Carbohydrate fractionation and elongation of lupin hypocotyl cell walls : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Botany at Massey University
The relationship between extensibility, growth rate and carbohydrate composition in different sections of lupin hypocotyl has been investigated. Although significant differences in extensibility were found, the carbohydrate composition of elongating and non-elongating regions were similar when delignified tissue was examined. However, it was subsequently found that the delignification removed all of the wall hydroxyproline, most of the arabinose, and much galactose and that all of these were higher in non-elongating than in elongating hypocotyl. The acid conditions of delignification caused about half of the loss of the sugars but did not cause the loss of hydroxyproline. Extraction of the hypocotyl cell walls with guanidinium thiocyanate and other denaturants, both before and after treatment with dilute acid or sodium methoxide in methanol did not dissolve the hydroxyproline, indicating that compounds containing this amino acids are sprobably covalently linked to insoluble wall constituents other than through acid labile arabinofuranose-hydroxyproline or ester links alone. 10% KOH extracted most of the wall hydroxyproline and hemicellulose largely as non-dialysable material. The hemicellulose thus extracted may be fractionated into hemicelluloses A and B and the latter into linear 1-4 linked polysaccharides and branched polysaccharides. Most of the hydroxyproline containing polymer is co-precipitated with the linear 1-4 linked hemicellulose-B arabinoxylan. When cell walls from elongating and non-elongating hypocotyl sections were compared the hemicellulose-B arabinoxylan fraction from the non-elongating wall had a much higher proportion or arabinose, galactose and hydroxyproline than the same polymer from elongating wall. Extraction of cell walls with 10% KOH at O°C removed about two thirds of the hemicellulose-B but little hydroxyproline. Subsequent treatment with 10% KOH at room temperature removed most of the hydroxyproline and remaining hemicellulose-B. The hemicellulose-B removed at room temperature showed the greatest increases in arabinose and galactose accompanying cessation of elongation. The polysaccharide extracted at 0°C is mainly xylan while that removed at room temperature contains large amounts of galactose and arabinose. The release of galactose at room temperature was accompanied by destruction or serine and appeared to parallel β-elimination of galactosylserine The kinetics of release of arabinose and galactose at room temperature differed. The above and other results are discussed particularly in relation to wall structure and a tentative model for the extensin-polysacchsride complex of lupin hypocotyl cell walls is proposed.