Photosynthetic activity of vegetable plants and its horticultural significance

I. Examination of the methods in measuring the photosynthesis with assimilation chamber

Abstract

This study was carried out to examine the apparatus in measuring the photosynthetic activity of vegetable plants and to discuss some problems in relation to the methods of measurement and plant materials.
Whole plant or single attached leaf was placed in the“Plexiglass”assimilation chamber in which the light intensity, the carbon dioxide concentration, the leaf temperature and the air humidity could be controlled over a wide range. The carbon dioxide concentration in the air of the inlet and outlet of the chamber was measured with infrared gas analyser, Hitachi-Horiba Type EIA-1 A (0-600ppm, 0-2, 000ppm).
In order to reduce the resistance against carbon dioxide transfer at boundary layer of leaf surface, an air circulating fan was installed in the plant assimilation chamber. The leaf assimilation chamber was also well constructed to distribute the air uniformly over entire leaf surface.
Photosynthetic activity of the tomato plant in the assimilation chamber increased with increased air supply up to the order of 0.8l/cm² leaf area/hr. Without operating air circulating fan, however, air supply of 2.0l/cm²/hr was not enough for adequate photosynthesis in the plant. This would suggest that the resistance of boundary layer of leaf surface against carbon dioxide transfer was one of the serious problems to achieve the sufficient photosynthesis in the assimilation chamber.
As the gas analyser used in this study has maximum error of 1 per cent of the carbon dioxide concentration over a given range, it is of considerable importance to define the lowest limit of air supply. In this apparatus the photosynthetic activity should be measured with the air supply of approximately 1.0l/cm²/hr and with carbon dioxide depletion of not more than 20 per cent of inlet air.
The photosynthetic activity of the tomato plant was almost independent of the leaf temperature at a normal carbon dioxide supply. However, in the atmosphere enriched with carbon dioxide, the rate of photosynthesis was greatly influenced by the leaf temperature within the range of 15°C to 35°C.
Considerably large standard deviation of photosynthetic activity was found in the tomato plants which were placed under natural conditions and sampled immediately before the measurement. Therefore, the plant materials should be selected in the evening and kept under controlled conditions for certain hours in the dark so as not to disturb the internal equilibrium.
When the photosynthetic rate was measured in a single cucumber leaf, the rate was slightly increased by darkening of young leaves outside assimilation chamber. By lowering the root temperature from 18°C to 13°C, the photosynthetic rate in the leaf dropped to 76 per cent of the normal. The steady state of the photosynthesis under given conditions was obtained after about 50 minutes of the exposure to the light. So it is suggested that the leaves outside assimilation chamber and root should be kept under the controlled conditions during the measurement of photosynthesis.