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Relationship of Photosynthetic Rate to Growth and Fruiting of Cotton, Soybean, Sorghum, and Sunflower¹

Cotton (Gossypium hirsutum L.), sorghum (Sorghum bicolor L.), soybean (Glycine max L.), and sunflower (Helianthus annuus L.) plants were exposed during daylight hours to an atmosphere enriched with CO₂ to 630 ppm (v:v) (HiCO₂) in a glasshouse. Temperature was trolled continuously to produce a daily maximum of 35 C and a minimum of 21 C. Days were cloudless and long, May to August, in Phoenix, Ariz. Average CO₂ exchange rate (CER) increased 15% for cotton, 2% for sorghum, 41% for soybean, and 7% for sunflower compared to the CER of these species at 330 ppm CO₂ (LoCO₂). The increase in CER was not statistically significant for sorghum and sunflower. Measurements of relative growth rate
and net assimilation rate
showed the growth rate of all species increased during the juvenile stage (10d-30d) in the enriched atmosphere.
and
were not higher in the enriched atmosphere after the juvenile stage, but cotton and soybean plants maintained their larger size and greater absolute growth.

Final dry weights of cotton and soybean increased 110 and 380%, respectively, in HiCO₂ compared to LoCO₂. Lint yield of cotton was increased 180% by HiCO₂ on a per-plant basis and 88% on a unit leaf area basis. Became of growth fimitations imposed by terminal flowers, the final size of sorghum and sunflower plants in HiCO₂ was not significantly larger than in LoCO₂ These results imply that selection for CER will not effectively increase yield of determinate species such as sunflower and sorghum, but may effectively increase yield of indeterminate species such as cotton and soybean, if a sensitive assay for CER can be found.

Key Words: Relative growth rate • Net assimilation rate • CO₂ enrichment • Determinate species • Indeterminate species • Gossypium hirsutum L. • Glycine max L. • Sorghum bicolor L. • Helianthus annuus L.

² Plant physiologists, Western Cotton Res. Lab., ARS, USDA, 4135 E. Broadway Rd., Phoenix, AZ 85040.

Received for publication May 27, 1977.