350 



FACTORS AFFECTING PHOTOSYNTHESIS 



incident light the greater the average rate of photosj^nthesis per unit of leaf 

 area. The total photosynthesis per tree therefore increases progressively w^ith 

 increased illumination up to the maximum possible sunlight intensity. 



The intensity of the sunlight incident on the earth's surface varies from 

 hour to hour and from season to season, as well as with meteorological condi- 

 tions. Clouds, fogs, dust, atmospheric humidity, etc. all influence the in- 

 tensity of the radiation which reaches the surface of the earth. The exposure 

 and pitch of a slope are also factors influencing the intensity of the light 

 impinging upon a given location, and are particularly of importance in hilly 



0200, 1 1 1 1 1 1 ^-1 1 1 



017; 



200 



400 600 



LIGHT INTENSITY FOOT-CANDLES 



Fig. 88. Relation between different light intensities and rate of photosynthesis of 

 wheat plants at three different carbon dioxide concentrations. Data of Hoover, et al. 



(1933)- 



or mountainous country. Other conditions being equal the intensity of sun- 

 light also increases with increase in altitude. Most variations in the intensity 

 of natural light are accompanied by variations in light quality of greater or 

 lesser magnitude. Usually, however, under out-of-doors conditions variations 

 in the intensity component of light are of greater physiological influence than 

 the accompanying variations in the quality component. 



Ecologically one of the most important factors causing different species 

 of plants to be exposed to differences in light intensity is the effect of taller 

 plants in shading those of lesser stature. Some species will thrive and photo- 

 synthesize efficiently only in fully exposed locations, others can complete their 

 normal life cycles in intensely shaded habitats. 



