Photosynthetic responses of cottonwood seedlings grown in glacial through future atmospheric [CO2] vary with phosphorus supply

Plants often exhibit proportionately larger photosynthetic responses to the transition from glacial to modern [CO2] than from modern to future [CO2]. Although this pattern may reflect increased nutrient demand with increasing [CO2], few studies have examined the role of nutrient supply in regulating responses to the range of [CO2] from glacial to future [CO2]. In this study, we examined the effects of P supply (0.004-0.5 mM) on photosynthetic responses of Populus deltoides (cottonwood) seedlings to glacial (200 μmol mol-1), modern (350 μmol mol-1) and future (700 μmol mol-1) [CO2]. The Asat (light-saturated net photosynthetic rates at the growth [CO2]) response to future [CO2] decreased with decreasing P supply such that there was no response at the lowest P supply. However, P supply did not affect Asat responses to an increase from glacial to modern [CO2]. Photosynthetic capacity [e.g., final rubisco activity, apparent, maximal Rubisco-limited rate of photosynthesis (Vcmax), apparent, maximal electron transport-limited rate of photosynthesis (J max)], stomatal conductance (gs) and leaf P generally increased with increasing P supply but decreased with increasing [CO 2]. Measures of carbohydrate sink capacity (e.g., leaf mass per unit leaf area, leaf starch) increased with both increasing P supply and increasing [CO2]. Changes in Vcmax and gs together accounted for 78% of the variation in Asat among [CO2] and P treatments, suggesting significant biochemical and stomatal controls on photosynthesis. However, Asat responses to increasing [CO 2] did not reflect the changes in the carbohydrate sink capacity. These results have important implications because low P already constrains responses to increasing [CO2] in many ecosystems, and our results suggest that the P demand will increasingly affect Asat in cottonwood as [CO2] continues to increase.