COMBINING ABILITY AND GENE ACTION FOR FRUIT YIELD AND HEAT TOLERANCE IN TOMATO (Lycopersicon lycopersicum Mill.) UNDER HEAT STRESS CONDITIONS

Habu S.H. Heat stress causes significant fruit yield loss in tomato (Lycopersicon lycopersicum Mill.). Breeding tomato varieties hybrids tolerant to high temperature will reduce fruit yield losses due to heat stress in Nigeria. Combining ability and gene action for fruit yield and heat tolerance was studied under heat stress conditions in tomato. The experiments were carried out at National Horticultural Research Institute, Bagauda Station farm (11°33 ́N; 8°23 ́E) and the Institute for Agricultural Research farm, Samaru (1111 ́N; 0738’E) between July to October, 2014 rainy season. Two heat tolerant and four susceptible tomato genotypes were crossed in a half diallel mating design. The results of combining ability analysis indicated that, both additive and non-additive actions were important for the inheritance of the traits. However, SCA variance components were higher than GCA variance components, indicating preponderance of dominance gene action for genetic control of the majority traits. The average degree of dominance values revealed over-dominance gene action for the most traits. The parent Icrixina was the best general combiner for the majority of the traits among the parents, while Petomech × Roma Savana and Icrixina × Rio Grande were the most desirable cross combinations for fruit yield per plant and percentage fruit set. Overall results indicated that hybrid vigor exploitation could be harnessed to produce high yielding and heat tolerant tomato hybrid under rainy season.


Habu S.H.
Heat stress causes significant fruit yield loss in tomato (Lycopersicon lycopersicum Mill.). Breeding tomato varieties hybrids tolerant to high temperature will reduce fruit yield losses due to heat stress in Nigeria. Combining ability and gene action for fruit yield and heat tolerance was studied under heat stress conditions in tomato. The experiments were carried out at National Horticultural Research Institute, Bagauda Station farm (11°33´N; 8°23´E) and the Institute for Agricultural Research farm, Samaru (11 0 11´N; 07 0 38'E) between July to October, 2014 rainy season. Two heat tolerant and four susceptible tomato genotypes were crossed in a half diallel mating design. The results of combining ability analysis indicated that, both additive and non-additive actions were important for the inheritance of the traits. However, SCA variance components were higher than GCA variance components, indicating preponderance of dominance gene action for genetic control of the majority traits. The average degree of dominance values revealed over-dominance gene action for the most traits. The parent Icrixina was the best general combiner for the majority of the traits among the parents, while Petomech × Roma Savana and Icrixina × Rio Grande were the most desirable cross combinations for fruit yield per plant and percentage fruit set. Overall results indicated that hybrid vigor exploitation could be harnessed to produce high yielding and heat tolerant tomato hybrid under rainy season.

Introduction:-
Tomato is widely produced and consumed. It ranks second high priority vegetable throughout the world after potato (FOASTAT, 2005). It is very rich in vitamins, minerals, essential amino acids, sugars and dietary fibers. It contains a high level of lycopene, an antioxidant that reduces the risk related to several cancers and neurodegenerative diseases (Srivinasan et al., 2010). The optimum temperatures for tomato cultivation are between 25 and 30 o C during photoperiod and 20 o C during the dark period (Camejo et al., 2005). An increase of 2-4 o C over the optimal temperature had been adjudged to adversely affect gamete development, while inhibiting the ability of pollinated flowers to develop into fruits and thus reduced fruit yield (Peet et. al., 1997, Sato, et. al., 2001Firon, et. al., 2006). Tomato is commonly grown during harmattan under irrigation and the rainy season in Nigeria and high temperature during the rainy season usually causes a substantial reduction of fruit size, increase in flower abortion and decrease in fruit set which reduced fruit yield. However, the presently cultivated tomato varieties in the country are sensitive to high temperature, which limits tomato production and causes a shortage of market supply and high cost during the rainy season. Consequently, there is need for producing high yield open pollinated or tomato hybrids that can thrive well and set fruit under high temperature environment. Combining ability analysis is a fundamental technique in understanding the genetic potential of parents and their hybrids. It also provides information on gene action and effects controlling the inheritance of quantitative traits which help in formulating an effective breeding program. Cheema et al. (2003) reported significant GCA and SCA variance for most of the studied traits in the heat tolerant lines of tomato, revealing the importance of both additive as well as non-additive in the controlling the traits. Hazra and Ansary (2008) studied genetics of heat tolerance for floral and fruit set to high temperature and reported gene action to be predominantly non-additive. This study was conducted to identify the best parental combination with superior fruit yield and heat tolerance of tomato under heat stress conditions.

