Chemical Composition of Turnip Roots Stored or Intermittently Grown at Low Temperature

Field and greenhouse studies were conducted to investigate the effects of low temperature on the starch, sugar, ascorbic acid, and glucosinolate (GS) concentration in turnip (Brassica rapa ssp. rapifera (Metzg.) Sinsk) roots. Field-harvested roots were stored at 0C for 2 and 4 weeks. In the greenhouse, plants were grown at 0 to 12C for parts of 11 days before harvest. Cold-stored roots decreased in both starch and total sugar concentration (sucrose, fructose, and glucose) when compared to freshly harvested roots. Greenhouse-grown plants subjected to low temper- atures had roots with a similar starch content but with a higher concentration of total sugars than the control. In both experiments, the cold treatments induced a slight but significant increase in root sucrose concentration. The ascorbic acid concentration of roots was not affected by low temperature. In both the field and greenhouse studies, low temperature did not change the total concentration of the eight major GSs identified in peeled root "and peel tissues, but did alter the concentration of specific GSs. Turnips are popularly believed to retain good eating quality when stored under optimal conditions (0C, 95% relative hu- midity). However, preliminary investigations have indicated that long-term storage of large roots of certain turnip cultivars, such as 'White Lady', resulted in appreciable flavor change (V.I.S. and Y. K., unpublished data). This observation is not surprising since variation in storage potential commonly exists among cul- tivars of other horticultural crops (Lidster et al., 1988; Ministry of Agriculture, Fisheries and Food, 1979) and low-temperature- induced quality changes involving carbohydrates and GSs have been shown to occur in various cruciferous crops (Berard and Chong, 1984; Guffy and Hicks, 1984; Peirce, 1987; Suzuki and Cutcliffe, 1981). In spite of the economic importance of turnip, no reports have focused on the influence of low temperature on root quality. Low temperatures, besides affecting the sensory characteristics of the root, might also change other compounds that could ul- timately lead to poor acceptance by consumers. This study was undertaken to determine the influence of low temperature on the starch, sugar, ascorbic acid, and GSs of a) field-grown tur- nip roots subjected to short-term refrigerated storage, and b) roots of intact greenhouse-grown plants before harvest. The first experiment was conducted since large turnip roots (> 350 g) are occasionally harvested, detopped, and stored for short periods before being marketed. Since we obtained conflicting reports from turnip growers whether low night temperatures before har- vest influence the quality of turnip roots, the second experiment was initiated.