Intraspecific variation for CO2 compensation point and differential growth among variants in a C3-C4 intermediate plant.

CO2 compenstation point (Γ), the concentration of CO2 at which photosynthesis and respiration are at equilibrium, is a commonly used diagnostic for the C4 photosynthetic pathway, since it reflects the reduced photorespiration that is a property of C4 photosynthesis. Geographic variation for Γ was examined within Flaveria linearis, a C3-C4 intermediate species. Collections from four widely separated Floridian populations were propagated in a greenhouse and measured for Γ. Little differentiation among populations was found, but significant within-population variation was present. Temperature is a hypothesized selective agent for the C4 photosynthetic pathway. To test this hypothesis, plants exhibiting a range of Γ were cloned and placed in growth chambers at 25°C and 40°C. After 7 weeks, Γ valves were remeasured and plants were harvested and weighed. There was a poor correlation between initial and final measures of Γ for a given genotype (r=0.38, P>0.1). Broad sense heritability for Γ was computed to be 0.10. At 25°C, there was no relationship between final size and Γ. At 40°C, more C4-like plants, as indicated by their low Γ, had grown larger. Differences in relative growth rate were attributable more to differences in net assimilation rate than in leaf area ratio. Taken together, these results demonstrate that although significant plasticity exists in the amount of photorespiration in this C3-C4 species, high temperature appears to be an effective selective agent for the reduction of photorespiration and the enhancement of C4-like traits.