High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C plants exit leaves in the dark. In the light, C species use a small portion of xylem-transported CO for leaf photosynthesis. However, it is not known if xylem-transported CO will exit leaves in the dark or be used for photosynthesis in the light in Kranz-type C plants. Cut leaves of Amaranthus hypochondriacus were placed in one of three solutions of [NaH CO ] dissolved in KCl water to measure the efflux of xylem-transported CO exiting the leaf in the dark or rates of assimilation of xylem-transported CO * in the light, in real-time, using a tunable diode laser absorption spectroscope. In the dark, the efflux of xylem-transported CO increased with increasing rates of transpiration and [ CO *]; however, rates of C in A. hypochondriacus were lower compared to C species. In the light, A. hypochondriacus fixed nearly 75% of the xylem-transported CO supplied to the leaf. Kranz anatomy and biochemistry likely influence the efflux of xylem-transported CO out of cut leaves of A. hypochondriacus in the dark, as well as the use of xylem-transported CO * for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz-type C species over C species.
Determining the fate of CO respired in woody tissues is necessary to understand plant respiratory physiology and to evaluate CO recycling mechanisms. An aqueous C-enriched CO solution was infused into the stem of 3-4 m tall trees to estimate efflux and assimilation of xylem-transported CO via cavity ring-down laser spectroscopy and isotope ratio mass spectrometry, respectively. Different tree locations (lower stem, upper stem and leafy shoots) and tissues (xylem, bark and leaves) were monitored in species with tracheid, diffuse- and ring-porous wood anatomy (cedar, maple and oak, respectively).
Laboratory of Plant Ecology, Department of Plants and Crops, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium.
Respired CO2 in woody tissues can build up in the xylem and dissolve in the sap solution to be transported through the plant. From the sap, a fraction of the CO2 can either be radially diffuse to the atmosphere or be assimilated in chloroplasts present in woody tissues. These processes occur simultaneously in stems and branches, making it difficult to study their specific dynamics.
Traditionally, leaves were thought to be supplied with CO for photosynthesis by the atmosphere and respiration. Recent studies, however, have shown that the xylem also transports a significant amount of inorganic carbon into leaves through the bulk flow of water. However, little is known about the dynamics and proportion of xylem-transported CO that is assimilated, vs simply lost to transpiration.
