Root-derived bicarbonate assimilation in response to variable water deficit in Camptotheca acuminate seedlings.

Water deficit is one of the key factors that limits the carbon (C) assimilation and productivity of plants. The effect of variable water deficit on recently root-derived bicarbonate assimilation in Camptotheca acuminate seedlings was investigated. Three-month-old seedlings were subjected to three water regimes, well-watered (WW), moderate stress (MS), and severe stress (SS) induced by polyethyleneglycol, in conjunction with relatively high (H) and low (L) natural 13C-abundance of NaHCO3-labeled treatments in hydroponics for 14 days. The δ13C of the newly expanded leaves in H were generally more enriched in heavy isotopes than were those in L, indicative of the involvement of bicarbonate in aboveground tissues. The C isotope fractionation of newly expanded leaves relative to air (∆13Cair-leaves) ranged from 17.78 to 21.78‰ among the treatments. The ∆13Cair-leaves under the MS and SS treatments in H were both more negative than was that in L. A linear regression between Ci/Ca and ∆13Cair-leaves in both L and H were different from the theoretical regression. On the basis of the two end-member mixing model, the proportion of fixed CO2 supplied from bicarbonate contributing to the total photosynthetically inorganic C assimilation were 10.34, 20.05 and 16.60% under the WW, MS, and SS treatments, respectively. These results indicated that the increase in water deficit decreased the atmospheric CO2 gain but triggered a compensatory use of bicarbonate in C. acuminate seedlings.