Seminar Details

What is keeping respired CO2 within tree stems?




Boaz Hilman - Dept. of Earth Sciences, The Hebrew University of Jerusalem


Tree stem respiration is an important, yet poorly understood, component of the terrestrial carbon cycle. CO2 efflux that has been used to estimate respiration rate was questioned, due to CO2 dissolution and transport in the xylem stream. The ratio of CO2 efflux to O2 influx is defined as ARQ (apparent respiratory quotient). Since carbohydrates are probably the prime respiration substrate in plants, ARQ in tree stems is predicted to be 1.0. ARQ measured using stem chambers was found to be well below 1.0 in various ecosystems in Israel, USA, Panama, and Spain. Low ARQ values may indicate transport of respired CO2 in the xylem stream. However, incubations of detach stem tissues in Panama and Spain that yielded similar ARQ values to the stem chamber values, and little influence of natural variations in transpiration stream on ARQ, suggest that the removal of the respired CO2 is not via the xylem stream. Alternative pathway that may remove respired CO2 is local re-fixation. In preliminary experiment we have found indications to fixation by the enzyme phosphoenolpyruvate carboxylase (PEPC). Although PEPC was reported to occur in tree stems, it part in carbon cycling in C3 trees is unknown.

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