13C Labeling kinetics of sucrose in glumes indicates significant refixation of respiratory CO2 in the wheat ear

Abstract

Photosynthesis by the vegetative structures of the ear (i.e. glumes) of wheat (Triticum aestivum L.) may draw on two sources of CO₂: atmospheric CO₂ and CO₂ originating from respiration within the ear. We exposed wheat plants to a changed C isotope composition (δ) of one of the two sources, atmospheric CO₂, to assess the contributions of atmospheric and respiratory CO₂ to ear photosynthesis by following the labeling kinetics of water-soluble carbohydrates (WSC; fructose, glucose, sucrose, fructan) in glumes. Experiments included sampling during diurnal cycles and after extended exposures (7 and 14 d). The labeling kinetics of sucrose in the flag leaf and grains was also determined. Significant diurnal changes in sucrose content (depletion during dark and accumulation during light periods) were observed in flag leaves and in glumes. In flag leaves (but not in glumes) the sucrose accumulating during the light period had a d that was close to the value expected if atmospheric CO₂ (with changed δ) was the sole source of CO₂ for photosynthesis. The δ of sucrose in glumes did not saturate after extended exposure to the labeling CO₂ indicating the utilization of unlabeled (respiratory) CO₂ for photosynthesis (i.e. refixation). Short-term labeling indicated that 73% of the sucrose accumulating in glumes after the start of labeling came from fixation of respiratory CO₂. Stable C isotope discrimination during glume photosynthesis is discussed.