Carbon Dioxide Limitations of Plant Growth in Tube Culture, with Special Reference to Legume-Nodulation Studies

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Abstract
The results clearly implicate restricted CO2 diffusion as a major factor in limiting photosynthesis within stoppered test tubes. The main barrier to CO2 diffusion is not the reduction in the diffusion constant, D, caused by the density of the stopper, but the value A/L, the ratio of cross-sectional area of plug to length of plug in tube. Although the main function of the stopper, be it cotton wool or cap, is to prevent contamination by unwanted microorganisms, it also prevents turbulent exchange of CO2 between the external atmosphere and the atmosphere around the plant. Hence, plants in nodulation-effectiveness studies should be grown so that high rates of growth are maintained (preferably with an exponential increase in dry weight), and that the experiments should not be terminated before the most effectively nodulated plants have fixed at least 5 times as much nitrogen as was originally present in the seeds. This does not preclude the use of the "Thornton technique" for such studies, provided due recognition is given to the size of the tubes, and the size of the plants. Furthermore, there is nothing to suggest that the use of this technique to culture plants for counting nodule bacteria (Brockwell 1963) is unsatisfactory.

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