Experimental researches on vegetable assimilation and respiration. XIX.-The effect of variations of carbon dioxide supply upon the rate of assimilation of submerged water plants

Abstract
A study of the relation between CO2 concentration and the rate of assimilation is reported in which particular attention was paid to the method of supplying the CO2. An apparatus is described in which CO2 and sodium bicarbonate solutions could be passed over a plant at various speeds under controlled conditions of light intensity and temp. If 400 cc. of solution were passed through the apparatus per hr., NaHCO3 gave a faster rate of assimilation than a pure solution of CO2 at an equal partial pressure, providing no other factor was limiting. With 600 cc. per hr., at low light intensity and consequently with slow assimilation, the 2 solutions gave equal rates. At a higher light intensity, the bicarbonate again gave the faster rate of assimilation. It is concluded that the form of the curve relating CO2 concentration to the rate of photosynthesis depends on the conditions of CO2 supply. The alterations in the curve are due to changes in the diffusion resistance in the external solution. The importance of the diffusion phase in measuring rates of assimilation is emphasized. With due regard for this factor, the curves relating assimilation rate to CO2 concentration and light intensity were redetermined and found to be smooth curves, probably of hyperbolic form. In dilute solutions, where alkalinity is not great enough to be injurious, NaHCO3 solutions are satisfactory for determining assimilation rates, since there appears to be no direct assimilation of HCO3 ions.

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