Quantification of photosynthetic inorganic carbon utilisation via a bidirectional stable carbon isotope tracer

The amount of bicarbonate utilised by plants is usually ignored because of limited measurement methods.Accordingly, this study quantified the photosynthetic assimilation of inorganic carbon（CO;and HCO;） by plants. The net photosynthetic CO;assimilation（PN）, the photosynthetic assimilation of CO;and bicarbonate（PN’）,the proportion of increased leaf area（fLA） and the stable carbon isotope composition(δ;C) of Orychophragmus violaceus（Ov） and Brassica juncea（Bj）under three bicarbonate levels（5, 10 and 15 mm NaHCO;）were examined to determine the relationship among PN,PN’ and fLA. PN’, not PN,changed synchronously with fLA.Moreover, the proportions of exogenous bicarbonate and total bicarbonate（including exogenous bicarbonate and dissolved CO;-generated bicarbonate） utilised by Ov were 2.27 % and 5.28 % at 5 mm bicarbonate, 7.06 % and13.28 % at 10 mm bicarbonate, and 8.55 % and 17.31 % at15 mm bicarbonate, respectively. Meanwhile, the proportions of exogenous bicarbonate and total bicarbonate utilised by Bj were 1.77 % and 3.28 % at 5 mm bicarbonate,2.11 % and 3.10 % at 10 mm bicarbonate, and 2.36 % and3.09 % at 15 mm bicarbonate, respectively. Therefore, the dissolved CO;-generated bicarbonate and exogenous bicarbonate are important sources of inorganic carbon for plants.