Maximum carboxylation rate estimation with chlorophyll content as a proxy of rubisco content

The maximum carboxylation rate (Vcmax) is a key parameter in determining
the plant photosynthesis rate per unit leaf area. However, most
terrestrial biosphere models currently treat Vcmax as constants changing
only with plant functional types, leading to large uncertainties in
modelled carbon fluxes. Vcmax is tightly linked with
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Here we
investigated the relationship between leaf chlorophyll content and Rubisco
(Chl-Rub) within a winter wheat paddock. With chlorophyll as a proxy of
Rubisco, a semi-mechanistic model was developed to model Vcmax25. The
Chl-Rub relationship was validated using measurements in a temperate
forest in Canada. The results showed that Rubisco was strongly correlated
with chlorophyll (R2 = 0.96, p < 0.001) for winter wheat, due to
the absorption of light energy by chlorophyll and the amount of CO2
catalysed by Rubisco are tightly coupled. Incorporating the Chl-Rub
relationship into the semi-mechanistic model, the root mean square error
of modelled Vcmax25 was the lowest among all estimation models. The slopes
of Chl-Rub relations were almost identical for the winter wheat and
temperate forest, demonstrating the potential for using leaf chlorophyll
content as a proxy of leaf Rubisco in modelled Vcmax25 at large spatial
scales. We anticipate that improving Vcmax25 estimates over time and space
will reduce uncertainties in terrestrial biosphere models and improve the
estimates of global carbon budgets.