Abstract: Better parent heterosis was studied in direct and reciprocal crosses using 9 early and late okra cultivars which belonged to the Early-early, Early-late, Late- early and Late-late okra flowering types. The ANOVA for length of pod, circumference of pod, number of locules per pod, number of seeds per pod, Number of pods per plant, weight of 1000 seeds, density of seeds, weight of pods per plant, days to flower opening, plant height and circumference of stem at 10 cm above soil level, showed almost very highly significant differences-an indication that the cultivars are genetically diverse. Very highly significant, narrow and intermediate, heterosis was recorded for most of the direct and reciprocal crosses, showing that selections could be made from the hybrids to meet desired local okra qualities. A cross between an Early-late and Late-early parents, using early okra as the maternal parent, gave rise to a stable viable bridge hybrid which outperformed the better parent in many respects thereby overcoming the hitherto strong barrier to gene flow in interspecific hybridization studies involving the two okra types and also indicating the existence of maternal effects. Consequently with this development, the window has been opened for possible accelerated transfer of several desirable genes from late okra types to many promising but vulnerable early okra types. This could result to minimizing the further erosion of such early okra germplasm still in the custody of the local farmers.
INTRODUCTION
Okra is one of the many important kinds of economic vegetable crops where the fresh edible pods provide human supplementary vitamins such as C, A, B Complex, Iron, Calcium, Proteins and many others (Benjawan et al., 2007a, b). Okra, Abelmoschus esculentus (L.) Moench (early okra) and Abelmoschus callei (A. Chev.) Stevils (late okra or West African okra) are members of the Malvaceae family. Early okra is an annual plant while late okra has a perennial tendency (Singh and Bhatnager, 1976). Early okra and late okra are synonymous with the Soudanais and Guineen okra types described by Martin (1982) and Siemonsma (1982). 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, 1976; Martin, 1982; Siemonsma, 1982). Martin et al. (1981) distinguished late okra from the common okra on the basis of the following five characteristics; red leaf vein, very late flowering habit, reduced number of sub-calyx bracts, pods mounted at right angles to the stem and large numbers of seeds per pod.
Njoku (1958) coined the names early and late okra when his studies in photoperiodism, in Nigeria, showed that early okra has a critical daylength (CDL) of 12.5 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 September, when natural daylength shortens considerably. Sensitivity to photoperiodism is a very important expression of genetic diversity in okra in West Africa. Understanding and exploiting the phenomenon of photoperiodism in sensitive plants, like okra, can bring about accelerated gene transfer as demonstrated by Udengwu (1998).
Fatokun et al. (1979) identified three agronomic types of okra from their collection of over 300 okra types. They categorized the plants essentially on the basis of flowering habit and plant size. Type A plants were early flowering and small in stature. Type B were medium flowering and fairly robust while type C were late flowering with very robust growth. When they carried out crosses between members of group C and those of group A and B, they got F1 hybrid plants that were essentially sterile with fruits bearing less than ten well developed seeds as against an average of 60 seeds in normal fruits. Percentage germination of the seeds ranged from 4 to 18%. These observations according to them suggest the existence of a barrier to gene flow between members of group C and other groups. This barrier they stated is likely to retard the progress to be made in transferring some of the desirable characteristics like resistance to okra leaf mosaic virus from the members of group C to those of other groups. They therefore called for further research to identify the factors responsible for sterility in these crosses.
Okra research, especially in developed countries, has for long been concentrated on Early okra, with little attention paid to Late or West African okra despite the fact that it contains many desirable genes not found in Early or conventional okra (Singh and Bhatnager, 1976; Martin, 1982; Siemonsma, 1982).
In their own studies, Hamon and Hamon (1991) reported that interspecific hybrids between Abelmoschus esculentus and A. caillei can be obtained artificially, but at experimental stations and in the field very low rates of cross fertilization are observed. In addition, the sterility of the F1 hybrids makes their genetic participation in subsequent generations unlikely. Some other workers like Martin (1982) and Siemonsma (1982) have also highlighted this existence of barrier to gene flow between the two okra types. Over the years little or nothing has been done towards overcoming this barrier to gene flow between the two okra types and this has led to the loss of promising early okra germplasm that were highly susceptible to okra viral diseases. Such erosion of invaluable germplasm could have been prevented if genes for resistance to okra viral diseases, which abound in late okra types, were promptly transferred to these susceptible early okra types. Achieving this entails involving different species with diverse genetic make-ups in hybridization studies, which may result to the formation of stable hybrids apart from exploiting hybrid vigour. Crop diversity has important implications for genetic vulnerability and potential for crop improvement (Mikel and Kolb, 2008).
