Cheng, W., Sims, D. A., Luo, Y., Coleman, J. S., Johnson, D. W. (2000). Photosynthesis, respiration and net primary production of sunflower stands in ambient and elevated atmospheric CO2 concentrations: an invariant NPP:GPP ratio?, Global Change Biology, 6 (8), 931-941, 10.1046/j.1365-2486.2000.00367.x