Much of the yield improvement bred into modern cotton (Gossypium hirsutum L.) genotypes has come from improvements in the harvest index. An alternative strategy for yield enhancement might be the identification or development of cotton lines with superior photosynthetic rates. In several species, smaller leaftypes and greater specific leaf weights (SLW) have often been correlated with higher photosynthetic rates per unit leaf area. Thus, our objectives were to determine if gas exchange differences existed between cotton leaftype isolines and to determine whether these differences could be explained by leaf biochemical or anatomical characteristics.

Field studies were conducted in 1989 and 1990 on the cotton genotypes, DPL 50 and the super okra, okra, and normal leaftype isolines of MD 65-11. CO₂-exchange rates (CER) were measured throughout both growing seasons on the youngest fully expanded, fully sunlit leaf in each plot. Leaf chlorophyll concentration, soluble protein concentration, and SLW were periodically determined each year. Anatomical studies, immuno-gold labeling, and quantification of ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) by immuno-diffusion were performed on 1990 leaf samples.

The CER of the super okra and okra leaftype isolines consistently exceeded that of the normal leaftype isolines. On average, super okra and okra had a 19 % and 8 % greater chlorophyll concentration and a 9 % and 4 % higher SLW respectively, than the normal leaftype. Super okra and okra leaf extracts tended to have a higher soluble protein concentration. When expressed on a fresh weight basis or per unit area of the chloroplast, the Rubisco concentration did not differ between genotypes. The genotypes did not differ in the relative volumetric tissue percentages of air space or of palisade, spongy mesophyll, vascular, and epidermal cells. However, both super okra and okra were found to have 42 % thicker leaves than the normal leaftype. A thicker leaf, higher chlorophyll concentration, and greater SLW indicates a greater density of photosynthetic machinery in the super okra and okra leaftype isolines. Alternative production practices that increase the leaf area per unit land area, might better utilize the higher photosynthesizing okra and super okra leaves.