What is the fate of xylem‐transported CO 2 in Kranz‐type C 4 plants?

Summary High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C 3 plants exit leaves in the dark. In the light, C 3 species use a small portion of xylem‐transported CO 2 for leaf photosynthesis. However, it is not known if xylem‐transported CO 2 will exit leaves in the dark or be used for photosynthesis in the light in Kranz‐type C 4 plants. Cut leaves of Amaranthus hypochondriacus were placed in one of three solutions of [NaH 13 CO 3 ] dissolved in KCl water to measure the efflux of xylem‐transported CO 2 exiting the leaf in the dark or rates of assimilation of xylem‐transported CO 2 * in the light, in real‐time, using a tunable diode laser absorption spectroscope. In the dark, the efflux of xylem‐transported CO 2 increased with increasing rates of transpiration and [ 13 CO 2 *]; however, rates of 13 C efflux in A. hypochondriacus were lower compared to C 3 species. In the light, A. hypochondriacus fixed nearly 75% of the xylem‐transported CO 2 supplied to the leaf. Kranz anatomy and biochemistry likely influence the efflux of xylem‐transported CO 2 out of cut leaves of A. hypochondriacus in the dark, as well as the use of xylem‐transported CO 2 * for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz‐type C 4 species over C 3 species.