| Abstract: | The relation of the net uptake or output of carbon dioxide bydetached leaves of Pelargonium zonale or wheat to ambient carbondioxide concentration was investigated at two or three lightintensities. The statistics of fitted curves were used as datain analysis of variance. For Pelargonium the results were consistentwith a hypothesis of a ‘straight balance’ betweena rate of photosynthetic uptake proportional to concentrationand a constant rate of respiratory production. For wheat, however,the curve was steeper near  than at lower carbon dioxide levels;this disproved the ‘straight balance’ hypothesisand was consistent with an increase in carbon dioxide fixationor a reduction in output over a limited concentration rangeon either side of . The characteristics of light respiration, possibly carbon dioxidedependent, were investigated further in an experiment on theeffect of oxygen concentrationupon r values for two species(P. zonale and Hydrangea sp.) at two light intensities; theywere also studied by measuring the ‘burst’ of carbondioxide output when leaves were darkened after illuminationat four different light intensities and at two temperatures. was linearly related to oxygen tension up to 610 mm partialpressure of mercury and a fivefold increase in light intensityhad only a very small effect indicating that photoxidation wasnot important. The carbon dioxide burst on darkening showedrelations to temperature and previous light intensity quitedifferent from those of which should be proportional to lightrespiration. These results, therefore, do not support the viewthat the burst represents the persistence of an enhanced lightrespiration. |
