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Variation in sugar content and composition of carrot storage roots at harvest and during storage
Scientia Horticulturae 85 (2000) 1±19
Variation in sugar content and composition of carrot storage roots at harvest and during storage Terhi Suojala* Agricultural Research Centre of Finland (MTT), Plant Production Research, Horticulture, Toivonlinnantie 518, FIN-21500 PiikkioÈ, Finland Accepted 25 October 1999
Abstract Changes in the soluble sugar content of carrot storage roots (cvs. Fontana and Panther) at harvest and during storage were studied at an experimental site and on vegetable farms in three successive years. The later the harvest, the higher the content of soluble sugars, especially sucrose, tended to be, but on farms in 1996, there was a clear decline in dry matter and sugar content due to frost injury in ®eld experiments. Even in uninjured carrot stands, changes in sugar content and composition did not coincide with a cessation of yield increase or changes in storability. Total sugar and sucrose contents at the beginning of the harvest period were higher in the colder year, 1996, than in the warmer years, 1995 and 1997. Despite some differences from earlier studies, the compositional changes observed here during storage followed the general pattern of increasing hexose and decreasing sucrose contents. Frost injury, however, resulted in post-harvest changes in sugars which were different from earlier results. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Daucus carota; Frost; Maturity; Quality; Sucrose
1. Introduction The major ecological function of the carrot storage root is to act as a temporary reserve for the production of a ¯owering stem after appropriate stimuli (Hole, 1996). The existence of storage compounds is also the primary reason for the utilisation of the carrot in human nutrition. In commercial production, these * Tel.: 358-2-477-2200; fax: 358-2-477-2299. E-mail address: terhi.suojala@mtt.® (T. Suojala)
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 3 3 - 8
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T. Suojala / Scientia Horticulturae 85 (2000) 1±19
compounds provide the chemical energy reserves needed for long-term storage and shelf life. Most of the compounds stored in the vacuoles of the parenchyma cells are soluble sugars. These account for 34±70% of the dry weight of the storage root (Goris, 1969a; Ricardo and Sovia, 1974; Nilsson, 1987a). Sucrose is the predominant transport and storage sugar at maturity (Daie, 1984), but its proportion is affected by genotype and environment (Goris, 1969a; Phan and Hsu, 1973; Ricardo and Sovia, 1974; Nilsson, 1987a). At 30±50 days after sowing, the sucrose concentration starts to increase faster than that of hexoses (fructose, glucose), resulting in a higher sucrose to hexose ratio (SteingroÈver, 1981; Hole and McKee, 1988). Several authors have attempted to de®ne the changes in sugar content and the sucrose to hexose ratio as a measure of biochemical or physiological maturity (Goris, 1969b; Phan and Hsu, 1973; Fritz and Weichmann, 1979; Le Dily et al., 1993). The usage of sugar composition as the indicator of maturity has been questioned. For carrots grown in Sweden (Nilsson, 1987a), sucrose accumulation continued up to the ®nal harvest, and Nilsson concluded that ``maturity'' should only be considered as a reduction in metabolic activity in an environment no longer favourable for growth. Rosenfeld (1998) noted that, neither the ratio of sugars nor any other chemical variable indicated the presence of maturity and could not be used as an indicator of the optimal stage of harvest. Physiological maturity has been considered the stage at which a plant or plant part continues to develop even if detached (Watada et al., 1984). It has been noted that, it is not usually observed in organs like roots, foliage, stems and tubers. Horticultural maturity refers to the stage of development at which a plant or a plant part can be used for a particular purpose (Watada et al., 1984). In carrot, maturity in this sense would require a satisfactory outer, sensory and nutritional quality combined with an adequate potential for storage and shelf life. The question is whether sugar composition bears any relation to the horticultural maturity of carrot. During storage, the common trend in sugar composition is for the hexose content to increase and the sucrose content to decrease, especially during the ®rst months (Phan et al., 1973; Nilsson, 1987b; OldeÂn and Nilsson, 1992; Le Dily et al., 1993). Most experiments record only minor changes in the total sugar content. Nilsson (1987b) found that compositional changes occurred irrespective of harvest date and the differences were maintained throughout the storage period. The objective of this study was to investigate changes in the sugar content and composition of carrot during the harvest period and storage as chemical energy reserves and possible indicators of horticultural maturity. The results are discussed in terms of a northern climate, which is characterised by a relatively short growing period and a need for long-term storage.
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2. Materials and methods Material for analyses was obtained from experiments conducted at the Vegetable Experimental Site of the Agricultural Research Centre of Finland at KokemaÈki and on farmers' ®elds in southern Finland. Two cultivars were used in the experiments, Fontana F1 (Bejo Zaden, Netherlands), which is a late variety cultivated for the processing industry, and Panther F1 (Sluis and Groot, Netherlands), an earlier, fresh-market variety. At the Vegetable Experimental Site, a split-plot experiment with cultivar in whole plots and harvest time in subplots was conducted in 1996 and 1997. A randomised complete block design with four replicate blocks was used, but sugar analyses were performed on only three replicates. The farm experiments with cv. Fontana in 1995 and 1996 were located in the same production area, Huittinen. Farms with cv. Panther in 1996 and 1997 were located in two other areas, Forssa and Laitila. The number of farms and harvest dates are given in Table 1. In 1995, the farm was treated as a random block factor. In 1996 and 1997, each farm had its own randomised complete block design with three replicates. Samples for analysing sugar composition after storage in the 1995 and 1996 yields were stored in woven polypropylene bags (75 g mÿ2) in batches of approximately 10 kg. Carrots from different locations were kept in the same store, which differed in the two years. Storage conditions were as follows: in 1995±1996, temperature 0 0.58C, relative humidity 80±85%; in 1996±1997, temperature 0 18C, relative humidity 85±95%. Details of the experiments are given by Suojala (1999a,b). 2.1. Weather conditions Meteorological data were obtained from the nearest meteorological stations of the Finnish Meteorological Institute (Table 2). At KokemaÈki, the meteorological station is situated at the experimental site. Its data were also used for farms at Huittinen. Data from Jokioinen were used for farms at Forssa and data from Mietoinen for farms at Laitila. The distance between meteorological stations and the experimental ®elds did not exceed 40 km. The weather differed markedly from one experimental year to another (Table 2). 1995 and 1997 were warmer than the long-term average, but in 1996, the monthly mean temperatures were lower than the long-term average, except in August and October. Night frosts were exceptionally hard in 1996, and severe frost injured the carrot ®elds on 21st September, when the temperature at ground level dropped below ÿ88C at the measuring sites closest to the farm experiments. Plants were injured on farms but not at the Vegetable Experimental Site. In 1997, frosts did not cause damage before late October, when carrots in the last harvest at the Vegetable Experimental Site were exposed to permanent frost.
