Composition and Structure of Starch from Taproots of Contrasting Genotypes of Medicago sativa L.

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Abstract
The objective of this study was to examine the composition and branch chain lengths of alfalfa (Medicago sativa L.) taproot starch during starch utilization and reaccumulation in response to defoliation. Genotypes were propagated vegetatively and well-estblished plants were sampled at defoliation and at weekly intervals thereafter. Starch granules from root tissues were dispersed in dimethyl sulfoxide and starch components separated using gel permeation chromatography. Root starches also were debranched enzymatically, and branch chain lengths were examined. Results indicate that, irrespective of starch concentration, starch from taproots of the high starch genotype was composed of approximately 80% high molecular weight starch with I2-KI absorbance characteristics similar to amylopectin. The remaining 20% of the starch was low molecular weight with I2-KI absorbance characteristics similar to amylose. Starches of the low starch genotype contained approximately 85% high molecular weight polysaccharide at high root starch concentrations (> 50 grams per kilogram). At low root starch concentrations (> 10 grams per kilogram), starch from the low starch genotype had nearly equal proportions of low and high molecular weight polysaccharide. The I2-KI absorbance properties of the low molecular weight starches from roots of the low starch genotype indicated that some branching may be present. The distribution of chain lengths from amylopectin did not change during starch degradation and reaccumulation for the high starch genotype. In the low starch genotype, the proportion of low molecular weight branches having a degree of polymerization between 1 and 30 was decreased at the very low starch concentrations observed on the 14th day of regrowth. Higher concentrations and/or quantities of starch in roots of the high starch genotype were not associated with greater rate of herbage growth, when compared to the low starch genotype.