5i6 



RESPIRATION 



exchanges occurring between a green plant organ and its environment in the 

 absence of light are usually less than those which generally take place — but 

 in the opposite direction — in its presence. 



Since at low intensities light is the limiting factor in photosynthesis it is 

 evident there should be a certain light intensity at which the rate of photosyn- 

 thesis and the rate of respiration in a leaf or other chlorophyllous organ are 

 exactly equal. At this light intensity, often called the compensation point, 

 the volume of carbon dioxide being released in respiration is exactly equal to 

 the volume being consumed in photosynthesis, while the converse is true for 

 oxygen. The light intensity corresponding to the compensation point varies 

 greatly with different species of plants. The compensation point for any 

 one species is also influenced by various environmental factors, especially tem- 

 perature, and is markedly affected by the conditions to which the leaves or 

 other photosynthetic organs have been exposed during their development. 



Burns (1923) studied what he termed the "minimum light requirement" 

 of a number of species of forest trees. This was done by enclosing the tops 

 of 5'^oung potted trees under a bell jar and sealing off the soil surface. He 

 then determined, for each species, the light intensity at which no change 

 occurred in the volume of carbon dioxide in the bell jar during a three hour 

 exposure, which would represent a condition under which the rates of respira- 

 tion and photosynthesis are equal (Table 51). This light intensity would 

 therefore represent essentially the compensation point for the aerial organs 

 of the tree considered as a unit. 



TABLE 51 MINIMUM LIGHT REQUIREMENT (lIGHT INTENSITY AT WHICH PHOTOSYNTHESIS 



EQUALS respiration) OF FOREST TREE SPECIES (dATA OF BURNS, I923) 



