Carbon isotope composition of biochemical fractions isolated from leaves ofBryophyllum daigremontianumberger, a plant with crassulacean acid metabolism: Some physiological aspects related to CO2dark fixation

摘要

Isotope analysis of the biochemical fractions isolated quantitatively from young and mature leaves ofBryophyllum daigremontianumBerger have been carried out before and after a dark period of accumulation of organic acids. The mature leaf is enriched in13C compared to the young leaf. The δ13C values of the different leaf constituents vary between the δ13C values of C4plants (-11‰) and those of C3plants (-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 in13C, and the major portion of the CO2is the result of the complete breakdown (respiration) of the soluble sugars rich in12C. The existence of two independent sugar pools leads to the assumption that there are two separate glycolytic pathways. The13C enrichment of the stored products of the young leaves in the day seems to be the result of a weak discrimination for13C by ribulose diphosphate carboxylase, which reassimilates to a great extent the CO2released from malat