ABSTRACT

Four field experiments were conducted at the Teaching and Research Farm of the Department of Crop Science, University of Nigeria, Nsukka to: (i) improve the photoperiodic response, especially in local (late) maturing varieties of Okra; (ii) estimate heterosis and heritability for improved yield and productivity; (iii) determine combining ability and gene effects of 13 genotypes of okra; (iv) estimate gene effects of quantitative traits; (v) determine inheritance pattern of qualitative traits for improved fruit production. The experimental materials used were three local varieties (Ele Ndu, Ele Ogwu, Ele Uhie) collected from local farmers in Nsukka LGA, Enugu State; and ten improved varieties (LUDU V., Esculentus V, Jokoso, Agwu Early, TAE 38-Dwarf, V-21 Ivra, Clemson spineless, LD88 V., V.35 and NHE 47-4 V) from the National Institute for Horticultural Research and Training (NIHORT), Okigwe, Imo State. General combining ability of the parents and specific combining ability of the hybrids were estimated using Griffings’ model 1 method 2 in a 5 x 5 diallel crosses. Chi square statistic was used to test for the inheritance pattern and degree of significance in traits of interest. ‘Ele Uhie’ parent showed significantly (p < 0.05) higher branch length, number of fruits/branch, number of fruits/plant, number of flowers/plant, plant height at maturity and total fruit yield/ hectare (123.7 cm, 8.31, 29.76, 9.03, 237.66 cm and 32.56 t/ha, respectively) than the other parents. ‘Agwu early’ parent had lower days to 50% germination, days to flower bud initiation, days to anthesis, days to 50% flowering, days to first fruiting, days to fruit maturity and plant height at maturity (4.00, 22.21, 47.13, 48.31, 51.10, 52.28 and 57.8 cm). ‘Ele Uhie’ showed significantly (p < 0.05) higher 100 seed weight (5.19 g), dry fruit weight (11.46 g), fruit girth (11.10 cm), fruit weight (34.5 g), number of ridges/pod (11.33) and number of seeds/pod (120.57). The cross ‘UHIE x LD88’ had significantly (p < 0.05) higher number of branches/plant (7), number of fruits/branch (8.64), number of fruits/plant (35.63), number of flowers/plant (10.16) and total fruit yield/hectare (38.25 t/Ha) . Higher negative better parent heterosis values were observed in the cross, ‘UHIE x CLM’ in days to 50% germination (-46.32), days to flower bud initiation (-49.26), days to anthesis (-46.54), days to 50% flowering (-45.12), days to first fruiting (-45.12) and days to fruit maturity (-35.05). The cross ‘OGW x LD88’ had higher positive BPH value in total fruit yield/hectare (161.26). Narrow sense heritability in total fruit yield showed values above average in all the crosses except ‘AGW x CLM’, ‘AGW x LD88’, ‘CLM x LD88’, ‘OGW x CLM’ and ‘UHIE x CLM’. ‘Ele Uhie’ was the best general combiner for number of branches/plant (0.65), number of fruits/branch (2.01), number of fruits/plant (2.37), number of fruits/stem (3.75) and total fruit yield (9.97). ‘Ele Uhie’ was the best general combiner in days to flower bud initiation (-7.18), days to anthesis (-7.46), days to 50% flowering (-8.11), days to first fruiting (-7.46) and days to fruit maturity (- 8.43). The cross, ‘UHIE x LD88’ was the best specific combiner for number of fruits/branch (3.31), number of fruits/branch (0.7414), number of fruits/plant (9.34), number of flowers/plant (2.53) and total fruit yield (10.26). The gene effect for total fruit yield showed significant positive additive, dominance and additive x dominance gene effects occurred in all the crosses with the exception of ‘UHIE x AGW’, ‘UHIE x CLM’

and ‘UHIE x LD88’. The relative proportion of additive gene effect was important for high heritability estimates recorded. This suggests that its contribution to the inheritance of earliness is ideal for developing desired hybrids. The medium-long and green with pink patched fruits predominated in the F2 populations and the observed ratio mostly fitted the expected ratio of 3:1. The hybrid ‘UHIE x LD88’ with significantly higher yield potential has the potential for commercial cultivation after further selection.

