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Abstract

F₁ hybrids between high sugar and acid lines of tomato (Lycopersicon esculentum Mill.) were rated higher in sweetness, ‘tomato-like’, and overall flavor intensity than the high-acid parent common to the crosses. Titratable acidity and soluble solids content were responsible for most of the observed differences in sweetness. The results indicate that improved tomato flavor can be achieved by genetically enhancing sugar and acid content. Rapid gains in flavor quality might be achieved in horticulturally acceptable types through F₁ hybrids.

Abstract

Male-sterile, male-fertile, exserted stigma, and exserted stigma with positional sterile (ps) genotypes of tomato (Lycopersicon esculentum Mill.) were pollinated at daily intervals from one day before to 2 days after anthesis (A^-1, A⁰, A⁺¹, A⁺², respectively) in 2 field experiments and one greenhouse experiment. A male-sterile with gene ms-10 ³⁵ had greater seeds per pollination than other genotypes in all 3 experiments. Seeds per pollination produced in the greenhouse was greater than that of a similar field experiment at all pollination stages except A^-1, where seed production was similar. Seeds per pollination was less at A^-1 than at later pollination stages. This was due to significant differences in seeds per fruit in all experiments and percentage of fruit set in 2 of 3 experiments. There was no selfing in the 2 male-sterile genotypes in any experiment and no selfing for exserted stigma with ps in the field experiments. There was about 2% selfing with ps at A⁺² in the greenhouse. Selfing with stigma exsertion alone ranged from 2% to 22% in the field and from less than 1% to about 5% in the greenhouse. The relationship of selfing with pollination stage was not clear for the exserted stigma genotype.

Crosses were made between tomato (Lycopersicon esculentum Mill.) inbreds susceptible to races T2 and T3 of bacterial spot (Xanthomonas vesicatoria and Xanthomonas campestris pv. vesicatoria, respectively) and accession PI 114490 with resistance to races T1, T2, and T3. Resistance to race T2 was analyzed using the parents, F₁, and F₂ generations from one of the crosses. The F₁ was intermediate between the parents for disease severity suggesting additive gene action. The segregation of F₂ progeny fit a two-locus model (χ² = 0.96, P = 0.9-0.5) where four resistance alleles are required for a high resistance level, two or three resistance alleles provide intermediate resistance, and zero or one resistance allele results in susceptibility. The narrow sense heritability of resistance to T2 strains was estimated to be 0.37 ± 0.1 based on F₂ to F₃ parent-offspring regression. A second cross was developed into an inbred backcross (IBC) population to facilitate multilocation replicated testing with multiple races. Segregation for T2 resistance in the inbred backcross population also suggested control was by two loci, lending support to the two-locus model hypothesized based on the F₂ segregation. To determine if the same loci conferred resistance to the other races, selections for race T2 resistance were made in the F₂ and F₃ generations and for race T3 resistance in the F₂ through F₄ generations. Six T3 selections (F₅), 13 T2 selections (F₄'s that diverged from seven F₂ selections), and control lines were then evaluated for disease severity to races T1, T2, and T3 over two seasons. Linear correlations were used to estimate the efficiency of selecting for resistance to multiple races based on a disease nursery inoculated with a single race. Race T1 and race T2 disease severities were correlated (r ≥ 0.80, P< 0.001) within and between years while neither was correlated to race T3 either year. These results suggest that selecting for race T2 resistance in progeny derived from crosses to PI 114490 would be an effective strategy to obtain resistance to both race T1 and T2 in the populations tested. In contrast, selection for race T3 or T2 will be less likely to result in lines with resistance to the other race. PI 114490 had less resistance to T3 than to T2 or T1. Independent segregation of T2 and T3 resistance from the IBC population derived from PI 114490 suggests that T3 resistance is not controlled by the same genes as T2 resistance, supporting the linear correlation data.

Several procedures for evaluating the resistance of tomato (Lycopersicon esculentum Mill.) to bacterial wilt were used to account for diversity in strains of Pseudomonas solanacearum Smith and to approximate resistance under field conditions. Five strains of P. solanacearum from Florida and one from North Carolina were inoculated onto 19 tomato genotypes and one tomatillo (Physalis ixocarpa Brot.) genotype using a stem-puncture technique. Genotypes were also transplanted as seedlings into naturally infested soil. Resistance was evaluated by comparing the response of each genotype to the susceptible cultivars Bonny Best and Sunny. With the stem-puncture technique, the mean incidence of disease ranged from 30% with the strain from North Carolina to 94% with a strain from northern Florida. Significant differences in the resistance of genotypes and pathogenicity of strains were observed. However, no interaction between strain and genotype was observed. Using naturally infested soil, the mean incidence of disease was 51% and significant differences in the resistance of genotypes were also observed. Hawaii 7997, Hawaii 7998, and CRA 66 had the lowest incidence of disease, regardless of inoculation method. The results indicate that assessing pathogen diversity and using a combination of resistance screening techniques can facilitate the evaluation of many genotypes, account for potential regional variability in the pathogen, and differentiate levels of field resistance to tomato bacterial wilt.

The major components of flavor in tomato (Lycopersicon esculentum Mill.) and other fruit are thought to be sugars, acids, and flavor volatiles. Tomato overall acceptability, tomato-like flavor, sweetness, and sourness for six to nine tomato cultivars were analyzed by experienced panels using a nine-point scale and by trained descriptive analysis panels using a 15-cm line scale for sweetness, sourness, three to five aroma and three to seven taste descriptors in three seasons. Relationships between sensory data and instrumental analyses, including flavor volatiles, soluble solids (SS), individual sugars converted to sucrose equivalents (SE), titratable acidity (TA), pH, SS/TA, and SE/TA, were established using correlation and multiple linear regression. For instrumental data, SS/TA, SE/TA, TA, and cis-3-hexenol correlated with overall acceptability (P = 0.05); SE, SE/TA (P≤0.03), geranylacetone, 2+3-methylbutanol and 6-methyl-5-hepten-2-one (P = 0.11) with tomato-like flavor; SE, pH, cis-3-hexenal, trans-2-hexenal, hexanal, cis-3-hexenol, geranylacetone, 2+3-methylbutanol, trans-2 heptenal, 6-methyl-5-hepten-2-one, and 1-nitro-2-phenylethane (P≤0.11) with sweetness; and SS, pH, acetaldehyde, aceton, 2-isobutylthiazole, geranlyacetone, β-ionone, ethanol, hexanal and cis-3-hexenal with sourness (P≤0.15) for experienced or trained panel data. Measurements for SS/TA correlated with overall taste (P=0.09) and SS with astringency, bitter aftertaste, and saltiness (P≤0.07) for trained panel data. In addition to the above mentioned flavor volatiles, methanol and 1-penten-3-one significantly affected sensory responses (P = 0.13) for certain aroma descriptors. Levels of aroma compounds affected perception of sweetness and sourness and measurements of SS showed a closer relationship to sourness, astringency, and bitterness than to sweetness.