OOS 50-5 - Rising CO2 shifts the balance between carbon and nutrient limitation of growth

Friday, August 10, 2012: 9:20 AM
A105, Oregon Convention Center
James D. Lewis, Louis Calder Center - Biological Station and Department of Biological Sciences, Fordham University, Armonk, NY
Background/Question/Methods

Atmospheric concentrations of carbon dioxide ([CO2]) over the past 650,000 years have ranged from 180 - 280 µmol mol-1, but in the past 150 years have risen rapidly to 389 µmol mol-1. This 40% increase is similar in magnitude to the increase in [CO2] projected for the 21st century, and understanding the factors that regulated plant responses to past increases in [CO2] may improve our ability to predict responses to future [CO2]. However, studies often observe proportionately larger growth and physiological responses to the transition from glacial to modern [CO2] than from modern to future [CO2]. This pattern may reflect several factors, including that the relative effect of short-term increases in [CO2] on photosynthesis in C3 plants is highest at low [CO2], declining with rising [CO2]. This decline in photosynthetic response as [CO2] rises may reflect a transition from [CO2] limitation to nutrient limitation. In this study, the interactive effects of phosphorus supply (0.004 – 0.5 mM) and rising atmospheric [CO2] from glacial (280 µmol mol-1) to future (700 µmol mol-1) concentrations on growth and physiology were examined on Populus deltoides (cottonwood) seedlings grown under well watered conditions.

Results/Conclusions

Increasing [CO2] from glacial to modern increased growth 25% across P treatments, reflecting reduced [CO2] limitations to photosynthesis and increased Asat. Conversely, the growth response to future [CO2] was very sensitive to P supply. Future [CO2] increased growth 80% in the highest P supply but only 7% in the lowest P supply, reflecting P limitations to Asat, leaf area and LAR, compared to modern [CO2]. Changes in the maximal Rubisco-limited rate of photosynthesis (Vcmax) and stomatal conductance (gs) together accounted for 78% of the variation in Asat among [CO2] and P treatments, suggesting significant biochemical and stomatal controls on photosynthesis. These results suggest P demand will increasingly affect growth and photosynthesis in cottonwood as [CO2] continues to rise, reflecting a transition from CO2 limitation to nutrient limitation.