A Rapid Method for the Measurement of Rates of Photosynthesis using 14CO2

An apparatus for measuring the rates of photosynthesis of leavesis described. In principle, the method consisted of interruptingthe flow of air over a leaf for 15sec, during which time aircontaining ¹⁴C dioxide was passed over it. The amount of¹⁴Cassimilated by the leaf was then measured. The results werecompared with those obtained using an infra-red gas analyser.The principal cause of the discrepancies between the resultsappeared to be preferential fixation of ¹²C by the leaves andloss of respiratory ¹²C at low light intensities. There wasa linear relation between the results obtained by the two methodsand thus, using the ¹⁴C method, the rates of net photosynthesiscould be estimated. By fitting a set of neutral-density filtersover the chamber that enclosed the leaf, the rates of net photosynthesiswere determined for several light intensities simultaneously.Examples are given of curves showing the relation between lightintensity and net photosynthesis for leaves of Beta and Phaseolusplants.