MARTIN, SUSAN S. USDA-ARS, 1701 Centre Avenue, Fort Collins CO 80526. - Late-season accumulation of sucrose and oligosaccharides in sugarbeet (Beta vulgaris L.).
In the U.S., sugarbeet usually is planted in the spring and harvested
as late as possible in the fall to allow maximum growth and yield
potential. Processing companies use polarimetry as a rapid means of
analysis of sucrose at the time of delivery of each harvested
truckload of sugarbeets, and grower payments are based on their
sugarbeets' sucrose content as well as tonnage. In some years,
processing laboratory sucrose data appear to show an unexpected,
rather sudden late-season increase in sucrose percentage in the roots.
Much confusion has existed about this apparent late-season
accumulation of sucrose, particularly with regard to the effects of
temperature and precipitation. A phytochemical and physiological
analysis of the potential for accumulation of sucrose and several
related oligosaccharides shows that the accumulation of raffinose
(galactosyl sucrose) and higher galactosyl homologs can contribute
significantly to the optical rotation of sugarbeet extracts in late
season. Raffinose synthesis involves the action of a cold-induced
enzyme, galactinol synthetase, providing a rational basis for the
widely-held (though incorrect) belief that "a light frost causes
sugaring-up," or a rapid and significant accumulation of sucrose
in the root. Thus, a plausible phytochemical explanation of
"apparent" late-season sucrose accumulation now is
possible.
Key words: Beta vulgaris, galactinol synthetase, polarimetry, raffinose, sucrose