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 offersvast 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 beof 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.
