Cell wall changes in ripening kiwifruit: 13C solid state NMR characterisation of relatively rigid cell wall polymers

Cell wall material was isolated from the outer pericarp of kiwifruit at harvest and at several ripening stages following a postharvest ethylene treatment. Solid state 13C NMR spectra showed no evidence for changes in the nature of the cellulose crystallites or the polysaccharides adhering to crystallite surfaces even in cell wall material isolated from fruits in which cell wall dissolution was extreme. Nuclear spin relaxation experiments showed that pectin retained in the cell wall became ‘softened’ in the early stages of ripening, prior to solubilisation later in ripening. Chemical data showed that this ‘softening’ of pectin was not accompanied by any marked change in its chemical composition. Fruit firmness was directly proportional to the amount of non-cellulosic matter that remained sufficiently rigid to respond to the cross-polarisation NMR pulse sequence. The results support the idea that pectin solubilisation in ripening fruit may in part be a consequence of cell wall swelling rather than a direct cause of it.