Photosynthesis: A Simple Demonstration of the Emerson Enhancement Effect

Abstract

Photosynthesis is perhaps the single most important living process. However, the rapid advances within the last 25 years have resulted in a dearth of simple experiments that can demonstrate the basis of our present understanding. The experiment described here illustrates the operation of two separate light reactions in photosynthesis by showing the Emerson enhancement effect. The efficiency of photosynthesis has been found to vary as plants are illuminated with monochromatic light of different wavelengths. One common finding has been that the efficiency of photosynthesis decreases with the use of longer wavelengths of light. The decline in photosynthetic rate occurs in spite of strong light-absorption by chlorophyll. Robert Emerson (1957) and his colleagues discovered that the low rate of photosynthesis in long-wavelength light could be overcome, or even surpassed, by simultaneously illuminating the plants with a second beam of shorter-wavelength light. The rate of photosynthesis in the combined light beams was greater than could be expected by adding the rates found when either beam was provided alone. This relative excess of photosynthesis is known as the Emerson enhancement effect. One conclusion that Emerson reached on the basis of his discovery was that photosynthesis involves two photochemical processes. The exact, detailed explanations of how these two processes are achieved by the chlorophyll and accessory pigments have been modified considerably since Emerson published his work. Howevrer,