Information regarding levels of diversity in different germplasm would help to identify sources for broadening improved breeding pools and in seeking genes and alleles that have not been tapped in modern breeding (Mikel and Kolb, 2008; Warburton et al., 2008). According to Kuleng et al. (2006), Moncada et al. (2007) and Abdel-Kuleung (2009), genetic materials for breeding should have high genetic variability to maximize the gains from selection and provide insight into diversity and relatedness of natural populations which are necessary for crop improvement.
Reports on the involvement of the two okra types in heterotic studies for the exploitation of hybrid vigour, hardly exists. Among genetic phenomena ever discovered, few have had greater beneficial effects on man than heterosis. According to Hallauer (1997), heterosis as one of the most widely used plant breeding terms has made a dramatic impact on the development of breeding methods and high yields for many field crops, vegetables, ornamentals and tree species during the 20th century. Through exploitation of heterosis production costs could be drastically reduced by increasing yield levels while enhancing input use efficiency (Pingali, 1997). According to Biswas et al. (2005), both positive and negative heterosis is useful in crop improvement, depending on the breeding objectives; in general, positive heterosis is desired for yield and negative heterosis for earliness. In the words of Mather and Jinks (1977), in all cases of negative heterosis, whether mid-parent or better parent, the implication is that the F1 hybrids tended towards the lower parent for the character. High heterosis can be expected in crosses showing high levels of genetic variability (Chandra-Shekara et al., 2007).
The essence of this present study was to primarily involve the selected early and late okra cultivars, belonging to different flowering groups, in heterotic studies, during shorter day photoperiod, with a view to identifying hybrids with desirable, exploitable qualities, for the genetic improvement of the crop, to meet local demands. This is with a special bias towards identifying stable bridge types between the two okra types that could help overcome the barrier to gene flow that exists between them and which can in follow-up studies, facilitate the transfer of, desirable genes among, as well as between, the two okra types which hitherto had been problematic.
MATERIALS AND METHODS
Green house planting of parents: The following nine parental cultivars, Kano dwarf, Lady finger, Ogbu oge, Obimo girdle and Ogba mkpe (Early Okra types) and Ogolo, Alanwangboho, Ebi ogwu and Oru ufie (Late okra types) selected from four identified groups of okra, after preliminary studies, based on their flowering habits under natural photoperiods during both the shorter and longer days were involved in direct and reciprocal crosses in the Botanic garden, University of Nigeria, Nsukka between September 1989 and January, 1990. Table 1 gives the summary of the flowering habits of all the cultivars during the natural longer days (March to July) as well as during the natural shorter days (August to February).
Three pre-germinated seeds were sown in medium sized black polythene bags measuring 12 cm in diameter and 25 cm deep, filled with a mixture of top garden soil, poultry manure and river sand in the ratio of (3:1:1).
| Table 1: | Mean time taken by 9 okra cultivars for flower buds initiation and flower opening during the April planting (Under rain fed condition and longer photoperiod) and September planting (Under irrigation and shorter photoperiod) |
The seeds of the early okra cultivars were planted 10 days earlier than those of late okra, because Njoku (1958) and Nwoke (1980) had shown that when both okra types are grown at 113/4 h photoperiod, early okra flowers opened 10 days earlier than those of late okra. The bags were placed on the wooden raised platforms in the green house and the plants were maintained at 10 h controlled photoperiod throughout the experiment; since Udengwu (1998) had shown that if crosses involving early and late okra cultivars were done under 10 h controlled photoperiod, up to three generation of plants could be handled in one year as against only one generation possible under natural photoperiod.