4
Coding of harvest
H1 H2 H3 H4 H5 H6
Vegetable Experimental Site, KokemaÈki
Farm experiments `Fontana' (8)
`Fontana' (5)
`Panther' (8)
`Panther' (8)
1995
1996
1996
1997
1996
1997
12 September 26 September 10 October
11 September 25 September 8 October 22 October
10±11 September 23±24 September 7±9 October 21±22 October
9±11 September 23±24 September 7±8 October 21±22 October
5 September 16 September 26 September 7 October 16 October 28 October
8 September 18 September 29 September 9 October 20 October 30 October
104±120
113±128
106
117
Growing time from sowing to H1 (days) 107±130 114±120
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Table 1 Harvest dates in different experiments (number of farms is given in parentheses)
Table 2 Monthly mean temperature and precipitation for sites and years of experiments compared with long-term averages KokemaÈki (618160 N, 228150 E)
1996
1961±1990
1995
9.4 14.3 15.8 14.2 9.4 4.7
8.7 16.1 15.2 14.4 10.3 8.0
35 47 80 83 65 58
74 90 42 72 29 74
Mean temperature (8C) May 8.8 June 13.1 July 13.9 August 17.0 September 8.3 October 6.4 Precipitation (mm) May 65 June 52 July 136 August 14 September 20 October 56
1997 7.7 16.1 17.8 17.8 10.0 2.4 16 101 141 44 78 47
1996
1997
8.5 13.2 14.1 16.7 8.3 6.9
7.8 16.3 17.9 17.9 10.5 2.6
57 49 111 26 27 52
14 85 91 17 121 52
Mietoinen (608380 N, 218520 E) 1961±1990
1996
9.5 14.4 15.8 14.2 9.6 5.0
8.9 13.1 14.5 17.8 9.5 7.4
33 48 71 77 65 54
65 37 93 17 26 49
1997 7.8 16.3 18.8 18.7 11.0 3.2 17 58 132 25 137 69
1961±1990 9.5 14.5 16.2 14.8 10.3 5.8 33 44 75 85 70 62
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Jokioinen (608490 N, 238300 E)
5
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2.2. Samples and analyses Samples for sugar analyses were taken at the time of harvest. In 1995 and 1996, samples were also taken after storage, usually three times in association with analyses of storage losses. For cv. Panther, only the yield of three farms was sampled twice during storage. In 1996 and 1997, 20 roots were sampled from each plot. In 1995, one sample comprised only ®ve carrots but the mean of three replicate samples was used in data analysis. Subsamples from each of 20 (15 in 1995) carrots were combined and grated, frozen and stored at ÿ208C until analysis. Soluble sugars were analysed at the Laboratory of Food Chemistry of the Agricultural Research Centre of Finland. Fructose, glucose and sucrose were assayed by gas±liquid chromatography (GLC) employing the method of Li and Schuhmann (1980) with the modi®cations of Haila et al. (1992). A Perkin±Elmer Autosystem GLC equipped with a ¯ame ionisation detector and a capillary column NB 30 (column length 25 m, ID 0.32 mm and ®lm thickness 0.25 mm) was employed. Total soluble sugars were calculated as the sum of fructose, glucose and sucrose. Results are expressed as percent of fresh weight. In addition, the ratio of sucrose to hexose (fructose glucose) is presented. 2.3. Statistical analysis In 1995, the last harvest was not performed on one of the farms. In 1996, samples were not taken from the last harvest of cv. Fontana in autumn. One observation was lacking from the farm experiments in 1996. The results were analysed by mixed models using the SAS MIXED procedure (Littell et al., 1996). The experiment at the Vegetable Experimental Site was analysed by a mixed model for split-plot design with cultivar, harvest time and their interaction as ®xed factors, and block, whole plot and subplot errors as random factors. In the farm experiments in 1995, the model included harvest time as a ®xed factor and farm as a random block factor. In 1996 and 1997, the experimental design was more complicated. Results of the two cultivars were analysed separately. The model for cv. Panther included production area, harvest time and their interaction as ®xed factors and experimental block, farm, interaction farm harvest time and error term as random factors. Block was nested within farm and farm within area. The model for cv. Fontana in 1996 was reduced from the previous model by omitting the effect of area and its interaction with harvest time. The effect of harvest time was further studied by estimating the difference between the nth harvest and the mean of the following harvests. The signi®cance of the estimated difference was investigated with the aid of 95% con®dence intervals (CI). If the 95% CI of the difference includes zero, the difference is not statistically signi®cant at the 5% level. Statistical analysis of the
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data for 1996 and 1997 is described in more detail by Suojala (1999a). For results after storage, the effect of storage time and its interactions were included in the models as described by Suojala (1999b). 3. Results 3.1. Effect of harvest time 3.1.1. Vegetable Experimental Site, KokemaÈki In 1996, the effect of harvest time was dependent on the cultivar (Fig. 1). Standard error of mean (SEM) was also determined for each case. Changes in fructose and glucose concentrations were small in both cultivars. The sucrose concentration increased, but the accumulation was more pronounced in cv. Panther. The total sugar content increased, especially in cv. Panther, whereas in cv. Fontana, the changes were not as clear. The sucrose to hexose ratio increased during the harvest period in cv. Panther, but did not change in cv. Fontana. The average concentrations of fructose and glucose were higher and sucrose concentration and sucrose to hexose ratio were lower in cv. Fontana than in cv. Panther. Sugars comprised 44±52% of dry matter in cv. Fontana and 42±53% in cv. Panther. In 1997, the total sugar contents of the two cultivars changed in different ways (Fig. 1). For cv. Fontana it did not change but for cv. Panther it increased signi®cantly after H4. The glucose concentration decreased slightly and the sucrose concentration increased clearly, resulting in a higher sucrose to hexose ratio towards the end of the harvest period. As in 1996, the cultivars differed in their sugar concentrations; cv. Fontana having a higher concentration of fructose, glucose and total sugars and a lower sucrose to hexose ratio than cv. Panther. Sugars accounted for a smaller proportion of dry matter than in the previous year: 42±48% in cv. Fontana and 37±43% in cv. Panther. 3.1.2. Farm experiments Ð cv. Fontana In 1995, the total sugar content did not change during the 4 week harvest period but the sugar composition did (Fig. 2): the fructose and glucose concentrations decreased and the sucrose concentration increased. The sucrose to hexose ratio, therefore, rose from 0.6 to 0.9 in 4 weeks. Sugars accounted for 47±48% of dry matter. In 1996, the changes in soluble sugars differed from those in 1995 (Fig. 2). The total sugar and sucrose contents decreased by about 1% unit between H2 and H3, but the fructose and glucose concentrations did not change during the harvest period. Therefore, the sucrose to hexose ratio fell from 1.6±1.7 in H1 and H2 to 1.1 in H3. At the same time, the proportion of sugars in dry matter declined from 53% in H1 and 52% in H2 to 45% in H3.
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Fig. 1. Contents of soluble sugars and the sucrose to hexose ratio (S/H ratio) at different harvest dates at the Vegetable Experimental Site at KokemaÈki in 1996 and 1997. Probability values are given for the effects of cultivar (C) and harvest time (H) and for their interaction (CH).
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Fig. 2. Contents of soluble sugars and the sucrose to hexose ratio (S/H ratio) at different harvest dates on vegetable farms (cvs. Fontana and Panther) in 1995±1997. Figures are means of 3±8 farms. Probability values are given for the effect of harvest time (H), and in cv. Panther, for the effect of area (A) and their interaction (AH).
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3.1.3. Farm experiments Ð cv. Panther In 1996, the changes were similar to those in cv. Fontana: the sucrose and total sugar contents decreased, especially after H2 (Fig. 2), but the changes differed slightly in the two production areas. At Forssa, the concentrations started to decrease after H2, whereas at Laitila, the concentrations decreased straight after H1 but there was no further change after H3. The average sucrose and total sugar contents were higher at Laitila than at Forssa, due to the higher concentrations in H1 and H2. The fructose and glucose concentrations were similar in both areas and decreased slightly during the harvest period. Owing to the drop in sucrose concentration, the sucrose to hexose ratio decreased, but at a slower rate at Forssa than at Laitila. At Forssa, sugars accounted for 47% of dry matter in H1 and 51% in H2, falling to 45% in H3 and 42% in H4. At Laitila, the drop was even larger: from 55% in H1 to 44% in H4. In 1997, the sucrose and total sugar contents increased similarly in both areas, especially between the ®rst two harvests (Fig. 2). The total sugar content did not change after H2, but the sucrose concentration increased up to H3 and the sucrose to hexose ratio up to H4. The fructose and glucose concentrations decreased up to H3 but the changes were slight. Sugars comprised 38% of dry matter in H1 and 46±48% in later harvests. 3.2. Variation in sugar content between years The effect of growing season on the total sugar content or sucrose to hexose ratio is dif®cult to assess due to the unusual weather in autumn 1996. The results from the ®rst harvest date, when the plants had not been exposed to frosts, show that the total sugar content and the sucrose to hexose ratio were higher in 1996 than in the warmer years, 1995 and 1997 (Fig. 2). The same trend applied to both cultivars. At the Vegetable Experimental Site, the same effect was observed in the total sugar content, which was, on average, lower in the warmer year, 1997 (Fig. 1). However, the sucrose to hexose ratio was higher in 1997 than in 1996 in both cultivars. As well as between years, the sugar content varied between farms. Differences in the total sugar content at H1 could be more than 2% units. The variation between farms was not related to the thermal time from sowing to harvest. 3.3. Effect of storage At KokemaÈki in 1996, the changes in the course of storage were dependent on cultivar and harvest time, and cultivars were analysed separately (Fig. 3). In cv. Fontana, the overall trend showed an increase in fructose and glucose contents, especially in H1, H2 and H3, and a decrease in sucrose and total sugar contents and the sucrose to hexose ratio. In cv. Panther, the interaction between harvest time and storage time was more pronounced: in H1 and H2, fructose and glucose
T. Suojala / Scientia Horticulturae 85 (2000) 1±19
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Fig. 3. Changes in sugar composition during storage at the KokemaÈki experiment in the yield of 1996. First points refer to the values at harvest. Probability values are given for the effects of harvest time (H) and storage time (S) and for their interaction (HS).
