Does growth irradiance affect temperature dependence and thermal acclimation of leaf respiration? Insights from a Mediterranean tree with long-lived leaves

Understanding the response of leaf respiration (R) to changes in irradiance and temperature is a prerequisite for predicting the impacts of climate change on plant function and future atmospheric CO₂ concentrations. Little is known, however, about the interactive effects of irradiance and temperature on leaf R. We investigated whether growth irradiance affects the temperature response of leaf R in darkness (R_dark) and in light (R _light) in seedlings of a broad-leaved evergreen species, Quercus ilex. Two hypotheses concerning R_dark were tested: (1) the Q ₁₀ (i.e. the proportional increase in R per 10°C rise in temperature) of leaf R_dark is lower in shaded plants than in high-light-grown plants, and (2) shade-grown plants exhibit a lower degree of thermal acclimation of R_dark than plants exposed to higher growth irradiance. We also assessed whether light inhibition of R_light differs between leaves exposed to contrasting temperatures and growth irradiances, and whether the degree of thermal acclimation of R_light is dependent on growth irradiance. We showed that while growth irradiance did impact on photosynthesis, it had no effect on the Q₁₀ of leaf R _dark. Growth irradiance had little impact on thermal acclimation when fully expanded, pre-existing leaves were exposed to contrasting temperatures for several weeks. When R_light was measured at a common irradiance, R_light/R_dark ratios were higher in shaded plants due to homeostasis of R_light between growth irradiance treatments and to the lower R_dark in shaded leaves. We also showed that R_light does not acclimate to the same degree as R_dark, and that R _light/R_dark decreases with increasing measuring and growth temperatures, irrespective of the growth irradiance. Collectively, we raised the possibility that predictive carbon cycle models can assume that growth irradiance and photosynthesis do not affect the temperature sensitivity of leaf R_dark of long-lived evergreen leaves, thus simplifying incorporation of leaf R into such models.