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INTRODUCTION

Okra (Abelmoschus spp.) is one of the most significant vegetable crops in the Malvaceae family and is very popular in the Indo-Pak subcontinent (Kumar et al., 2010). It belongs to the genus Abelmoschus and the species esculentus. Okra is an upright annual, herbaceous plant with a hibiscus-like flower. It is a direct-sown vegetable with duration of 51-68 days for the early maturing varieties and 70-120 days for the late maturing varieties with partly deep taproot system. Its stem is semi-woody and sometimes pigmented with a green or reddish tinge of colour.

According to Martin (1982) and Siemonsma (1982), okra can be broadly classified into two main varieties, namely: Early Okra [Abelmoschus esculentus (L.) Moench] and Late or West African Okra [Abelmoschus callei (A. Chev.) Stevils]. Early Okra is similar to exotic Okra cultivars which are found in several okra growing regions of the world while Late Okra is restricted in distribution to the most humid parts of West Africa (Singh and Bhatnager, 1975; Martin, 1982; Siemonsma, 1982). Njoku (1958) coined the names early and late Okra when his studies on photoperiodism in Nigeria, showed that Early Okra has a Critical Day Length (CDL) of 12.50 h and so can flower at any time of the year while Late Okra with CDL of 12.25 h can only flower later in the year, around August-September, when natural day length shortens considerably.

Worldwide production of okra as fruit vegetable is estimated at six million tonnes/year. In West Africa, it is estimated at 500,000 to 600,000 tonnes/year (Burkil, 1997; Farinde et al., 2007). It is one of the most important vegetable crops in Nigeria, accounting for 5.5% of the total vegetables cropped (Sorapong, 2012). Nigeria is the second largest producer of okra in the world with an estimated 0.72 million tonne produced annually, which represents 15% of total world annual production (Gulsen et al., 2007). It is a common household vegetable crop, cultivated primarily for its fresh pods and leaves. It is often called “a perfect villager’s vegetable” because of its rich source of dietary fibre, minerals, and vitamins; often recommended by nutritionists in cholesterol controlling and weight reduction programmes (Holser and Bost, 2004; Sanjeet et al., 2010). Its mucilage is suitable for medicinal and industrial applications (Adetuyi et al., 2012).

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In crop improvement programme, the success rests upon isolation of valuable gene combinations as determined in the form of lines with high combining ability, lines which produce good progenies on crossing being of immense value to the plant breeder (Adiger et al., 2013). Diallel analysis provides good information on the genetic identity of genotypes as well as being one of the most powerful tools for characterizing the genetic architecture of plants (Obiadalla-Ali et al., 2013). Similarly, diallel crosses have been used in genetic research to determinate the inheritance of a trait among a set of genotypes and to measure combining ability and gene action for earliness, yield and yield components in Okra (Yan and Kang, 2003).

Okra research, especially in developed countries, has for long been concentrated on early or conventional okra, with little attention paid to late or West African varieties. Though, they are photoperiod sensitive with wide variations in days to flowering and fruiting, they also contain many desirable genes not found in early or conventional Okra (Singh and Bhatnager, 1975; Martin, 1982; Siemonsma, 1982; Udengwu, 2008). Sensitivity to photoperiodism is a very important expression of genetic diversity in Okra in West Africa (Udengwu, 2008). The photoperiodic nature apparently makes it difficult to cultivate the late maturing varieties twice during a growing season. Early cultivation in February/March often results in excessive vegetative growth as compared with planting in July/August (Adeniji, 2003).

