EFFECTS OF ATMOSPHERIC CO_2 CONCENTRATION ON PHOTOSYNTHETIC PERFORMANCE OF C_3 AND C_4 PLANTS

CHEN X. M., ALM D. M. and HESKETH J. D. Effects of atmospheric CO2 concentration on photosynthetic performance of C3 and C4 plants. BIOTRONICS 24, 65-72, 1995. Four C3 and C4 plant species (soybean, sunflower, pigweed, and corn) were grown in the field to test photosynthetic performance at ambient (350,umol CO2 mol- 1 air or ppm) and enriched (700 ppm) atmospheric CO2 (Ca), particularly after plants were switched from the 700 to 350 ppm treatment and back. Comparing plants in a CO2 enriched atmosphere with those at ambient, leaf water content increased for pigweed and corn, but was the same or less for the C3 plants; leaf mass per unit area (kg m -2) was less for C4 plants and greater for C3 plants, for the same comparison. At 700 ppm steady -state photosynthetic gas exchange (Pn) values were the same for C4 species and were greater for C3 species, while stomatal conductances (gs) were reduced in all comparisons, compared to plants at ambient CO2 levels. Short term Pn values taken immediately after switching treatments were not significantly different from long term values for the same treatment, except for pigweed, where in both cases Pn after switching was 73% of the long term values. In all tested species, gs values were lowered when measured directly after being transferred to the other treatment. Continual gas exchange measurements, for up to 90 min after a transfer from long term CO 2 enrichment to ambient and back, indicated that the short term Pn value for pigweeq, as pointed out above in the switching experiments, had not equilibrated to the change in treatment whereas that of corn had. Pn at 700 ppm continued at a rapid pace throughout the day for the C3 species, that for the C4 species being the same as at 350 ppm. Pn declined over the course of the day for the same light levels for all species and treatments, except for pigweed at 350 ppm. The decrease in Pn at 700 ppm suggested nonstomatal control in the C3 species.