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Fig. 4. Changes in sugar composition during storage in farm experiments in 1995±1996. First points refer to the values at harvest. Probability values are given for the effects of harvest time (H) and storage time (S) and for their interaction (HS).
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concentrations increased at the beginning of storage, after which they declined. No large effects were found in carrots from later harvests. The sucrose content decreased at the beginning of storage, after which it again increased in carrots from all harvests. These changes were re¯ected in the total sugar content and the sucrose to hexose ratio, which at ®rst decreased but later increased again. The sucrose content remained lowest in carrots from H1 throughout storage. On farms in 1995±1996, the beginning of storage was characterised by an increase in fructose and glucose contents and a decrease in the sucrose content and the sucrose to hexose ratio (Fig. 4). At the end of storage, the sucrose content increased again and the fructose content declined. The changes in fructose and glucose concentrations were uniform in carrots from all harvest dates, but there was a signi®cant interaction between harvest and storage time in sucrose content and the sucrose to hexose ratio, which were highest in carrots from H2 after storage. The total sugar content was little affected by storage time. In 1996, the effect of storage time was the opposite of that in the previous year and there was a highly signi®cant interaction between harvest and storage time in most variables. In cv. Fontana, the fructose content decreased until February, after which the fructose and glucose concentrations started to increase. The sucrose and total sugar contents and the sucrose to hexose ratio decreased, especially in the carrots from H1 and H2 at the beginning of storage. Cv. Panther showed a similar difference between early and late harvests. In carrots from H1 and H2, the fructose and glucose concentrations increased and the sucrose content and sucrose to hexose ratio decreased, while the changes were reversed in carrots from H3 and H4. The total sugar content was not clearly in¯uenced by storage. 4. Discussion 4.1. Sugar composition at harvest Changes in sugar content and composition during the harvest period were dependent on the year and location. On farms in 1995 and 1997 and at the Vegetable Experimental Site in 1996 and 1997 the trend in sugar composition followed the general patterns reported in earlier studies, with an increase in the sucrose content and sucrose to hexose ratio. Fructose and glucose concentrations changed only very little during the harvest period, and any trends found were towards lower values. In 1996, however, when severe frosts injured the plants on farms, accumulated storage carbohydrates were used in the production of new leaves a few weeks later. Thus, by destroying part of the foliage, frost caused the storage root to become a ``source'' for assimilates. A similar effect has been noted in defoliated tap roots in which sucrose stored in vacuoles is used in the formation of new
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leaves (Sturm et al., 1995). Frost-injured carrots from H6 at KokemaÈki in 1997 did not start new growth and showed no decline in sugar content. The range of total sugar content in different harvests and years, 4.5±7.5% of fresh weight, was similar to the values obtained in earlier studies conducted at northern latitudes (Balvoll et al., 1976; Evers, 1989; Hogstad et al., 1997) and more southerly locations (Lester et al., 1982; Le Dily et al., 1993). However, higher total sugar contents and sucrose to hexose ratios have been measured at high temperatures in a controlled climate (Rosenfeld, 1998b,c) and under ®eld conditions, especially after a long growing period (Weichmann and KaÈppel, 1977; Fritz and Weichmann, 1979; Le Dily et al., 1993). On the basis of this and other studies in northern Europe (Balvoll et al., 1976; Evers, 1989; Hogstad et al., 1997), it seems likely that a relatively short and cool growing period reduces the accumulation of sucrose under northern conditions. In farm experiments, the total sugar content and the sucrose to hexose ratio were, on average, lower in the warm growing seasons, 1995 and 1997, than in the cooler season, 1996. High sugar content has previously been associated with lower temperatures (Evers, 1989; Rosenfeld et al., 1998a). Nevertheless, other studies report high sugar concentrations at higher temperatures (Nilsson, 1987b; Hogstad et al., 1997; Rosenfeld et al., 1998b), and that chemical composition was in¯uenced more by temperature than light (Rosenfeld et al., 1998c). These results seem to be in direct contrast to the ®ndings of this study, at least in terms of temperature. The high sugar content in early harvests in 1996 may be partly explained by the low precipitation in August and September, as drought has been reported to increase the sucrose content of carrot (Barnes, 1936; Dragland, 1978). Some differences in sugar composition between the two cultivars were found at the Vegetable Experimental Site. Cv. Fontana, which is regarded as a late cultivar in Finland, had a lower sucrose to hexose ratio than had cv. Panther, the earlier cultivar. Moreover, the changes during harvest period were slightly different in the two cultivars. In farm experiments, however, the sugar composition varied more between 1995 and 1996 in cv. Fontana than between the two cultivars in 1996 (Figs. 2 and 4). Despite the large variation in sugar composition, changes during the harvest period do not provide evidence for the existence of either physiological or horticultural maturity. As the roots were still growing, while the contents of total sugar and sucrose decreased in 1996, the results do not show a clear relation between root growth and sucrose accumulation. Even under ``normal'' growing conditions, neither the changes in sucrose or total sugar content nor in the sucrose to hexose ratio coincided with the cessation of growth (Table 3). Similarly, compositional changes do not seem to be related to storage ability, which improved each year the later was the harvest, with the exception of the very late harvest, H6, at KokemaÈki in 1997 (Suojala, 1999b). Therefore, the compositional changes cannot be used to predict the storage root growth or storage performance of the yield.
Table 3 Harvest dates after which yield increase, improvement in storability or accumulation (on farms in 1996: decline) of sugars were statistically nonsigni®cant at 5% probability level (harvest dates are compared by estimating the difference between nth harvest and the mean of the following harvests)
`Fontana'
`Fontana'
`Panther'
`Panther'
1995
1996
1996
1997
1996
1997
Fresh yield
Not measured
H3
H3
H4
H4
Dry matter yield
Not measured
nsb
H3
H4
H4
Storability Total sugar content
H2 nsb
H2 H3a
Forssa: H3 Laitila: H2 Forssa: H2 Laitila: nsb H3 H3
H2 H2
H5 nsb
Sucrose content
H2
H3a
H3
Sucrose to hexose ratio
H2
H3a
Forssa: H4a Laitila: H3 H3
H4 `Fontana': 0c `Panther': H3 H4 `Fontana': 0c `Panther': H3
H5
H4a
H5
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Vegetable Experimental Site, KokemaÈki
Farm experiments
a
Final harvest. ns no statistically signi®cant effect of harvest time. c 0 no clear trend, although harvest time effect is signi®cant. b
15
16
