The structure of starch from seeds and leaves of the fructan-accumulating ryegrass, Lolium temulentum L.

Abstract

Starch granules isolated from leaves and seeds of Lolium temulentum L. were roughly spherical with an irregular perimeter having mean diameters of 1.7 and 4.1 μm respectively. On a dry mass basis, 47 % of the seed and maximally 4 % of the leaf were starch. Starch structures were deduced from a combination of data obtained using acid-hydrolysis/Smith degradation, glycosyl-linkage analysis/GC-MS, iodine complex formation, isoamylase digestion, high performance size exclusion chromatography and high performance anion exchange chromatography/pulsed amperometry. The mean degree of polymerisation (DP) of seed starch was 4800 ( M r = 778 kDa) and consisted entirely of a highly branched, amylopectin-like polymer with an average chain length of DP = 46 ± 6 and an average of 104 chains per molecule. Leaf starch was 98.8 % by mass of a highly-branched, amylopectin-like polymer of DP = 5850 ( M r 947 kDa) containing on average, 136 chains of mean DP = 43 ± 6. The remaining 1.2 % of the leaf starch was isoamylase-resistant, of high M r (5850 kDa) and was designated amylose. Both starches were unusual in that the chain lengths were longer by a factor of 2, than is normal for amylopectins and also because they contained little or no long 1,4-linked chains, as determined by isoamylase digestion. HPAEC of isoamylase fragments from leaf starch revealed a bi-modal distribution of branch chain lengths which was not evident in the seed starch. With the exceptions of this bi-modality and the trace of amylose in the leaf starch, the structure of the starches from the two organs were very similar. Seed starch exhibited an anomalous reaction with iodine. The chromogen was deep blue with a λ max = 608 nm, indicative of the presence of long amylose chains. On digestion with isoamylase, all of the mass of starch was converted to low molecular mass with an iodine λ max shifted to 567 nm. Hence, short 1,4-linked glucan chains which were contiguous through 1,6-linkages formed blue chromogens and can give the appearance of amylose.