To achieve 10 h day length, thick black khaki hoods were lowered to completely cover the plants at 18:00 h every evening and raised at 08:00 h every morning. Ten days after release of the two cotyledon-like leaves, thinning was carried out leaving only one seedling per bag. There were three replications with a total of ten plants for each cultivar per replicate giving a total of 270 plants. Watering was done twice daily, morning and evening. When the flowers of the plants started opening, emasculation and crosses were done following the methods as reported by Udengwu (2007). Table 2 gives the summary of the direct and reciprocal crosses which were carried out in the three broad categories CI, CII and CIII (Table 2). CI crosses were essentially among the early-early, mid-early and late-early okra types (Table 1). The CII crosses which were the main target crosses were carried out essentially between groups B and C cultivars that were considered potential bridge cultivars between the early and late okra types based on preliminary studies. The CIII crosses were just among late okra cultivars. The choice of the parents for this study was based on preliminary hybridization studies involving okra from the diverse categories with special interest on parents for the late early and early late okra types crosses.
| Table 2: | Summary Of direct and reciprocal crosses made for the heterosis studies |
Field planting of F1 hybrids alongside the parents for heterosis studies: The F1 pre-germinated seeds from the fourteen crosses together with the nine parents were planted in the field in the Botanic garden in August 1989, since Nwoke (1980) and Udengwu (1998, 2007) had shown that both the vegetative and reproductive growths of both okra types were almost identical during the shorter photoperiods under natural conditions. The last column of Table 1 shows the flowering habit of both okra types during the shorter days. Well cured chicken droppings were applied at the rate of 1.6 m th-1. The plantings were done on beds measuring 8x4 m. Three seeds of each of the parents and hybrids were randomly planted on the ridges using table of random numbers with 30x30 cm bed spacing. When the seedlings got properly established the plants were thinned down to one plant per stand. There were a total of 230 plants per plot with three replications. Both the leaf borer Podogrica uniforma and fruit borer Earias biplaga were controlled by weekly foliar spray of Vetox-85 at the rate of 7 kg ha-1. Data for the following important agronomic parameters were collected at the appropriate time during the growth of the plants-length of pod, circumference of pod, Number of locules per pod, number of seeds per pod, number of pods per plant, weight of 1000 seeds, density of seeds, weight of pods per plant, days to flower opening, plant height and circumference of stem at 10 cm above soil level. Data collected were used to calculate better parent heterosis (BP) using the formula reported by Biswas et al. (2005).
The classification of range of heterosis as, narrow level (<50%), intermediate level (50-100%) and high level (>100%) followed the report of Veerendra et al. (2007). The heterotic values were tested for significance to establish the differences if the F1 hybrid means from their respective better parents (BP) by applying t-test as described by Steele and Torrie (1981).
RESULTS
Analysis of variance of 11 characters: Table 3 shows the summary of analysis of variance for the eleven characters studied which include: Length of pod, diameter of pod, number of locules per pod, number of seeds per pod, weight of pods per plant, number of pods per plant, weight of 1000 seeds, density of seeds, days to flower opening, plant height and circumference of stem at 10 cm above soil level. The results showed that the block effects were essentially non-significant with the exception number of seeds per pod and days to flower opening which were very highly significant (<0.001) and highly significant (p<0.01), respectively. The mean Square for all the eleven treatments were very highly significant (p<0.001).
Days to flower opening: For the CI crosses involving the early okra groups, two direct crosses, P1xP2 and P1xP5 showed negative, very highly significant (p<0.001) but narrow heterosis for this parameter. Their reciprocal crosses were both positive but non significant. For the CII crosses involving the early and late okra parents, these two direct crosses P3xP9 and P4xP6 showed zero and 4% heterosis respectively while their reciprocal crosses showed negative, narrow (<50% heterosis), insignificant heterosis for the trait. The other four direct and reciprocal crosses, with the exception of P9xP4, showed negative, very highly significant (p<0.001), narrow heterosis. Both the direct and reciprocal crosses in CII, involving the late parents, showed negative, very highly significant (p<0.001), narrow heterosis for days to flower opening.
Plant height: The direct cross, P1xP2, in the CI category, expressed positive but insignificant narrow heterosis for this trait. The reciprocal, P2xP1 expressed negative, non significant, narrow heterosis for this trait. The direct and reciprocal crosses for P1xP5 expressed narrow, negative, very significant (p<0.01) and very highly significant (p<0.001) heterosis, respectively, for plant height. All the crosses involving the early and late parents, in the CII category, with the exception of P4xP6, showed non significant narrow positive and negative heterosis. The direct and reciprocal crosses, in CIII, involving the late okra parents showed negative, very highly significant (p<0.001), narrow heterosis for plant height.