There is every need to develop early flowering and consequently early maturing genotypes of okra with improved fruit yield so as to meet the ever increasing demands for the crop. Also, an understanding and subsequent improvement in the photoperiodic response of okra, especially the West African varieties is necessary in organizing a systematic breeding programme to improve yield and associated traits of the crop. Hence, the objectives of the study were:

·       To improve the photoperiodic response, especially in local or late maturing varieties of Okra

·       To estimate heterosis and heritability for improved yield and productivity

·       To determine combining ability

·       To estimate gene effects of quantitative traits

·       To determine inheritance pattern of qualitative traits for improved fruit production

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LITERATURE REVIEW

Okra (Abelmoschus spp.) also called lady's fingers (American), gumbo (West Indies), bhindi (India) or okra, is a traditional vegetable crop well known for its rounded and ridged nature of fruits (Kumar et al., 2010). It is mainly an allopolyploid (hybrid polyploid) plant.

Origin and Distribution

The genus Abelmoschus originated in South-East Asia. Okra (Abelmoschus spp.) specifically originated somewhere around Ethiopia, and was cultivated by the ancient Egyptians by the 12th Century BC. Its cultivation spread throughout Middle East and North Africa (Tindall, 1983; Lamont, 1999 and Reddy, 2010). It is one of the most important vegetable crops, grown throughout the tropical and subtropical parts of the world. It is widespread in tropical, subtropical and warm temperate regions, but is particularly popular in West Africa, India, the Philippines, Thailand and Brazil. Abelmoschus esculentus has been reported from the whole of tropical Africa, whereas West African Okra [Abelmoschus caillei (A.Chev.) Stevels] is restricted to the humid and per humid climates of Africa (Siemonsma and Kouamé, 2004). It is one of the oldest cultivated crops and presently grown in many countries and is widely distributed from Africa to Asia, Southern Europe and America (Sorapong, 2012).

Growth and Development

Okra is erect, variable in branching, with many short branches that are connected to the thick semi-woody stem. The plant attains heights of 0.4 m - 0.80 m in early maturing varieties to 1.5 m-2 m in others (Udengwu, 2008). It is generally an upright annual plant. Its stem is rounded, erect, and variable in branching. The stems also bear leaves that are lobed and are generally hairy, some reaching up to 12cm in length. Leaves are cordate (heart-shaped), simple or palmately lobed and veined. The woody stem bear leaves that are generally hairy, some reaching up to 12 inches in length. Okra leaves are dark green in color and resemble a maple leaf (Kumar et al., 2010). Flower is axillary and solitary. Okra plants are characterized by indeterminate growth. Flowering is continuous but highly dependent upon biotic and abiotic stress. The plant bears its first flower one to two months after sowing i.e. improved varieties. The fruit is a capsule and grows quickly after flowering. The greatest increase in fruit length, height and diameter occurs during 4th to 6th day after pollination (Tripati et al., 2011).

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Floral and fruit biology

Okra flowers are 4-8 cm in diameter, with five white to yellow petals, often with a red or purple spot at the base of each petal and the flower withers within one day. The flower structure combines hermaphroditism and self compatibility. Flower bud appears in the axil of each leaf, above 6th to 8th leaf depending upon the cultivar. The crown of the stem at this time bears 3-4 underdeveloped flowers but later on during the period of profuse flowering of the plant there may be as many as 10 undeveloped flowers on a single crown. The flower buds, especially those of the conventional varieties take about 22-26 days from initiation to full bloom. The style is surrounded by a staminal column which may bear more than 100 anthers. The pollen may come in contact with the stigmas through a lengthening of the staminal column or through insect foraging (Thakur and Arora, 1986; Abdelmageed, 2010). Thus the flowers of okra are self fertile. The pollen grain is large with many pores, and every pore is a potential tube source; therefore, many tubes can develop from one pollen grain (Tripati et al., 2011).

The fruit is an elongated, conical or cylindrical capsule, comprising for the most part, five cavities containing ovules. The fruit is actually long pod and generally ribbed, developing in the leaf axil and spinelesss in cultivated kinds. The fruit is normally yellowish green to green, but is sometimes purple or whitish green. The pods are the edible portion, which are harvested while still tender and immature. They grow rapidly into long (10-30 cm) and narrow (5-10 cm) pod with a tip that is either pointed like a beak or blunt (Aladele et al., 2008).

