**Figure S1.** Representative plot of net CO_{2} assimilation (*A*_{net}, *µ*mol CO_{2} m^{−2} s^{−1}) versus irradiance (*µ*mol photons m^{−2} s^{−1}) to illustrate the Kok effect and related calculations. Data are from a well-watered, ambient 'CO_{2}'-grown replicate plant measured in February 2009 of the experiment. Solid symbols show measured rates of *A*_{net} over the 0–100 *µ*mol photons m^{−2} s^{−1} range, with rates of leaf respiration in darkness (*R*_{dark} = 1.17 *µ*mol CO_{2} m^{−2} s^{−1}) shown. The break from linearity at irradiances below 40 *µ*mol photons m^{−2} s^{−1} (dotted line) is shown, with a linear regression fitted (r^{2} = 0.992 for this replicate) to values between 40 and 100 *µ*mol photons m^{−2} s^{−1} to estimate apparent rates of leaf *R* in the light (*R*_{light} ‘apparent’, ◊ = 0.21 *µ*mol CO_{2} m^{−2} s^{−1}) at the *y*-axis intercept. Actual rates of *R*_{light} (□ = 0.22 *µ*mol CO_{2} m^{−2} s^{−1}) that take into account changes in internal CO_{2} concentration (*c*_{i}) that occur as irradiance declined (Kirschbaum & Farquhar 1987) are also shown 'assuming infinite internal conductance (*g*_{i})'. For this replicate, measurements of *A*_{net} were also made under saturating irradiance (1800 *µ*mol photons m^{−2} s^{−1}), yielding rates of *A*_{sat} of 21.96 *µ*mol CO_{2} m^{−2} s^{−1}, with the underlying rates of carboxylation (*V*_{c}) and oxygenation (*V*_{o}) being 26.32 and 8.28 *µ*mol CO_{2} m^{−2} s^{−1}, respectively (calculated using Eqns 2 and 3 in the main text). At an irradiance of 100 *µ*mol photons m^{−2} s^{−1}, the corresponding values were: *A*_{100} = 4.24 *µ*mol CO_{2} m^{−2} s^{−1}, *V*_{c100} = 5.08 *µ*mol CO_{2} m^{−2} s^{−1}, and *V*_{o100} = 1.22 *µ*mol CO_{2} m^{−2} s^{−1}. Finally, for this replicate, actual *R*_{light} values when assuming *g*_{i} = 0.012*A*_{sat} were 0.26 *µ*mol CO_{2} m^{−2} s^{−1} (Eqn 5 in main text). For parameters above, *Γ** was assumed to be 36.9 and 38.6 *µ*L L^{−1} when assuming *C*_{i} and *C*_{c}, respectively (Von Caemmerer *et al*. 1994).

**Figure S2.** Rates of leaf respiration in the light (*R*_{light}, *µ*mol CO_{2} m^{−2} s^{−1}) calculated assuming that internal conductance (*g*_{i}) is infinite plotted against the corresponding *R*_{light} values assuming an ‘estimated’ *g*_{i} (*g*_{i} = 0.012*A*_{sat}; Evans & Von Caemmerer 1996). For the latter, we calculated *c*_{c} at any given irradiance according to: *c*_{c} = *c*_{i} − (*A*_{net}/*g*_{i}), with *R*_{light} then being estimated via application of the Kirschbaum & Farquhar (1987) correction procedure after replacing *c*_{i} with *c*_{c}. For these calculations, *Γ** was assumed to be 36.9 and 38.6 *µ*L L^{−1} when assuming *C*_{i} and *C*_{c}, respectively (Von Caemmerer *et al*. 1994). Data are shown for all growth 'CO_{2}' and water availability treatments, using data collected over the 4 month experimental period (December–March). Values shown are individual replicates. The dashed line shows the 1:1 relationship. See Supporting Information Table S2 for treatment averages of the ‘estimated’ *g*_{i} values and corresponding *R*_{light} values.

**Figure S3.** Rates of photosynthetic electron transport (*J*, *µ*mol m^{−2} s^{−1}) calculated assuming that internal conductance (*g*_{i}) is infinite plotted against the corresponding *J*-values assuming an ‘estimated’ *g*_{i} (*g*_{i} = 0.012*A*_{sat}; Evans & Von Caemmerer 1996). For the latter, we calculated *c*_{c} at any given irradiance according to: *c*_{c} = *c*_{i} − (*A*_{net}/*g*_{i}), with *J* then being estimated after replacing *c*_{i} with *c*_{c}. For these calculations, *Γ** was assumed to be 36.9 and 38.6 *µ*L L^{−1} when assuming *C*_{i} and *C*_{c}, respectively (Von Caemmerer *et al*. 1994). Data are shown for all growth 'CO_{2}' and water availability treatments, using data collected over the 4 month experimental period (December–March). Values are shown for individual replicates of *J* calculated at two irradiances (100 and 1800 *µ*mol m^{−2} s^{−1} PFFD). The dashed line shows the 1:1 relationship.

**Figure S4.** Rates of carboxylation (*V*_{c}) and oxygenation (*V*_{o}) by Rubisco (*µ*mol m^{−2} s^{−1}) calculated assuming that internal conductance (*g*_{i}) is infinite plotted against the corresponding *J*-values assuming an ‘estimated’ *g*_{i} (*g*_{i} = 0.012*A*_{sat}; Evans & Von Caemmerer 1996). For the latter, we calculated *c*_{c} at any given irradiance according to: *c*_{c} = *c*_{i} − (*A*_{net}/*g*_{i}), with *J* then being estimated via application of the Kirschbaum & Farquhar (1987) correction procedure after replacing *c*_{i} with *c*_{c}. For these calculations, *Γ** was assumed to be 36.9 and 38.6 *µ*L L^{−1} when assuming *C*_{i} and *C*_{c}, respectively (Von Caemmerer *et al*. 1994). Data are shown for all growth 'CO_{2}' and water availability treatments, using data collected over the 4 month experimental period (December–March). Values are shown for individual replicates calculated at two irradiances (100 and 1800 *µ*mol m^{−2} s^{−1} PFFD). The lines show the 1:1 relationship.

