MULTI-LOCATION ENVIRONMENTAL STUDIES OF YIELD AND ANTIOXIDANT PROPERTIES OF SEVEN VARIETIES OF SWEET POTATO (IPOMOEA BATATAS (L) LAM)

ABSTRACT

The  chronic problem of fluctuations  in environmental conditions coupled with the  lack  of cultivar evaluations over a wide range of environments, have limited the rapid development of elite sweet potato (Ipomoea batatas (L) Lam.) cultivars across different agro-ecological locations of Nigeria. To evaluate the multi-locational environmental performance of sweet potato over three agro-ecolological locations, (Mbano, Nsukka and Umudike) in Eastern Nigeria, seven local elite  sweet  potato  varieties Ex-Onyunga, Centina,  NRSP/05/022, PYT/09/057, PYT/09/0x2, PYT/09/013, and Tis 87/0087 were used in yield and yield components, nutritional and antioxidant, as well as genotype x environment interaction, using the instrumentality of G x E biplot analysis, studies. The experiments were set out in a randomized complete block design (RCBD) with three replications in each of the study areas. The result of the analysis of variance showed a significant (p<0.05) genotypic effect and a genotype by location interactions. The biplot analysis showed that genotypes (varieties) responded differently with respect to prevailing environmental conditions in a particular location. The  variety NRSP/09/022 had significant overall best performance in seven out of the seventeen traits studied and Nsukka was the best environment for seven traits (stand count at harvest, saleable tubers, dry matter content, ash content, starch content, water absorption capacity and bulk density). Umudike was best environment for eight traits (leaf biomass, saleable tuber weight, total tubers, un-saleable tuber weight, moisture content, total flavonoid, total carrotenoid and scavenging activity. Mbano was the least environment and was best in two characters (un-saleable tubers and total phenolic compounds. The biplot showed that NRSP/09/022 was best in the following yield components; saleable tubers, saleable tuber weight, total tubers and un-saleable tuber weight, as well as in, ash content, bulk density, moisture content and water absorption capacity. Tis 87/087 (check variety) was best in leaf biomass, and showed significant (p<0.05) values in saleable tubers, saleable

tuber weight, and stand count at harvest and total tubers. Variety Ex-Oyunga was best for total phenolic compounds and total flavonoid. Variety Centinna was best in total carotenoid based on the  mean  yield.  Following this result, NRSP/09/22, and Tis87/087 can be  selected as best varieties and Umudike and Nsukka as the best locations.

CHAPTER ONE

1.1     INTRODUCTION

Sweet potato (Ipomoea batatas (L) Lam.), belongs to the family Convolvulaceae and originated from South America from where it was introduced to Europe between 1565 and 1573 (Adams, 2005).  It  is  a  hexaploid,  and  usually  considered the  only  species of Ipomoea of economic importance. Horton and Sawyer (1985) as cited by Babatunde et al., (2007) reported that globally, sweet potato is a very important food crop after rice, wheat and corn. Also, of the world’s root and tuber crops, sweet potato ranks third after Irish potato and cassava (Ikeorgu,

2003). It is the only crop among the root tuber crops that had a positive per capita annual rate of increase in production in sub-Saharan Africa FAOSTAT, (2007). Sweet potato has a high yield potential that may be realized within a relatively short growing season and it can adapt to a wide range of ecological conditions ranging from 0.2000-0.4000 meters above the sea level and can grow in areas between 30N and 30S of the equator (Hahn and Hozyo, 1984) as cited in Belehu, (2003). Sweet potato has become an important staple food in the East African region, after banana  and  maize  (FAO,  1991).  Small-scale  farmers  produce  it  under  traditional  farming systems where it serves as food to humans and livestock and for industrial use, but generally, yields vary with cultivar, disease resistance, location and production practices (Onwueme, 1978).

The importance of sweet potato is increasing in Nigeria’s farming and food systems because it is easy to plant, matures quickly and has enormous industrial and economic potentials (Chukwu, 1999). It is equally a crop with huge promise for food security in developing countries. Also  it  has  high  nutritional  energy  qualities,  and  the  leaves  are  consumed  as  vegetables (Holloway, 1993; Chukwu, 1999). In Kwara State of Nigeria, particularly in Offa Local Government Area (LGA), sweet potato has enjoyed some traditional sentiments of the people resulting to the harvest season being celebrated with feasting and cultural dances (Agbo and Ene,

1993).

Despite  the  unlimited  promises  that  this  staple  crop  has,  its  production  has  been

insufficient in Nigeria and there is need to improve on it through increased efficiency (Ndukwu,

2010). This insufficiency could be enhanced by proper education of farmers on the use of developed techniques and adequate use of materials in response to better information (Idiong,

2006). With the difficulties encountered in developing countries in adopting improved technologies due to resource poverty, efficiency has become a very significant challenge in increasing productivity and therefore requires concerted effort across disciplines to improve on this existing menace (Ali and Chandry, 1990).

