The Monomolecular and Rectangular Hyperbola as Empirical Models of the Response of Photosynthetic Rate to Photon Flux Density, with Applications to Three Veronica Species

The properties of the rectangular hyperbola and monomolecular functions, with respect to the photosynthesis/photon flux density (PFD) relationship, are discussed, and the shortcomings of the former are highlighted. Both models were fitted to data acquired from three closely related Veronica species of contrasting ecology. The non-linear regression algorithms give estimates, with standard errors, of light saturated photosynthetic rate, light compensation point, dark respiration rate, and photochemical efficiency at low PFD. While the rectangular hyperbola gave almost as good a fit to the data as the monomolecular for each species, the light saturated photosynthetic rate estimate given by the former was always unacceptably high in comparison with that indicated by the obvious trend of the data. Moreover, this tendency was accentuated if near-saturating PFDs were removed from data sets, and there was a tendency for the fitting algorithm to become unstable. No such problems were encountered with the monomolecular function, and it is suggested that this be used whenever a simple empirical model is required to analyze photosynthesis/PFD data.