Linking photosynthesis and leaf N allocation under future elevated CO ₂ and climate warming in Eucalyptus globulus

Highlight

Photosynthetic acclimation of Eucalyptus globulus leaves to elevated CO ₂ was underpinned by reduced leaf N and Rubisco contents; the opposite occurred under warming, coinciding with growth resumption in spring.

Abstract

Leaf-level photosynthetic processes and their environmental dependencies are critical for estimating CO ₂ 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 CO ₂ [eC; ambient CO ₂ (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 V _cmax (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 V _cmax 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 [CO ₂] climates.