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The effect of bicarbonate (HCO3−) on the growth and development of plants varies by species. To better understand inorganic carbon and nitrogen assimilation changes of karst-adaptable plants under different HCO3− treatments, we conducted experiments on seedlings and in vitro plantlets of Orychophragmus violaceus (Ov). We found that the vital photosynthesis potential (as measured by net photosynthetic rate, actual photochemical efficiency of photosystem-II, photochemical quenching coefficient, and the instantaneous carbon isotope ratio of 3-phosphoglycerate) was consistent under different HCO3− treatments of Ov. Bicarbonate’s lack of effect on carbon assimilation of Ov may be related to carbonic anhydrase in Ov converting HCO3− to H2O and CO2. In this way, Ov could prevent HCO3− ion toxicity and high pH from harming its growth and development under HCO3− stress. This study also found that high HCO3− concentrations could promote nitrogen assimilation and utilization of Ov through changes in related indexes (foliar nitrogen isotope fractionation ratio, stable nitrogen isotope assimilation ratio, foliar stable nitrogen isotope fractionation, nitrate nitrogen utilization efficiency, and nitrate utilization share) under different HCO3− treatments. Bicarbonate has different effects on photosynthesis and on inorganic nitrogen assimilation of Ov, which may be connected to photosynthesis providing electrons for nitrate/nitrite reduction through the photosynthetic chain.

Natural nitrogen isotope composition (δ15N) is an indicator of nitrogen sources and is useful in the investigation of nitrogen cycling in organisms and ecosystems. δ15N is also used to study assimilation of inorganic nitrogen. However, the foliar δ15N of intact plants, which is a consequence of nitrate assimilation occurring in the roots and shoots, is not suited for studying nitrate assimilation in cases where nitrate is the sole nitrogen source. In this study, Orychophragmus violaceus (Ov) and Brassica napus (Bn) plantlets, in which nitrate assimilation occurred in the leaves, were used to study the relationship between foliar δ15N and nitrate assimilation. The plantlets were grown in vitro in culture media with different nitrate concentrations, and no root formation occurred for the plantlets during the multiplication stage. Nitrogen isotope fractionation occurred in both the Ov and the Bn plantlets under all treatments. Furthermore, the foliar nitrogen content of both the Ov and Bn plantlets increased with increasing nitrate concentration. Foliar nitrogen isotope fractionation was negatively correlated with foliar nitrogen content for both the Ov and Bn plantlets. Our results suggest that the foliar nitrogen isotope fractionation value could be employed to evaluate nitrate assimilation ability and leaf nitrate reductase activity. Moreover, high external nitrate concentrations could contribute to improved foliar nitrogen content and enhanced nitrate assimilation ability.