Assimilation and release of internal carbon dioxide by woody plant shoots

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Abstract
This study was undertaken to determine whether tree stems can reassimilate internal CO2 produced by respiration or whether this CO2 is evolved and could possibly interfere with measurements of leaf gas exchange. Radioactive CO2 was added to the stem transpiration stream of slash pine seedlings (Pinus elliottii Engelm.) and the distribution of 14C studied in shoots and needles exposed to dark and light conditions.Photosynthesis decreases the amount of internal CO2 evolved. Large amounts of 14CO2 from the transpiration stream are incorporated into organic compounds of needles and stems, primarily into ethanol-soluble sugars and organic acids, and in time, small amounts of 14C occur in the ethanol-insoluble materials.These results indicate that respiratory CO2 transported in the transpiration stream of woody plants can be reused in photosynthesis or possibly other metabolic processes. Internal CO2 is also evolved to the atmosphere in large amounts, but related research indicates it diffuses primarily out of the stem tissue not the needles. The evolved CO2 supplied from stems does not significantly affect short term measurements of needle gas exchange in pine seedlings.

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