Gas Exchange Differences and Comparative Anatomy among Cotton Leaf‐type Isolines

Abstract

Photosynthesis rate could be a selection criterion for plant breeders, especially if lines with superior photosynthesis could be identified and coupled with those lines with suitable partitioning of photosynthate between reproductive and vegetative growth. Establishing why certain lines may differ in photosynthetic rate could provide additional tools for selection. Photosynthesis and leaf anatomy of the youngest fully expanded leaf in field plots of the super okra, okra, and normal leaftype isolines of the cotton (Gossypium hirsutum L.) genotype ‘MD 65− 11’, which differ in leaf size and degree of Iobing, were compared with the normal leaf genotype ‘DPL 50’. Super okra and okra averaged 24 and 22% greater leaf CO2‐exchange rates (CER), respectively, than the normal leaf isoline in both 1989 and 1990. These differences were associated primarily with greater specific leaf weight (SLW) and leaf chlorophyll concentration of the super okra and okra leaf types. Water use efficiency [WUEg = CER/leaf stomatal conductance (gs)] was and 40% greater for super okra and okra, respectively, during 1990 compared to normal leaf. Leaves of the super okra and okra isolines were 42% thicker than normal leaf which contributed to this greater SLW. The relative tissue percentage of various cell types and mesophyil surface area per unit leaf volume did not differ among genotypes. Rubisco concentrations per unit stroma area were similar among chloroplasts of the different genotypes. Genotypic differences in CER are likely due to a greater concentration of photosynthetic apparatus per unit leaf area caused by leaf thickness differences, however the possibility of differences in the rate of dark respiration or photores piration cannot be discounted.