Tomato (Lycopersicon esculentum Mill) is an important and widely grown vegetable around the world and belongs to the family of solanacae. Tomato plants can be grown under a wide range of climatic conditions, they are extremely sensitive to hot and wet conditions. El-Ahmadi (1979) also said that fruit setting in tomato is interrupted at temperature above 26 ᵒC and 20 ᵒC day/night and is often completely arrested at temperature above 38/27 ᵒC day/night. According to El-mahdy (1990), very recently, Bangladesh agricultural research institute (BARI) has strengthened the program for year round tomato variety development and already succeeded to develop some heat tolerant hybrids. Difference existed among the cultivars in their ability to transmit their fruit setting ability under high temperatures to their immediate hybrid progenies. Rick (1969) said  that tomato is a tropical day neutral plant and is mainly self pollinated but a certain percentage of cross pollination also occurs, it is a warm season crop reasonably tolerant to heat and drought. So far efforts of many vegetables breeders from both public and private sector have resulted in spectacular improvement in yield and quality characters. As a result of these efforts, hundreds of new cultivars have been developed since 50years to meet the diverse needs and varied situation and climate under which tomato is grown. The development of a meaningful breeding programme needs information on the nature of gene action controlling the economic characters and other characters of importance. Knowledge of genetic architecture of the characters under improvement is essential for adopting appropriate breeding procedures. Such knowledge leads the plant breeder to develop new commercial varieties of the crop. Hybrid bred for heat tolerance might have better performance over any open pollinated varieties but should be evaluated under particular situation according to Yordanovs (1983). Veershety (2004) stressed the information on variation attributable to genetic difference and also on the relationship among various quantitative traits is fundamentally significant in crop improvement programme. Viredelwala (1987) reported that heat tolerance is controlled by largely recessive genes and inherited in a complex fashion with low heritability which are typical of polygenetic traits. They also suggested that heat tolerance genes are easily influence by environment. In another observation (AVRDC, 1998), pointed out that heat tolerance in tomato may not be as complex as had been reported previously by Viredelwala (1987). In the face of mounting pressure, sustainable advance in tomato productivity and product is perhaps salutary to realise nutritional security particularly in areas with high temperature stress, so as to meet the ever increasing demand for this vegetable, there is a need for improvement and to develop superior stable and heat tolerance varieties and also for development of hybrid with better yield and quality. One of the methods to improve yield and quality is heterosis breeding which was first observed by Hendrick and Booth (1968) for higher yield. Chaudhary (1965) emphasized the extensive utilization of heterosis to step up tomato production. Heterosis manifestation in tomato is in the form of tolerance to biotic and abiotic stresses according to Yadav (1989). The efficiency of selection and development of improved hybrids solely depends on the general combining ability and specific combining ability expressed for yield and related characters. Hence, combining ability which is important in the development of breeding procedures is of notable use in crop hybridization either to exploit heterosis or to combine the favourable fixable genes. Mahesh (2006), reported that in breeding larger fruited cultivars for hot conditions, additional problems such as fruit cracking, black shoulder and rough blossom ends become major obstacle. Thus it is difficult to fix all desired traits in a single line. According to Hari (1997), that tomato is considered as an important source of vitamin A, C and minerals. Apart from these, lycopene is valued for its anti-cancer property and acts as an anti-oxidant and scavenger of free radicals which is often associated with carcinogenesis as reported by Bose (2002). Lycopene may also interfere with oxidative damage to DNA and lipoproteins inhibit the oxidative LDL (low density lipoprotein) cholesterols. Tomatoes are being used in sandwish, salads and processed products like paste, puree, juice and ketchup. The fruits are mainly consumed either raw or in the preparation of food. Although tomato production in Nigeria is constrained by high temperature and relative humidity limiting it production to the short harmatan period. The study is conducted to estimate the magnitude of heterosis in F1 hybrids and general combining ability on heat tolerant tomato.

