Carbon isotope composition of biochemical fractions isolated from leaves of Bryophyllum daigremontianum berger, a plant with crassulacean acid metabolism: Some physiological aspects related to CO2 dark fixation.

Isotope analysis of the biochemical fractions isolated quantitatively from young and mature leaves of Bryophyllum daigremontianum Berger have been carried out before and after a dark period of accumulation of organic acids. The mature leaf is enriched in (13)C compared to the young leaf. The δ(13)C values of the different leaf constituents vary between the δ(13)C values of C4 plants (-11‰) and those of C3 plants (-27‰). During the dark period, the two types of leaves store organic acids with δ(13)C values of ≃-15‰ and lose insoluble sugars, including starch with a δ(13)C value of ≃-12‰. Furthermore, young leaves store phosphorylated compounds with δ(13)C values of ≃-11‰ and lose weakly polymerised sugars with δ(13)C values of ≃-18‰. These results led to the formulation of a hypothesis of the origin of the two substrates of β-carboxylation: phosphoenolpyruvate arises from the glycolytic breakdown of the insoluble sugars rich in (13)C, and the major portion of the CO2 is the result of the complete breakdown (respiration) of the soluble sugars rich in (12)C. The existence of two independent sugar pools leads to the assumption that there are two separate glycolytic pathways. The (13)C enrichment of the stored products of the young leaves in the day seems to be the result of a weak discrimination for (13)C by ribulose diphosphate carboxylase, which reassimilates to a great extent the CO2 released from malate accumulated in the night.