THE INFLUENCE OF LIGHT AND CARBON DIOXIDE ON PHOTOSYNTHESIS

Abstract

1. An optical system is described which furnishes an intensity of 282,000 meter candles at the bottom of a Warburg manometric vessel. With such a high intensity available it was possible to measure the rate of photosynthesis of single fronds of Cabomba caroliniana over a large range of intensities and CO2 concentrations. 2. The data obtained are described with high precision by the equation KI = p/(p2max. – p2)½ where p is the rate of photosynthesis at light intensity I, K is a constant which locates the curve on the I axis, and pmax. is the asymptotic maximum rate of photosynthesis. With CO2 concentration substituted for I, this equation describes the data of photosynthesis for Cabomba, as a function of CO2 concentration. 3. The above equation also describes the data obtained by other investigators for photosynthesis as a function of intensity, and of CO2 concentration where external diffusion rate is not the limiting factor. This shows that for different species of green plants there is a fundamental similarity in kinetic properties and therefore probably in chemical mechanism. 4. A derivation of the above equation can be made in terms of half-order photochemical and Blackman reactions, with intensity and CO2 concentration entering as the first power, or if both sides of the equation are squared, the photochemical and Blackman reactions are first order and intensity and CO2 enter as the square. The presence of fractional exponents or intensity as the square suggests a complex reaction mechanism involving more than one photochemical reaction. This is consistent with the requirement of 4 quanta for the reduction of a CO2 molecule.