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Megan Bartlett - Carbon Dioxide and Plant Physiology

Harvard University, Cambridge, MA

Abstract of Research

Elevated carbon dioxide induces a larger growth rate and total biomass in most plant species, and increases the efficiency of photosynthesis and nutrient use in the plant and changes nutrient availability in the environment. As a result, rising carbon dioxide levels are expected to increase vegetation growth, but it is unclear whether changes in nutrient status will be able to sustain these greater growth rates. For example, nitrogen undergoes substantial changes in demand in plants exposed to elevated CO2. Increased photosynthetic efficiency allows a reduction in levels of photosynthetic enzyme rubisco, so the nitrogen that it was using can be reallocated to other plant functions, decreasing tissue nitrogen levels. However, it is not clear whether this decrease is due to reduced need for nitrogen, or an inability to take up adequate nitrogen; and whether it is primarily caused by a change in the plant's uptake rate, or a loss of nitrogen availability in the environment.
     In order to investigate these questions, rates of ammonium uptake were measured for plants of the marsh species Scirpus olneyi grown for 3 weeks in elevated and ambient CO2, and in low (0.2 mmol/l) and high levels (1 mmol/L) of the phytotoxin hydrogen sulfide (H2S). H2S is a microbial metabolic product whose concentration has been shown to increase in response to elevated CO2, and to interfere with ammonium uptake in several marsh species. The experimental plants were grown hydroponically in a low salinity seawater solution to eliminate CO2 effects on soil or soil microbes.  They were then tested for uptake rate (assessed as a 24 hour depletion in the nitrogen concentration of a 1 ppm nutrient solution), rubisco activity (as Vcmax, the level of photosynthesis at different CO2 concentrations), and growth rate.
     There was a significant decrease in uptake rates for the elevated CO2 plants, which matched their reduction in rubisco activity (Vcmax). These results showed that plants are able to reduce their uptake in accordance with their demand, independently of environmental factors. There was no H2S effect, although there was an interaction between high sulfide/elevated CO2 that increased rubisco activity and lessened the decrease in uptake rate. It appears that H2S, independent of CO2 level, does not have a strong interaction with ammonium uptake of this species. There was no significant effect on growth rate for any of the treatments. The mechanisms for the CO2-induced reduction, the apparent H2S tolerance, and high sulfide/elevated CO2 interaction are unknown, but they are hypothesized to involve changes in root energy status and assimilation enzyme activity.

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Funding provided by the National Science Foundation – Research Experience for Undergraduates (REU)