Comparative photorespiration in Amaranthus, soybean and corn.

A tracer technique was used to measure photorespiration in Amaranthus lividus, soybean and corn. Under a light intensity of 40 Wm(-2) (400-700 nm) efflux of tracer carbon dioxide from Amaranthus into air was comparable to that from soybean over a 30-min period and 10 times that from corn. Initial rates of efflux of tracer into air from Amaranthus were higher than from soybean and 9 times that from corn. Efflux of CO2 from Amaranthus over 30 min in 120 Wm(-2) was only 5 times that of corn and the initial rate was only one third that of soybean. Though total efflux from soybean was similar at the two light intensities, the initial rate was slightly higher under 120 Wm(-2). For Amaranthus and soybean, pure oxygen doubled total efflux of CO2 and substantially increased the initial rate compared with CO2-free air whereas there was no effect on corn. A comparison of the light and dark curves suggests that light and dark respiration had different substrates. The results are interpreted in terms of the recycling of photorespiratory CO2.