Materials and Methods:-
Two heat tolerant (Icrixina and Rio Grande) and four heat susceptible tomatoes (Tima, Tropimech, Petomech and Roma Savana) were crossed using half diallel mating design. The resultant 15 hybrids, 6 parents and 4 checks (Roma VF, UC82 B, Thorgal F 1 and Jaguar F 1 ) were evaluated at National Horticultural Research Institute, Bagauda experimental farm (11°33´N; 8°23´E) in the Sudan savannah and Institute for Agricultural Research farm, Samaru (11 0 11´N; 07 0 38'E) Guinea savannah ecological zones of Nigeria in a 5 × 5 partially balanced lattice design with three replications between July to October, 2014 rainy season to synchronize flowering stage with heat period (September and October) as shown in Table 4. The plot size was 2 × 2m and 1m alleys. Seedlings of tomatoes were raised in nursery on 17 th July, 2014 and transplanted to the field on three rows at inter-row spacing of 60cm and intra-row of 50cm on 17 th August, 2014. Fertilizer (N.P.K 15:15:15) was split and applied at the rate of 45kgN, 45kg P 2 O 5 and 45kgK 2 O/ha and Urea (46%) at the rate of 64.4kgN/ha at two and five weeks after transplanting, respectively. All agronomic practices were kept uniform in both locations. Growth and yield data were taken randomly on five centered plants for observations and measurements leaving the plants on either end of the plot to avoid the border effect. Data were recorded for agronomic traits (plant height, days to 50% flowering, number of branches per plant, number of clusters per plant, number of flowers per cluster, number of flowers per plant, number of fruits per cluster, number of fruits per plant, average fruit weight, fruit length, fruit diameter, fruit shape index, fruit yield per plant and percentage fruit set) with corresponding physiological traits such as (leaf chlorophyll content and canopy temperature depression). The leaf chlorophyll content and canopy temperature depression were measured using SPAD chlorophyll meter (SPAD 502plus. Konica Minolta, Tokyo, Japan) and handheld infrared Thermometer (Spectrum technologies, Inc. U.S.A), respectively. Canopy temperature depression was estimated using equation 1. The data from each location was subjected to analysis of variance separately to detect the significance of genotypic variations (Gomez and Gomez, 1984) before a combined analysis of variance. Diallel analysis was used for fixed effects, according to Griffin (1956) method 2 model 1. DIALLEL-SAS05 which is a comprehensive software program for Griffin's and Gardner-Eberhart analysis (Manjit et al., 2005) was used to estimate combining ability variance and effects. The average degree of dominance value was calculated using equation 4 according to Peyman et al. (2012) and classified according to Lagervall, (1961) as follows: 0 = no dominance, less than unity = partial dominance, 1 = complete dominance and greater than unity = overdominance. When the inbreeding coefficient (F) of parents equal to zero (no inbreeding). The additive and non-additive variances were estimated using equations 2 and 3 as follows:

