Photon Flux, Chlorophyll Content, and Photosynthesis Under Natural Conditions

The photon flux in the photosynthetic region from the sun and sky is calculated for a range of conditions and shown to vary by about 10%. A mean figure of 1.2 x 10¹ ⁹ photons cal— ¹ (400—700 nm) is assumed. This waveband is taken to include 45% of the total incident short wave radiation. The relation between photon flux and the available photosynthetic machinery, in terms of chlorophyll content, is calculated. It seems that the normal chlorophyll content of leaves is adequate to absorb the available photon flux under most natural conditions, and that the chlorophyll content of plant communities is not, in itself, likely to be a limiting factor in photosynthetic production. High chlorophyll content of plant communities is a concomitant of large leaf area index, increasing radiant energy absorption, and total dry matter production.