The Content Variations of Main Nutrients in Storage Root Expanding Stages of Sweet Potato and Their Mutual Relationships

Meng KOU,Yungang ZHANG,Yaju LIU,Xin WANG,Wei TANG,Hui YAN,Daifu MA,Qiang LI*

1.Jiangsu Xu Huai Area of Xuzhou Institute of Agricultural Sciences,Jiangsu Xuzhou Sweet Potato Research Center,Key Laboratory of Sweet Potato Genetic Improvement,Chinese Academy of Agricultural Sciences,Xuzhou 221131,China;2.School of Life Science,Xuzhou Normal University,Xuzhou 221116,China

In recent years,food and clothing problems of urban and rural residents in China have been solved,and dietary structure is also optimized gradually,and people’s nutritional needs obtain basic satisfaction.However,there are more people still facing insufficient intakes of trace elements,namely potential threat of“invisible starvation”.According to the statistics,occurrence frequency of iron-deficiency anemia in China is between 15% and 20%[1].In 3-12-year old of children,deficiency of vitamin A accounts for 9.3%,and marginal deficiency rate of vitamin A reaches 45.1%[2].Micro nutrient deficiency may cause blindness and brain damage,and affect mental and physical function development,thereby hindering rapid economic and social development in China.

It is an effective way solving invisible starvation to cultivate and promote the crops rich in trace elements,including VA,iron and zinc.Under the support of international biological enhancementproject,China biological enhancement project was started in 2004,which subsidizes the researches of four crops including sweet potato.βcarotene is prerequisite matter synthesizing VA,and sweet potato is rich in β-carotene, especially orangefleshedsweetpotato.We studied content changes of trace elements(β-carotene,iron and zinc)in storage root of orange-fleshed sweet potato during expanding stage,and aimed to understand change characteristics of nutritive compositions in storage root of sweet potato during different growth stages,explore interrelationship among quality traits,and provide scientific basis for quality improvement of sweet potato.

Materials and Methods

Test materials

The tested sweet potato(Ipomoea batatas(L.)Lam.)was Xuyu Potato 34 which was rich in carotene and was bred by 06-2-9/4-3-218 hybridization in Jiangsu Xuzhou Sweet Potato Research Center. Both methanol and acetonitrile were chromatographic purity,and β-carotene standard sample was bought from Sigma Company.

Test method

On June 16,2012,sweet potato was planted in experimental field of Sweet Potato Institute,China Academy of Agricultural Sciences.It was sandy loam soil,and common highyield cultivation measures were used.We dug root for sampling and investigation in 40,70,100 and 125 days after planting.Partial root samples were madeintodrypowder,which was stored in-70℃of ultra low temperature freezer to measure β-carotene,iron,zinc and other traits.

Measuring items and methods

Branching number of single plant,the longest vine length,fresh weights of stem and leaf,rates of large and medium sweet potato,fresh sweet potato yield were investigated in the field,while dry matter rates of stem,leaf and root were determined in the laboratory.Extraction and determination methods of β-carotene referred to that of Wu Xinet al.[3],while iron and zinc contents were measured by atomic absorption spectrometry[4],and starch content,protein content,carbohydrate content and component determination referred to the method of Maet al[5].

Statistical analysis of test data

Excel 2007 and DPS7.5 statistical analysis software was used for data processing,and ANOVA was used for variance analysis.LSDwas used for multiple comparison to test significant difference among treatments.

Results and Analysis

Variations of β-carotene and protein contents in storage root of orange-fleshed sweet potato during growing process

Seen from Fig.1,β-carotene content quickly rose in 40-70 days after planting,and reached the maximum(29.61 mg/kg)in the 70thday.At this time,it was key period of β-carotene accumulation.From the 70thday to harvest,β-carotene contentslowly declined.During whole growth period,β-carotene showed single-peak fluctuation trend.Protein content showed first declining and then rising tendency(Fig.2).During 40-100 days,protein content slowly declined and reached the minimum(1.48% )in the 100th day,and then slowly rose until harvest.

Variations of starch and carbohydrate substance contents in storage root of orange-fleshed sweet potato during growing process

Fig.3 displayed that starch content showed first rising and then declining change curve.Starch content slowly rose during 40-70 days and reached the maximum(19.99% )in the 70thday,and then slowly declined.When harvesting,starch content was the minimum.Seen from Fig.4,change curve of carbohydrate substance was just contrary to that of starch,and showed first declining and then rising fluctuation.Here,fluctuation range of sucrose was larger,while that of fructose and glucose was smaller,and they all reached the maxima during harvest period.

