Contribution and consequences of xylem-transported CO₂ assimilation for C₃ plants

Traditionally, leaves were thought to be supplied with CO₂ 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 CO₂ that is assimilated, vs simply lost to transpiration. Cut leaves of Populus deltoides and Brassica napus were placed in either KCl or one of three [NaH13CO₃] solutions dissolved in water to simultaneously measure the assimilation and the efflux of xylem-transported CO₂ exiting the leaf across light and CO₂ response curves in real-time using a tunable diode laser absorption spectroscope. The rates of assimilation and efflux of xylem-transported CO₂ increased with increasing xylem [13CO₂*] and transpiration. Under saturating irradiance, rates of assimilation using xylem-transported CO₂ accounted for c. 2.5% of the total assimilation in both species in the highest [13CO₂*]. The majority of xylem-transported CO₂ is assimilated, and efflux is small compared to respiration. Assimilation of xylem-transported CO₂ comprises a small portion of total photosynthesis, but may be more important when CO₂ is limiting.