Influence of starch deficiency on photosynthetic and post-photosynthetic carbon isotope fractionations

Abstract

Carbon isotope (13C) fractionations occurring during and after photosynthetic CO 2 fixation shape the carbon isotope composition (δ 13 C) of plant material and respired CO 2 . However, responses of 13C fractionations to diel variation in starch metabolism in the leaf are not fully understood. Here we measured δ 13 C of organic matter (δ 13 COM), concentrations and δ 13 C of potential respiratory substrates, δ 13 C of dark-respired CO 2 (δ 13 CR), and gas exchange in leaves of starch-deficient plastidial phosphoglucomutase (pgm) mutants and wild-type plants of four species (Arabidopsis thaliana, Mesembryanthemum crystallinum, Nicotiana sylvestris, and Pisum sativum). The strongest δ 13 C response to the pgm-induced starch deficiency was observed in N. sylvestris, with more negative δ 13 COM, δ 13 CR, and δ 13 C values for assimilates (i.e. sugars and starch) and organic acids (i.e. malate and citrate) in pgm mutants than in wild-type plants during a diel cycle. The genotype differences in δ 13 C values could be largely explained by differences in leaf gas exchange. In contrast, the PGM-knockout effect on post-photosynthetic 13C fractionations via the plastidic fructose-1,6-bisphosphate aldolase reaction or during respiration was small. Taken together, our results show that the δ 13 C variations in starch-deficient mutants are primarily explained by photosynthetic 13C fractionations and that the combination of knockout mutants and isotope analyses allows additional insights into plant metabolism.