Five genotypes, including two cultivars of Arachis hypogaea L. and three wild species of Arachis, were tested for their photosynthetic capacity at atmospheric CO₂ concentrations and for photorespiration in CO₂-free air. Photosynthetic response to CO₂ concentrations in the range of 50 to 600 ppm was also tested. Diffusive resistance (DRH₂O) of the adaxial surface of the five genotypes was measured with a diffusive resistance porometer. Several other leaf characteristics related to CO₂ exchange were measured.

There was a linear increase in net photosynthesis (Pn) for four of the five genotypes as CO₂ concentration was increased from 50 to 600 ppm. The increase in Pn of an A. hypogaea genotype from Tanganyika appeared to be progressively less at CO₂ concentrations near 600 ppm. The florunner cultivar of A. hppogaea had the highest Pn at CO₂ concentrations of 300 ppm and above; A. pintoi had the lowest. Photorespiration as measured by CO₂ evolution into CO₂-free air averaged about 4 mg CO₂ dm ^-2 hr ^-1 and did not differ among genotypes. Dark respiration was higher in leaves of wild species than in the two genotypes of A. hypogaea.

Diffusive resistance of A. hypogaea, A. pintoi and A. sp. (glabrata?) leaves remained constant from 9 a.m. to 3 p.m. EST and then increased up to the last measurement at 10 p.m. The DRH₂O of A. hypogaea and A. pintoi were similar during the daytime and ranged from 1.5 to 5.0 sec cm ^-1 between 9 a.m. and 3 p.m. A. villosulicarpa and A. sp. (glabrata?) had higher DRH₂O values during the same time period, ranging from 5 to 12 sec cm^-1. At 10 p.m. DRH₂O of A. hypogaea was 84 sec cm^-1 compared to only about 20 sec cm^-1 for A. pintoi. Net photosynthesis of leaves of the five Arachis genotypes was not closely related to DRH₂O nor leaf characteristics including chlorophyll content, stomatal frequency, leaf nitrogen content or specific leaf weight.