Though Nigeria with an estimated production quota of 3.49 million metric tons ranks first in Africa, followed by Uganda her contribution to global sweet potato production is only about

0.2 percent despite her wide arable land and favorable environmental conditions (FAO 2007). This low global potato output by Nigeria could be as a result of poor breeding programs, varieties and farming systems employed during farming and inconsistencies in performance of varieties over locations. There is therefore no gain-saying that it is necessary to evolve varieties with  high  yield  potentials and  consistent  performances over  diverse environments, because productivity of a variety is the function of its adaptability while the adaptability is dependent on fitness (stability) and flexibility. Evolution of more high yielding and stable varieties will as a matter of fact give us a global standing in production and utilization of sweet potato.

To further buttress the foregoing issues, it  has been demonstrated that micronutrient enrichment traits are available within genomes of the major staple food crops, including sweet potato. Research to identify accessions high in different nutritional qualities (dry matter, protein, starch, sucrose, ß-carotene, Fe, Zn, Ca and Mg) has been initiated by Center for International Potato (CIP), for germplasm in genebank and breeding which is evident in Suda et al., (2003). Such characterization needs to be done for the germplasm from the Nigerian region. Sweet potato  in  relation  to  other  vegetables  has  nutritional  potentials  with  endowed  medicinal properties for healthy living, like antioxidant activities. Antioxidants are substances or nutrients present in our food which can increase cellular defense and help prevent oxidative damage to cellular component of our bodies in the hope of maintaining health and preventing diseases such as cancer and coronary heart disease (Mervat and Hanan, 2009). In addition to many other uses in medicine and industrial field, it is used in manufacturing cosmetics and food preservation and colorant. Namiki (1990) cited by Mervat and Hanan (2009) reported that the synthetic antioxidants have restricted use in food, as they are suspected to be carcinogenic. The importance of searching for and exploiting natural antioxidants has increased greatly in recent years Patil et al. (2009). While, antioxidant is a class of plant secondary metabolites, the plant kingdom offers vast array of valuable natural polyphenolic compounds, several isolated plant constituents as

well as crude extracts of vegetables and fruits; which have been recognized to possess beneficial effects against free radicals in biological systems as antioxidants (Yi Fang and Xianzona, 2002).

National Root Crops Research Institute (NRCRI) Umudike and the International Institute for Tropical Agriculture (IITA) Ibadan have made a great effort to evolve new varieties that will meet  the  commercial, subsistence and  nutritional needs of the  nation.  Unfortunately, these varieties  have  not  received  a  wide  evaluation to  ascertain their  yield  potentials  and  agro- morphological characteristics across diverse environmental conditions. To this effect, Nwankwo et al. (2010) stated that morpho-agronomic characterization provides information that could be of immense support to a plant breeder and other users in the efficient use of newly acquired materials.  They  equally  opined  that  an  unevaluated  and  uncharacterized  accession  in  a germplasm is a resource lying waste; emphasizing the need for the breeder to evaluate and characterize  his  new collections to  ascertain their  breeding worth. Gruneberg et  al.  (2005) maintained  that  the  identified  and  characterized  accessions could  be  promoted  as  superior varieties to farmers or used as parents in a comprehensive breeding program for improved nutrition in sweet potato varieties without negatively impacting crop yields.

Apart from identifying varieties with good morpho-agronomic characteristics such as leaf biomass, good tuber yield which are rich in nutrients, there is need to understand the genotype x environment (G x E) as well as the stability of the nutrient traits across diverse environments to guide future choice and use of appropriate breeding strategies for the improvement of sweet potato (Gruneberg et al., 2005). Such an understanding would also allow making informed choices regarding which locations and input systems to be used in breeding efforts for improved nutrient varieties and new sweet potato populations.

In recent times, ideas have shifted from seeing sweet potato as a crop with high calories to other veritable nutritional potentials such as β-carotene which is a precursor of vitamin A and polyphenolic compounds which have anti-oxidative properties. Access to sweet potato varieties with such qualities will not only boost the nutrition of the consumers but also positions it as a global staple food. Consequently, evaluating and stabilizing these nutrients in this crop is a step in the right direction.

Stability of β-carotene (a micro nutrient) in sweet potato cultivars has been reported (Manrique and Hermann, 2002). African orange flesh sweet potato (OFSP) farmer varieties are a potential source of β-carotene whose genetic diversity and distinctiveness are not understood.

This is crucial if such varieties are to be maximally utilized for breeding. Understanding the foregoing can  help  to  tackle  the  problem of vitamin  A deficiencies in  diets  of Nigerians. According to Nwankwo et al., (2010), the World Health Organization (WHO) reported that vitamin A is a major factor contributing to an estimated 1 to 3million child death each year mainly in Africa and Southeast Asia. Malnutrition such as vitamin A deficiency can cause blindness, low intelligent quotient (IQ) to children and make them prone to numerous illnesses. Despite this precarious situation, no new orange fleshed varieties which are very rich sources of vitamin A has been introduced officially into Nigerian farming system till date in relation to their stability in the environment.