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4.2. Sugar composition during storage During storage, the compositional changes in farm crops in 1995 followed the common trend, with an increasing hexose content and a decreasing sucrose content. However, the sucrose content increased once again at the end of storage. In 1996, the typical pattern of increasing hexose concentration occurred only at the end of storage, and in cv. Panther, only in carrots from H1 and H2. Similarly, the sucrose content decreased only in carrots from H1 and H2 in both cultivars. At KokemaÈki in 1996, the overall trend was in accordance with earlier results, with some variation between the harvest times. Completely opposite results were obtained for the carrots from H3 and H4 in farm experiments in 1996. In cv. Panther in particular, the fructose and glucose concentrations decreased and sucrose concentrations increased during storage. The unusual trends observed in the late harvests in 1996 are probably due to frost injuries, which reduced the sucrose content at harvest. The total sugar content of the injured carrots changed very little during storage, and hence most of the sucrose accumulation during storage was due to the recombination of hexoses to sucrose. Accumulation of sucrose might be related to ageing of the carrot roots, which was promoted by the frost injuries. Rutherford (1981) mentions that some recombination of reducing sugars to sucrose occurred after prolonged periods of storage, which was also observed in the yield from 1995 at the end of storage. Similarly, Svanberg et al. (1997) noticed that in early cultivars, which were more mature at the time of harvest, sucrose increased in relation to hexoses during storage. Greater accumulation of hexoses during storage has been related to vernalisation (Nilsson, 1987b). The knowledge on the vernalisation requirements of carrot (Atherton et al., 1990; Craigon et al., 1990) suggest that vernalisation is likely to proceed under normal growing conditions in Finland and to be enhanced in a cooler season. This was not, however, supported by our results which showed less hexose accumulation in store after a cooler season. Changes during storage do not appear to contribute to the storage potential, since the total amount of sugars, that is the source of energy for maintaining the metabolism of the plant, does not decrease much during cold storage and storage losses usually increase linearly with time (Suojala, 1999b). Compositional changes might indicate storage quality by affecting the behaviour of carrots after they have been taken from storage, an issue that has not been extensively studied. 5. Conclusions Under normal growing conditions, changes in sugar composition of carrot during the harvest period followed the general patterns reported in earlier studies,
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with an increasing sucrose content and a decreasing hexose content towards the end of the season. The changes were not, however, related to root growth or storage ability. Severe frosts in one season caused a clear decline in sucrose and total sugar content and an unusual post-harvest development of sugar composition. The total sugar content was higher in the colder year, 1996, than in the warmer years, 1995 and 1997. The results do not indicate that changes in sugar composition are directly related to either the horticultural maturity of the carrot storage root at northern latitudes or to the development of storage potential in the post-harvest period. Acknowledgements The author wishes to thank the personnel of the Laboratory of Food Chemistry (MTT) for performing the sugar analyses and Profs. Irma Voipio (University of Helsinki) and Risto Tahvonen (MTT, Horticulture) for their comments on the manuscript. References Atherton, J.G., Craigon, J., Basher, E.A., 1990. Flowering and bolting in carrot. I. Juvenility, cardinal temperatures and thermal times for vernalization. J. Hort. Sci. 65, 423±429. Balvoll, G., Apeland, J., Auranaune, J., 1976. Chemical composition and organoleptic quality of carrots grown in South- and North-Norway. Forsking og Forsùk i Landbruket 27, 327± 337. Barnes, W.C., 1936. Effects of some environmental factors on growth and color of carrots. Agricultural Experimental Station Memoir 186. Cornell University, pp. 1±36. Craigon, J., Atherton, J.G., Basher, E.A., 1990. Flowering and bolting in carrot. II. Prediction in growth room, glasshouse and ®eld environment. J. Hort. Sci. 65, 547±554. Daie, J., 1984. Characterization of sugar transport in storage tissue of carrot. J. Am. Soc. Hort. Sci. 109, 718±722. Dragland, S., 1978. Nitrogen- og vassbehov hos gulrot. Forsking og Forsùk i Landbruket 29, 139± 159. Evers, A.-M., 1989. Effects of different fertilization practices on the glucose, fructose, sucrose, taste, taste and texture of carrot. J. Agric. Sci. Finl. 61, 113±122. Fritz, D., Weichmann, J., 1979. In¯uence of the harvesting date of carrots on quality and quality preservation. Acta Hort. 93, 91±100. Goris, A., 1969a. Les sucres de la racine de carotte cultiveÂe (varieÂte Nantaise demi-longue): variations climatiques et saisonnieÁres, reÂpartition dans les tissus, modi®cation au cours du stockage. Qual. Plant. Mater. Veg. 18, 283±306. Goris, A., 1969b. MeÂtabolism glucidique de la racine de carotte cultiveÂe (varieÂte Nantaise demilongue) au cours du cycle veÂgeÂtatif de la plante. Qual. Plant. Mater. Veg. 18, 307±330. Haila, K., Kumpulainen, J., HaÈkkinen, U., Tahvonen, R., 1992. Sugar and organic acid contents of vegetables consumed in Finland during 1988±1989. J. Food Compos. Anal. 5, 100±107.
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Hogstad, S., Risvik, E., Steinsholt, K., 1997. Sensory quality and chemical composition in carrots: a multivariate study. Acta Agric. Scand. Sect. B, Soil Plant Sci. 47, 253±264. Hole, C.C., 1996. Carrots. In: Zamski, E., Schaffer, A.A. (Eds.), Photoassimilate Distribution in Plants and Crops. Marcel Dekker, New York, pp. 671±690. Hole, C.C., McKee, J.M.T., 1988. Changes in soluble carbohydrate levels and associated enzymes of ®eld-grown carrots. J. Hort. Sci. 63, 87±93. Le Dily, F., Villeneuve, F., Boucaud, J., 1993. Qualite et maturite de la racine de carotte: in¯uence de la conservation au champ et au froid humide sur la composition biochimique. Acta Hort. 354, 187±199. Lester, G.E., Baker, L.R., Kelly, J.F., 1982. Physiology of sugar accumulation in carrot breeding lines and cultivars. J. Am. Soc. Hort. Sci. 107, 381±387. Li, B., Schuhmann, P., 1980. Gas±liquid chromatographic analysis of sugars in ready-to-eat breakfast cereals. J. Food Sci. 45, 138±141. Littell, R.C., Milliken, G.A., Stroup, W.W., Wol®nger, R.D., 1996. SAS1 System for Mixed Models. SAS Institute, Cary, NC, 633 pp. Nilsson, T., 1987a. Growth and chemical composition of carrots as in¯uenced by the time of sowing and harvest. J. Agric. Sci. (Camb.) 108, 459±468. Nilsson, T., 1987b. Carbohydrate composition during long-term storage of carrots as in¯uenced by the time of harvest. J. Hort. Sci. 62, 191±203. OldeÂn, B., Nilsson, T., 1992. Acid and alkaline invertase activities in carrot during root development and storage. Swed. J. Agric. Res. 22, 43±47. Phan, C., Hsu, H., 1973. Physical and chemical changes occurring in the carrot root during growth. Can. J. Plant Sci. 53, 629±634. Phan, C., Hsu, H., Sarkar, S.K., 1973. Physical and chemical changes occurring in the carrot root during storage. Can. J. Plant Sci. 53, 635±641. Ricardo, C.P.P., Sovia, D., 1974. Development of tuberous roots and sugar accumulation as related to invertase activity and mineral nutrition. Planta 118, 43±55. Rosenfeld, H.J., 1998. Maturity and development of the carrot root (Daucus carota L.). Gartenbauwissenschaft 63, 87±94. Rosenfeld, H.J., Samuelsen, R.T., Lea, P., 1998a. Relationship between physical and chemical characteristics of carrots grown at northern latitudes. J. Hort. Sci. Biotechnol. 73, 265± 275. Rosenfeld, H.J., Samuelsen, R.T., Lea, P., 1998b. The effect of temperature on sensory quality, chemical composition and growth of carrots (Daucus carota L.) I. Constant diurnal temperature. J. Hort. Sci. Biotechnol. 73, 275±288. Rosenfeld, H.J., Samuelsen, R.T., Lea, P., 1998c. The effect of temperature on sensory quality, chemical composition and growth of carrots (Daucus carota L.) II. Constant diurnal temperatures under different seasonal light regimes. J. Hort. Sci. Biotechnol. 73, 578±588. Rutherford, P.P., 1981. Some biochemical changes in vegetables during storage. Ann. Appl. Biol. 98, 538±541. SteingroÈver, E., 1981. The relationship between cyanide-resistant root respiration and the storage of sugars in the taproot in Daucus carota L. J. Exp. Bot. 32, 911±919. Sturm, A., SebkovaÂ, V., Lorenz, K., Hardegger, M., Lienhard, S., Unger, C., 1995. Developmentand organ-speci®c expression of the genes for sucrose synthase and three isoenzymes of acid ûfructofuranosidase in carrot. Planta 195, 601±610. Suojala, T., 1999a. Cessation of storage root growth of carrot in autumn. J. Hort. Sci. Biotechnol. 74, 475±483. Suojala, T., 1999b. Effect of harvest time on the storage performance of carrot. J. Hort. Sci. Biotechnol. 74, 484±492.
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Svanberg, S.J.M., Nyman, E.M.G., Andersson, R., Nilsson, T., 1997. Effects of boiling and storage on dietary ®bre and digestible carbohydrates in various cultivars of carrot. J. Sci. Food Agric. 73, 154±245. Watada, A.E., Herner, R.C., Kader, A.A., Romani, R.J., Staby, G.L., 1984. Terminology for the description of developmental stages of horticultural crops. HortScience 19, 20±21. Weichmann, J., KaÈppel, R., 1977. Harvesting dates and storage-ability of carrots (Daucus carota L.). Acta Hort. 62, 191±196.
