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

The unloading of sucrose in the apical part of the hypocotyl of Ricinus com munis L. seedlings was measured by C-13-nuclear magnetic resonance (NMR) sp ectroscopy. The cotyledons of the seedling were immersed in 5 mM Mes buffer containing 100 mM C-13-labeled sucrose. At intervals of 70-90 min, (C-NMR) -C-13 spectra with broadband decoupling and nuclear Overhauser enhancement were acquired in vivo. The spectra showed growing C-13-resonances of the la beled positions in the sucrose molecule reaching steady-state labeling with in 7-8 h. The specific C-13 labeling of sucrose in the G(1)-position change d from 0.38 in the supplied sucrose solution to 0.16 in the sucrose extract ed from the hypocotyl piece at the end of the experiment (13 h). Labeling o f 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 parenchy ma, respectively, and less to continuous starch degradation and conversion to sucrose. The report gives an example of the application of C-13-NMR spec troscopy in assimilate allocation studies.