Previous studies (Negeve and Bouwkamp, 1993; Ravindran et al., 1995) on several traits have shown that sweet potato is sensitive to environmental variation, despite wide adaptability to harsh growing conditions. Consequently, research aimed at identifying varieties that are stable with respect to a given location is a welcome development. It is known that G x E interactions serves a useful purpose when carrying out such a meaningful task as it helps to identify stable varieties under different environmental conditions. Of additional importance, especially to multi- trait breeding objectives of the micronutrients in sweet potato, is the understanding of the genetic correlations of the target  quality traits.  All these  information are currently lacking. In sub Saharan Africa (SSA), breeding for ß carotene rich cultivars has been faced with moderate rates of acceptability (due to low dry matter) and high susceptibility to viruses and drought by the introduced Orange Fleshed Sweet Potato (OFSP) varieties. At the same time, Center for International Potatoes (CIP) and partners in the region have identified what are considered as African ß-carotene rich  farmer varieties,  which  are  more  adapted and  are  looked upon as important gene pool to enhance the breeding objectives for quality sweet potato in Africa. However, the genetic variation and distinctiveness of this group of OFSP farmer varieties are not well understood. This knowledge is important for efficient rationalization and utilization of this germplasm, designing appropriate plant breeding programs, as well as in making choice of parent genotypes for population development (Zhang et al., 1998; LaBonte et al., 1997).

Despite  its  growing  importance and  known potential as  food, animal  feed and  raw material source as well as the cultural sentiments it commands in some areas, records of sweet potato production, processing and marketing in Nigeria’s food system are scanty. Sweet potato has the potential for food security as well as serving as a cash crop. Sweet potato has the potential for food security as well as serving as a cash crop. In Nigeria, sweet potato cultivation

in  commercial quantities  is  still restricted to  few places despite wide arable  land  that  can

comfortably support the growth and productivity of the crop. Onwueme (1978)  reported that the areas of potato (both Irish and Sweet potatoes) cultivation in Nigeria as at 1971 were the middle- belt and the riverine states even though new awareness are ongoing and positive light are coming forth

. Estimates of sweet potato production in Nigeria varied widely among different sources. The report of the presidential task force on alternate formulation of livestock feeds in 1992; sets the National output at 530,000 tons for 1990; while FAO (2002) estimated that 143,000 tons of sweet potatoes were produced in 1990. Despite this low production quoted by FAO, it still represents a remarkable increase in the production of sweet potato in 1990 than in previous years, with output growing by nearly ten times over the decade. This points out that the crop has all it takes to perform wonderfully if efforts could be doubled (Tewe et al., 2003).

In Nigeria, meeting the food and nutrients needs of the ever increasing population has been a huge task for every succeeding government. How well this objective is achieved is often used to judge the performance of any Government. Therefore, anchoring any project to achieving such a disturbing objective is a great relieve to such a government. Tewe et al. (2003) submitted that the International Institute for Tropical Agriculture (IITA), Ibadan and the National Root Crops research Institute (NRCRI), Umudike have reported high agronomic yield potentials of sweet potato as a food security crop in Nigeria. However, this high yield potential is yet to be converted into increased output under the present farming system. One of the reasons identified for the failure to achieve sweet potato production in Nigeria is the bad agronomic system of cultivation. To this effect, Tewe et al. (2003) also pointed out that sweet potato is usually grown in crop mixture that has negative effects on its output.

In Nigeria sweet potato cultivation is challenged by its restriction to few places and its production is mainly for home consumption which could primarily be as a result of ignorance of the yield potentials even in marginal soil, adaptability of the crop over a range of environmental factors, cultural practices, nutritional potentials, and medicinal uses etc. These challenges are controlled by breeders through evaluating genotypes in several environments to ensure that they select varieties with high and stable performance over wide range of environments and places and also through plant hybridization, genetic engineering and distribution of improved varieties to rural farmers.

This research work is therefore poised to:

1.  Evaluate the yield potentials of the seven sweet potato varieties from National Root Crops Research Institute Umudike; Ex-oyunga, Centina, NRSP/05/022, PYT/09/057, PYT/09/0x2, PYT/09/013 and TIS 87/0087 on the three selected agro- ecological zones; Mbano, Nsukka and Umudike in South Eastern Nigerisa.

2.  Evaluate the effect of genotype and environment as well as the relative effect of the genotype and the environment interaction on the seven varieties of sweet potato.

3. Evaluate the antioxidant properties of the varieties of sweet potatoes under investigation over the three selected environments.

4.  Determine the possible interactions between environment and genotype in the antioxidant properties of the cultivars.

5.  Correlate the performances of the varieties in the different environments with the available meteorological data of the agro-ecological zones. 6.  Carry out a proximate analysis on the varieties to determine their other nutritional potentials.

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