354 FACTORS AFFECTING PHOTOSYNTHESIS 



has important effects upon the growth and development of plants which will 

 be discussed in Chap. XXXIII. 



The effect of rapid alternations of light and dark in increasing the amount 

 of photosynthesis per unit of light, and the theoretical implications of this 

 effect, have already been discussed in the preceding chapter. 



5. The Induction Period. — As shown by McAlister (1937), Smith 

 (1937) and others, when a plant is illuminated the rate of photosynthesis 

 is at first low and gradually increases until it reaches a more or less constant 

 value. The length of this "induction period" varies from a minute or two 

 to more than an hour, depending upon the species and environmental condi- 

 tions. The existence of such an induction period has important theoretical 

 implications which must be considered in any postulated mechanism of photo- 

 synthesis. 



6. Solarization. — Intense illumination inhibits the accumulation of starch 

 in leaves, a phenomenon which is called solarization. For example Holman 

 (1930) showed that bean leaves exposed to light of about 6800 foot-candles 

 readily accumulate starch, but that in light of about twice this intensity 

 starch accumulated only when the length of the exposure period was relatively 

 short and then only in relatively small quantities. This effect is believed to 

 involve the process of photosynthesis rather than the secondary reaction of 

 starch synthesis. At the present time there is no entirely satisfactory explana- 

 tion for the phenomenon of solarization. 



Temperature Effects on Photosynthesis. — The determination of the 

 effect of temperature upon photosynthesis in terrestrial plants is complicated 

 by the fact that the leaf temperatures of such plants are seldom the same as 

 atmospheric temperatures. Whenever leaves are exposed to direct illumina- 

 tion, which is almost invariably the situation when photosynthesis is occurring 

 at a rapid rate, their temperatures exceed those of the surrounding atmosphere. 

 It is therefore difficult if not impossible to maintain the temperature of the 

 leaves of land plants at a desired value while they are exposed to light of 

 any considerable intensity. Evaluation of the effect of the temperature of 

 leaves upon photosynthesis is possible only if direct measurement is made of 

 the actual leaf temperature. For this reason many of the more critical studies 

 of the effect of temperature upon photosynthesis have been made with sub- 

 merged water plants, in which a close thermal equilibrium is maintained be- 

 tween the plant body and the surrounding water. 



I. Temperature Limits of Photosynthesis. — Photosynthesis can take place 

 over a wide range of temperatures. It has been reported to occur in some 

 species of conifers at temperatures as low as "35° C. and in some kinds 

 of lichens at —20° C. It is said that many temperate zone evergreens are 