Reproductive Pattern in Okra

Anthesis takes place at the end of the night. The flower is open at dawn, remains open all morning and closes in the middle of the afternoon (Al Ghzawi et al., 2003). Its pollen grains are very large and echinate, 156 μm in diameter with spines over 20 μm in length (Vaissière and Vinson, 1994) so that pollination with both self and cross pollen is possibly achieved by insects (Hamon and Koechlin, 1991b; Al Ghzawi et al., 2003). It is wilted in the evening and the petals usually fall off the next day along with the staminal column, consequently providing the insect with the possibility of efficiency in the process between 7:00 and 10: 00am (Azo’o et al., 2011).

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In okra, stigmas are exposed to allopollination at anthesis, and only anthers in the upper ring come into contact with stigmas. The moment of contact depends on the initial distance between these and the rate of elongation of the stamen sheath. All the wild Abelmoschus species bear fruits with five carpels (Reddy et al., 2012). This character is still present in many cultivars. Mulcahy (1983) stressed that flowers of plants making preferential autogamy tend to self pollinate before or during opening. Exposure, of still receptive stigmas, to potential pollinators occurs only after self pollination. This makes easier to understand observed differences between cultivars. Because of the sticky pollens, allo-fertilization in okra is necessarily entomophilous i.e. pollinated by insects. The absence of systematic self-fertilization in Okra cultivars combined with a progressive self pollination mechanism can be used in plant breeding for at least two reasons - the emasculation operation is required for controlled hybrids. The removing of one hundred stamens from each flower is difficult and produces a stress and some varieties do not withstand this action very well. As a consequence, a high abscission level has been observed in numerous crosses (Hamon, 1988).

Climate and Soil Requirements

Okra requires a long, warm and humid growing period. It can be successfully grown in hot humid areas. It is sensitive to frost and extremely low temperatures. For normal growth and development a temperature between 24°C and 28°C is preferred. At 24°C the first flower bud may appear in the third leaf axil while at 28°C it may appear in sixth leaf axil. This higher position is not necessarily accompanied with a delay in time because at higher temperatures the plants grow faster and the higher position is reached earlier. For seed germination, optimum soil moisture and a temperature between 25°C and 35°C is needed with fastest germination observed at 35°C. Beyond this range the germination will be delayed and weak seeds may not even germinate (Tripati et al., 2011). Loose, friable, well manured loam soils are desirable. A pH of 6.0–6.8 is ideally-suited. All soils need to be pulverized, moistened and enriched with organic matter before sowing.

Economic Importance of Okra

The economic importance of Okra cannot be overemphasized. It contains carbohydrate, proteins and vitamin C (Adebooye and Oputa, 1996). The essential and nonessential amino acids that

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okra contains are comparable to that of soybean. Hence it plays a vital role in human diet. For consumption, young immature fruits are important fresh fruit –vegetable that can be consumed in different forms. They could be boiled, fried or cooked. In Nigeria, Okra is usually boiled in water resulting in slimy soups and sauces, which are relished. The fruits also serve as soup thickeners (Schippers, 2000). The leaf buds and flowers are also edible. Okra seed could be dried. The dried seed is a nutritious material that can be used to prepare vegetable curds, or roasted and ground to be used as coffee additive or substitute. Okra leaves are considered good cattle feed, but this is seldom compatible with the primary use of the plant (Holser and Bost, 2004, Sanjeet et al., 2010).

Okra mucilage is suitable for industrial and medicinal applications (Akinyele and Temikotan 2007). Industrially, okra mucilage is usually used for glace paper production and also has a confectionery use. Okra has found medical application as a plasma replacement or blood volume expander (Savello et al. 1980, Markose and Peter 1990, Lengsfeld et al. 2004, Adetuyi et al. 2008, Kumar et al. 2010).

Combining Ability and gene action in Okra

Griffing (1956) used diallel mating system to detect General Combining Ability (GCA) and Specific Combining Ability (SCA) in crop genotypes. General combining ability is due to genes, which are largely additive in their effects while specific combining ability is due to the genes with dominance or epistatic effect. The concept of general and specific combining ability (Sprague and Tatum, 1942) helps the breeder to assess the combining ability effects of the parents and thus select superior combiners for heterosis breeding (Adiger