دانلود رایگان مقاله خواص تفاوت بافت در دیواره سلولی پلی ساکارید میوه گلابی

ABSTRACT

Cell walls from flesh, parenchyma cells, stone cells and skin were isolated from ripe and overripe Pyrus communis L. cv “De Cloche” using the phenol-buffer method. Pear polysaccharides were solubilized from cell walls by sequential extractions with aqueous solutions of ammonium oxalate, Na2CO3, and increasing concentration of NaOH, to explore overripening impact. Cell walls were also differentiated using MIR spectral data. Stone cells contained high levels of xylose and lignin while parenchyma cells had high levels of glucose, uronic acids and arabinose. Sequential extractions revealed that pear pectins had highly branched rhamnogalacturonans and were extremely methylated. Xylans were the main hemicelluloseespecially for stone cells. Cellulose represented about half of all cell walls. This heterogeneous composition of pear affected differently cell wall evolutions and properties. Thus, overripening involved a decrease in arabinose and a loss of pectic side chains mostly from parenchyma cells. Changes in hemicellulose and cellulose were minor.

4. Conclusion

Pear cell walls were very heterogeneous compared to other fruits due to the presence of two types of cells in the flesh: parenchyma cells and stone cells with different ultrastructures and compositions. The use of non destructive methods as infrared spectroscopy and scanning electron microscopy allowed a differentiation between different pear tissues. Chemically, the main difference was a higher pectin content (uronic acid, arabinose, rhamnose and galactose) in the parenchyma cells and a higher hemicellulose, cellulose and lignin in the stone cells. Thus, stone cells were harder and stiffer than parenchyma cells. The skin cell wall was also distinct, notably by its higher proportion of lignin and cellulose, and lower pectins. The clearest difference after overripening was found in the proportion of arabinose and in less extent galactose, which decreased in ammonium oxalate fraction of all overripe tissues, indicating the breakdown of arabinogalactan side chain presumably from rhamnogalacturonans. However, the (Arabinose + galactose) to rhamnose ratio decreased most in parenchyma cells (and whole flesh). Stone and skin were less affected by overripening due to their richness in cellulose and lignin. Therefore, for parenchyma cells (and whole flesh), the loss of arabinan/galactan side chains could modify physical properties of cell walls by increasing their pore size. Thus, the cell wall might become a much more open structure allowing enhanced accessibility of binding side for other polymers such as procyanidins, especially after overripening. (Brummell, 2006; Watrelot, Le Bourvellec, Imberty, & Renard, 2014). Acknowledgements The authors thank Isabelle Bornard (INRA, Avignon) for microscopy sessions, Line Touloumet, Marielle Boge and Romain Bott (INRA, Avignon) for their technical help. We gratefully acknowledge Henk Schols and Suzanne Broxterman (Wageningen University, Netherlands).We thank also Régis Aubry (Clécy, France) for the supply of pears. Marwa Brahem was supported by a PhD grant(RB3C-2476)from the Ministry of Higher Education and Scientific Research of Tunisia.