Analysis of Combining Ability:-
The combined analysis of variance for combining ability of the traits under heat stress is given in Table 1. The result showed that mean squares due to location were highly significant (P<0.01) for most of the traits except number of flowers per cluster, number of fruits per cluster, average fruit weight, fruit length, fruit shape index and significant (P<0.05) for fruit diameter, indicating that the conditions in both locations were not the same, which was possibly why the genotypes behave differently in both locations regarding these traits. Highly significant (P<0.01) mean squares due to parents were observed for days to 50% flowering, average fruit weight, fruit length, fruit diameter, fruit shape index and significant for number of fruits per plant, showing that the parents were influenced by change in locations. Dagade et al. (2015) reported highly significant mean squares due to parents for fruit weight, fruit length, fruit diameter and highly significant was also observed for number of fruits per plant (Enang et al., 2015) and 91 50% flowering (Zengin et al., 2015). The mean squares due to parents vs. cross were not significant for all traits, with exception of number of fruits per cluster which recorded significant difference at (P <0.05) revealing that the hybrids formed were better than at least one of the parents with regard to this trait. Parents vs. cross measure average heterosis for non-additive gene effect. The mean squares due to crosses revealed high significance (P<0.01) for the all studied traits except plant height, number of branches per plant, average fruit weight, percentage fruit set, leaf chlorophyll content and canopy temperature depression suggesting that parents can be used to develop suitable hybrids regarding these traits. The results corroborated with the findings of Dagade (Table 2) were greater than unity for all traits, except for number of branches per plant, number of clusters per plant, number of fruits per clusters, average fruit weight, fruit length, fruit shape index, percentage fruit set and canopy temperature depression, revealing the existence of overdominance gene action. Hazra and Ansary, (2008) reported overdominance and partial dominance gene action for most of the characters influencing heat tolerance.

Combining Ability Effects:-
The gca effects for the traits across locations (Table 3) showed that none of the parent was the best general combiner for all traits indicating genetic variability among the parents. However, the parent Icrixina recorded high significance (P<0.01) and positive gca effects for the majority of the traits. Among the parents, Rio Grande was considered to be a good combiner for average fruit weight, fruit length, fruit diameter and fruit shape index. These parents were good combiner for the traits and could be utilized for hybrid breeding in an individual location. The result for sca effects showed that, Rio Grande × Petomech and Tima × Roma Savana had significant positive sca effect for days to 50% flowering, while, the hybrids Icrixina × Tropimech, Tima × Petomech, Tropimech × Petomech and Petomech × Roma Savana expressed significant negative sca effect for days to 50% flowering, highly desirable for early flowering. Among the cross combinations, only Petomech × Roma Savana showed significant positive sca effects for a number of clusters per plant, number of fruits per plant, percentage fruit set and fruit shape index. However, Rio Grande × Roma Savana and Tima × Tropimech recorded significant sca effects for number of flowers per plant. Rio Grande × Roma Savana and Rio Grande × Tima expressed significant sca effect for average fruit weight and fruit length respectively. The high sca effects manifested by these crosses for the traits were possibly due to one of the best general combiner parents involved (Icrixina and Rio Grande). The hybrids, Petomech × Roma Savana and Icrixina × Rio Grande were good cross combinations in a desirable direction for fruit yield per plant and percentage fruit set. These hybrids could be selected and utilized for improving tomato yield under high temperature. The results were similar to findings of Hannan et al. (2007), Yashavantakumar (2008) and Saleem et al. (2013). Number of branches per plant, number of flowers per plant, fruit diameter, leaf chlorophyll content and canopy temperature depression revealed non-significant sca effects among hybrids. Singh and Narayanan (2004) observed that sca effect does not contribute much to the improvement of self-pollinated crop like tomato.

Conclusion:-
The present study revealed SCA variance components were higher than GCA variance components for the major traits, suggesting the preponderance of non-additive gene action. Considering GCA to SCA ratios and average degree of dominance values, hybrid vigor could be exploited to develop high yielding heat tolerant hybrid tomato that can thrive well under rainy season. The parent Icrixina was considered as the best general combiner for the major traits. Hybrids Icrixina × Rio Grande and Petomech × Roma Savana were considered as the most desirable Cross combinations for fruit yield per plant and percentage fruit set.