EVALUATION OF GAMMA RADIATION-INDUCED MUTANT LINES OF SESAME (SESAMUM INDICUM L.) FOR GENETIC IMPROVEMENT OF SOME DESIRABLE TRAITS

ABSTRACT

The creation, evaluation and selection of mutants with desirable traits have served as source of genetic variability for breeding programmes of many crops. Thus, eleven mutant lines of sesame developed through induced gamma irradiation were evaluated in the M4 generation alongside the three parental varieties [NCRIBEN 04E (check-1), NCRIBEN 01M (check-2) and NCRIBEN 03L (check-3)]. The mutant lines and their respective checks were laid in a randomized complete block design (RCBD) with three replicates each. All the parameters were studied following standard procedures. The results of vegetative parameters revealed that mutant lines ML-6 (123.83 cm) and ML-8 (121.59 cm) had highest plant height at maturity with 5 % level of significance while ML-7 had the highest number of branches per plant. There was no significant difference (P ˃ 0.05) in number of days to 50 % flowering. ML-2 (77.30), ML-3 (105.33), ML-7 (151.00) and ML-8 (163.00) produced higher number of capsules per plant than their respective checks. Mutant lines with improved capsule characteristics comprised of ML-2 (2-3 capsules per leaf axil), ML-6 (1-2 capsules per leaf axil), ML-9 (1-2 capsules per leaf axil), ML-10 (2-3 capsules per leaf axil) and ML-11 (1-3 capsules per leaf axil). All the M4 mutants had adequate pollen viabilities (over 80 %). The highest pollen germinability was recorded at 20 % sucrose concentration for all the mutant lines. Suboblate shaped pollens with 10-13 colpi were observed in all the mutant lines and the checks. Significant increase was observed in oil contents of ML-2 (40.32 %) and ML- 10 (40.05 %) over their check groups. All mutants derived from Check-1 [ML-1 (22.83 %), ML-2 (28.63 %), ML-3 (27.41 %)] and Check-2 [ML-4 (23.72 %), ML-5 (22.85 %), ML-6 (23.41 %) and ML-7 (32.83 %)] showed significant enhancements in protein content of the seeds with ML-7 (32.83 %) having the highest value. Similarly, all mutants showed significantly higher (P ˂ 0.05) moisture contents than their checks except ML-8 (3.38 %). ML-10 (0.303 %) had the highest tannin value while the least oxalate and phytate contents were recorded in ML-1 (2.545 %) and Check-2 (0.532 %), respectively. All the mutants obtained from check-1 and check-3 showed significantly higher free fatty acid except ML-8 (1.13 %). Result on oil composition revealed that the physical properties of oil obtained from all the mutant lines and  checks are within the acceptable limits by Codex. The variability observed in morphological, yield, seed nutritional composition and oil properties of the M4 lines reflects the existence of genetic diversity as a result of gamma irradiation and suggests potential genetic improvements for sesame. Mutant lines with higher pollen viability could be used as male parents in controlled pollinations for increasing the productivity of sesame and further improvement of the crop.

CHAPTER ONE

1.0 INTRODUCTION

1.1 Background to the Study

Sesame (Sesamum indicum L.), also referred to as the queen of oil seed crops is said to be the most traditional and the oldest oil seed crop that is valued for its high-quality oil seeds (Sruba and Amitava, 2017). According to Rizki et al. (2015), sesame plants are considered to have originated from Africa and have been utilized extensively for thousands of years as a seed of worldwide significance for edible oil, cake, paste and for confectionary purposes. Sesame oil can be used for cooking, salad oils and margarine, manufacture of soaps, pharmaceuticals, insecticides, perfumes and paints (Tadese and Misgana, 2017). Sesame oil is preferred as cooking oil among many Nigerian families (Falusi  et al., 2001) and is increasingly becoming a popular seed oil (Biabani and Pakniyat, 2008). In Nigeria, sesame is locally called by different names; ‗Ridi‘ in Hausa, ‗Esso‘ in Nupe, ‗Eeku‘ in Yoruba and Ekuku‘ in Igbo (Muhammad, 2018).