Variation in sugar content and composition of carrot storage roots at harvest and during storage Terhi Suojala* Agricultural Research Centre of Finland (MTT), Plant Production Research, Horticulture, Toivonlinnantie 518, FIN-21500 PiikkioÈ, Finland Accepted 25 October 1999
Abstract Changes in the soluble sugar content of carrot storage roots (cvs. Fontana and Panther) at harvest and during storage were studied at an experimental site and on vegetable farms in three successive years. The later the harvest, the higher the content of soluble sugars, especially sucrose, tended to be, but on farms in 1996, there was a clear decline in dry matter and sugar content due to frost injury in ®eld experiments. Even in uninjured carrot stands, changes in sugar content and composition did not coincide with a cessation of yield increase or changes in storability. Total sugar and sucrose contents at the beginning of the harvest period were higher in the colder year, 1996, than in the warmer years, 1995 and 1997. Despite some differences from earlier studies, the compositional changes observed here during storage followed the general pattern of increasing hexose and decreasing sucrose contents. Frost injury, however, resulted in post-harvest changes in sugars which were different from earlier results. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Daucus carota; Frost; Maturity; Quality; Sucrose
1. Introduction The major ecological function of the carrot storage root is to act as a temporary reserve for the production of a ¯owering stem after appropriate stimuli (Hole, 1996). The existence of storage compounds is also the primary reason for the utilisation of the carrot in human nutrition. In commercial production, these * Tel.: 358-2-477-2200; fax: 358-2-477-2299. E-mail address: terhi.suojala@mtt.® (T. Suojala)
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 3 3 - 8
2
T. Suojala / Scientia Horticulturae 85 (2000) 1±19
compounds provide the chemical energy reserves needed for long-term storage and shelf life. Most of the compounds stored in the vacuoles of the parenchyma cells are soluble sugars. These account for 34±70% of the dry weight of the storage root (Goris, 1969a; Ricardo and Sovia, 1974; Nilsson, 1987a). Sucrose is the predominant transport and storage sugar at maturity (Daie, 1984), but its proportion is affected by genotype and environment (Goris, 1969a; Phan and Hsu, 1973; Ricardo and Sovia, 1974; Nilsson, 1987a). At 30±50 days after sowing, the sucrose concentration starts to increase faster than that of hexoses (fructose, glucose), resulting in a higher sucrose to hexose ratio (SteingroÈver, 1981; Hole and McKee, 1988). Several authors have attempted to de®ne the changes in sugar content and the sucrose to hexose ratio as a measure of biochemical or physiological maturity (Goris, 1969b; Phan and Hsu, 1973; Fritz and Weichmann, 1979; Le Dily et al., 1993). The usage of sugar composition as the indicator of maturity has been questioned. For carrots grown in Sweden (Nilsson, 1987a), sucrose accumulation continued up to the ®nal harvest, and Nilsson concluded that ``maturity'' should only be considered as a reduction in metabolic activity in an environment no longer favourable for growth. Rosenfeld (1998) noted that, neither the ratio of sugars nor any other chemical variable indicated the presence of maturity and could not be used as an indicator of the optimal stage of harvest. Physiological maturity has been considered the stage at which a plant or plant part continues to develop even if detached (Watada et al., 1984). It has been noted that, it is not usually observed in organs like roots, foliage, stems and tubers. Horticultural maturity refers to the stage of development at which a plant or a plant part can be used for a particular purpose (Watada et al., 1984). In carrot, maturity in this sense would require a satisfactory outer, sensory and nutritional quality combined with an adequate potential for storage and shelf life. The question is whether sugar composition bears any relation to the horticultural maturity of carrot. During storage, the common trend in sugar composition is for the hexose content to increase and the sucrose content to decrease, especially during the ®rst months (Phan et al., 1973; Nilsson, 1987b; OldeÂn and Nilsson, 1992; Le Dily et al., 1993). Most experiments record only minor changes in the total sugar content. Nilsson (1987b) found that compositional changes occurred irrespective of harvest date and the differences were maintained throughout the storage period. The objective of this study was to investigate changes in the sugar content and composition of carrot during the harvest period and storage as chemical energy reserves and possible indicators of horticultural maturity. The results are discussed in terms of a northern climate, which is characterised by a relatively short growing period and a need for long-term storage.
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3
2. Materials and methods Material for analyses was obtained from experiments conducted at the Vegetable Experimental Site of the Agricultural Research Centre of Finland at KokemaÈki and on farmers' ®elds in southern Finland. Two cultivars were used in the experiments, Fontana F1 (Bejo Zaden, Netherlands), which is a late variety cultivated for the processing industry, and Panther F1 (Sluis and Groot, Netherlands), an earlier, fresh-market variety. At the Vegetable Experimental Site, a split-plot experiment with cultivar in whole plots and harvest time in subplots was conducted in 1996 and 1997. A randomised complete block design with four replicate blocks was used, but sugar analyses were performed on only three replicates. The farm experiments with cv. Fontana in 1995 and 1996 were located in the same production area, Huittinen. Farms with cv. Panther in 1996 and 1997 were located in two other areas, Forssa and Laitila. The number of farms and harvest dates are given in Table 1. In 1995, the farm was treated as a random block factor. In 1996 and 1997, each farm had its own randomised complete block design with three replicates. Samples for analysing sugar composition after storage in the 1995 and 1996 yields were stored in woven polypropylene bags (75 g mÿ2) in batches of approximately 10 kg. Carrots from different locations were kept in the same store, which differed in the two years. Storage conditions were as follows: in 1995±1996, temperature 0 0.58C, relative humidity 80±85%; in 1996±1997, temperature 0 18C, relative humidity 85±95%. Details of the experiments are given by Suojala (1999a,b). 2.1. Weather conditions Meteorological data were obtained from the nearest meteorological stations of the Finnish Meteorological Institute (Table 2). At KokemaÈki, the meteorological station is situated at the experimental site. Its data were also used for farms at Huittinen. Data from Jokioinen were used for farms at Forssa and data from Mietoinen for farms at Laitila. The distance between meteorological stations and the experimental ®elds did not exceed 40 km. The weather differed markedly from one experimental year to another (Table 2). 1995 and 1997 were warmer than the long-term average, but in 1996, the monthly mean temperatures were lower than the long-term average, except in August and October. Night frosts were exceptionally hard in 1996, and severe frost injured the carrot ®elds on 21st September, when the temperature at ground level dropped below ÿ88C at the measuring sites closest to the farm experiments. Plants were injured on farms but not at the Vegetable Experimental Site. In 1997, frosts did not cause damage before late October, when carrots in the last harvest at the Vegetable Experimental Site were exposed to permanent frost.
