The purpose of this project is to determine whether trees can internally recycle and reuse the CO2 released from root and stem respiration. The CO2 concentration in the xylem of tree stems is very high, relative to atmospheric CO2 concentration, often in the range of 5 to 10%, and sometimes exceeding 20% (200,000 umol mol-1). This CO2 appears to be derived largely from woody tissue respiration, although a portion may also come from the soil. Using a mass balance approach to account for both internal and external fluxes of CO2 released by stem respiration, it has been reported that as much as 50% of the carbon respired by woody tissues over a 24 hour period can remain within trees. This CO2 dissolves in xylem water and can be transported upward in the transpiration stream. However, the ultimate fate of the transported CO2 is unknown. This project addresses the idea that much of the respired CO2 remaining in the stem may be transported upward and refixed by photosynthetic cells in the stem, branches and leaves, providing a previously unrecognized mechanism to recover and recycle carbon. Recovery of a portion of the carbon released by respiration of woody tissues would improve the carbon economy and growth of trees and would help compensate for the carbon costs of constructing and maintaining a large stem and branch support system for the leaves. Two objectives will be addressed in this study: 1) Determine how much internally-transported CO2 is refixed by photosynthetic cells in stems, branches and leaves, and 2) Develop a diurnal carbon balance for trees that accounts for photosynthetic use of both internal and external sources of CO2. Two experiments will address these objectives using a variety of methodology including stable isotopes, CO2 microelectrodes, and CO2 gas exchange techniques.
There are a number of benefits to society from this project. First, results will improve our understanding of how carbon is used by trees. If this project successfully demonstrates for the first time that there is an internal pathway for recycling carbon in trees, it will revise our understanding of the carbon cycle in trees and forest ecosystems. Since forests are a key component of the global carbon cycle, this has particular relevance now. Second, this project will promote teaching, training and learning in a number of ways. Targeted recruitment of a graduate student will help improve participation in science education by underrepresented groups. New K-12 educational experiences will be created by integrating this project with the educational programs of the Global Forest Foundation and the American Forest Foundation. Third, infrastructure for research and education will be enhanced by stimulating the use of new instrumentation in plant biology, in particular, the use of microelectrodes for measuring CO2 concentrations and innovative use of stable isotope techniques.