Circumference of stem: All the CI crosses involving the early parents exhibited negative, narrow heterosis with only P2xP1 and P5xP1 direct crosses showing very significant (p<0.01) heterotic effects. For the CII crosses involving the early and late parents, all the crosses showed negative, narrow heterotic effects. While the cross P4xP6 showed very highly significant (p<0.001) heterotic effect, the crosses P9xP3 and P5xP9 showed highly significant (p<0.01) and significant (p<0.05) heterotic effects respectively. For the CIII crosses involving the late okra types, while the direct cross, P7xP8, showed negative, very highly significant (p<0.001) heterotic effects, its reciprocal, showed highly significant (p<0.01) negative heterosis.
| Table 3: | Analysis of variance for 11 characters in the parents and F1 hybrids showing mean squares only |
| *, **, *** Significant at 0.05, 0.01 and 0.001 levels of probability, respectively. NS: Not significant | |
Number of pods per plant: For the trait, number of pods per plant, all the CI crosses involving the early okra parents expressed narrow, non significant heterotic effects. While all the other crosses showed positive heterotic effects, P2xP1 showed negative heterotic effect. For the CII crosses involving early and late okra parents, all the hybrids exhibited positive, narrow heterosis, with only P6xP4 and P9xP4 showing significant (p<0.05) and very significant (p<0.01) heterotic effects. The CIII crosses involving the late parents showed narrow, positive, non significant heterosis for the direct cross, P7xP8, while the reciprocal cross showed narrow, negative, non significant heterotic effect.
Weight of 1000 seeds: The CI crosses showed that for P1xP2, both the direct and reciprocal crosses produced negative narrow heterotic effects. While the direct cross showed very highly significant (p<0.001) heterosis, the reciprocal cross showed only significant heterosis (p<0.05). For the cross P1xP5, the heterotic effect was positive, narrow and significant (p<0.05) while the reciprocal showed positive, narrow but insignificant heterosis. In the CII crosses involving the early and late parents, all the crosses showed, narrow, negative but very highly significant (p<0.001) heterotic effects. The same was the case with the CIII crosses involving the late parents.
Density of seeds: The results of the P1xP2 direct and reciprocal crosses, involving the early okra parents in the CI category, showed that all the hybrids expressed negative, narrow but very highly significant (p<0.001) heterotic effects. In the cross P1xP5, both hybrids from the direct and reciprocal crosses showed non significant, narrow heterosis, with the effect of the direct cross being negative while that of the reciprocal cross was positive. For the crosses involving the early and late okra parents in the CII category, with the exception of the cross, P5xP9 and its reciprocal which showed 8.05 and 76.04% negative, significant (p<0.05) and highly significant (p<0.01) heterosis, respectively, all the other crosses in this category, showed negative, intermediate (50-100%), very highly significant (p<0.001) heterotic effects. Both the direct and reciprocal crosses for the CIII category involving the late parents showed negative, narrow, non significant heterotic effects
Weight of pods per plant: For the CI crosses involving the early okra parents, the direct cross, P1xP2 hybrid showed non significant, narrow, positive heterotic effect while the reciprocal showed intermediate (50-100%) negative, very highly significant (p<0.001) heterosis. For the cross P1xP2 and its reciprocal, their hybrids expressed positive, intermediate, very highly significant (p<0.001) and positive, narrow, very highly significant (p<0.001) heterotic effects respectively. For the CII crosses, involving the early and late okra parents, with the exception of the cross, P5xP9, which showed positive, narrow, non significant heterosis, all the crosses expressed intermediate, negative, very highly significant (p<0.001) heterotic effects. The responses of the crosses between the late parents in CIII is indicative of the fact that all the hybrids expressed, negative, narrow, very highly significant (p<0.001) heterotic effects.
Length of pod: With the exception of the cross P1xP2 which showed positive, narrow, very highly significant (p<0.001) heterosis in the CI crosses involving only the early parents, all the other crosses expressed non significant, narrow heterosis with that of P2xP1, showing negative heterotic effect. In the CII category, all the hybrid manifested negative heterosis with only the cross, P4xP6 and its reciprocal showing intermediate, very highly significant (p<0.001) heterosis. The rest of the crosses except P5xP9 and its reciprocal which showed highly significant (p<0.001) and significant (p<0.05) heterotic effects respectively, showed very highly significant (p<0.001) heterotic effects. The direct and reciprocal crosses in the CIII category involving the late parents, showed narrow, negative and very highly significant heterosis for length of pod.
