Leaf respiration of snow gum in the light and dark. Interactions between temperature and irradiance

Abstract

We investigated the effect of temperature and irradiance on leaf respiration (R, non-photorespiratory mitochondrial CO(2) release) of snow gum (Eucalyptus pauciflora Sieb. ex Spreng). Seedlings were hydroponically grown under constant 20 degrees C, controlled-environment conditions. Measurements of R (using the Laisk method) and photosynthesis (at 37 Pa CO(2)) were made at several irradiances (0-2,000 micromol photons m(-2) s(-1)) and temperatures (6 degrees C-30 degrees C). At 15 degrees C to 30 degrees C, substantial inhibition of R occurred at 12 micromol photons m(-2) s(-1), with maximum inhibition occurring at 100 to 200 micromol photons m(-2) s(-1). Higher irradiance had little additional effect on R at these moderate temperatures. The irradiance necessary to maximally inhibit R at 6 degrees C to 10 degrees C was lower than that at 15 degrees C to 30 degrees C. Moreover, although R was inhibited by low irradiance at 6 degrees C to 10 degrees C, it recovered with progressive increases in irradiance. The temperature sensitivity of R was greater in darkness than under bright light. At 30 degrees C and high irradiance, light-inhibited rates of R represented 2% of gross CO(2) uptake (v(c)), whereas photorespiratory CO(2) release was approximately 20% of v(c). If light had not inhibited leaf respiration at 30 degrees C and high irradiance, R would have represented 11% of v(c). Variations in light inhibition of R can therefore have a substantial impact on the proportion of photosynthesis that is respired. We conclude that the rate of R in the light is highly variable, being dependent on irradiance and temperature.

Figure 1

Example of the effect of irradiance on net CO₂ exchange (μmol CO₂ m⁻² s⁻¹) versus p_i of a single leaf at three temperatures: 6°C (A), 15°C (B), and 25°C (C). Measurements were also conducted at 10°C, 20°C, and 30°C (not shown). The symbols represent the irradiances under which each set of measurements was made (in μmol photons m⁻² s⁻¹). Lines represent the linear regressions at each irradiance.

Figure 2

Effect of temperature on Γ_*. ○, Γ_* values calculated using the intercept of three linear regressions of net CO₂ exchange data versus p_i (e.g. Fig. 1) for leaves of 20°C-grown plants exposed to 15°C, 20°C, 25°C, and 30°C (e.g. Fig. 1, B and C). The three linear regressions used to calculate Γ_* were for 100, 200, and 400 μmol photons m⁻² s⁻¹ for all temperatures except 30°C, where 200, 400, and 800 μmol photons m⁻² s⁻¹ were used. Values represent the mean of three individual leaves (±se); where the se values are not visible, they are smaller than the shown symbol. The erroneous Γ_* values for leaves exposed to 6°C and 10°C are shown for comparison (●); it was not possible to accurately calculate the Γ_* values at 6°C and 10°C because the common regression intercept for measurements at three irradiances yielded a negative R value. The solid line represents the temperature dependence of Γ_* of spinach calculated from the data of Jordan and Ogren (1984) using our estimate of Γ_* at 25°C (4.31 ± 0.04 Pa).

Figure 3

Relationship between R and irradiance at various temperatures. Values are ±se; n = 3. Values of R were calculated using the linear regressions of net CO₂ exchange versus P_i at each irradiance (e.g. Fig. 1), our estimate of Γ_*25 (4.31 Pa), and the temperature dependence of Γ_* given in Equation 3.

Figure 4

Determining the effect of different Γ_* values on the relationship between R and irradiance at 6°C using the temperature dependence of Γ_* given in Equation 3 (±se n = 3). Three different estimates of Γ_* at 6°C were used in the calculations.

Figure 5

Effect of irradiance on the relationship between temperature and R. Values for 0 to 100 μmol photons m⁻² s⁻¹ are shown in A and C, whereas B and D show values for 200 to 2,000 μmol photons m⁻² s⁻¹. Values of R were calculated using the linear regressions of net CO₂ exchange versus P_i at each irradiance (e.g. Fig. 1), our estimate of Γ_*25 (4.31 Pa), and the temperature dependence of Γ_* given in Equation 3. A and B show the absolute rates of leaf respiration, while C and D show rates in the light as a percentage of those in darkness.

Figure 6

Relationship between temperature and the Rubisco carboxylation rate (ν_c) (A), the ratio of photorespiratory CO₂ release to Rubisco carboxylation (B), and the ratio of non-photorespiratory respiration to Rubisco carboxylation (R/ν_c) (C). Rates of ν_c, photorespiration, and R at each temperature and irradiance were calculated as described in the “Materials and Methods.” The line in B is fitted to all of the data; variations in photorespiration at a particular temperature were due to variations in p_i.