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

Area of Science:

• Plant Physiology
• Biochemistry
• Plant Biology

Background:

• Traditionally, atmospheric CO2 was considered the sole source for leaf photosynthesis.
• Recent research indicates xylem transports inorganic carbon into leaves via water flow.
• The fate of this xylem-transported carbon (assimilation vs. transpiration loss) remains unclear.

Purpose of the Study:

• To quantify the assimilation and efflux of xylem-transported inorganic carbon in plant leaves.
• To investigate the dynamics of xylem-transported CO2 under varying conditions.
• To determine the proportion of total photosynthesis contributed by xylem-transported carbon.

Main Methods:

• Used cut leaves of Populus deltoides and Brassica napus.
• Applied solutions of [NaH13CO3] to measure 13CO2 assimilation and efflux.
• Employed tunable diode laser absorption spectroscopy for real-time measurements under light and CO2 response curves.

Main Results:

• Xylem-transported CO2 assimilation and efflux increased with xylem [13CO2] and transpiration rates.
• Under saturating light, xylem-transported CO2 contributed approximately 2.5% to total assimilation.
• The majority of xylem-derived inorganic carbon was assimilated, with minimal efflux compared to respiration.

Conclusions:

• Xylem-transported inorganic carbon is primarily assimilated by leaves.
• While a small fraction of total photosynthesis, xylem-transported CO2 may play a crucial role when atmospheric CO2 is limited.
• This finding highlights a previously underestimated carbon pathway in plants.