Circumference of pod: All the crosses in the CI category involving only early parents produced hybrids that expressed negative, narrow but very highly significant (p<0.001) heterotic effects. For the CII crosses involving both the early and late parents, all the crosses expressed negative, narrow and very highly significant (p<0.001) heterosis with the exception of the cross, P3xP9 and its reciprocal whose levels of significance were (p<0.05) and (p<0.01), respectively. For the CIII crosses, involving only the late parents, both the direct and reciprocal crosses manifested narrow, negative but very highly significant (p<0.001) heterotic effects.
Number of locules per pod: All the crosses in the CI category involving only the early parents, exhibited negative, narrow heterotic effects with the cross, P1xP5 and its reciprocal, showing very highly significant (p<0.001) and significant (p<0.05) heterotic effects, respectively. The effects of the other crosses were not significant. For the CII crosses, all the hybrids showed negative, narrow and very highly significant (p<0.001) heterotic effects, with the exception of the reciprocal cross P9xP3, which expressed significant (p<0.05) heterosis. For the CIII crosses involving only the late okra parents, both the direct and reciprocal crosses, P7xP8, showed negative, narrow and very highly significant (p<0.001), heterotic effect.
Number of seeds per pod: With the exception of the cross, P1xP2, in the CI category involving only early okra parents, which manifested non significant, narrow heterosis, all the other crosses expressed, narrow, negative and very highly significant (p<0.001) heterotic effect for the trait, number of seeds per pod. For the CII crosses, involving early and late okra parents, all the crosses, expressed, intermediate, negative and very highly significant heterotic effect with the exception of the cross, P5xP9, which showed narrow heterotic effect(<50%). The CIII crosses involving the late okra parents expressed, intermediate, negative and very highly significant (p<0.001) heterosis for number of seeds per pod.
DISCUSSION
The very highly significant difference, (p<0.001), among the 11 characters studied, Table 3, can be attributed to the fact that there were clear cut genotypic differences among the parents and their hybrids, which were phenotypically expressed. This occurrence of very highly significant genotypic differences among the parents and their hybrids is in line with the views of Rao (1972), Kuleung et al. (2006), Moncada et al. (2007), Shengbin et al. (2007) and Abdel-Moneam (2009), who observed that the success of any plant breeding programme is dependent to a very large degree on the genetic diversity of the available germplasm. Similarly Joshi (1979) and Lee et al. (2008) opined that involving genetically diverse parents in crossing could be advantageous in the sense that it would provide an opportunity for bringing together gene constellations of divergent origin. Isolation in space and time results in locking up genes in different constellations, these should be unshackled and brought together. Continuing they noted that geographic diversity could mean genetic diversity and therefore parents with diverse geographical origins should be joined together in hybridization.
Flowering habit appears to have a very strong link with plant geography and genetic diversity. Just like late okra which is restricted in distribution to the equatorial rainforest region of West Africa; Lonn et al. (2006) reported that the late-flowering Gymnadenia conopsea have smaller habitat amplitude with preference for wetter habitats than the early-flowering type. Furthermore, West Africa being the centre of diversity for late okra as reported by Harlan (1971) might account for the fact that it is the source of many useful genes not found in the Early okra type. This is similar to the report of Lee et al. (2008) who opined that South Korea is a major centre of diversity for wild soybeans and potentially a source of useful genes not found in other parts of the world.
Fatokun et al. (1979) posited that the existence of a wide range of variation in many of the important highly heritable characters such as leaf shape and size, plant stature, branching habit, fruit colour, hairiness and size and number of fruits per plant offers an opportunity to quickly and effectively develop improved varieties of okra suitable for various growing regions within the country. Similar to the views of Lee et al. (2008), definitely the involvement of parents from other states in the country in this study must have contributed to the observed level of significance even in crosses involving only Early okra. The very highly significant and highly significant probabilities for block effect in the characters: - number of seeds per fruit and days to flower opening, could be attributed to differences in pollination patterns and rates of development of initiated flower buds. It was often observed that flower on the plants in blocks at the extreme ends of the experimental areas received more sunlight and insect visitors than those on the inside blocks.
