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

We investigated the effect of temperature and irradiance on leaf respiration (R, non-photorespiratory mitochondrial CO₂ release) of snow gum (Eucalyptus pauciflora Sieb. ex Spreng). Seedlings were hydroponically grown under constant 20°C, controlled-environment conditions. Measurements of R (using the Laisk method) and photosynthesis (at 37 Pa CO₂) were made at several irradiances (0-2,000 μol photons m^-2 S^-1) and temperatures (6°C-30°C). At 15°C to 30°C, substantial inhibition of R occurred at 12 μmol photons m^-2 s^-1, with maximum inhibition occurring at 100 to 200 μmol 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°C to 10°C was lower than that at 15°C to 30°C. Moreover, although R was inhibited by low irradiance at 6°C to 10°C, it recovered with progressive increases in irradiance. The temperature sensitivity of R was greater in darkness than under bright light. At 30°C and high irradiance, light-inhibited rates of R represented 2% of gross CO₂ uptake (V(c)), whereas photorespiratory CO₂ release was approximately 20% of V(c). If light had not inhibited leaf respiration at 30°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.