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 C-abundance of NaHCO-labeled treatments in hydroponics for 14 days. The δC 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 (∆C) ranged from 17.78 to 21.78‰ among the treatments. The ∆C under the MS and SS treatments in H were both more negative than was that in L. A linear regression between Ci/Ca and ∆C 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 CO 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 CO gain but triggered a compensatory use of bicarbonate in C. acuminate seedlings.

Abstract:To determine whether the bicarbonate in karst limestone soil could be used as a new carbon resource for biomass production by the catalysis of carbonic anhydrase (CA), a simulative karst drought stress experiment was designed and performed. Three plants used for biofuel energy,, and Euphorbia lathyris L. (El), were grown under simulated karst drought stress. In response to drought stress, the photosynthesis of the three energy plants was inhibited, but their CA activity increased. The hypothesis was confirmed by plant physiological and stable isotope techniques. The obtained results showed that plant biomass was produced with atmospheric CO 2 as well as bicarbonate under drought stress. Bicarbonate use was proportional to the CA activity of the plants. With high CA activity over a long period, El had the highest proportional bicarbonate use compared to Ov and Bj, reaching 26.95%. Additionally, a new method is proposed for the screening of plants grown for energy in karst habitats.