Intraspecific measurements of photorespiration.

The relative magnitudes of (a) CO(2) compensation concentration, (b) zero CO(2) intercept of the CO(2) response curve, (c) O(2) suppression of net photosynthesis, (d) differential (12)CO(2) and (14)CO(2) uptake, and (e) (14)CO(2) efflux into CO(2)-free air were determined in the dry bean (Phaseolus vulgaris L.) varieties Michelite-62 (M-62) and Red Kidney (RK). In comparing the two varieties for each of the above processes, there were three categories of response, M-62 > RK, M-62 = RK, and M-62 < RK. Since these processes did not give the same relative difference for the two varieties being studied, it was concluded that these phenomena cannot validly be used to estimate the magnitude of photorespiration, although they do identify its presence. The results suggest that photorespiration is but one component of O(2) inhibition of net photosynthesis and that photorespiration itself has two or more component metabolic pathways.