Pollen selection in breeding tomato (lycopersicon esculentum Mill.) for adaptation to low temperature

Summary F₂ and BC₁ progenies from crosses between tomato (Lycopersicon esculentum) varieties differing for growth capacity at low temperature were produced under controlled conditions by hand pollination under two temperature regimes (22°C D/15°C N, normal temperature (N), and 15°C D/8°C N, low temperature (L)) with pollen formed under both regimes, resulting in four pollination treatments: NN, NL, LN and LL. Vegetative growth of the offspring was compared under a rather low temperature regime (19°C D/10°C N). Populations from different treatments within the progeny of a particular F₁ often differed significantly for average dry weight of 7 weeks old plants, the average of the NN population always being higher. Variances for dry weight were sometimes larger for LN populations, but this never resulted from a larger number of vigorous plants. Differences between populations within each progeny seemed to result in part from differences in the conditions for embryo development. Pollen selection at low temperature did not appear to be efficient for sporophyte breeding in this experiment.     

Effect of three anthocyaninless genes on germination in tomato (Lycopersicon esculentum Mill.) I. Seed germination under optimal conditions

The effect of three mutant genes affecting anthocyanin biosynthesis – anthocyaninless of Hoffmann (ah), anthocyaninwithout (aw) and baby lea syndrome (bls) – on tomato germination capacities at 24 ° C was investigated. The study was performed on nine anthocyanin containing lines – Apedice, Apeca, Ailsa Craig, Monfavet 167, Monfavet 168, Por, Piernita, VF 36 and VFNT cherry and 13 isogenic/near isogenic lines (IL/NIL) from them: 4 differing for gene ah, 3 differing for gene aw and 6 differing for gene bls). In the majority of the anthocyaninless IL/NILs germination began earlier than that in the wild type lines. Significant differences were observed in the time to 50% germination between the anthocyanin-containing and the anthocyaninless IL/NILs except for the most rapidly germinating line VFNT cherry. Seed weight, water uptake, effect of testa removal and presence or absence of maternal effects in F1 germination responses were investigated in order to elucidate the causes of the more rapid germination in the anthocyaninless IL/NILs. The increased amount of water uptake by the anthocyaninless lines, the shorter treatment time necessary for their testa removal, the important maternal effects in the genetic variability of the time to 50% germination and the differences in germination capacities between the wild type and the mutant IL/NILs after testa removal, indicated that the seed coat and the endosperm had to be affected by the effects of the three genes in a way that caused enhanced germination. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic Analysis of Resistance to Phytophthora Root Rot in Tomato (Lycopersicon esculentum Mill.)

The resistant accession, LA1312, and the susceptible cultivar ‘Peto 343′, were crossed to develop F₁, F₂ and BC₁ populations for genetic analysis of resistance in tomatoes to Phytophthora parasitica Dastur, the causal agent of Phytophthora root rot. There was no maternal effect on resistance. Generation means analysis indicated that tolerance to Phytophthora root rot was under genetic control with both simple (additive and dominance) and digenic interaction (additive × additive and additive × dominance) effects contributing to the total genetic variation among generation means. Weighted least square regression analysis indicated that the majority (ca. 96 %) of the genetic variation could be explained by simple additive effects alone. Narrow sense heritability was estimated as 0.22. Based on effective factor formulae, at least five effective factors controlled the resistance. Implications for breeding procedures are discussed.     

Inheritance of resistance to fruit rot (Phytophthora parasiticaDast.) in tomato (Lycopersicon esculentum Mill.)

Summary Fruit rot disease caused by Phytophthora parasiticaDast. is a limiting factor in tomato production in Himachal Pradesh. 30 to 60 per cent fruits are damaged by this disease. Crosses were made between EC 54725 (Lycopersicon pimpinellifolium), a small tyuited type, resistant to fruit rot and four highly susceptible tomato commercial cultivars (Gola, Sioux, S12, and Lalmani). Studies of F₁'s, F₂'s and back crosses indicated that EC 54725 carries a dominant gene imparting resistance to fruit rot.     