For the purposes of accelerated breeding, as was demonstrated by Udengwu (1998), as well as production of high quality germplasm, for okra improvement, the current studies were carried out during the shorter photoperiods of August-December. Fatokun et al. (1979) had noted that for good quality seed production the late cropping season (August to November) in Southern Nigeria should be used. It should however be pointed out that the performances of the hybrid and the parents as currently reported are bound to differ when evaluated during the longer photoperiod (early planting, March-April). This is because though early and late okra types are short day plants, early okra has a Critical Day Length (CDL) of 12½ h while late okra has a CDL of 12¼ h, with the result that late okra can only flower later in the year when daylength shortens considerably (Njoku, 1958; Oyolu, 1977; Nwoke, 1980; Martin et al., 1981; Udengwu, 1998). The longer day photoperiod plantings unfortunately produces low quality germplasm which could retard breeding programmes with the two okra types (Fatokun et al., 1979).
When daylength shortens from September to December, both the vegetative and reproductive behaviour of both okra types are identical as demonstrated by Njoku (1958), Nwoke (1980) and Udengwu (1998); but during the longer days, late okra takes much longer time for both vegetative and reproductive growth thereby imposing limitations on hybridization studies between the two. Incidentally, according to Joshi et al. (1958), increase either in height or in number of branches or in both does not appear to contribute substantially towards increased yield. In the case of fruit characters, increase in number of fruits in the hybrids was observed to be significant. Fruit size was bigger only in one of the ten combinations. This increase in the number of fruits has been mainly responsible for the increased yields. Singh et al. (1974) analysed 30 varieties of okra and found yield most highly correlated with weight of fruit and number of fruits per plant. To a lesser extent yield is related to height, branching, stem diameter, leaves per plant and other characteristics which suggest large size and vigour of plant.
For number of pods per plant it was interesting that all the crosses in the CII category showed positive heterosis, with P9xP4 showing intermediate highly significant heterosis. It was intriguing to watch almost all the flowers produced on these hybrid plants producing fruits that were however shrunken with very few viable seeds with the exception of P5xP9. The crosses P1xP2, P1xP5 along side P5xP9 were selected for greater yield of pods per plant. For weight of pods per plant, the crosses P1xP2, P1xP5 and P5xP9 were equally selected for greater yield of pods per plant. Partap et al. (1979) opined that selection in okra should be based on stem diameter, flower number per plant, fruit number per plant and per branch, fruit length and fruit weight.
For length of pod P1xP2, P1xP5 and P8xP7 were selected for production of shorter fruits which is a desirable character for okra consumers in Southern Nigeria (Fatokun et al., 1979). For circumference of pod P1xP2, P1xP5, P5xP9 and P7xP8 were selected for production of smaller fruit size which is a desirable fruit quality in South Eastern Nigeria.
For number of locules per pod, P1xP2, P1xP5, P5xP9 and P7xP8 were selected for production of fruits with small number of locules per pod which contributes to smaller fruit size as well as fewer numbers of seeds - a desirable quality when okra is grown for soup making in West Africa. Woolfe et al. (1977) had shown that okra seeds do not contribute to the mucilaginous character of okra fruit, which is a primary quality of okra in the region where the mucilaginous character in soups prepared with okra fruits makes the swallowing of often large molded carbohydrate balls of food smooth and faster. It is also believed that okra soup has a soothing and calming effect on troubled stomachs. Further research in this direction may result to the production of seedless okra, with perhaps enhanced mucilage character.
For number of seeds per pod the value of the very highly significant negative heterosis for the CII category which ranged from -79.54 to -97.20 is indicative of very few numbers of seed per pod for these interspecific hybrids. This contrasts with the -41.04 value for P5xP9 showing that though seed content was less than that of the better parent, it was much more than that of the other hybrids in its category. In fact it was more than even the seeds produced for the late x late parents. It was therefore selected for further seed production. For days to flower opening P1xP2, P1xP5, P5xP9 and P7xP8 were selected for production of plants with earlier flowering habits. For plant height, P1xP2 as well as P5xP9 was selected. For circumference of stem only P5xP9 was selected because it is the bridge type.
