Monday, August 3, 2009: 2:50 PM
Santa Ana, Albuquerque Convention Center
Background/Question/Methods The C4 photosynthetic pathway mostly suppresses RuBP oxygenation and subsequent photorespiration, which is regarded as a wasteful reaction of Rubisco. In contrast, C3-C4 intermediate photosynthesis does not suppress photorespiration, but employs a mechanism that permits these species to recapture some of the CO2 that would be lost to photorespiration in C3 leaves. This pathway is believed to be most important in hot, dry environments where levels of RuBP oxygenation are high and C3 carbon assimilation is consequently very inefficient. C4 photosynthetic species derive corollary ecological advantages from suppression of RuBP oxygenation such as increased water-use and nitrogen-use efficiencies, but it is unclear whether C3-C4 species do so as well. Results from such studies are inconsistent and dependent upon measurements of a few (3-5) individuals. In this study, we test the hypothesis that water-use efficiency (WUE) and nitrogen-use efficiency (NUE) in intermediates will increase with C4-cycle activity.
We included fourteen species of Flaveria (Asteraceae): two C3 species, three C4 species, and nine C3-C4 species with a gradient of C4-cycle activities (3-95% CO2 fixation via the C4 pathway). Plants were fertilized using one of five nitrogen concentrations (1, 2, 4, 8, 16 mmol) to provide a gradient of photosynthetic capacities. Reduced photosynthetic capacity results in reduced stomatal conductance (gs), and at least fifteen individuals of each species were measured to determine the response of gs to light-saturated CO2 assimilation rate (Amax), an index of WUE. Leaves were sampled for Rubisco, chlorophyll, and total nitrogen contents to assess NUE.
Results/Conclusions We found that C3-C4 species with <80% C4-cycle activity exhibited WUE values that were not significantly different from the C3 species, while the more C4-like intermediates exhibited WUE values that were not significantly different from the C4 species. The response of Amax to leaf Rubisco content showed a similar pattern, with a reduction in Rubisco content to C4 levels occurring in species with >80% C4-cycle activity. Responses of Amax to chlorophyll content and total leaf nitrogen were also similar; an increase in NUE to C4 levels occurred when C4-cycle activity was greater than 80%.
These results demonstrate that substantial C4-cycle activity is required to provide increased WUE and NUE in C3-C4 species. This also implies that WUE and NUE were probably not important factors in selecting for C3-C4 photosynthesis. This points to more efficient CO2 assimilation through refixation of photorespired CO2 as the primary, and perhaps only, ecological advantage of C3-C4 photosynthesis in Flaveria.
We included fourteen species of Flaveria (Asteraceae): two C3 species, three C4 species, and nine C3-C4 species with a gradient of C4-cycle activities (3-95% CO2 fixation via the C4 pathway). Plants were fertilized using one of five nitrogen concentrations (1, 2, 4, 8, 16 mmol) to provide a gradient of photosynthetic capacities. Reduced photosynthetic capacity results in reduced stomatal conductance (gs), and at least fifteen individuals of each species were measured to determine the response of gs to light-saturated CO2 assimilation rate (Amax), an index of WUE. Leaves were sampled for Rubisco, chlorophyll, and total nitrogen contents to assess NUE.
Results/Conclusions We found that C3-C4 species with <80% C4-cycle activity exhibited WUE values that were not significantly different from the C3 species, while the more C4-like intermediates exhibited WUE values that were not significantly different from the C4 species. The response of Amax to leaf Rubisco content showed a similar pattern, with a reduction in Rubisco content to C4 levels occurring in species with >80% C4-cycle activity. Responses of Amax to chlorophyll content and total leaf nitrogen were also similar; an increase in NUE to C4 levels occurred when C4-cycle activity was greater than 80%.
These results demonstrate that substantial C4-cycle activity is required to provide increased WUE and NUE in C3-C4 species. This also implies that WUE and NUE were probably not important factors in selecting for C3-C4 photosynthesis. This points to more efficient CO2 assimilation through refixation of photorespired CO2 as the primary, and perhaps only, ecological advantage of C3-C4 photosynthesis in Flaveria.