Genetic basis of physiological traits related to salt tolerance in tomato, Lycopersicon esculentum Mill.

Genetic relationships between salt tolerance and expression of various physiological traits during vegetative growth in tomato, Lycopersicon esculentum Mill., were investigated. Parental, F₁, F₂ and backcross progeny of a cross between a salt tolerant (PI174263) and a salt sensitive tomato cultivar (‘UCT5’) were evaluated in saline solutions with electrical conductivity of 0.5 (non-stress) and 20 dS/m (salt stress). Absolute growth, relative growth, tissue ion content, leaf solute potential and the rate of ethylene evolution were measured. Growth of both parents was reduced under salt stress; however, the reduction was significantly less in PI174263 than ‘UCT5’, suggesting greater salt tolerance of the former. Under salt stress, leaves of PI174263 accumulated significantly less Na⁺ and Cl^? and more Ca²⁺ than leaves of ‘UCT5’. Across parental and progeny generations, growth under salt stress was positively correlated with leaf Ca²⁺ content and negatively correlated with leaf Na⁺ content. In contrast, no correlation was observed between growth and either leaf solute potential or the rate of ethylene evolution under salt stress. Generation means analysis indicated that under salt stress both absolute and relative growth and the Na⁺ and Ca²⁺ accumulations in the leaf were genetically controlled with additivity being the major genetic component. The results indicated that the inherent genetic capabilities of PI174263 to maintain high tissue Ca²⁺ levels and to exclude Na⁺ from the shoot were essential features underlying its adaptation to salt stress and that these features were highly heritable. Thus, tissue ion concentration may be a useful selection criterion when breeding for improved salt tolerance of tomato using progeny derived from PI174263.     

Inheritance of quality traits in processing tomato (Lycopersicon esculentum Mill.)

Summary The genetic basis of pH, colour and soluble solids in processing tomato is examined. In a first experiment, aimed at identifying the base populations with which to start selection, parents, F₁ and F₂ progenies of an 8 × 8 diallel cross without reciprocals were tested.The results indicate that additive, dominance and additive x additive epistatic effects were noticeable for the three characters. Higher order interlocus interactions were also detected.As to the soluble solids, a trait for which higher order epistatic effects were less marked, the breeding potential of the cross combinations was assessed by calculating the expected mean values at the F generation. Some cross populations having C33 or C35 as parent appeared to be superior.A second experiment was conducted to collect data on the heritability of the foregoing qualitative traits by utilizing the regressions of F₃ offspring on F₂ parents. The low heritability coefficients observed would suggest the ineffectiveness of individual selection in early generations.In both the F₂ and F₃ generations rather low correlation coefficients among the three quality traits were observed.     

Genetic analysis of salt tolerance during vegetative growth in tomato, Lycopersicon esculentum Mill.

Breeding for salt tolerance in tomato has been impeded by insufficient knowledge of the genetic control of tolerance. The genetic basis of salt tolerance during vegetative growth was investigated by growing a salt-tolerant (PI174263) and a salt-sensitive tomato cultivar (UCT5) and their F₁, F₂ and backcross progeny in saline solutions with electrical conductivity of 0.5 (control) and 20 dS/m (salt-stress). The relative salt-tolerance of each generation was determined as the percentage of growth (i.e. dry matter production) under salt-stress relative to growth under control conditions. In all generations, shoot growth was significantly reduced by salt stress. The reduction was largest in UCT5 (56.1%) and smallest in the F₁ (27.4%), followed by PI174263 (32.3%). Analysis of the absolute and relative growth under salt-stress indicated that genes contributing to vigour might be different from genes conferring tolerance. Generation means analyses of the absolute and relative growth indicated that the majority of the genetic variation among generations were due to simple (additive and dominance) genetic effects; nonallelic interactions, although significant, were far less important. Partitioning of the total genetic variance by weighted least-square regression analysis and variance component analysis indicated that 88% or more of the variation was due to additive genetic effects. A moderate estimate of narrow sense heritability (0.49 ± 0.09) was obtained for shoot DW under salt-stress treatment. The results indicate that tomato salt-tolerance during vegetative growth can be improved by breeding and selection.     

