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 CO(2) concentrations. 2. The data obtained are described with high precision by the equation KI = p/(p(2) (max.) - p(2))((1/2)) where p is the rate of photosynthesis at light intensity I, K is a constant which locates the curve on the I axis, and p(max.) is the asymptotic maximum rate of photosynthesis. With CO(2) concentration substituted for I, this equation describes the data of photosynthesis for Cabomba, as a function of CO(2) concentration. 3. The above equation also describes the data obtained by other investigators for photosynthesis as a function of intensity, and of CO(2) 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 CO(2) 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 CO(2) 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 CO(2) molecule.
Highlights
Abundant evidence has accumulated to show that the primary reactions in the photosynthetic mechanism involve a cyclical process consisting of a photochemical reaction and a temperature-sensitive reaction ("dark" or "Blackman reaction")
One of the principal difficulties connected with previous studies on the effect of light intensity has been the inability to achieve a high intensity of illumination without serious temperature disturbance
Since a small change in respiration rate has a large effect on measurements made at low photosynthesis rates, the respiration value used in correcting rate of photosynthesis was that obtained at the beginning of a run
Summary
Abundant evidence has accumulated to show that the primary reactions in the photosynthetic mechanism involve a cyclical process consisting of a photochemical reaction and a temperature-sensitive reaction ("dark" or "Blackman reaction"). Starting with the system as a cyclical process, several investigators (Stoll, 1932, 1936; Franck, 1935; Gaffron and Wolff, 1936) have recently considered certain reactions as possibly being involved in photosynthesis. These discussions have revolved for the most part about the properties shown by chlorophyll in vitro and on quantum yields and the energies involved in possible reactions, and have neglected quantitative treatment of the reaction kinetics. Many schemes have been proposed for the kinetics of the
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