Photosynthesis, respiration, and net primary production of sunflower stands in ambient and elevated atmospheric CO2 concentrations: an invariant NPP : GPP ratio?

The effect of elevated CO2 on photosynthesis, respiration, and growth effic iency of sunflower plants at the whole-stand level was investigated using a whole-system gas exchange facility (the EcoCELLs at the Desert Research In stitute) and a C-13 natural tracer method. Total daily photosynthesis (CPP) , net primary production (NPP), and respiration under the elevated CO2 trea tment were consistently higher than under the ambient CO2 treatment. The ov erall level of enhancement due to elevated CO2 was consistent with publishe d results for a typical C3 plant species. The patterns of daily GPP and NPP through time approximated logistic curves under both CO2 treatments. Regre ssion analysis indicated that both the rate of increase (the parameter 'r') and the maximum Value (the parameter 'k') of daily GPP and NPP under the e levated CO2 treatment were significantly higher than under the ambient CO2 treatment. The percentage increase in daily GPP due to elevated CO2 varied systematically through time according to the logistic equations used for th e two treatments. The GPP increase due to elevated CO2 ranged from approxim ately 10% initially to 73% at the peak, while declining to about 33%, as pr edicted by the ratio of the two maximum values. Different values of percent age increase in GPP and NPP were obtained at different sampling times. This result demonstrated that one-time measurements of percentage increases due to elevated CO2 could be misleading, thereby making interpretation difficu lt. Although rhizosphere respiration was substantially enhanced by elevated CO2, no effect of elevated CO2 on R: P (respiration:photosynthesis) was fo und, suggesting an invariant NPP:GPP ratio during the entire experiment. Fu rther validation of the notion of an invariant NPP:GPP ratio may significan tly simplify the process of quantifying terrestrial carbon sequestration by directly relating total photosynthesis to net primary production.