The Naturally Varying Energy Environment and Its Effects Upon Net Photosynthesis

Abstract

Measurements of meteorological parameters were made at St. Paul, Minnesota, on four different types of days. The data obtained were analyzed by an energy—balance technique which yielded daily trends of light intensity and temperature for individual plant leaves. The computed leaf temperatures were verified experimentally. Together with the light—intensity data they were then used to construct typical net photosynthetic rate curves which would have been characteristic of maize and sorghum growing under these conditions. On a clear—acool day the net photosynthetic rates of maize and sorghum both rose gradually to a sharp peak near solar noon and then declined. On a clear, warm day, however, net photosynthesis in sorghum rose rapidly to establish an enduring high rate throughout the major portion of the day, while maize showed a midday slump with photosynthetic rates depressed almost to zero. On a cool, cloudy day both species had only a nominal net CO2 fixation and a day with broken cumulus clouds resulted in an oscillation of the net photosynthetic rates of both species between quite high and medium values.