Abstract
The rate of photorespiration of Douglas-fir seedlings was measured under different light intensities by: (1) extrapolating the curve for CO(2) uptake in relation to atmospheric CO(2) content to zero CO(2) content, and (2) measuring CO(2) evolution of the plants into a CO(2)-free airstream. Different results, obtained from these techniques, were believed to be caused by a severe restriction of the photosynthetic activity when the latter was used. With the first method, CO(2) evolution was lower than the dark respiration rate at low light intensity. For all temperatures studied (6 degrees , 20 degrees , 28 degrees ) a further increase in light intensity raised the CO(2) evolution above dark respiration before it leveled off. The rate of CO(2) evolution was stimulated by increase in temperature at all light intensities. With the CO(2)-free air method, CO(2) evolution in the light was less than dark respiration at all light intensities.