Sesamum indicum L. (2n = 26) is a self-pollinated crop that belongs to the family Pedaliaceae (Falusi et al., 2001). The genus Sesamum consists of 36 species, most of which are wild with S. indicum being the most commonly cultivated species (Purselglove, 1974; Falusi, 2006; Abejide et al., 2013). Sesame is primarily grown for its oil rich seeds which are used for oil extraction (El Khier et al., 2008; Olaleye et al., 2018). Approximately, half of the seeds weight is its oil [International Plant Genetic Resource Institute and National Bureau of Plant Genetic Resources (IPGRI and NBPGR, 2004)]. Sesame oil contains two major constituents; sesamolin (0.3 – 0.5 %) which yields a powerful antioxidant (sesamol) on hydrolysis that gives excellent stability to oil and sesamin (0.5 – 1.0 %), both of which are absent in other fixed oils (Pathak et al., 2018; Saha, 2018). Anilakumar et al. (2010) reported that sesame seeds contain 43.3 – 44.3 % oil and around 39.0 % of oil present in sesame consists of monounsaturated fatty acid, 46.0 % polyunsaturated fatty acid and 14.0% saturated fatty acid. According to Savant and Kothekar (2011), the oil content of sesame seeds and its fatty acid compositions are greatly influenced by both the genetic makeup as well as environmental conditions during oil accumulation. Oil characterization is a vital parameter used in determining the quality of oil seed crops (Mohammed, 2019).

According to Food and Agricultural Organization (FAO), (2019), Nigeria is the world‘s fourth largest producer of sesame seed with an annual production of about 550,000 tons placed after Tanzania (805,691 tons), Myanmar (764,320 tons), and India (751,000 tons). The  productivity  is  however  relatively  low  (global  production  of  4.04  million  tons annually) compared to other oil seed crops (Hota et al., 2016). The major yield constraints are lack of novel hybrids, narrow genetic base, low harvest index, lack of shattering resistance,  and  longer  days  to  maturity  and  prevalence  to  abiotic  and  biotic  stress conditions (Kumari et al., 2016).

Genetic variability as a result of induced mutations by various mutagens has contributed to modern plant breeding and has played a major role in the development of superior plant varieties (Kharkwal and Shu, 2009; Audu et al., 2018). Mutation breeding has played a key role in the improvement of self-pollinated crops with limited genetic variability (Girija and Dhanavel, 2013). It has been used for the improvement of cowpea by Dhanavel et al. (2008), black gram by Thilgavathi and Mullianathan (2009), wheat by Sirvastava et al. (2011), rye by Jong-jin et al. (2012), Sorghum by Murali et al. (2013), rice by Omorigei etal. (2014), sesame by Aliyu et al. (2017), in pepper by Yafizhan and Herwibawa (2018) and in so many other crops.

In sesame plant, Muhammad (2018) studied the M2 lines of gamma-irradiated sesame seeds and revealed desirable traits like multicapsule per leaf axil and multicarpellate capsules. Similar studies of Mohammed (2019) on M3 lines of the same mutants of sesame revealed lower oxalate and peroxide values and a rise in free fatty acid. Further research on these lines might lead to the discovery of more desirable traits. Similarly, there is a need to further test for the stability of the observed desirable traits.

1.2 Statement of the Research Problem

In the recently released lists of mutant varieties throughout the world by International Atomic Energy Agency  (IAEA)  (2019), no  single mutant  variety of sesame has  been released in Nigeria despite the fact that Nigeria remains one of the major producers of sesame. This might be due to lack of continuity in mutation breeding programmes.

Although studies of Muhammad (2018) on M2 generation of these mutants has revealed promising desirable traits like multicapsule per leaf axil and multicarpellate capsules, the stability of these traits is yet to be ascertained. Similarly, reductions in oxalate and peroxide values as well as a rise in free fatty acid in the M3  lines (Mohammed, 2019) are not yet ascertained.

Information on irradiation induced changes in pollen viability and germinability that will eventually affect the yield attributes of sesame is scanty (Falusi et al., 2013). In addition, there is dearth of information concerning the nutritional composition and oil properties of sesame cultivars grown in Nigeria in general as well as mutant lines and breeder‘s lines.

1.3 Aim and Objectives of the Study

The aim of this research was to evaluate the fourth mutant (M4) generation of sesame for genetic improvement of some identified desirable traits.

The objectives of this study were to determine the:

i.        vegetative and yield parameters of M4  lines

ii.       pollen parameters of M4  lines

iii.     proximate composition and anti-nutritional factors of M4  sesame seed

iv.      quantitative and qualitative attributes of M4  sesame seed oil

1.4 Justification for the Study

For many decades, mutagenesis has been successfully utilized as a tool for inducement of genetic variability in many crops, giving room for the isolation of mutants with desirable characters of economic importance and as a result, many new cultivars have been directly or indirectly released in the world (Diouf et al., 2010). The success of this work will thus ensure continuity in the evaluation of the mutant lines which will eventually lead to varietal release.

In mutagenesis, many phenotypic traits displayed by mutant lines in their early generations are not true and might be due to certain environmental factors. Thus, further evaluation of the mutants will determine whether they are true mutants or not.

Evaluation of pollen parameters of the mutant lines will add more information to the few existing literature. The extent of pollen viability and germinability will indicate how effective the mutant lines would be as a male parent or pollinator and can be used in future breeding programmes.

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