**Figure S5.** Plots of rates of leaf respiration in the light (*R*_{light}) '(a) and (b)' and the ratio of *R*_{light} to that in darkness (*R*_{light}/*R*_{dark}) '(c) and (d)' against corresponding rates of photorespiration (i.e. oxygenation rate by Rubisco) at 1800 *µ*mol m^{−2} s^{−1} PPFD (*V*_{o1800}) '(a) and (c)', the carboxylation rate by Rubisco at 1800 *µ*mol m^{−2} s^{−1} PPFD (*V*_{c1800}) '(b) and (d)' in ambient (circles) and elevated (triangles) CO_{2} treatments. Open symbols represent the droughted plants and closed symbols represent the well-watered plants. The regression equation and associated statistics are summarized in Table 3. Values shown were calculated assuming infinite internal conductance (*g*_{i}). Similar plots of *R*_{light} and *R*_{light}/*R*_{dark} ratios against rates of *V*_{c} and *V*_{o} 100 *µ*mol m^{−2} s^{−1} PPFD are shown in Fig. 4 (main text). Data shown are for the two months where *R*_{light} and light-saturated photosynthesis were measured on the same leaf (i.e. February and March only). In December and January, data on light-saturated photosynthesis were measured using adjacent leaves to those used for *R*_{light} measurements; consequently, only data from February and March are shown in this figure.

**Figure S6.** Comparison of linear relationships between *R*_{light} and photorespiration (i.e. oxygenation rate of Rubisco) at 100 *µ*mol m^{−2} s^{−1} (*V*_{o100}) for calculations made assuming that internal conductance (*g*_{i}) is infinite (left-hand panel) and assuming an ‘estimated’ *g*_{i} (*g*_{i} = 0.012*A*_{sat}; Evans & Von Caemmerer 1996). For the latter, we calculated *C*_{c} at any given irradiance according to: *C*_{c} = *C*_{i} − (*A*_{net}/*g*_{i}), with *R*_{light} then being estimated via application of the Kirschbaum & Farquhar (1987) correction procedure after replacing *C*_{i} with *C*_{c}. Similarly, *V*_{o100} values were calculating using *J-*values that employed *C*_{i} or *C*_{c} values. For these calculations, *Γ** was assumed to be 36.9 and 38.6 *µ*L L^{−1} when assuming *C*_{i} and *C*_{c}, respectively (Von Caemmerer *et al*. 1994). Data are shown for the four growth 'CO_{2}' and water availability treatments, using data collected over the 4 month experimental period (December–March). Values shown are individual replicates.

**Table S1.** Means ± standard error per month and per treatment (*n* = 3) of *Eucalyptus saligna* leaves for the following variables: area-based rates of leaf respiration the light (*R*_{light}) and dark (*R*_{dark}), carboxylation rates of Rubisco at 100 and 1800 *µ*mol m^{−2} s^{−1} PPFD (*V*_{c100} and *V*_{c1800}), oxygenation rates of Rubisco at 100 and 1800 *µ*mol m^{−2} s^{−1} PPFD (*V*_{o100} and *V*_{o1800}), net photosynthesis at 100 and 1800 *µ*mol m^{−2} s^{−1} PPFD (*A*_{100} and *A*_{sat}), ratios of leaf *R* (in the light and in the dark) to ‘gross’ rates of light-saturated photosynthesis (where ‘gross’ *A* = *A*_{sat} plus *R*_{light}), total soluble sugars, mass-based leaf nitrogen concentration (N), mass-based leaf phosphorus concentrations (P) and leaf mass per surface area (LMA). Note: gas exchange rates are for leaves measured in the mid-late morning at the prevailing ambient air temperature of each month.

**Table S2.** Overview of the effect of assumed internal conductance (*g*_{i}) on calculated rates of leaf *R* in the light (*R*_{light}; *µ*mol CO_{2} m^{−2} s^{−1}) for trees grown under ambient/elevated 'CO_{2}' and well-watered/droughted conditions. *R*_{light}/*R*_{light} ratios are shown in parentheses. *R*_{light} values were calculated on the assumption of an infinite internal conductance (*g*_{i}) (i.e. *C*_{i} = *C*_{c}) and ‘estimated’ *g*_{i} (i.e. *g*_{i} = 0.012*A*_{sat}) (Evans & von Caemmerer, 1996). For the latter, we calculated *C*_{c} at any given irradiance according to: *C*_{c} = *C*_{i} − (*A*_{net}/*g*_{i}), with *R*_{light} then being estimated via application of the Kirschbaum & Farquhar (1987) correction procedure after replacing *C*_{i} with *C*_{c}. For these calculations, *Γ** was assumed to be 36.9 and 38.6 *µ*L L^{−1} when assuming *C*_{i} and *C*_{c}, respectively (Von Caemmerer *et al*. 1994). Data shown are averages over the 4 month experimental period (December–March).

**Table S3.** Results from linear regression analysis of relationships between area-based respiration and month-to-month variations in leaf temperature (*T*_{leaf}). *R*_{light} represents the non-photorespiratory respiration in illuminated leaves (assuming infinite internal conductance) and *R*_{dark} represents the mitochondrial respiration in dark-adapted leaves. See Fig. 2 in main text for treatment/monthly average data.

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