Almost in all the eleven parameters studied the performance of the hybrids from the direct cross P5xP9 (with early okra as the maternal parent) was better than that of the reciprocal cross P9xP5 (using late okra as the maternal parent) showing evidence of maternal effect. The hybrid of P5xP9 having shown good stability in subsequent generations, resulted in overcoming this barrier to geneflow. Since P9 as a late okra cultivar crosses freely with other late okra cultivar and P5 an early okra crosses freely with other early okra cultivars, desirable traits like resistance to okra mosaic and leaf curl diseases may now be transferred from late okra to P5 and subsequently to other susceptible early okra cultivars and likewise other desirable traits like supernumerary inflorescence can also be transferred from other early okra cultivars to P5 and then to P9 and other late okra cultivars.
The reasons for the success recorded with the bridge types, with early okra as the maternal parent is suspected to be connected with the chromosome numbers within each of the four okra groups (Table 1), probably showing a wide variation. Bringing two plants from two groups with chromosome numbers that are close could be the possible reason for the success of the bridge hybrid. This view is supported by the assertion of Martin (1982) who commenting on the wide variation in chromosome number of okra stated that the possibility of more than one chromosome number within a species cannot be ruled out. Also after a review of all the available cytotaxonomic information on okra, Joshi and Hardas (1976) concluded that okra is probably not a single but a polytypic complex which exhibits both high polyploidy and hybridity and of which the parental wild species are yet undetermined. Even basic chromosome numbers are unknown. Additionally, Martin et al. (1981) noted that West African or Guineen okra varieties are a distinctive group with about 194 chromosomes as compared to the 130 of typical okra (Singh and Bhatnagar 1976). This group is not homogenous within itself and appears to consist of a body of variation not formerly recognized by plant breeders. Follow up cytological or flow cytometric analysis may provide more reasons for this observed maternal effect. Obviously, without overcoming this barrier to gene flow, no breeding progress can be made between the two okra types because of infertility. Other workers, like Shengbin et al. (2007), have also battled similar problems as can seen from their report that poor fertility is the main barrier for utilizing heterosis between two rice (Oryza sativa L.) subspecies, indica and japonica.
A general overview of the performance of the parents and hybrids showed that mostly narrow and intermediate negative heterosis were expressed by the CI, CII and CIII categories of crosses. Based on previous works done by other workers, positive heterosis was desirable in a number of parameters while negative heterosis was desirous in a few other parameters. Negative heterosis in the crosses involving the CII group, early x late was expected since they constitute the interspecific hybrids with known barrier to gene flow. Interestingly enough some of the crosses in the CII group expressed heterosis in the desired direction though. With the exception of the cross P5xP9 however others produced seeds that had very low germination percentages and may not be very useful for the exploitation of the expressed heterosis. This is in line with the observations of Shengbin et al. (2007).
Overall this result, on one hand, agrees with the findings of Lonn et al. (2006), to the extent that their studies revealed that the late and early flowering groups of Gymnadenia conopsea showed that the degree of introgression between flowering-time groups was associated with fruit set failure, which was higher in the late flowering plants and lower in the early flowering plants. Additionally they noted that there seems to be a strong barrier against introgression into the late flowering group which is kept genetically distinct and less diverse, while the early flowering group is diverse, includes two subgroups and seems to benefit from gene flow. On the other hand, however, their report differs from this report since they could not indicate the potential effects of hybridization between the late-flowering and the early-flowering groups on fruit set and flowering time.
Incidentally the cross P5xP9 showed positive non significant narrow heterosis for number of pods per plant but negative narrow heterosis for weight of pods per plant. As Partap and Dhankar (1980) stated, even when the heterosis value was not significant but per se performance was comparatively high then selection might also be executed for such characters taking into consideration the gene action present. P5xP9 was therefore selected for the two parameters. Equally of special interest was the heterosis for density of seeds and weight of pods per plant in CII category. Though all the hybrids expressed negative intermediate heterosis for these parameters P5xP9 expressed narrow heterosis of -8.05 and -12.03 showing that the seeds and fruits it produced though less dense and lighter than the better parent, but still much denser and heavier than all other seeds and fruits in that category. The density of its seeds is comparable to that of crosses in the CI and CIII categories, while its pod weight per plant ranks only second to P1xP2 in the CI category, which is an excellent performance. It was again selected for these parameters.
