Sucrose unloading in the hypocotyl of the Ricinus communis L. seedling measured by 13 C-nuclear magnetic resonance spectroscopy in vivo

Abstract

The unloading of sucrose in the apical part of the hypocotyl of Ricinus communis L. seedlings was measured by 13C-nuclear magnetic resonance (NMR) spectroscopy. The cotyledons of the seedling were immersed in 5 mM Mes buffer containing 100 mM 13C-labeled sucrose. At intervals of 70–90 min, 13C-NMR spectra with broadband decoupling and nuclear Overhauser enhancement were acquired in vivo. The spectra showed growing 13C-resonances of the labeled positions in the sucrose molecule reaching steady-state labeling within 7–8 h. The specific 13C labeling of sucrose in the G1-position changed from 0.38 in the supplied sucrose solution to 0.16 in the sucrose extracted from the hypocotyl piece at the end of the experiment (13 h). Labeling of starch (and other insolubles) in the hypocotyl piece was ca. 0.10. It is proposed that the decreased specific labeling of unloaded sucrose is mostly due to the separate local pools of sucrose in the cortex and pith parenchyma, respectively, and less to continuous starch degradation and conversion to sucrose. The report gives an example of the application of 13C-NMR spectroscopy in assimilate allocation studies.