4
Coding of harvest
H1 H2 H3 H4 H5 H6
Vegetable Experimental Site, KokemaÈki
Farm experiments `Fontana' (8)
`Fontana' (5)
`Panther' (8)
`Panther' (8)
1995
1996
1996
1997
1996
1997
12 September 26 September 10 October
11 September 25 September 8 October 22 October
10±11 September 23±24 September 7±9 October 21±22 October
9±11 September 23±24 September 7±8 October 21±22 October
5 September 16 September 26 September 7 October 16 October 28 October
8 September 18 September 29 September 9 October 20 October 30 October
104±120
113±128
106
117
Growing time from sowing to H1 (days) 107±130 114±120
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Table 1 Harvest dates in different experiments (number of farms is given in parentheses)
Table 2 Monthly mean temperature and precipitation for sites and years of experiments compared with long-term averages KokemaÈki (618160 N, 228150 E)
1996
1961±1990
1995
9.4 14.3 15.8 14.2 9.4 4.7
8.7 16.1 15.2 14.4 10.3 8.0
35 47 80 83 65 58
74 90 42 72 29 74
Mean temperature (8C) May 8.8 June 13.1 July 13.9 August 17.0 September 8.3 October 6.4 Precipitation (mm) May 65 June 52 July 136 August 14 September 20 October 56
1997 7.7 16.1 17.8 17.8 10.0 2.4 16 101 141 44 78 47
1996
1997
8.5 13.2 14.1 16.7 8.3 6.9
7.8 16.3 17.9 17.9 10.5 2.6
57 49 111 26 27 52
14 85 91 17 121 52
Mietoinen (608380 N, 218520 E) 1961±1990
1996
9.5 14.4 15.8 14.2 9.6 5.0
8.9 13.1 14.5 17.8 9.5 7.4
33 48 71 77 65 54
65 37 93 17 26 49
1997 7.8 16.3 18.8 18.7 11.0 3.2 17 58 132 25 137 69
1961±1990 9.5 14.5 16.2 14.8 10.3 5.8 33 44 75 85 70 62
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Jokioinen (608490 N, 238300 E)
5
6
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2.2. Samples and analyses Samples for sugar analyses were taken at the time of harvest. In 1995 and 1996, samples were also taken after storage, usually three times in association with analyses of storage losses. For cv. Panther, only the yield of three farms was sampled twice during storage. In 1996 and 1997, 20 roots were sampled from each plot. In 1995, one sample comprised only ®ve carrots but the mean of three replicate samples was used in data analysis. Subsamples from each of 20 (15 in 1995) carrots were combined and grated, frozen and stored at ÿ208C until analysis. Soluble sugars were analysed at the Laboratory of Food Chemistry of the Agricultural Research Centre of Finland. Fructose, glucose and sucrose were assayed by gas±liquid chromatography (GLC) employing the method of Li and Schuhmann (1980) with the modi®cations of Haila et al. (1992). A Perkin±Elmer Autosystem GLC equipped with a ¯ame ionisation detector and a capillary column NB 30 (column length 25 m, ID 0.32 mm and ®lm thickness 0.25 mm) was employed. Total soluble sugars were calculated as the sum of fructose, glucose and sucrose. Results are expressed as percent of fresh weight. In addition, the ratio of sucrose to hexose (fructose glucose) is presented. 2.3. Statistical analysis In 1995, the last harvest was not performed on one of the farms. In 1996, samples were not taken from the last harvest of cv. Fontana in autumn. One observation was lacking from the farm experiments in 1996. The results were analysed by mixed models using the SAS MIXED procedure (Littell et al., 1996). The experiment at the Vegetable Experimental Site was analysed by a mixed model for split-plot design with cultivar, harvest time and their interaction as ®xed factors, and block, whole plot and subplot errors as random factors. In the farm experiments in 1995, the model included harvest time as a ®xed factor and farm as a random block factor. In 1996 and 1997, the experimental design was more complicated. Results of the two cultivars were analysed separately. The model for cv. Panther included production area, harvest time and their interaction as ®xed factors and experimental block, farm, interaction farm harvest time and error term as random factors. Block was nested within farm and farm within area. The model for cv. Fontana in 1996 was reduced from the previous model by omitting the effect of area and its interaction with harvest time. The effect of harvest time was further studied by estimating the difference between the nth harvest and the mean of the following harvests. The signi®cance of the estimated difference was investigated with the aid of 95% con®dence intervals (CI). If the 95% CI of the difference includes zero, the difference is not statistically signi®cant at the 5% level. Statistical analysis of the
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7
data for 1996 and 1997 is described in more detail by Suojala (1999a). For results after storage, the effect of storage time and its interactions were included in the models as described by Suojala (1999b). 3. Results 3.1. Effect of harvest time 3.1.1. Vegetable Experimental Site, KokemaÈki In 1996, the effect of harvest time was dependent on the cultivar (Fig. 1). Standard error of mean (SEM) was also determined for each case. Changes in fructose and glucose concentrations were small in both cultivars. The sucrose concentration increased, but the accumulation was more pronounced in cv. Panther. The total sugar content increased, especially in cv. Panther, whereas in cv. Fontana, the changes were not as clear. The sucrose to hexose ratio increased during the harvest period in cv. Panther, but did not change in cv. Fontana. The average concentrations of fructose and glucose were higher and sucrose concentration and sucrose to hexose ratio were lower in cv. Fontana than in cv. Panther. Sugars comprised 44±52% of dry matter in cv. Fontana and 42±53% in cv. Panther. In 1997, the total sugar contents of the two cultivars changed in different ways (Fig. 1). For cv. Fontana it did not change but for cv. Panther it increased signi®cantly after H4. The glucose concentration decreased slightly and the sucrose concentration increased clearly, resulting in a higher sucrose to hexose ratio towards the end of the harvest period. As in 1996, the cultivars differed in their sugar concentrations; cv. Fontana having a higher concentration of fructose, glucose and total sugars and a lower sucrose to hexose ratio than cv. Panther. Sugars accounted for a smaller proportion of dry matter than in the previous year: 42±48% in cv. Fontana and 37±43% in cv. Panther. 3.1.2. Farm experiments Ð cv. Fontana In 1995, the total sugar content did not change during the 4 week harvest period but the sugar composition did (Fig. 2): the fructose and glucose concentrations decreased and the sucrose concentration increased. The sucrose to hexose ratio, therefore, rose from 0.6 to 0.9 in 4 weeks. Sugars accounted for 47±48% of dry matter. In 1996, the changes in soluble sugars differed from those in 1995 (Fig. 2). The total sugar and sucrose contents decreased by about 1% unit between H2 and H3, but the fructose and glucose concentrations did not change during the harvest period. Therefore, the sucrose to hexose ratio fell from 1.6±1.7 in H1 and H2 to 1.1 in H3. At the same time, the proportion of sugars in dry matter declined from 53% in H1 and 52% in H2 to 45% in H3.
8
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Fig. 1. Contents of soluble sugars and the sucrose to hexose ratio (S/H ratio) at different harvest dates at the Vegetable Experimental Site at KokemaÈki in 1996 and 1997. Probability values are given for the effects of cultivar (C) and harvest time (H) and for their interaction (CH).
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9
Fig. 2. Contents of soluble sugars and the sucrose to hexose ratio (S/H ratio) at different harvest dates on vegetable farms (cvs. Fontana and Panther) in 1995±1997. Figures are means of 3±8 farms. Probability values are given for the effect of harvest time (H), and in cv. Panther, for the effect of area (A) and their interaction (AH).
10
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3.1.3. Farm experiments Ð cv. Panther In 1996, the changes were similar to those in cv. Fontana: the sucrose and total sugar contents decreased, especially after H2 (Fig. 2), but the changes differed slightly in the two production areas. At Forssa, the concentrations started to decrease after H2, whereas at Laitila, the concentrations decreased straight after H1 but there was no further change after H3. The average sucrose and total sugar contents were higher at Laitila than at Forssa, due to the higher concentrations in H1 and H2. The fructose and glucose concentrations were similar in both areas and decreased slightly during the harvest period. Owing to the drop in sucrose concentration, the sucrose to hexose ratio decreased, but at a slower rate at Forssa than at Laitila. At Forssa, sugars accounted for 47% of dry matter in H1 and 51% in H2, falling to 45% in H3 and 42% in H4. At Laitila, the drop was even larger: from 55% in H1 to 44% in H4. In 1997, the sucrose and total sugar contents increased similarly in both areas, especially between the ®rst two harvests (Fig. 2). The total sugar content did not change after H2, but the sucrose concentration increased up to H3 and the sucrose to hexose ratio up to H4. The fructose and glucose concentrations decreased up to H3 but the changes were slight. Sugars comprised 38% of dry matter in H1 and 46±48% in later harvests. 3.2. Variation in sugar content between years The effect of growing season on the total sugar content or sucrose to hexose ratio is dif®cult to assess due to the unusual weather in autumn 1996. The results from the ®rst harvest date, when the plants had not been exposed to frosts, show that the total sugar content and the sucrose to hexose ratio were higher in 1996 than in the warmer years, 1995 and 1997 (Fig. 2). The same trend applied to both cultivars. At the Vegetable Experimental Site, the same effect was observed in the total sugar content, which was, on average, lower in the warmer year, 1997 (Fig. 1). However, the sucrose to hexose ratio was higher in 1997 than in 1996 in both cultivars. As well as between years, the sugar content varied between farms. Differences in the total sugar content at H1 could be more than 2% units. The variation between farms was not related to the thermal time from sowing to harvest. 3.3. Effect of storage At KokemaÈki in 1996, the changes in the course of storage were dependent on cultivar and harvest time, and cultivars were analysed separately (Fig. 3). In cv. Fontana, the overall trend showed an increase in fructose and glucose contents, especially in H1, H2 and H3, and a decrease in sucrose and total sugar contents and the sucrose to hexose ratio. In cv. Panther, the interaction between harvest time and storage time was more pronounced: in H1 and H2, fructose and glucose
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11
Fig. 3. Changes in sugar composition during storage at the KokemaÈki experiment in the yield of 1996. First points refer to the values at harvest. Probability values are given for the effects of harvest time (H) and storage time (S) and for their interaction (HS).