Genetic analysis of low-temperature tolerance during germination in tomato, Lycopersicon esculentum Mill.

The genetic basis of low-temperature tolerance during germination of tomato seed was investigated using two approaches. First, a cold-tolerant (PI 120256) and a cold-sensitive tomato cultivar (UCT5) and their reciprocal F₂, F₃ and BC₁ progeny (total of 10 generations) were evaluated for germination at a low (11 ± 0.5°C) and a high (control) temperature 20 ±0.5° C) Weighted least-square regression analysis indicated that in the low-temperature treatment most of the variation resulted from additive genetic effects, and dominance and epistatic interactions were nonsignificant. Partitioning of the total genetic variance into those attributable to the effects of embryo, endosperm, testa and the cytoplasm indicated that additive effects of endosperm and embryo could individually account for 80% and 77% of the total variance, respectively. In the control treatment, greater than 60% of the variation could be explained by individual additive effects of endosperm or embryo and ? 27% of the variation could be explained by embryo dominance effects. Across generations, there was a positive correlation (r = 0.78, P < 0.01) between germination in the control and low-temperature treatments and there were no significant genotype × temperature interactions. The results indicate the presence of similar or identical genes with predominantly additive effects on germination under both low and high temperatures. In the second approach, the effectiveness of directional phenotypic selection to improve tomato cold tolerance during germination was evaluated by selecting (in an F₂ population of the same cross) the fastest germinating seeds under low temperature and comparing the germination of the selected F₃ progeny with germination of an unselected F₃ population. The results indicated that selection was highly effective and significantly improved germination performance of the progeny; a realized heritability of 0.74 was obtained for low-temperature tolerance during germination. It is concluded that in these tomato lines germination under low temperature is genetically controlled, with additivity being the major genetic component, and thus the trait can be improved by phenotypic selection.     

Inheritance of tolerance of the tomato (Lycopersicon esculentum Mill.) to metribuzin herbicide

Summary The inheritance of tolerance in the tomato (Lycopersicon esculentum Mill.) to metribuzin 4-amino-6-tert- butyl-3-(methylthio)-as-triazine-5(4H)-one herbicide was investigated. A biossay using a nutrient solution culture in controlled environment growth cabinets was used to evaluate progeny from tolerant by susceptible tomato cultivar crosses for response to metribuzin. Fireball and Vision were used as tolerant cultivars and Heinz 1706 as a susceptible cultivar. Parental, F₁, F₂, and some backcross generations were studied. Visual phytotoxicity, seedling height, and seedling dry weight were recorded.The bioassay was effective for classification of the segregating generations. F₂, and progeny variance analysis suggested that the inheritance of tolerance to metribuzin was controlled by one major gene with modifiers. Heritability values of 0.58 to 0.72 indicated a potential for cultivar improvement through plant breeding.     

Mapping of quantitative trait loci conferring resistance to bacterial wilt in tobacco (Nicotiana tabacum L.)

Tobacco bacterial wilt (TBW) is one of the most serious tobacco diseases in the world. Studies have shown that tobacco resistance to TBW is quantitatively inherited. This study aimed to map quantitative trait loci (QTL) conferring TBW resistance. An F_2 : 3 population containing 237 lines was developed from a cross between two flue‐cured tobacco cultivars, ‘Yanyan 97’ (YY97; moderately resistant to TBW) and ‘Honghua Dajinyuan’ (HD; highly susceptible to TBW), and a linkage map consisting of 201 simple sequence repeats (SSR) markers and spanning a total length of 2326.7 cM was constructed based on the population. Field experiments were conducted 2011 and 2012, and disease symptoms were investigated three times in each year. The phenotypic data were analysed either separately or jointly for QTL mapping using the software QTLNetwork 2.1. Eight QTL with significant main effects were mapped on chromosomes 2, 6, 12, 17 and 24. A major QTL (qBWR17a) was detected on chromosome 17, which explained up to 30% of the phenotypic variation. The results can facilitate marker‐assisted selection (MAS) in TBW resistance breeding programme.     

