440 PLANT GROWTH AND PLANT COMMUNITIES 



ratio is 2 to 3 per cent for crop plants grown under optimal or near- 

 optimal conditions (Wassink, 1953, 1959; Brown et a]., 1957). The 

 value is the same also for forests (Ilellmers and Bonner, 1959), for 

 algae grown in outdoor tanks ( Van Oorschot, 1955 ) , and in fact for all 

 plants thus far investigated. The efficiency of plants as converters of 

 solar energy appears, therefore, to be remarkably uniform tl.roughout 

 the plant kingdom. 



Why do plants convert and store so little of the incident solar 

 energy? Why is their efficiency 2 per cent instead of some other num- 

 ber? The factors that determine the efficiency of photosynthsis have 

 gradually become clearer during the past ten years, and we can try 

 today to enumerate and assess them. 



In the first place, a minimum of approximately ten quanta of en- 

 ergy is required to reduce one molecule of carbon dioxide to the level 

 of plant material. Since ten quanta in the wave lengths absorbed by 

 chlorophyll possess an energy content of roughly 540 large calories per 

 mole, and since only 105 large calories per mole arc stored in the re- 

 duction of COi to plant material, we must conclude that the photo- 

 synthetic act itself possesses an inherent efficiency of approximately 20 

 per cent (Emerson, 1958). This efficiency may in fact be approximated 

 by higher plants or by algae grown imder lo\\' light-intensities, where 

 the incident light energy is the limiting factor (Went, 1957; Gaastra, 

 1958; \m\ Oorscliot, 1955; and others ) . 



In the second place, the photosynthetic rate of leaves becomes 

 saturated at intensities of light well below that of fidl sunlight. Thus 

 the photosynthetic rate of sugar beet leaves reaches its maximum at an 

 intensity about one-fifth that of full sunlight (Gaastra, 1958). The 

 absorption of light b\' leaves, however, is linear with light-intensity. As 

 the intensity incident on a leaf increases beyond that sufficient to cause 

 light-saturation, light-absorption continues to increase while the photo- 

 synthetic rate remains constant. As light-intensity increases, the over- 

 all efficiency of the leaf steadily decreases. A leaf photosynthesizing in 

 full sunlight should, on the basis of the above information, exhibit a 

 maximum photosynthetic efficiency of 0.2 ( quantum efficiency ) X 0.2 

 (intensity at saturation as a fraction of full sunlight) = 0.04, or ap- 

 proximately 4 per cent. This calculated efficiency is not, however, ob- 

 tainable imless the leaf is provided with CO2 in a concentration greater 

 than that of air. The photosynthetic rate of leaves, as well as of algal 

 cultures, is normalK- limited by the range of the CO2 concentrations in 

 air. The rate of photosynthesis of leaves in air ma\', in fact, be increased 

 by a factor of roughly t\\o by increasing the CO2 concentration to a 

 saturating value (Thomas, 1949, and others). So the over-all effectixc- 

 n(\ss of the leaf in air and in full sunlight is onK' about 2 per cent, both 



