 

 of curves such as those shown in Fig. 91. As indicated in this figure the 

 initial rate of apparent photosynthesis increases with increase in temperature 

 up to at least 40° C. This initial maximum rate is established soon after 

 the plant is brought to each temperature. The rate of photosynthesis at any 

 given temperature depends, however, not only on the temperature but also 

 on the length of time that the plant has already been at that temperature. 

 There is a marked tendency for the rate of photosynthesis to diminish with 

 time. At 25° C. and even at 30° C. this effect is scarcely perceptible. At 

 35° C, however, the diminution in photosynthetic rate with time is marked, 

 and at 40° C. it is so pronounced that within a relatively short period, in this 

 experiment, the rate of photosynthesis had declined to nearly a zero value. 

 In other words the higher the temperature, the shorter the period of time 

 for which the maximum rate of photosynthesis can be maintained, and the 

 more rapid the decline in rate after it is once initiated. 



The diminution in the rate of photosynthesis with time, particularly marked 

 at higher temperatures, is evidence of the progressively increasing limiting 

 effect of some internal factor, generally called the "time factor." The exact 

 nature of this "time factor" is unknown but a number of suggestions have 

 been made concerning the possible mechanism of this effect: 



