Thermal acclimation of photosynthesis in black spruce [Picea mariana (Mill.) B.S.P.]

We investigated the thermal acclimation of photosynthesis and respiration in black spruce seedlings [Picea mariana (Mill.) B.S.P.] grown at 22/14°C [low temperature (LT)] or 30/22°C [high temperature (HT)] day/night temperatures. Net CO₂ assimilation rates (A_net) were greater in LT than in HT seedlings below 30°C, but were greater in HT seedlings above 30°C. Dark and day respiration rates were similar between treatments at the respective growth temperatures. When respiration was factored out of the photosynthesis response to temperature, the resulting gross CO ₂ assimilation rates (A_gross) was lower in HT than in LT seedlings below 30°C, but was similar above 30°C. The reduced A _gross of HT seedlings was associated with lower needle nitrogen content, lower ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) maximum carboxylation rates (V_cmax) and lower maximum electron transport rates (J_max). Growth treatment did not affect V _cmax: J_max. Modelling of the CO₂ response of photosynthesis indicated that LT seedlings at 40°C might have been limited by heat lability of Rubisco activase, but that in HT seedlings, Rubisco capacity was limiting. In sum, thermal acclimation of A_net was largely caused by reduced respiration and lower nitrogen investments in needles from HT seedlings. At 40°C, photosynthesis in LT seedlings might be limited by Rubisco activase capacity, while in HT seedlings, acclimation removed this limitation.