The measurement of soluble solids by refractometry is a widely used method to determine the quality of marketable plant parts. For instance, soluble solids concentration was used as a measure of sugar content in sugar cane (Mamet, 1999), sugar beet (Campbell, 2002), muskmelon (Beaulieu et al., 2003), tomato (Stommel et al., 2005), and table beet (Feller and Fink, 2004). In contrast to laboratory methods, refractometry measurements of soluble carbohydrates are both quick and inexpensive. Recently Wilson et al. (2002) and Drost (2005) suggested the use of a refractometer for measuring root soluble carbohydrates in asparagus. The authors stated that growth of spears and ferns during the crop's annual cycle is associated with a characteristic pattern of depletion and accumulation of soluble carbohydrates in storage roots. Detecting deviations from the normal pattern could help to diagnose crop problems, such as unwanted excessive depletion of carbohydrates incited by harvest periods that are too long. Frese and Dambroth (1987) noted that refraction readings from plant sap that contains different types of sugars must be interpreted with caution, because each sugar has a different refraction index. The estimation of total sugar content would be especially error prone if both sugar distribution of samples is not constant and the sugars differ strongly in their specific refractive index increment (SRI), which describes how much the refractive index of a solution changes with respect to the concentration of the solute.
It is not known whether the composition of and variability in storage carbohydrates of asparagus roots permits accurate assessment with a refractometer. The objective of our study was to determine how accurately soluble carbohydrates in storage roots of asparagus can be assessed using refractometry.
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