CARBOHYDRATE METABOLISM OF VITIS VINIFERA: HEMICELLULOSE

Abstract

The more recent studies on the composition of hemicellulose have led to other theories as to its physiological function besides those of reserve food and structural reinforcement. O'Dwyer (31) has suggested that since hemicellulose is closely related to pectins it serves as an intermediary form in a synthesis of lignin from pectic compounds. The support for this theory is founded on the supposed decrease of pectin during the period of tissue maturation and on the apparently successful attempts to convert pectins to hemicelluloses by mild alkaline hydrolysis (5), by hydrolysis with water under pressure (21), and by mild oxidation (28). The last reaction is presumably analogous to conversion in situ in the plant. Although it has been possible to demonstrate the transformation of pectin into hemicellulose, there is no support for the further transformation of hemicellulose into lignin (32). Since the structure of lignin has not been established, the theory of the transformation of hemicellulose into lignin must be considered as speculative. Kerr and Bailey (17) in their microchemical investigations of the cell wall, moreover, found no reason to believe that pectins disappear with cell maturation. A number of investigators have isolated a small amount of pectin substances from mature wood. The apparent percentage decrease in pectin accompanying maturation appears to be the result of an increasing proportion of other constituents. Difficulties of extraction engendered by mechanical protection or perhaps by a physico-chemical union afforded by the later deposits may possibly have decreased the analytical yield of pectin. Hence positive evidence that the absolute amount of pectin decreases is lacking. Large variations of any of the constituents, notably that of the labile carbohydrate reserves, cause an inverse change in the percentage of other components. This is illustrated by the hemicellulose content of the fruit of the apple (13, 24), the peach (27), and the pear (22). In each case with maturation a decreasing hemicellulose content was found on a percentage basis which led to the opinion that hemicellulose is utilized as a reserve food in the development of the fruit. Where the hemicellulose content has been expressed on a single fruit basis, however, there is a steady increase in the