 

Materials and Methods

The genetic materials used for the study consist of 6 parents and 8 crossed seeds, aggregate of 14 entries generated during 2013 cropping season. These were evaluated in a Randomized Complete Block Design (RCBD) in two locations, Yola and Mubi locations 0f Adamawa State, The crossing nursery was established in June to October 2013 at the teaching and research Farm of the Department of Crop Production and Horticulture Modibbo Adama University of Technology, Yola. After normal land preparation, 8 beds of 2m x 2m each was made. Each bed was planted with four stands of testers and 12 stands of lines. Giving a total of 16 plants per bed. Crosses were made between the four exotic breeding lines (Roma VF, UC28B GoldenRoma and RioGrandei) and the two non- commercially cultivated heat tolerant cultivars as testers (Cherry and Currant) thereby generating 8 crosses. These crosses were made by emasculating the flowers of the lines by removal of the anther from the female plants in the evening. Emasculation involved selecting of flowers that was just about to show yellow color, The anther cone was carefully removed making sure the stigma and style do not break. Leaving intact the sepals, pistilate and the pedicel parts. One day before maturation of the anthers, tomatoes are ready for pollination at this stage. The resultant 8F₁s progenies along with six parental cultivars were evaluated at both Yola and Mubi locations of Adamawa State during the dry season of 2013/2014 in a Randomized Complete Block Design (R.C.B.D) with three replications.

 

Results and Discussion

Mean squares from the general analysis of variance for 8 characters in tomato combined across locations are presented in Table 1. The result indicated that, there was no significant difference among the locations for all characters measured except for percentage damaged fruits. Highly significant difference was observed for all characters among the entries except for weight of fruits per plant. Similar results were also observed for parents and for crosses.  The location by entries variance was noticed to be high for plant height at 60 days after transplanting as well as percentage damaged fruits, number of flower cluster and number of days to final harvest. It is also clear that location by parents interaction was highly significant in characters such as plant height at 60 days after transplanting, percentage damaged fruit, number of flower cluster, number of fruits  per plant  and  number of days to final harvest. With regards to location by crosses, all characters showed highly significant differences except for number of trichome. Similar results were also observed for parents by crosses. The GCA variance were  highly significant in characters such as plant height at 60 days after transplanting, number of leaves at 60 days after transplanting, number  of fruit per plant and number of flower cluster.  However, SCA variance showed significant differences for all characters except number of flower cluster, trichome count and number of days to final harvest. The GCA/SCA variance ratios were less than unity with respect to trichome count and number of fruits per plant. While it was positive and greater or equal to unity for all the remaining characters. The estimate of general combining ability effects of parents combined across locations for all the characters studied as presented in Table 2 reveals that currant  was the highest general combiner, because it showed significant positive GCA effects with respect to all the characters except percentage damaged fruits where negative values are preferable. Cherry, the second high general combiner had significant positive GCA effects in all characters studied except weight of fruits per plant. This genotype however, recorded negative GCA effects for percentage damaged fruits. Among the lines, Roma vf is the best general combiner, because it recorded positive GCA effects for plant height, number of leaves at 60 days after transplanting, number of fruits per plant, number of flower clusters and high negative GCA effect for number of days to final harvest which shows earliness. It was closely followed by UC28B as it has high positive GCA effects for number of leaves at 60 days after transplanting, number of fruits per plant and number of flower clusters, it also recorded high negative GCA effect with respect to percentage damaged fruits. The remaining lines are the least general combiners because they have low GCA effects. The estimates of higher parents heterosis for agronomic characters in tomato combined across locations as presented in Table 3 indicates that Four hybrids expressed positive higher parents heterotic values for plant height; Currant x RioGrandei had the highest heterotic value followed by Currant x UC28B.  For weight of fruits per plant, three hybrids recorded positive higher parents heterotic values, with Currant x GoldenRoma recording the highest heterotic value, followed by Cherry x Romavf and Currant x UC28B. For number of leaves per plant at 60 DAT. Two hybrids recorded positive heterotic values, the highest positive higher parents heterotic value for these character was exhibited by Cherry x RioGrandei. Three hybrids recorded positive higher parents heterotic values for number of fruits per plant. Among these Cherry x RioGrandei showed the highest heterotic value, followed by Cherry x GoldenRoma. Three hybrids showed positive higher parents heterotic values for number of flower clusters, Cherry x GoldenRoma had the highest heterotic values. All the hybrids recorded negative heterotic value for percentage damaged fruits. The highest was shown by Cherry x UC28B, closely followed by Currant x UC28B while the lowest was recorded by Cherry x RioGrandei. Four hybrids expressed negative higher parent heterotics values for number of days to final harvest. Among these, Currant x UC28B and Cherry x Romavf had higher negative heterotics values for these characters.

