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Light and Carbon Assimilation by Plant Communities¹

In general, the shape of the response curves of leaf CO₂-assimilation rate plotted against light intensity is predictable from the CO₂-assimilation rate in intense light. Of four factors limiting photosynthesis in intense light, the one causing differences between tropical grasses and other species has received much attention recently. Two other factors, associated with diffusion of CO₂, have been well defined; little is known about a fourth factor. Four hypotheses are discussed. The photosynthetic rate of a crop stand may be defined as the product of the amount of light intercepted and the efficiency of the intercepting tissue. Interception depends on solar angle and stand geometry. Efficiency is often constant over considerable periods of time (several examples are discussed). This indicates, in species whose leaves exhibit hyperbolic light response curves, either (a) that there are other important light-intercepting tissues, e.g., stems and fruit, with different photosynthetic light response curves, or (b) that the manner in which light is received by the leaves changes. In the second case, it is implied that leaves move.

² Assistant Plant Breeder, Arizona Agricultural Experiment Station, Tucson, Ariz., and Soil Scientist, Soil and Water Conservation Research Division, USDA, ARS, Boll Weevil Research Laboratory, State College, Miss., respectively.

Received for publication July 15, 1966.

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