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

Abstract

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 ¹³C compared to the young leaf. The δ¹³C values of the different leaf constituents vary between the δ¹³C values of C₄ plants (-11‰) and those of C₃ plants (-27‰). During the dark period, the two types of leaves store organic acids with δ¹³C values of ≃-15‰ and lose insoluble sugars, including starch with a δ¹³C value of ≃-12‰. Furthermore, young leaves store phosphorylated compounds with δ¹³C values of ≃-11‰ and lose weakly polymerised sugars with δ¹³C 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 ¹³C, and the major portion of the CO₂ is the result of the complete breakdown (respiration) of the soluble sugars rich in ¹²C. The existence of two independent sugar pools leads to the assumption that there are two separate glycolytic pathways. The ¹³C enrichment of the stored products of the young leaves in the day seems to be the result of a weak discrimination for ¹³C by ribulose diphosphate carboxylase, which reassimilates to a great extent the CO₂ released from malate accumulated in the night.