Inheritance of growth characters of tomato (Lycopersicon esculentum Mill.) under low energy conditions

Summary To study the mode of inheritance of W, RGR, NAR, LAR, SLA and LWR, the growth of 15 tomato genotypes and 104 of their F1's was analyzed. The plants were grown at a 19°C day temperature and a 10°C night temperature under a light intensity of 24 Wm^-2 visible radiation and a daylength of 8 hours. Combining ability analyses of variance revealed highly significant differences in GCA values between parents for all characters studied and significant SCA values for some of the characters. GCA values for NAR and LAR, and for NAR and SLA were strongly negatively correlated. A strong positive correlation existed between GCA values for LAR and SLA. The possibilities for improving growth under low energy conditions by using growth characters in breeding are discussed.Abbreviations W dry weight of the plant - RGR relative growth rate - NAR net assimilation rate - LAR leaf area ratio - SLA specific leaf area - LWR leaf weight ratio - MANOVA multivariate analysis of variance - GCA general combining ability - SCA specific combining ability     

Genetics of resistance to early blight (Alternaria solani Sorauer) in tomato (Lycopersicon esculentum L.)

The genetic nature of early blight resistance in tomato was studied in three crosses at seedling and adult plant stages. A six generation mean analysis of the cross Arka Saurabh (susceptible) × IHR1939 (resistance) and its reciprocal cross revealed that the resistance to early blight was conferred by recessive polygenes at both seedling and adult plant stages. This polygenic early blight resistance revealed the importance of additive and additive × additive gene effects at seedling stage and higher magnitude of dominance and dominance× dominance gene effects at adult plant stage. Evaluation of parents, F₁, F₂ and backcross generations of IHR1816 (resistance) × IHR1939 (resistance) revealed that the early blight resistance genes in IHR1816 (Lycopersicon esculentum NCEBR-1) and IHR1939 (Lycopersicon pimpinellifolium L4394) are independent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variation of 2-Tridecanone level in tomato plant leaflets and resistance to two mite species (Tetranychus sp.)

The wild tomato accession Lycopersicon hirsutum f. glabratum ‘PI 134417’ is a promising source of resistance to arthropods in tomato. It is self compatible and intercrosses easily with the cultivated species Lycopersicon esculentum. It is resistant to several arthropods, including spider mites, and these resistances have been related to the presence of high contents of 2-tridecanone (2-TD) and other methyl ketones in the leaves. The objective of this study was to evaluate the contribution of 2-TD concentration in the leaflets of Lycopersicon for resistance to the spider mites Tetranychus urticae (Koch, 1836) and T. ludeni Zacher, 1913. Plants of two segregating generations (F₂) were evaluated through the thumbtack bioassay (Weston & Snyder, 1990) in two different assays. Both experiments included as checks L. esculentum ‘TSWV-547’ (mite susceptible), L. hirsutum f. glabratum ‘PI 134417’ (resistant) and the interspecific hybrid F₁ (TSWV-547 × PI 134417). In the first experiment, 10 genotypes selected from F₂ (TSWV-547 × PI 134417), with contrasting 2-TD concentrations in the leaflets, were tested for resistance to T. urticae. In a second experiment, seven genotypes of the F₂ (TSWV-547 × BPX-318p1#230), also with contrasting 2-TD concentrations, were used to evaluate the resistance to T. ludeni. BPX-318p1#230 is a single plant selected from the segregating population F₂ (TSWV-547 × PI 134417) for its high 2-TD content. In both experiments, high negative correlations (r = − 0.88; − 0.91 and − 0.93 in the first experiment, and r = − 0.78, − 0.99 and − 0.96 in the second experiment) were found between the 2-TD content and average distance travelled by the spider mites after 60, 90 and 120 minutes, respectively. The level of resistance (repellency) to spider mites T. urticae and T. ludeni in the genus Lycopersicon appears to be mediated by the 2-TD levels present in the tomato leaflets. The similar results of both experiments showed that selection for high 2-TD content in leaflets can lead to genetic improvement in the direction of increased levels of spider mite resistance in tomato. This revised version was published online in August 2006 with corrections to the Cover Date.     

