Abstract
Leaf-level photosynthetic processes and their environmental dependencies are critical for estimating CO2 uptake from the atmosphere. These estimates use biochemical-based models of photosynthesis that require accurate Rubisco kinetics. We investigated the effects of canopy position, elevated atmospheric CO2 [eC; ambient CO2 (aC)+240 ppm] and elevated air temperature (eT; ambient temperature (aT)+3 °C) on Rubisco content and activity together with the relationship between leaf N and Vcmax (maximal Rubisco carboxylation rate) of 7 m tall, soil-grown Eucalyptus globulus trees. The kinetics of E. globulus and tobacco Rubisco at 25 °C were similar. In vitro estimates of Vcmax derived from measures of E. globulus Rubisco content and kinetics were consistent, although slightly lower, than the in vivo rates extrapolated from gas exchange. In E. globulus, the fraction of N invested in Rubisco was substantially lower than for crop species and varied with treatments. Photosynthetic acclimation of E. globulus leaves to eC was underpinned by reduced leaf N and Rubisco contents; the opposite occurred in response to eT coinciding with growth resumption in spring. Our findings highlight the adaptive capacity of this key forest species to allocate leaf N flexibly to Rubisco and other photosynthetic proteins across differing canopy positions in response to future, warmer and elevated [CO2] climates.
Highlights
Photosynthetic CO2 assimilation by the terrestrial biosphere constitutes the largest component of global CO2 fluxes
Vcmax was well correlated with leaf N in the upper canopy (Fig. 3C), while the Jmax/Vcmax ratio was constant across the various treatments (Table 3). These results indicate that in E. globulus Rubisco largely remained a constant fraction of leaf N across the elevated CO2, warming, and canopy position treatments, and that Vcmax in this species can be predicted from leaf N and the Rubisco fraction, while Jmax can be estimated from Vcmax
Rubisco catalytic parameters for E. globulus measured at 25 °C were similar to the widely used tobacco kinetics in C3 photosynthesis, and may provide a model for evergreen plantation trees, caution is needed in the general applicability of these parameters across different taxa and temperatures (Sharwood et al, 2016)
Summary
Photosynthetic CO2 assimilation by the terrestrial biosphere constitutes the largest component of global CO2 fluxes. A key component of the FvCB model is the parameter Vcmax, the maximum rate of carboxylation by Rubisco. Vcmax is recognized as the most critical parameter for modelling global primary productivity and projecting future global change (Rogers, 2014). This importance stems from estimates of Jmax often being extrapolated from a linear function of Vcmax (Walker et al, 2014) and that many global carbon models estimate Vcmax as a fraction of leaf N content (Friend, 2010). Eucalypts are important plants for both native forests and commercial plantations in Australia and worldwide
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