Heterosis and leaf curl virus resistance in rainy season tomato under North Indian conditions

Purpose of this study was to identify heterotic hybrids of tomato resistant to leaf curl virus disease. Eleven parental lines and their 55 one-way F1 crosses produced in a diallel mating design were evaluated for fruit yield, fruit quality, tomato leaf curl virus (TLCV) resistance and other important traits at Research Farm of the Department of Vegetable Science, Punjab Agricultural University, Ludhiana (India) during autumn of 2012. Parent 2-1 showed no TLCV symptoms throughout the growth period under natural epiphytotic conditions. The remaining lines showed mild to very severe disease infection. Seventeen of the crosses were completely free from TLCV symptoms under natural field conditions. Heterosis was worked out over mid-parent, better parent and commercial resistant check NS 524. The hybrids outperforming the best check (NS 524) included 102-13-6-1 × 2-1 for marketable (66.15%) and total fruit yield (63.12%), Punjab Chhuhara × 58-18-1-1 for dry matter content (43.89%), 58-11-1-1 × 10213-6-1 for fruit weight (34.14%), 102-1-6-1 × 115-1-8-1 for number of locules (-45.15%), 56-14-7-1 × 58-181-1 for lycopene content (61.06%), 102-1-6-1 × 58-18-1-1 for TSS content (19.32%), 58-18-1-1 × 55-26-1-1 for pericarp thickness (49.56%), 102-1-6-1 × 115-1-8-1 for polar diameter (32.07%) and 115-1-8-1 × 55-26-1-1 for equatorial diameter (19.07%). Based on the overall performance, four crosses viz., 102-8-5-1 × 7-5-1, 10213-6-1 × 55-26-1-1, 58-11-1-1 × 58-18-1-1 and 102-13-6-1 × 2-1 were identified as promising. These hybrids recorded fruit yield of 2.22, 2.29, 2.70 and 3.24 kg plant -1 , respectively, which was either at par with or significantly better than the resistant check NS 524 (1.95 kg plant -1 ). The hybrids also had acceptable fruit shape index (0.97-1.20) and fruit size (36.66-96.66 g).


INTRODUCTION
Tomato (Solanum lycopersicum L.) is a major vegetable crop grown throughout the world including tropical, subtropical and temperate regions.On global basis, it is planted on 4.4 million hectares with a total production of 161.79 million tonnes (Anonymous, 2012).India is the third largest tomato producer in the world after China and USA, accounting for about 8% of the world tomato production.It is cultivated over an area of about 0.905 *Corresponding author.E-mail: prashantumri@gmail.com.Tel: +919068980032.Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License million hectares with the total production of 19.10 million tonnes and productivity of 21.10 t ha -1 (Anonymous, 2013).Leaf curl virus is a serious disease of tomato and its incidence can reach up to 100% with yield losses often exceeding 90% (Saikia and Muniyappa, 1989).Tomato cultivation, especially in autumn season in North India and in summer season in South India is adversely affected due to high incidence of TLCV.The disease is caused by different species of genus Begomovirus (Family Geminiviridae), which are transmitted by whitefly (Bemisia tabaci Genn.).Managing the disease with pesticides has been a difficult task because of the rapid development of pest resistance to most insecticides (Horowitz et al., 2005).In recent years, the hybrid cultivars of tomato have become very popular with the growers due to their superior per se performance, which is manifested due to better plant vigour, faster growth and development, earliness, increased productivity and higher levels of resistance to biotic and abiotic stresses (Yordanov, 1983).Punjab Agricultural University has recommended three hybrids of tomato viz., TH 2312, TH 802 and TH 1 for cultivation in the state.However, all the three hybrids are susceptible to TLCV and are, therefore, not suitable for cultivation in the rainy season.Recently, the university has developed some breeding lines possessing high degree of resistance to TLCV.These lines will serve as the potential source for the development of TLCV resistant hybrids suitable for cultivation in rainy season.
Choice of parents for hybridization is one of the critical and most important tasks for plant breeders.The common approach for selecting the parents based on per se performance does not necessarily lead to desired results.Therefore, the present investigation was undertaken to identify tomato hybrids resistant to TLCV and suitable for cultivation in rainy season.

