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Light use efficiency in a non-nutrient limiting environment.
Klimas, Christie*,1, Potosnak, Mark2, Murthy, Ramesh1, Griffin, Kevin3, 1 Biosphere 2 Center, Oracle, AZ, USA2 N.C.A.R., Boulder, CO, USA3 Columbia University, Palisades, NY, USA
ABSTRACT- This experiment examined leaf level photosynthetic responses of Populus deltoides trees grown in the Intensive Forestry Mesocosm at the Biosphere 2 Center, to light at different canopy positions under two CO2 treatments: ambient (43 Pa) and elevated (120 Pa) CO2 conditions. A fully developed shade (interior) and sun (exterior) leaf was chosen at three canopy levels (high, middle, upper) for 5 trees in each CO2 treatment. PPF (photosynthetic photon flux) sensors were attached to branches adjacent to each leaf. Light curves coupled with maximum photosynthesis (Asat) measurements were done 3 times over a six-week experiment. After photosynthesis measurements, LMA and leaf nitrogen content were determined for the sample leaves. Maximum rates of photosynthesis were significantly higher under elevated CO2 conditions with a concurrent increase in daytime respiration. Photosynthesis was significantly higher in middle and upper canopy and exterior sunlit leaves under both ambient and elevated CO2 conditions. Vertical position also had a significant effect on Rday with leaves in the upper canopy having higher respiration values, and this effect was enhanced under elevated CO2. While elevated CO2 resulted in overall increased quantum efficiency (AQE) the effect of vertical position and shading on AQE showed no significant change between CO2 treatments. Mid-canopy leaves showed a significantly higher AQE than upper and lower canopy leaves in both CO2 treatments and this was mirrored by a lower light compensation point. The leaves at the low canopy position under elevated CO2 showed a significantly lower light compensation point than the ambient low leaves. Nitrogen allocation also differed between the CO2 treatments with the ambient bay having higher percent leaf nitrogen content. These leaf level enhancements translated into greater accumulated biomass in the elevated CO2 treatment. Elevated CO2 resulted in a significant improvement in light utilization irrespective of vertical profile and incident light intensity.
Key words: light, Biosphere 2 Center, nitrogen, photosynthesis