DOI: 10.5061/dryad.xpnvx0kbv
title: Maximum carboxylation rate estimation with chlorophyll content as a proxy of rubisco content

description: 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 &lt; 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.
description: Location----Field campaigns were conducted at a farmland
 ecosystem station in Shangqiu, Henan Province, China (34.27°N,
 115.4°E). Samples----Unfolded leaves at the top of the
 canopy were randomly selected for destructive sampling approximately once
 every 10 days, from a 30×30 m subset area within the larger field. On each
 sampling date, mean values of leaf chlorophyll, nitrogen, and Rubisco were
 calculated from 3 samples. Leaf chlorophyll and
 nitrogen----Leaf chlorophyll and nitrogen content was measured from the
 same leaves due to the small sampling mass required for laboratory
 analysis. The leaves sampled for chlorophyll measurements were packaged in
 plastic sample bags and transported into an ice box immediately after
 collection. About half an hour later, these samples were sent to the
 laboratory for leaf chlorophyll measurement. In the laboratory, leaf
 chlorophyll was extracted into a solution with absolute
 ethyl
 alcohol and formaldehyde mixed
 in a volume ratio of 1:1. The absorbance of the dissolving solution was
 measured at 663.8, 646.8 and 480 nm using a Shimadzu UV-1700
 spectrophotometer (Wellburn, 1994). The remaining samples for measuring
 nitrogen were dried in an oven at 80 ℃ for approximately 48 hours until a
 constant weight and then determined total leaf nitrogen by the Kjeldahl
 method (Wolf, 1982).  Rubisco----A whole leaf was
 needed to measure the leaf Rubisco content, as the analysis requires a
 sample exceeding 0.5g fresh weight. A portable chlorophyll meter SPAD
 (Konica Minolta Inc., Osaka, Japan) was used to observe the SPAD values of
 leaves, for which Rubisco were measured. From tial to tip of one leaf, 5
 SPAD values were measured evenly and averaged for representing the SPAD
 value of the whole leaf. SPAD values were converted into leaf chlorophyll
 content using an empirical model fitted using measured SPAD values and
 laboratory analysed chlorophyll content in the same field. The Rubisco
 content of leaves was determined by the method of Makino et al. (1985,
 1986). Vcmax25-----Carbon dioxide response curves
 (A/Ci) were measured using an LI-6800 portable infrared gas analyser
 (LI-COR, Lincoln, NE, USA). To measure more leaves on one day, 3 leaves
 were measured using the A/Ci response curve method, and additional 3
 measurements were conducted using the rapid response curve method
 (Stinziano et al., 2017). For the A/Ci response curve, PPFD was set at
 1800 μmol m<sup>-2</sup> s<sup>-1</sup>, and the
 stepwise CO<sub>2</sub> concentrations were 400, 200, 100, 50,
 400, 600, 800, 1000, 1200, 1500, and 1800 ppm, respectively. The function
 fitaci in ‘plantecophys’ R package was used to estimate
 Vcmax<sub>25</sub> for both the A/Ci response curve and fast
 response curve methods.