ABSTRACT
Heat stress causes significant fruit yield loss in tomato (Lycopersicon lycopersicum Mill.). Breeding of tomato hybrids tolerant to high temperature will reduce fruit yield losses caused by heat stress in Nigeria. The genetics of heat tolerance in tomato was studied using half diallel mating design of six parents comprising two heat tolerant and four heat susceptible varieties. The resultant 15 hybrids, with the 6 parents and 4 checks were evaluated at two locations: Bagauda and Samaru under heat stress condition between July-October, 2014 rainy season. Data collected on agronomic, heat tolerance and physiological characters were used to determine heterosis, combining ability, heritability and correlations under field conditions. Analysis of variance indicated highly significant (P < 0.01) differences among the genotypes for most traits. Significant level of heterosis were observed in crosses Icrixina “ Rio Grande, Icrixina “ Roma Savana, Icrixina “ Tima, Icrixina “ Petomech and Petomech “ Roma Savana which ranged from -551.00 to 80.82% and from -31.81 to 120.80% for fruit yield and percentage fruit set, respectively. Combining ability analysis revealed that both additive and non-additive gene action were important for the inheritance of the number of clusters per plant, number of flowers per plant, number of fruits per plant, fruit length, fruit yield per plant and percentage fruit set. However, the variance components due to SCA were higher in magnitude than the GCA variance components for all traits except number of fruits per cluster and fruit length, indicating preponderance of non-additive gene action in the inheritance of the traits. Estimates of gca effects revealed that the parent Icrixina was a good general combiner for all traits except plant height, average fruit weight, fruit diameter and leaf chlorophyll content. The estimates of sca effects of vii crosses indicated that Icrixina “ Rio Grande, Icrixina “ Tima, and Petomech “ Roma Savana were the most desirable cross combinations for fruit yield per plant and percentage fruit set indicating that they are more heat tolerant. Cell membrane thermostability and heat injury at 30oC, 40oC and 50oC exhibited significant variations among the genotypes for electrolyte leakage at flowering stage, suggesting that cell membrane thermostability was good index for screening and evaluating tomato genotypes for heat tolerance. The genotypes Icrixina, Icrixina “ Tropimech, Rio Grande “ Tropimech, Rio Grande “ Roma Savana and Rio Grande “ Petomech were heat tolerant. Broad sense heritability was high for all traits, while narrow sense heritability was low to moderate for most of the traits. Correlation results suggests that selection for number of flowers per plant, number of fruits per plant, leaf chlorophyll content and cell membrane thermostability would increase fruit yield per plant and percentage fruit set under high temperature, but with a reduction in days to flowering, average fruit weight and fruit size.Icrixina “ Rio Grande and Icrixina “ Tima gave higher yields per plant compared with the hybrids checks. Therefore, considering the predominance of non-additive gene action and large negative genetic association between percentage fruit set and fruit size, heterosis breeding could be exploited in developing heat tolerant tomato with acceptable fruit size.
