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PLANT PHYSIOLOGY , Vol 115, Issue 2 593-598, Copyright © 1997 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Molecular Rigidity in Dry and Hydrated Onion Cell Walls
M. A. Ha, D. C. Apperley and M. C. Jarvis
Chemistry Department, Glasgow University, Glasgow G12 8QQ, Scotland, United Kingdom (M.-A.H., M.C.J.)
Solid-state nuclear magnetic resonance relaxation experiments can provide
information on the rigidity of individual molecules within a complex
structure such as a cell wall, and thus show how each polymer can
potentially contribute to the rigidity of the whole structure. We measured
the proton magnetic relaxation parameters T2 (spin-spin) and T1p
(spin-lattice) through the 13C-nuclear magnetic resonance spectra of dry
and hydrated cell walls from onion (Allium cepa L.) bulbs. Dry cell walls
behaved as rigid solids. The form of their T2 decay curves varied on a
continuum between Gaussian, as in crystalline solids, and exponential, as
in more mobile materials. The degree of molecular mobility that could be
inferred from the T2 and T1p decay patterns was consistent with a
crystalline state for cellulose and a glassy state for dry pectins. The
theory of composite materials may be applied to explain the rigidity of dry
onion cell walls in terms of their components. Hydration made little
difference to the rigidity of cellulose and most of the xyloglucan shared
this rigidity, but the pectic fraction became much more mobile. Therefore,
the cellulose/xyloglucan microfibrils behaved as solid rods, and the most
significant physical distinction within the hydrated cell wall was between
the microfibrils and the predominantly pectic matrix. A minor xyloglucan
fraction was much more mobile than the microfibrils and probably
corresponded to cross-links between them. Away from the microfibrils,
pectins expanded upon hydration into a nonhomogeneous, but much softer,
almost-liquid gel. These data are consistent with a model for the
stress-bearing hydrated cell wall in which pectins provide limited
stiffness across the thickness of the wall, whereas the cross-linked
microfibril network provides much greater rigidity in other directions.
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