12
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Fig. 4. Changes in sugar composition during storage in farm experiments in 1995±1996. First points refer to the values at harvest. Probability values are given for the effects of harvest time (H) and storage time (S) and for their interaction (HS).
T. Suojala / Scientia Horticulturae 85 (2000) 1±19
13
concentrations increased at the beginning of storage, after which they declined. No large effects were found in carrots from later harvests. The sucrose content decreased at the beginning of storage, after which it again increased in carrots from all harvests. These changes were re¯ected in the total sugar content and the sucrose to hexose ratio, which at ®rst decreased but later increased again. The sucrose content remained lowest in carrots from H1 throughout storage. On farms in 1995±1996, the beginning of storage was characterised by an increase in fructose and glucose contents and a decrease in the sucrose content and the sucrose to hexose ratio (Fig. 4). At the end of storage, the sucrose content increased again and the fructose content declined. The changes in fructose and glucose concentrations were uniform in carrots from all harvest dates, but there was a signi®cant interaction between harvest and storage time in sucrose content and the sucrose to hexose ratio, which were highest in carrots from H2 after storage. The total sugar content was little affected by storage time. In 1996, the effect of storage time was the opposite of that in the previous year and there was a highly signi®cant interaction between harvest and storage time in most variables. In cv. Fontana, the fructose content decreased until February, after which the fructose and glucose concentrations started to increase. The sucrose and total sugar contents and the sucrose to hexose ratio decreased, especially in the carrots from H1 and H2 at the beginning of storage. Cv. Panther showed a similar difference between early and late harvests. In carrots from H1 and H2, the fructose and glucose concentrations increased and the sucrose content and sucrose to hexose ratio decreased, while the changes were reversed in carrots from H3 and H4. The total sugar content was not clearly in¯uenced by storage. 4. Discussion 4.1. Sugar composition at harvest Changes in sugar content and composition during the harvest period were dependent on the year and location. On farms in 1995 and 1997 and at the Vegetable Experimental Site in 1996 and 1997 the trend in sugar composition followed the general patterns reported in earlier studies, with an increase in the sucrose content and sucrose to hexose ratio. Fructose and glucose concentrations changed only very little during the harvest period, and any trends found were towards lower values. In 1996, however, when severe frosts injured the plants on farms, accumulated storage carbohydrates were used in the production of new leaves a few weeks later. Thus, by destroying part of the foliage, frost caused the storage root to become a ``source'' for assimilates. A similar effect has been noted in defoliated tap roots in which sucrose stored in vacuoles is used in the formation of new
14
T. Suojala / Scientia Horticulturae 85 (2000) 1±19
leaves (Sturm et al., 1995). Frost-injured carrots from H6 at KokemaÈki in 1997 did not start new growth and showed no decline in sugar content. The range of total sugar content in different harvests and years, 4.5±7.5% of fresh weight, was similar to the values obtained in earlier studies conducted at northern latitudes (Balvoll et al., 1976; Evers, 1989; Hogstad et al., 1997) and more southerly locations (Lester et al., 1982; Le Dily et al., 1993). However, higher total sugar contents and sucrose to hexose ratios have been measured at high temperatures in a controlled climate (Rosenfeld, 1998b,c) and under ®eld conditions, especially after a long growing period (Weichmann and KaÈppel, 1977; Fritz and Weichmann, 1979; Le Dily et al., 1993). On the basis of this and other studies in northern Europe (Balvoll et al., 1976; Evers, 1989; Hogstad et al., 1997), it seems likely that a relatively short and cool growing period reduces the accumulation of sucrose under northern conditions. In farm experiments, the total sugar content and the sucrose to hexose ratio were, on average, lower in the warm growing seasons, 1995 and 1997, than in the cooler season, 1996. High sugar content has previously been associated with lower temperatures (Evers, 1989; Rosenfeld et al., 1998a). Nevertheless, other studies report high sugar concentrations at higher temperatures (Nilsson, 1987b; Hogstad et al., 1997; Rosenfeld et al., 1998b), and that chemical composition was in¯uenced more by temperature than light (Rosenfeld et al., 1998c). These results seem to be in direct contrast to the ®ndings of this study, at least in terms of temperature. The high sugar content in early harvests in 1996 may be partly explained by the low precipitation in August and September, as drought has been reported to increase the sucrose content of carrot (Barnes, 1936; Dragland, 1978). Some differences in sugar composition between the two cultivars were found at the Vegetable Experimental Site. Cv. Fontana, which is regarded as a late cultivar in Finland, had a lower sucrose to hexose ratio than had cv. Panther, the earlier cultivar. Moreover, the changes during harvest period were slightly different in the two cultivars. In farm experiments, however, the sugar composition varied more between 1995 and 1996 in cv. Fontana than between the two cultivars in 1996 (Figs. 2 and 4). Despite the large variation in sugar composition, changes during the harvest period do not provide evidence for the existence of either physiological or horticultural maturity. As the roots were still growing, while the contents of total sugar and sucrose decreased in 1996, the results do not show a clear relation between root growth and sucrose accumulation. Even under ``normal'' growing conditions, neither the changes in sucrose or total sugar content nor in the sucrose to hexose ratio coincided with the cessation of growth (Table 3). Similarly, compositional changes do not seem to be related to storage ability, which improved each year the later was the harvest, with the exception of the very late harvest, H6, at KokemaÈki in 1997 (Suojala, 1999b). Therefore, the compositional changes cannot be used to predict the storage root growth or storage performance of the yield.
Table 3 Harvest dates after which yield increase, improvement in storability or accumulation (on farms in 1996: decline) of sugars were statistically nonsigni®cant at 5% probability level (harvest dates are compared by estimating the difference between nth harvest and the mean of the following harvests)
`Fontana'
`Fontana'
`Panther'
`Panther'
1995
1996
1996
1997
1996
1997
Fresh yield
Not measured
H3
H3
H4
H4
Dry matter yield
Not measured
nsb
H3
H4
H4
Storability Total sugar content
H2 nsb
H2 H3a
Forssa: H3 Laitila: H2 Forssa: H2 Laitila: nsb H3 H3
H2 H2
H5 nsb
Sucrose content
H2
H3a
H3
Sucrose to hexose ratio
H2
H3a
Forssa: H4a Laitila: H3 H3
H4 `Fontana': 0c `Panther': H3 H4 `Fontana': 0c `Panther': H3
H5
H4a
H5
T. Suojala / Scientia Horticulturae 85 (2000) 1±19
Vegetable Experimental Site, KokemaÈki
Farm experiments
a
Final harvest. ns no statistically signi®cant effect of harvest time. c 0 no clear trend, although harvest time effect is signi®cant. b
15
