Carbon Assimilation at Low Carbon Dioxide Levels

Abstract

The relation of the net uptake or output of carbon dioxide by detached leaves of Pelargonium zonale or wheat to ambient carbon dioxide concentration was investigated at two or three light intensities. The statistics of fitted curves were used as data in analysis of variance. For Pelargonium the results were consistent with a hypothesis of a ‘straight balance’ between a rate of photosynthetic uptake proportional to concentration and a constant rate of respiratory production. For wheat, however, the curve was steeper near Λ than at lower carbon dioxide levels; this disproved the ‘straight balance’ hypothesis and was consistent with an increase in carbon dioxide fixation or a reduction in output over a limited concentration range on either side of Λ. The characteristics of light respiration, possibly carbon dioxide dependent, were investigated further in an experiment on the effect of oxygen concentrationupon r values for two species (P. zonale and Hydrangea sp.) at two light intensities; they were also studied by measuring the ‘burst’ of carbon dioxide output when leaves were darkened after illumination at four different light intensities and at two temperatures. Λ was linearly related to oxygen tension up to 610 mm partial pressure of mercury and a fivefold increase in light intensity had only a very small effect indicating that photoxidation was not important. The carbon dioxide burst on darkening showed relations to temperature and previous light intensity quite different from those of Λ which should be proportional to light respiration. These results, therefore, do not support the view that the burst represents the persistence of an enhanced light respiration.