INTRASPECIFIC VARIATION FOR CO2 COMPENSATION POINT AND DIFFERENTIAL GROWTH AMONG VARIANTS IN A C-3-C-4 INTERMEDIATE PLANT

CO2 compensation point (Gamma), the concentration of CO2 at which photosynthesis and respiration are at equilibrium, is a commonly used diagnostic for the C-4 photosynthetic pathway, since it reflects the reduced photorespiration that is a property of C-4 photosynthesis. Geographic variation for Gamma was examined within Flaveria linearis, a C-3-C-4 intermediate species. Collections from four widely separated Floridian populations were propagated in a greenhouse and measured for Gamma. Little differentiation among populations was found, but significant within-population variation was present. Temperature is a hypothesized selective agent for the C-4 photosynthetic pathway. To test this hypothesis, plants exhibiting a range of Gamma were cloned and placed in growth chambers at 25 degrees C and 40 degrees C. After 7 weeks, Gamma valves were remeasured and plants were harvested and weighed. There was a poor correlation between initial and final measures of Gamma for a given genotype (r = 0.38, P > 0.1). Broad sense heritability for Gamma was computed to be 0.10. At 25 degrees C, there was no relationship between final size and Gamma. At 40 degrees C, more C,like plants, as indicated by their low Gamma, 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 C-3-C-4 species, high temperature appears to be an effective selective agent for the reduction of photorespiration and the enhancement of C-4-like traits.