MATERIALS AND METHODS
The present investigation was carried out at the Department of Vegetable Science, Punjab Agricultural University Ludhiana, India during 2011-12 and 2012-13.The material for the present study comprised of eleven parents viz., Punjab Chhuhara, 58-11-1-1 , 56-14-7-1, 102-13-6-1, 102-1-6-1, 102-8-5-1, 115-1-8-1, 58-18-1-1, 55-26-1-1, 7-5-1, 2-1 along with one resistant check hybrid NS-524.Fifty-five F1 crosses (excluding reciprocals) involving 11 parents were made in a diallel fashion during February-March 2012.The parents, except Punjab Chhuhara, were selectively chosen for their resistance to TLCV.The experimental material comprising 55 F1 hybrids, 11 parental lines and check NS-524 were sown and raised in plastic trays in July and transplanted in August in an Augmented Randomized Block Design.All the cultural practices were followed as per the recommendations in Package of Practices for Vegetable Crops (Anonymous, 2013a).To increase inoculum pressure, one row of TLCV susceptible cv.Punjab Chhuhara was planted after every fifth row of the experimental plots.Plants were examined at weekly intervals for TLCV symptoms.Disease spread and the severity of symptoms were recorded according to the disease severity scale described by Muniyappa et al. (1991).Data for various horticultural traits were recorded from five randomly selected plants excluding border plants.Observations were recorded on 10 economic traits viz., marketable yield (kg plant -1 ), total fruit yield (kg plant -1 ), equatorial diameter (cm), polar diameter (cm), dry matter (%), total soluble solids (%), lycopene (mg 100 -1 g of fresh weight), number of locules per fruit, pericarp thickness (cm) and average fruit weight (g).Mean performance of parents for different characters is presented in Table 1.

RESULTS AND DISCUSSION
There were significant differences in the performance of parents, hybrids and checks for fruit yield, horticultural traits and TLCV resistance.The genetic differences exhibited by the hybrid progenies could be exploited through heterosis breeding with the identification of superior performing F 1 hybrids.

Estimation of heterosis
The available literature on heterosis in tomato provided evidence that this phenomenon is not limited to crosspollinated crops.One of the theories of this performance was that natural cross-pollination predominated within the wild forms of tomato in the centers of their origin.Therefore, crosses between tomato lines might be considered as F 1 between inbred lines of a species, which is naturally cross-pollinating.The exploitation of heterosis phenomenon can prove a potential tool in the hands of plant breeders for the improvement of this crop.
The results pertaining to mean performance and percent heterosis over better parent, mid parent and NS-524 are reported in Table 2.

Number of locules per fruit
The locular proportion of tomato fruit contains more organic acids and less reducing sugars than the pericarp portion (Grierson and Kader, 1986).Locule number is also important from the fruit firmness point of view.

Screening of parents and hybrids for TLCV resistance
The data related to screening for TLCV under field conditions are given in Table 3.The parent 2-1 did not show any disease symptoms throughout the growth period under natural conditions where as Punjab Chhuhara and 102-1-6-1 showed severe disease infection.Of the remaining eight lines, seven showed mild and one showed moderate infection.Mild symptoms of TLCV were exhibited by the lines 58-11-1-1, 56-14-7-1, 102-13-6-1, 102-8-5-1, 115-1-8-1, 55-26-1-1 and 7-5-1, whereas, 58-18-1-1 showed moderate infection.In general, it was observed that the genotypes, which showed early appearance of TLCV, resulted in severe yield losses as compared to the genotypes which showed late appearance of TLCV.It was observed that out of fifty-five cross combinations, seventeen crosses were completely free from TLCV.Twenty-three cross combinations showed mild infection, thirteen moderate infection and two were susceptible.Tomato germplasm/hybrids were extensively screened by Rattan and Bindal (2002), Shekara et al. (2003), Castro et al. (2007) and Mazyad et al. (2007), and reported differential response of genotypes/hybrids against TLCV.
Significant differences that emerged from tomato hybrids on the incidence of disease could possibly be attributed to the fact that the whiteflies had affinity with some particular accessions than the other accessions and resulted in some hybrids being more susceptible to virus than the other accessions.The accessions that had less number of infected plants could be explained by late occurrence of TLCV infection related to whitefly population variation.Moreover, this difference in reaction could be due to the virus strain, vector genotype or altered feeding conditions of the vector.These genotypes were further used in the breeding programme for the development of tomato cultivars/hybrids resistant to TLCV.

Table 1 .
Mean performance of parental liness for different characters.