PHOTOSYNTHETIC ACCLIMATION AND PHOTOSYNTHATE PARTITIONING IN SOYBEAN LEAVES IN RESPONSE TO CARBON-DIOXIDE ENRICHMENT

Photosynthetic rates and photosynthate partitioning were studied in th ree-week-old soybean [Glycine max (L.) Merr. cv. Williams] plants expo sed to either ambient (35 Pa) or elevated (70 Pa) CO2 in controlled en vironment chambers. Ambient CO2-grown plants also were given a single 24 h treatment with 70 Pa CO2 1 d prior to sampling. Photosynthetic ra tes of ambient CO2-grown plants initially increased 36% when the measu rement CO2 was doubled from 35 to 70 Pa. Photosynthetic rates of the t hird trifoliolate leaf, both after 1 and 21 d of elevated CO2 treatmen t, were 30 to 45% below those of ambient CO2-grown plants when measure d at 35 Pa CO2. These reduced photosynthetic rates were not due to inc reased stomatal resistance and were observed for 2 to 8 h after plants given 1 d of CO2 enrichment were returned to ambient CO2. Initial and total ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activ ities, percent activation, Rubisco protein, soluble protein and leaf c hlorophyll content were similar in all CO2 treatments. Quantum yields of photosynthesis, determined at limiting irradiances and at 35 Pa CO2 , were 0.049 +/- 0.003 and 0.038 +/- 0.005 mol CO2 fixed per mol quant a for ambient and elevated CO2-grown plants, respectively (p < 0.05). Leaf starch and sucrose levels were greater in plants grown at 70 than at 35 Pa CO2. Starch accumulation rates during the day were greater i n ambient CO2-grown plants than in plants exposed to elevated CO2 for either 1 or 21 d. However, the percentage of C partitioned to starch r elative to total C fixed was unaffected by 1 d of CO2 enrichment. The above results showed that both photosynthetic and starch accumulation rates of soybean leaflets measured at 35 Pa CO2 were temporarily reduc ed after 1 and 21 d of CO2 enrichment. The biochemical mechanism affec ting these responses was not identified.