No acclimation: instantaneous responses to temperature maintain homeostatic photosynthetic rates under experimental warming across a precipitation gradient in Ulmus americana.

Past research has shown that plants possess the capacity to alter their instantaneous response of photosynthesis to temperature in response to a longer-term change in temperature (i.e. acclimate). This acclimation is typically the result of processes that influence net photosynthesis (A _net), including leaf biochemical processes such as the maximum rate of Rubisco carboxylation (V _cmax) and the maximum rate of photosynthetic electron transport (J _max), stomatal conductance (g _s) and dark respiration (R _d). However, these processes are rarely examined in the field or in concert with other environmental factors, such as precipitation amount. Here, we use a fully factorial warming (active heating up to +4 °C; mean = +3.1 °C) by precipitation (−50 % ambient to 150 % ambient) manipulation experiment in an old-field ecosystem in the north-eastern USA to examine the degree to which Ulmus americana saplings acclimate through biochemical and stomatal adjustments. We found that rates of A _net at ambient CO₂ levels of 400 µmol mol⁻¹ (A ₄₀₀) did not differ across climate treatments or with leaf temperatures from 20 to 30 °C. Canopy temperatures rarely reached above 30 °C in any treatment, suggesting that seasonal carbon assimilation was relatively homeostatic across all treatments. Assessments of the component processes of A ₄₀₀ revealed that decreases in g _s with leaf temperature from 20 to 30 °C were balanced by increases in V _cmax, resulting in stable A ₄₀₀ rates despite concurrent increases in R _d. Photosynthesis was not affected by precipitation treatments, likely because the relatively dry year led to small treatment effects on soil moisture. As temperature acclimation is likely to come at a cost to the plant via resource reallocation, it may not benefit plants to acclimate to warming in cases where warming would not otherwise reduce assimilation. These results suggest that photosynthetic temperature acclimation to future warming will be context-specific and that it is important to consider assimilatory benefit when assessing acclimation responses. [ABSTRACT FROM AUTHOR]