Linking photosynthesis and leaf N allocation under future elevated CO2 and climate warming in Eucalyptus globulus

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.

Keywords: Canopy position; Eucalyptus globulus; Rubisco kinetics; Vcmax; elevated CO2 and temperature; photosynthesis; whole-tree chambers..

Fig. 1.

Effect of elevated CO₂ and temperature on main photosynthetic parameters. Light-saturated rates of photosynthesis, A_sat25 (A), Rubisco content (B), and PSII-D1 protein content (C) in leaves of E. globulus trees (upper canopy on the left and lower canopy on the right) grown at ambient (aC, clear and checked columns) or elevated (eC, grey and black columns) atmospheric [CO₂], and at ambient (aT, clear and grey columns), or elevated (eT, checked and black columns) air temperature. Values represent averages of 2–3 biological replicates ±SE (n=3 except for eCaT, where n=2). A statistical summary is shown in Table 2.

Fig. 2.

Effects of elevated CO₂ and temperature on photosynthetic CO₂ response curves. The response of photosynthetic rates to chloroplastic [CO₂], C_c in the upper canopy leaves of E. globulus trees grown at ambient (aC) or elevated (eC) atmospheric [CO₂], and at ambient (aT) or elevated (eT) air temperature. Data points (filled circles) are the average (±SE) A–C_i curve measured at 25 ^°C for 2–3 biological replicates (n=3 except for eCaT, where n=2). Lines represent theoretical A–C_i curves modelled using the in vitro (solid lines) or in vivo (dashed lines) estimates of V_cmax as described in Table 1.

Fig. 3.

Effects of elevated CO₂ and temperature on the relationship between leaf photosynthesis and N. Relationships between leaf Rubisco (A) and PSII (B) contents and in vivo V_cmax (C) and J_max (D) with leaf N of E. globulus trees grown in whole-tree chambers. Values represent the means ±SE of 2–3 biological replicates for upper (open symbols) and lower (filled symbols) canopy leaves grown at aCaT (circles), eCaT (squares), aCeT (inverted triangles), and eCeT (upright triangles). The solid lines are linear fits of the data points.

Fig. 4.

Leaf nitrogen allocation of E. globulus in trees grown at ambient (aC) or elevated (eC) atmospheric [CO₂], and at ambient (aT) or elevated (eT) air temperature. Values represent averages of 2–3 biological replicates ±SE (n=3 except for eCaT, where n=2). The N percentages were calculated using data in Table 2 and Fig. 1. Other details are as described in the Materials and Methods.