GAS-EXCHANGE DIFFERENCES AND COMPARATIVE ANATOMY AMONG COTTON LEAF-TYPE ISOLINES

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 photosyntha te between reproductive and vegetative growth. Establishing why certai n lines may differ in photosynthetic rate could provide additional too ls for selection. Photosynthesis and leaf anatomy of the youngest full y expanded leaf in field plots of the super okra, okra, and normal lea f-type isolines of the cotton (Gossypium hirsutum L.) genotype 'MD 65- 11', which differ in leaf size and degree of lobing, were compared wit h the normal leaf genotype 'DPL 50'. Super okra and okra averaged 24 a nd 22% greater leaf CO2-exchange rates (CER), respectively, than the n ormal leaf isoline in both 1989 and 1990. These differences were assoc iated primarily with greater specific leaf weight (SLW) and leaf chlor ophyll concentration of the super okra and okra leaf types. Wateruse e fficiency [WUE(g) = CER/leaf stomatal conductance (g(5))] was 59 and 4 0% greater for super okra and okra, respectively, during 1990 compared to normal leaf. Leaves of the super okra and okra isolines were 42% t hicker than normal leaf which contributed to this greater SLW. The rel ative tissue percentage of various cell types and mesophyll surface ar ea per unit leaf volume did not differ among genotypes. Rubisco concen trations per unit stroma area were similar among chloroplasts of the d ifferent genotypes. Genotypic differences in CER are likely due to a g reater concentration of photosynthetic apparatus per unit leaf area ca used by leaf thickness differences, however the possibility of differe nces in the rate of dark respiration or photorespiration cannot be dis counted.