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4.2. Sugar composition during storage During storage, the compositional changes in farm crops in 1995 followed the common trend, with an increasing hexose content and a decreasing sucrose content. However, the sucrose content increased once again at the end of storage. In 1996, the typical pattern of increasing hexose concentration occurred only at the end of storage, and in cv. Panther, only in carrots from H1 and H2. Similarly, the sucrose content decreased only in carrots from H1 and H2 in both cultivars. At KokemaÈki in 1996, the overall trend was in accordance with earlier results, with some variation between the harvest times. Completely opposite results were obtained for the carrots from H3 and H4 in farm experiments in 1996. In cv. Panther in particular, the fructose and glucose concentrations decreased and sucrose concentrations increased during storage. The unusual trends observed in the late harvests in 1996 are probably due to frost injuries, which reduced the sucrose content at harvest. The total sugar content of the injured carrots changed very little during storage, and hence most of the sucrose accumulation during storage was due to the recombination of hexoses to sucrose. Accumulation of sucrose might be related to ageing of the carrot roots, which was promoted by the frost injuries. Rutherford (1981) mentions that some recombination of reducing sugars to sucrose occurred after prolonged periods of storage, which was also observed in the yield from 1995 at the end of storage. Similarly, Svanberg et al. (1997) noticed that in early cultivars, which were more mature at the time of harvest, sucrose increased in relation to hexoses during storage. Greater accumulation of hexoses during storage has been related to vernalisation (Nilsson, 1987b). The knowledge on the vernalisation requirements of carrot (Atherton et al., 1990; Craigon et al., 1990) suggest that vernalisation is likely to proceed under normal growing conditions in Finland and to be enhanced in a cooler season. This was not, however, supported by our results which showed less hexose accumulation in store after a cooler season. Changes during storage do not appear to contribute to the storage potential, since the total amount of sugars, that is the source of energy for maintaining the metabolism of the plant, does not decrease much during cold storage and storage losses usually increase linearly with time (Suojala, 1999b). Compositional changes might indicate storage quality by affecting the behaviour of carrots after they have been taken from storage, an issue that has not been extensively studied. 5. Conclusions Under normal growing conditions, changes in sugar composition of carrot during the harvest period followed the general patterns reported in earlier studies,
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with an increasing sucrose content and a decreasing hexose content towards the end of the season. The changes were not, however, related to root growth or storage ability. Severe frosts in one season caused a clear decline in sucrose and total sugar content and an unusual post-harvest development of sugar composition. The total sugar content was higher in the colder year, 1996, than in the warmer years, 1995 and 1997. The results do not indicate that changes in sugar composition are directly related to either the horticultural maturity of the carrot storage root at northern latitudes or to the development of storage potential in the post-harvest period. Acknowledgements The author wishes to thank the personnel of the Laboratory of Food Chemistry (MTT) for performing the sugar analyses and Profs. Irma Voipio (University of Helsinki) and Risto Tahvonen (MTT, Horticulture) for their comments on the manuscript. References Atherton, J.G., Craigon, J., Basher, E.A., 1990. Flowering and bolting in carrot. I. Juvenility, cardinal temperatures and thermal times for vernalization. J. Hort. Sci. 65, 423±429. Balvoll, G., Apeland, J., Auranaune, J., 1976. Chemical composition and organoleptic quality of carrots grown in South- and North-Norway. Forsking og Forsùk i Landbruket 27, 327± 337. Barnes, W.C., 1936. Effects of some environmental factors on growth and color of carrots. Agricultural Experimental Station Memoir 186. Cornell University, pp. 1±36. Craigon, J., Atherton, J.G., Basher, E.A., 1990. Flowering and bolting in carrot. II. Prediction in growth room, glasshouse and ®eld environment. J. Hort. Sci. 65, 547±554. Daie, J., 1984. Characterization of sugar transport in storage tissue of carrot. J. Am. Soc. Hort. Sci. 109, 718±722. Dragland, S., 1978. Nitrogen- og vassbehov hos gulrot. Forsking og Forsùk i Landbruket 29, 139± 159. Evers, A.-M., 1989. Effects of different fertilization practices on the glucose, fructose, sucrose, taste, taste and texture of carrot. J. Agric. Sci. Finl. 61, 113±122. Fritz, D., Weichmann, J., 1979. In¯uence of the harvesting date of carrots on quality and quality preservation. Acta Hort. 93, 91±100. Goris, A., 1969a. Les sucres de la racine de carotte cultiveÂe (varieÂte Nantaise demi-longue): variations climatiques et saisonnieÁres, reÂpartition dans les tissus, modi®cation au cours du stockage. Qual. Plant. Mater. Veg. 18, 283±306. Goris, A., 1969b. MeÂtabolism glucidique de la racine de carotte cultiveÂe (varieÂte Nantaise demilongue) au cours du cycle veÂgeÂtatif de la plante. Qual. Plant. Mater. Veg. 18, 307±330. Haila, K., Kumpulainen, J., HaÈkkinen, U., Tahvonen, R., 1992. Sugar and organic acid contents of vegetables consumed in Finland during 1988±1989. J. Food Compos. Anal. 5, 100±107.
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Hogstad, S., Risvik, E., Steinsholt, K., 1997. Sensory quality and chemical composition in carrots: a multivariate study. Acta Agric. Scand. Sect. B, Soil Plant Sci. 47, 253±264. Hole, C.C., 1996. Carrots. In: Zamski, E., Schaffer, A.A. (Eds.), Photoassimilate Distribution in Plants and Crops. Marcel Dekker, New York, pp. 671±690. Hole, C.C., McKee, J.M.T., 1988. Changes in soluble carbohydrate levels and associated enzymes of ®eld-grown carrots. J. Hort. Sci. 63, 87±93. Le Dily, F., Villeneuve, F., Boucaud, J., 1993. Qualite et maturite de la racine de carotte: in¯uence de la conservation au champ et au froid humide sur la composition biochimique. Acta Hort. 354, 187±199. Lester, G.E., Baker, L.R., Kelly, J.F., 1982. Physiology of sugar accumulation in carrot breeding lines and cultivars. J. Am. Soc. Hort. Sci. 107, 381±387. Li, B., Schuhmann, P., 1980. Gas±liquid chromatographic analysis of sugars in ready-to-eat breakfast cereals. J. Food Sci. 45, 138±141. Littell, R.C., Milliken, G.A., Stroup, W.W., Wol®nger, R.D., 1996. SAS1 System for Mixed Models. SAS Institute, Cary, NC, 633 pp. Nilsson, T., 1987a. Growth and chemical composition of carrots as in¯uenced by the time of sowing and harvest. J. Agric. Sci. (Camb.) 108, 459±468. Nilsson, T., 1987b. Carbohydrate composition during long-term storage of carrots as in¯uenced by the time of harvest. J. Hort. Sci. 62, 191±203. OldeÂn, B., Nilsson, T., 1992. Acid and alkaline invertase activities in carrot during root development and storage. Swed. J. Agric. Res. 22, 43±47. Phan, C., Hsu, H., 1973. Physical and chemical changes occurring in the carrot root during growth. Can. J. Plant Sci. 53, 629±634. Phan, C., Hsu, H., Sarkar, S.K., 1973. Physical and chemical changes occurring in the carrot root during storage. Can. J. Plant Sci. 53, 635±641. Ricardo, C.P.P., Sovia, D., 1974. Development of tuberous roots and sugar accumulation as related to invertase activity and mineral nutrition. Planta 118, 43±55. Rosenfeld, H.J., 1998. Maturity and development of the carrot root (Daucus carota L.). Gartenbauwissenschaft 63, 87±94. Rosenfeld, H.J., Samuelsen, R.T., Lea, P., 1998a. Relationship between physical and chemical characteristics of carrots grown at northern latitudes. J. Hort. Sci. Biotechnol. 73, 265± 275. Rosenfeld, H.J., Samuelsen, R.T., Lea, P., 1998b. The effect of temperature on sensory quality, chemical composition and growth of carrots (Daucus carota L.) I. Constant diurnal temperature. J. Hort. Sci. Biotechnol. 73, 275±288. Rosenfeld, H.J., Samuelsen, R.T., Lea, P., 1998c. The effect of temperature on sensory quality, chemical composition and growth of carrots (Daucus carota L.) II. Constant diurnal temperatures under different seasonal light regimes. J. Hort. Sci. Biotechnol. 73, 578±588. Rutherford, P.P., 1981. Some biochemical changes in vegetables during storage. Ann. Appl. Biol. 98, 538±541. SteingroÈver, E., 1981. The relationship between cyanide-resistant root respiration and the storage of sugars in the taproot in Daucus carota L. J. Exp. Bot. 32, 911±919. Sturm, A., SebkovaÂ, V., Lorenz, K., Hardegger, M., Lienhard, S., Unger, C., 1995. Developmentand organ-speci®c expression of the genes for sucrose synthase and three isoenzymes of acid ûfructofuranosidase in carrot. Planta 195, 601±610. Suojala, T., 1999a. Cessation of storage root growth of carrot in autumn. J. Hort. Sci. Biotechnol. 74, 475±483. Suojala, T., 1999b. Effect of harvest time on the storage performance of carrot. J. Hort. Sci. Biotechnol. 74, 484±492.
T. Suojala / Scientia Horticulturae 85 (2000) 1±19
19
Svanberg, S.J.M., Nyman, E.M.G., Andersson, R., Nilsson, T., 1997. Effects of boiling and storage on dietary ®bre and digestible carbohydrates in various cultivars of carrot. J. Sci. Food Agric. 73, 154±245. Watada, A.E., Herner, R.C., Kader, A.A., Romani, R.J., Staby, G.L., 1984. Terminology for the description of developmental stages of horticultural crops. HortScience 19, 20±21. Weichmann, J., KaÈppel, R., 1977. Harvesting dates and storage-ability of carrots (Daucus carota L.). Acta Hort. 62, 191±196.