Hamon and Hamon (1991) had observed that interspecific hybrids between Abelmoschus esculentus and A. caillei can be obtained artificially, but at experimental stations and in the field very low rates of cross fertilization are observed. In addition, the sterility of the F1 hybrids makes their genetic participation in subsequent generations unlikely. The success recorded with the bridge hybrid, P5xP9 however differs from the above report in that it has paved the way for the free flow of genes between Abelmoschus esculentus and A. caillei species which will now make it possible for the genes conferring resistance to the okra leaf curl as well as okra mosaic virus diseases among others, to be transferred from A. callei cultivars to promising A. esculentus germplasm that stood the risk of total loss because of their susceptibility to these very serious okra diseases.
From Table 4-6, it could be seen that the following parameters days to flower opening, plant height, circumference of stem, as well as fruit yield parameters of weight of pods per plant, length of pod, circumference of pod and number of locules per pod, all exhibited negative or non significant positive heterosis. This is a consequence of overall reduced vegetative growth, an established effect of growing okra plants under shorter photoperiods as had been demonstrated by Njoku (1958), Oyolu (1977), Nwoke (1980) and Udengwu (1998).
Interestingly as Fatokun et al. (1979) pointed out; although fruit size is an important yield component, an average Nigerian consumer prefers small sized fruits. A constraint has therefore been placed on increasing yield by breeding for larger fruits and the alternative is to breed for higher number of fruits per plant. Therefore, the current studies under the shorter photoperiod provide an excellent opportunity for not only discovering bridge hybrids but also to select hybrids that will meet local okra needs due to reduced fruit size.
Additionally studies under shorter photoperiod have other agronomic and economic advantages.
| Table 4: | Mean values of parents and F1 hybrids, percentage heterosis (BP) for days to flower opening, plant height and circumference of stem in a direct and reciprocal set of okra |
| *, **, *** Significant at 0.05, 0.01 and 0.001 levels of probability, respectively. NS: Not significant | |
| Table 5: | Mean values of parents and F1 hybrids, percentage heterosis (BP) for density of seeds and three yield parameters in direct and reciprocal set of okra |
| *, **, *** Significant at 0.05, 0.01 and 0.001 levels of probability, respectively. NS: Not significant | |
The less robust nature of okra plants during the shorter photoperiods provides the added advantage of not only planting more plants per unit area, but also results to less exploitation of soil nutrients because the roots of the plants do not go deep into the soil as against the longer photoperiod plantings that predisposes the late okra cultivars to develop deep tap roots, leading to not only heavy exploitation of soil nutrients but also to the plants developing perennial tendencies. Also, little or no expenses are incurred for pest and disease control during the shorter photoperiods because most of the pests and diseases of okra are known to have their peak period of attack during the longer photoperiods which corresponds to the rainy season.
| Table 6: | Mean values of parents and F1 hybrids, percentage heterosis (BP) for four pod characteristics in a direct and reciprocal set of okra |
| *, **, *** Significant at 0.05, 0.01 and 0.001 levels of probability, respectively. NS: Not significant | |
Equally of interest is the fact that with smaller plant size, less labour is involved in fruit harvesting because the plants hardly grow above 1.2 m as compared to a potential height of 4.5 m under the longer photoperiods, which often prompts okra farmers to decapitate the plants to reduce plant height; a practice aimed at preventing the breaking of the plants during fruit harvesting and also reducing labour cost.
Though studies of heterosis in okra under longer photoperiods was not part of this present report, but preliminary studies on heterosis under longer photoperiods indicate that performance under shorter photoperiods may have greater agronomic, economic and ecological benefits in terms of accelerated rate of hybridization to produce new hybrids with desirable characteristics as well as high quality okra germplasm; lower production cost due to little weeding, little or no pest and disease incidence, less labour input for fruit picking as well as less pollution of the environment due to little or no application of pesticides and insecticides.
ACKNOWLEDGMENTS
I am highly grateful to Prof. (Dr.) Ogugua C. Nwankiti, for his guidance and criticisms throughout the course of this work. Special thanks also go to Prof. E.E Ene-Obong, for his help with statistical analysis. I am also grateful to staff of the Botanic garden, University of Nigeria, Nsukka for all their assistance.