A Method for the Rapid Measurement of Photosynthesis

Abstract

This technique was developed to measure the relative photosynthetic capacities of genetically differing tobacco plants for selection in a breeding programme; it should also prove useful for other species. Its most important feature is the speed with which results are obtained, rates of up to 30 per hour being possible. Rapidity is essential if large populations are to be screened and where photosynthetic behaviour may drift during a prolonged screening programme. The procedure involved two phases. In the first the leaf samples were incubated in large numbers at atmospheric C02 concentration and constant temperature and illumination to establish a steady rate of photosynthesis. In the second phase they were transferred one by one to a leaf chamber under the same environmental conditions for photosynthetic rate measurements. Samples of 8 cm squares were cut from the lamina of tobacco leaves so as to exclude the mid vein. Each sample was floated under-surface uppermost on 15 ml of distilled water in a 10-cm-square plastic Petri-dish, large numbers of these dishes then being floated on a water bath at 24 °C under illumination of 23 500 lx from a bank of water-cooled tungsten lamps. The temperature of the water in the Petri-dishes remained within one degree of that of the water bath under these condi tions. After incubation for 1 h the dishes were removed for measurement of photo synthetic rate with an open circuit system and an infra-red C02 analyser. Air with an atmospheric C02 concentration from a compressed-air cylinder was fed to the leaf chamber via a flow meter at a rate controlled at 11 per minute by a 'Elowstat Minor' (G. A. Platon Ltd., Basingstoke, England). A vertical median section of the leaf chamber can be seen in Fig. 1. It was made of 16-mm-thick 'Plexiglas' and was designed to accommodate the leaf material floating in the Petri-dish. It was situated alongside the water bath and was illuminated by the same bank of lights. Cooling was by a water jacket in its base supplied with water drawn in a closed circuit from the main water bath. No water jacket was placed above the chamber because this was found to be unnecessary and would have caused condensation on the chamber roof and interference with light transmission. The Petri-dish containing the leaf sample was inserted and removed at one end of the chamber through a