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

The effect of elevated CO2 on photosynthesis, respiration, and growth ef®ciency of sun ̄ower plants at the whole-stand level was investigated using a whole-system gas exchange facility (the EcoCELLs at the Desert Research Institute) and a C natural tracer method. Total daily photosynthesis (GPP), net primary production (NPP), and respiration under the elevated CO2 treatment were consistently higher than under the ambient CO2 treatment. The overall level of enhancement due to elevated CO2 was consistent with published results for a typical C3 plant species. The patterns of daily GPP and NPP through time approximated logistic curves under both CO2 treatments. Regression 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 elevated CO2 treatment were signi®cantly 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 the two treatments. The GPP increase due to elevated CO2 ranged from approximately 10% initially to 73% at the peak, while declining to about 33%, as predicted by the ratio of the two maximum values. Different values of percentage 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 dif®cult. Although rhizosphere respiration was substantially enhanced by elevated CO2, no effect of elevated CO2 on R:P (respiration:photosynthesis) was found, suggesting an invariant NPP:GPP ratio during the entire experiment. Further validation of the notion of an invariant NPP:GPP ratio may signi®cantly simplify the process of quantifying terrestrial carbon sequestration by directly relating total photosynthesis to net primary production.