Highly significant difference observed in the mean squares for location among the genotypes for percentage damaged fruits is an indication that performance of the entries differ in difference locations in terms of temperature stress. Highly significant difference observed in the mean squares for parents and for crosses for all character except weight of fruit per plant indicates the existence of genetic variability between the parents and their respective crosses implying that the materials could be used for varietal improvement or could respond to selection pressure. This is in accordance with Falconer (1981) that the amount of improvement that is obtained by selection among a number of cross is dependent on the level of genetic variance between the crosses and the intensity of selection applied. Highly significant mean squares for location and entries interaction noticed on most characters further, suggested that difference genotypes used in this study, behaved differently in the respective locations with respect to agronomic characters such as plant height at 60 days after transplanting, number of flower cluster percentage damaged fruits and number of days to final harvest. Highly significant differences observed in all characters except trichome count for location x crosses and parents x crosses interaction had also suggested that the genotypes used in this study performed differently in different locations. This is similar to the report of Hannan and Kulkami (1998) on tomato. They found significant difference in genotypes by environmental interaction for number of fruits damaged by heat. They stated that genotypes and environmental effect is important for heat tolerance in tomato. This finding suggested that more than one test environment might be needed to obtain reliable information on the characters among the parents and crosses. This is also similar to the report of Kenga et al. (2004). Significant differences observed in the general combining ability (GCA) variances in all the agronomic characters under study except trichome count indicates the importance of additive gene actions in governing characters in tomato. This is in accordance with the report of Ghosh and Syamal (1993). The significant different observed in the specific combining ability in most characters under study except trichome count and number of days to final harvest indicate the important of non-additive gene action in controlling this characters in tomato. Less than unity GCA:SCA observed with respect to number of fruits per plant and trichome count suggest the preponderance of non-additive component of variances in the control of these characters, this also conformed to the findings of Rao (1978) in number of fruits per plant. Greater than unity ratio observed in most characters indicate additive gene actions while unity ratio observed in characters such as number of leaves at 60 days after transplanting and number of flower cluster indicates these characters are controlled by both gene actions.

 

The study reveals that cherry, currant, UC28 and Romavf has been identified as the best general combiners for all agronomic characters under study. The positive general combining ability shown by these genotypes for all characters under study except for percentage damaged fruits where negative is preferable, indicated that these genotypes are good testers and lines that had shown superiority of their hybrids where they are both used as parents or one of the parent. The negative general combining ability (GCA) shown by these genotypes in case of percentage damaged fruits is an indication of tolerance to heat damage. The parent with good general combining ability (GCA) also exhibited good perse performance. This is true with respect to cherry, Currant, UC28B and Romavf for all characters under study. This suggested that combining ability of parent used in this study can be judge accurately by their perse performance. Similar results were reported by Dhaliwal et al. (2000) in brinjal. Bad general combiners for all characters under study except for percentage damaged fruits were recorded by the lines Golden Roma and RioGrandei indicating that their hybrids performed below average. The high positive general combining ability (GCA) for percentage damaged fruits indicated their high level of susceptibility to heat. Ajjappalavara and Dharmatt (2008), noticed that the segregation of population to the inheritance of heat tolerance was 3 (tolerance): 1 (susceptible) ratio and suggested single gene inheritance for heat tolerance. Hannan et al. (2007) and Signh et al. (2001) had also observed some tomato genotypes with good general combining ability in number of flower clusters, plant height at 60 days after transplanting and number of fruits per plant. This study has identified potentials genotypes with high level of agronomic character because of the appreciable level of heterosis observed. Both positive and negative heterotic values were recorded from all the character under study. The reseasonabe positive heterosis over higher parents recorded by Currant x RioGrandei for plant height is an indication of tall plant. Negative higher parents heterosis recorded by Cherry x RioGrandei is an indication of short plant. The cross, Currant x GoldenRoma also recorded positive higher parents heterotic values for weight of fruits which indicates excellent yield. However, it does not always follow that high mean performance of crosses imply high heterosis of such crosses. Sometime such performance may be as a result of luxuriant growth; this is the same with positive heterosis recorded by Currant x RioGrandei for plant height. This is similar to work of Tadese et al. (2008). Though heterosis is minimal in self pollinated plant as reported by Romeo (1990). Hannan et al (2007) had observed positive higher parent heterosis in some tomato crosses with respect to number of leaves and plant height. All the crosses recorded negative heterosis over higher parents for percentage damaged fruits. This is desirable because it indicates high level of heat tolerance. These crosses therefore, had good tolerance to heat than their higher parents. Negative heterosis over parents recorded by crosses such as Currant x UC28B, Cherry x Romavf and Currant x Romavf for numbers of days to final harvest is an indication of earliness.

