Changes in Non-structural Carbohydrates in Tulip Bulb Scales during Cold Treatment and Greenhouse Forcing

Abstract

The energy and carbon needs for early shoot growth in tulips are mainly provided by reserve carbohydrates in bulbs. The cold-treatment of bulbs before greenhouse forcing enhances the breakdown and remobilization of reserve carbohydrates in bulb scales, and is necessary for proper shoot growth and flowering in tulips. Tulip bulbs are known to contain both starch and fructans as reserve carbohydrates. We evaluated several extraction solvents, including ethanol and distilled water, and several extraction temperatures to accurately determine the amounts of different types of non-structural carbohydrates in tulip bulb scales. Extraction with distilled water resulted in excessive solubilization of starch. For example, extraction at 70 °C solubilized more than 80% of starch to glucan polymers. On the other hand, 80% ethanol at 70 °C extracted all soluble sugars including fructans with no apparent solubilization of starch. The changes in non-structural carbohydrates in the outermost bulb scale of tulip (Tulipa gesneriana L. `Frankfurt') during 12 weeks of cooling at 8.8 °C followed by 5 weeks of greenhouse forcing were determined. Starch was the major carbohydrate in bulb scales consisting of ≈70% of the dry weight at the beginning of cold treatment. Starch content per scale decreased slightly during cold treatment, but rapidly after transferring to greenhouse. Sucrose and soluble fructan content per scale increased during cold treatment, then decreased after transferring to greenhouse. Glucose content per scale remained fairly constant during cooling and greenhouse forcing, while fructose content increased in the greenhouse.