What is the fate of xylem-transported CO2 in Kranz-type C4 plants?

Abstract

High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C3 plants exit leaves in the dark. In the light, C3 species use a small portion of xylem-transported CO2 for leaf photosynthesis. However, it is not known if xylem-transported CO2 will exit leaves in the dark or be used for photosynthesis in the light in Kranz-type C4 plants. Cut leaves of Amaranthus hypochondriacus were placed in one of three solutions of [NaH13CO3] dissolved in KCl water to measure the efflux of xylem-transported CO2 exiting the leaf in the dark or rates of assimilation of xylem-transported CO2* in the light, in real-time, using a tunable diode laser absorption spectroscope. In the dark, the efflux of xylem-transported CO2 increased with increasing rates of transpiration and [13CO2*]; however, rates of 13Cefflux in A. hypochondriacus were lower compared to C3 species. In the light, A. hypochondriacus fixed nearly 75% of the xylem-transported CO2 supplied to the leaf. Kranz anatomy and biochemistry likely influence the efflux of xylem-transported CO2 out of cut leaves of A. hypochondriacus in the dark, as well as the use of xylem-transported CO2* for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz-type C4 species over C3 species.