THE EFFECTS OF PRE-HARVEST FOLIAR SPRAY AND POST-HARVEST DIP OF ETHEPHON ON THE PHYSIOLOGY AND QUALITY OF ORANGE-FLESHED SWEET POTATO APOMUDEN DURING STORAGE

ABSTRACT

The postharvest storage of sweet potato roots in the tropics is a major challenge in the crop’s value chain as it deteriorates readily in the hot climate. Although prolonged storage in the cold chain has been shown to be feasible, technical and economic constraints in the tropics makes cold storage inaccessible to growers and retailers of sweet potato. It has been demonstrated in several studies that the plant hormone ethylene greatly influences metabolic changes in many types of horticultural produce postharvest and that controlled application can improve storage. In this research, the ethylene analogue ethephon was applied at four different concentrations as foliar spray on the Orange-Fleshed Sweet Potato Apomuden seven days before harvest and stored in the ambient. Untreated roots were also dipped in the same ethephon concentrations postharvest. Changes in the physiological and biochemical quality (preharvest treatment only) viz. weight loss, sprouting, decay, shrinkage, dry matter content, starch, sucrose, glucose, fructose and minerals concentration were studied. Preharvest foliar application of ethephon resulted in superior storage performance than postharvest root dip as it better reduced sprouting, improved resistance to weevil damage and shrinkage. Preharvest application of 500 ppm ethephon significantly reduced the dry matter loss compared to the control. Furthermore, carbohydrates, proteins and minerals were significantly more abundant in the ethephon treated roots.

CHAPTER ONE

INTRODUCTION

Background to the Study

Sweet potato (Ipomoea batatas (L.) Lam) is a member of the Convolvulaceae family which is grown for its fleshy storage roots. Sweet potato is reported to have originated in northwestern South America, arising from a hybrid cross or through karyotypic changes from an unidentified plant of the genus Ipomoea. It was introduced to Europe by the Spanish and spread to China, Japan, Malaysia and the Moluccas area. The Portuguese introduced it to India, Africa and Indonesia (O'Brien, 1972).

The root crop is grown annually though it is a perennial crop. It confers a wide range of health benefits that have recently enhanced its popularity. Sweet potato is one of the important food crops especially in developing countries (Woolfe, 1992). The white fleshed variety is the most cultivated in many developing countries and it is ranked among the five most important food crops (CIP, 2016). Sweet potato is among the world's most nutritious, yet under-exploited food crops (Srinivas, 2009).

With annual global production of more than 133 million tonnes, sweet potato is currently positioned as the fifth most essential food crop in developing countries after rice, wheat, maize, and cassava (Oke & Workneh, 2013). Sweet potato is among the widely grown root crops in Sub-Sahara Africa. It is specifically cultivated in countries surrounding the Great Lakes in Eastern, Central and parts of West Africa (Shonga, Gemu, Tadesse, & Urage, 2013). The cultivated area in Africa is about 3.2 million hectares with a projected output of 13.4 million tonnes of roots in 2005. It is mainly produced in marginal soils in low-input subsistence farming systems of developing countries where it is a major food crop (Woolfe, 1992).

Sweet Potato Cultivation and Storage Challenges

Despite its reputation as a food security crop, sweet potato roots are highly perishable if not well treated and properly stored. Particularly, damage by diseases and pests can lead to severe economic losses. The most common sweet potato pests include sweet potato weevil, white fly, wire worm and cricket (Theberge, 1985). These pests attack the roots both in the field and during storage. Some of the diseases that affect sweet potato include fusarium wilt, soil rot, black rot, root knot-nematode and other viral diseases (Jackson

Bohac, 2006). They all lead to decay and loss of economic value of sweet potato roots. Furthermore, temperatures above 15°C lead to more rapid sprouting and weight loss. The weight loss is due to increased respiration which uses up the stored food reserves in the roots. This restricts their shelf life and the distance over which sweet potato can be economically transported (Rees et al., 1998).

Maximum quality of sweet potatoes is mainly determined by the type of pretreatment that they are subjected to immediately after harvesting. This can also be determined by the growing conditions of the roots. Pre-harvest cultural practices during the growing season will later affect postharvest quality. Some factors such as the weather are impossible to control when the crop is planted in the open fields. Other growing factors like fertilizer application can be manipulated by a grower to ensure that a quality product goes into storage. The weather during the growing season, especially just before and during harvest, has a major effect on the postharvest quality. An extended drought, followed by heavy rain frequently accelerates growth, which often produces roots with delicate skin that are prone to growth cracks and attack by soil-borne pests.

Several studies have been conducted in the past to investigate the effective storage methods to prolong the shelf life and foster availability of the crop year round. Research on various methods of extending the shelf life of sweet potato has been ongoing. These include cold storage, modified atmosphere (MA) storage, pit storage and processing to chips. Other preservation methods involve the application of botanical extracts in the field prior to harvesting.

Some plant hormones like ethylene have been used to preserve both climacteric and non-climacteric produce. The effects of ethylene can extend or reduce the shelf life of agricultural produce depending on the growth stage, species, cultivar, type of tissue and the ethylene application regime (Abeles, 1969). Ethylene effects on the quality of climacteric fruits have been well documented and it is an important commercial hormone used for extending their shelf-life. Ongoing research on the effects of exogenous ethylene on low ethylene producing vegetables is largely focused on understanding its role in regulating dormancy, sprout growth and senescence in potatoes and onions (Amoah, 2014). The use of supplemental ethylene can help in quality retention of stored sweet potato and other agricultural produce (Cheema, Rees, Westby,

Taylor, 2008). In this research ethephon, as an ethylene precursor, was used to treat orange-fleshed sweet potatoes and its potential to preserve quality and shelf life of the roots was investigated on sprout control, storage decay, weight loss and shrinkage. In addition, the effects on the abundance of some nutritionally beneficial phytochemicals were studied.

Problem Statement

Sweet potato roots once harvested deteriorate rapidly mainly due to physiological, biochemical and microbial changes, which are accelerated by mechanical damage during harvesting, transportation and handling. In tropical weather conditions, the tuberous roots endure just 1-2 weeks and not more than 5 weeks under favorable storage conditions (Rees et al., 2003). This significantly reduces the economic and health prospects that could be derived from the production and utilization of sweet potato. The roots exhibit short dormancy and are susceptible to sprouting, decay and insect infestation during storage. The very short storage life after harvest has been recognized as a major hindrance to the cultivation of the crop. As a result, processing the roots into storable forms such as chips and flours are done to prolong the shelf life (Ayinde & Dinrifo, 2001). However, most consumers prefer fresh sweet potato roots to the processed ones. This has encouraged the production and marketing of fresh sweet potatoes but the fresh roots require suitable pretreatment and storage innovations to make it available over the entire year.

In many rural communities in the Central Region of Ghana, sweet potato production is a main income earner for some small-scale farmers. However, these farmers are hindered by the high perishability caused by sprouting and decay of the roots (Duku, 2005). The lack of good storage facilities compels the farmers to sell their sweet potatoes at the most reduced prices when there is surplus during the harvesting season. This has led to food insecurity and high economic losses from the monetary investments in sweet potato cultivation. This discourages more famers from taking up sweet potato farming (Birago, 2005).

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Item Type: Ghanaian Topic  |  Size: 137 pages  |  Chapters: 1-5

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