 

References

Bose P., Choudhary B., Punia R.S., Sangha H.S., 2002, Manifestation of hybrid vigour in F1 and its correlation in F2 generation of tomato (Lycopersicon esculentum Mill), Indian journal of horticulture, 22: 52-59

 

Chaudhary B., Punia R.S., and Sangha H.S., 1965, Manifestation of hybrid vigour in F1 and its correlation in F2 generation of tomato, Indian journal of Agriculture,21(2): 99-104

 

Dhaliwal M.S., Singh S., Cheema D.S., 2000, Estimating combining ability effects of the genetic male sterile lines of tomato for their use in hybrid breeding, Journal of plant breeding and genetics, 54: 199-205

 

Dharmatti P.R., Madalgeri B.B., Mannikeri I.M., Patil R. V., Girish Patil and Patil G., 2006, Genetic divergence studies in summer tomatoes. Karnataka journal Agric science, 19(2): 407-411

 

El-Ahmadi A.B., and Stevens M.A., 1979b, Genetics of high temperature fruit set in the tomato. Journal American society horticultural sci. 104: 691-696

 

El-Mahdy i., E-Metwally G., El-Fadly and Mazrouh A.Y., 1990, Inheritance of yield and fruit setting quality of some tomato crosses grown under heat stress conditions in Egypt. Journal Agri. Res. Tanta Uni. 16(3): 517-526

 

Falconer D.S., 1981, Introduction to quantitative genetics 2^nd edition longman New York

 

Ghosh P., and Syamal K., 1993, Combining ability studies with positional male sterile lines in tomato. Journal Maharashtra Agriculture University, 13: 261-262

 

Hannan K., Conti S., and Briggle L.W., 2007, Research on heterosis and components of phenotypic variance in long fruited tomato hybrids. Rivista Agronomica, 8:383-391

 

Hari L., 1997, Line x testers’ analysis in tomato (Lycopersicon esculentum Mill).ii heterosis South indian horticulture,24(2): 49-53

 

Hedrick U.P., and Booth N.O., 1968, Mendelian characters in tomatoes, Proc. American society horticultural science, 5: 19-25

 

Kenga R., Alabi S.O., and Gupta S.C., 2004, Combining ability studies in tropical sorghum (Sorghum bicolor (L.) Moench). Field crop research, 88: 251-260

 

Mahesh D.K., Apte Y.B., and Jadhan B.B., 2006, Studies on genetics divergence in tomato (Lycopersicon esculentum Mill). Crop research, 32(2): 401-403

 

Rao N., Rajput Y.S., and Singh A.K., 1978, Genetics divergence in tomato using non-hierarchical clustering approach, Vegetable Science., 25(2): 133-135

 

Rick C. M., 1969, Origins of cultivated tomato (Lycopersicon esculentum Mill).Current status of problem, Abstract, XI International congress, p-180

Romeo T. P., 1990, Genetics improvement of tomato, AVRDC, PP. 223-257.

 

Singh B., Singh S.K., Naresh R.K., Singh K.V., Bantnagar S.K., and Kumar A., 2011, General combining ability analysis of yield and its contributing traits in tomato (Lycopersicon esculentum Mill), Plant Achieves volume, 11(1): 201-204

 

Tadesse T., Tesse T., Ejeta G., 2008, Combining ability of introduced sorghum parental lines for major morpho-agronomic traits, Journal of SAT Agriculture research, 6: 1-7

 

Veershety, 2004, Studies on the variabity of characters association and genetic diversity in tomato (Lycopersicon esculentum Mill), M.sc. Thesis. Univ. Agric. Sci, Dharwad

 

Viredelwala H. A., Nandpuri K. S., and Singh S., (1987), Heterosis and combining ability in tomato, Vegetable Science,8: 120-129

 

Yadav W., Srivastava J.P., Hamveer S., Srivastava B.P., Verma H.P.S., and Singh H., 1989, Heterosis in relation to combining ability in tomato, Vegetable Science, 25(1): 43-47

 

Yordanaov., 1983, Parent selection in tomato based on the morpho-physiological traits. Horticulture Science, 14: 458