Variations of iron and zinc contents in storage root of orange-fleshed sweet potato during growing process

Iron content declined all the time.It quickly declined during 40-70 days,and then slowly declined after 70 days(Fig.5).During whole growth process,zinc content in storage root of orangefleshed sweet potato was straightly down (Fig.6).Moreover,iron content was always higher than zinc content.

Correlation study among major nutritional ingredients of orangefleshed sweet potato

During whole growth period,correlation among major quality traits in storage root of Xuyu Potato 34 was analyzed initially (Table 1).Result showed that sucrose content of fresh potato presented extremely significant negative correlation with β-carotene and significantly positive correlation with fructose;starch content presented extremely significant negative correlation with fructose and significantly positive correlation with zinc content;correlation among othernutritional qualities was smaller or did not reach significant level.

Table 1 Correlation coefficients among important quality traits in growth process

Discussion

Storage root of sweet potato mainly contains starch and soluble sugar[6-7],in which soluble sugar includes reducing sugar(glucose and fructose)and sucrose.Reducing sugar is original synthetic product of photosynthesis of higher plants,while sucrose is major form of organic matter transport in plant,and soluble sugar is major material or precursor substance of starch synthesis[8].When studying the relationship between soluble sugar content and starch accumulation in growth process of sweet potato,we found that starch content of storage rootshowed extremely significant negative correlation with soluble sugar,especially fructose,which was similar to prior research conclusion[9-10].The research result also showed that in prior period of sweet potato growth,soluble sugar(mainly sucrose)in leaf was quickly transported to storage root,and quickly transformed into starch and gradually accumulated;in middle and latter periods of growth,soluble sugar content started to gradually increase,especially sucrose quickly accumulated,while starch content slowly declined.Maybe it was caused by related enzyme activity of starch synthesis declining or amylase activity rising during the period[11].

Human body cannot synthesize vitamin A,which must be obtained from food.β-carotene is precursor substance of synthesizing VA,has the effects of protecting vision,preventing nyctalopia and sclerophthalmia[12-13],also has antioxidative effect[14],can enhance immunity and improve the anticancer ability of organism[15].Orangefleshed sweet potato receives more and more concerns because of rich in β-carotene,and is natural source of βcarotene.LiMingetal.explored change trend of β-carotene of stock variety during growth period under grafting situation,and result showed that β-carotene content change was divided into two types:significant(type I)and insignificant(type II)[16].Our research conclusion thought that change dynamic of β-carotene content in storage root of Xuyu Potato 34 belonged to type I,and it showed single-peak fluctuation curve of first rising and then declining.In biosynthetic pathway of plant isoprenoid,β-carotene is transformed from the first direct precursor substance GGPP,and GGPP is transformed from glucose molecule which is direct product of photosynthesis[17].In the research,it was found that change trend of β-carotene was contrary to that of soluble sugar content,and it showed extremely significant negative correlation with soluble sugar content,especially sucrose content.It showed that in the transformation process from sugar to β-carotene,their contents had a reciprocal relationship.

Besides rich in starch and soluble sugar,sweet potato also contains many kinds of vitamins and trace elements,such as iron and zinc,etc[18].Iron is essential trace element of human body,whose content is the highest.It combines with hemoglobin and myoglobin,joins in oxygen transport,storage and respiration[19],can enhance immunity,and plays important role in synthesizing cell pigment and many kinds of metal enzymes[20].Zinc exists in human body with the form of enzyme.It has important roles in promoting organism growth and development,maintaining sexualorgan function and improving organism immunity[21-22].So far,scholars mainly studied determination methods[23-24],contents[4]and region differences[25-26]of trace elements in sweet potato,but the understanding on change dynamics of iron and zinc contents was less.We found that iron and zinc contents in development process of storage root of orange-fleshed sweet potato de-clined allthe time,which maybe caused by that iron and zinc accumulation was lower than expanding rate of storage root.We also found that starch content was positively correlated with zinc content,which indicated that zinc content in storage root of sweet potato rich in starch may be higher.It provided possible channel for breeding of sweet potato variety rich in zinc.

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