New sources for high resistance of tomato to the tomato spotted wilt virus from Lycopersicon peruvianum

Mechanical inoculation and transmission by thrips in a growth chamber were used in order to screen Lycopersicon peruvianumand Lycopersicon chilense germplasm for tomato spotted wilt virus (TSWV) resistance. Two highly aggressive Spanish TSWV isolates (HA-931100 and T-941117), having different restrictotypes were used. L. peruvianum accessions PI-126935, PI-126944, CIAPAN 16, PE-18 and CIAPAN 17 showed high resistance to both isolates in mechanical and thrips transmission. Their resistance appears useful in breeding programmes.     

Inheritance of characters determining growth and development of tomato (Lycopersicon esculentum Mill.) under low energy conditions

Summary Diallel crosses were made between 15 tomato genotypes with varying performance under a low energy regime. It appeared that differences between genotypes for 11 vegetative and generative plant characters are determined mainly by additive genetic variation. For breeding cultivars adapted to low energy conditions, crossing genotypes with a high GCA for weight of trusses and fruits (WTF) and genotypes with a high GCA for weight of vegetative plant parts (WVP) seems most promising. Complications may arise from the negative correlation between GCA values for WTF and WVP, and the strong positive correlation between GCA values for fruit number and WTF, causing small fruits.     

Characterization 3333 of tomato (Solanum lycopersicum) accessions for resistance to phylotype I and phylotype II strains of the Ralstonia solanacearum species complex under high temperatures

Bacterial wilt of tomato caused by Ralstonia solanacearum species complex (RSSC) causes substantial yield losses in the tropics and subtropics. Disease management options by chemicals are limited, and host resistance is the cheapest and easiest means of control. However, sources of bacterial wilt resistance in tomato are limited. The disease often coincides with higher temperatures in the tropics, and resistance sources that are more heat stable are particularly valuable for breeding of tropically adapted tomato cultivars. The objectives of this study were to identify tomato accessions that demonstrate relatively high bacterial wilt resistance under high temperatures and to identify accessions that may possess QTLs other than Bwr-6 and Bwr-12 (two major disease resistance QTLs against bacterial wilt), which could be exploited in future breeding. Sixty-seven tomato entries reported as bacterial wilt resistant were evaluated in a greenhouse against one strain each of phylotype I (Pss4) and phylotype IIB (Pss1632) of the RSSC (average temperature ≥29°C). Of those, five and 19 were homozygous for Bwr-6 and Bwr-12, respectively, and six were homozygous for both QTLs. Bwr-12 contributed to resistance against phylotype I strain but not against the phylotype II strain. Bwr-6 contributed to resistance against both phylotype strains. Entries with both QTLs as a group performed relatively better against the phylotype I strain. Entry “94T765-24-79”, which lacked Bwr-6 and Bwr-12, demonstrated relatively high resistance against the phylotype II strain and may carry new QTL/s. As new bacterial wilt resistance QTLs are mapped and markers designed, pyramiding multiple bacterial wilt resistance QTLs into new varieties should be straightforward, thereby increasing the chances of obtaining stable resistance.     

Inheritance of resistance to corky root in tomato (Lycopersicon esculentum Mill.)

Summary Material derived fromLycopersicon glandulosum and resistant to corky root was released to breeding establishments in 1965. The next year an investigation was started into the inheritance of the resistance.The method used in the resistance test is described. A detailed account is given of the causes of the variation observed in infection, the consequential problems encountered in the study of the inheritance of the resistance, and the possibilities of reducing the variation.After the resistant basic lines have been found to be pure, the material is shown to carry one major gene for resistance. This gene appears to have a very low degree of dominance; when heterozygous, its expression appears to be highly influenced by the environment.     

In vitro pollen germination and tube growth of tomato (Lycopersicon esculentum Mill.) and its relation with plant growth

Summary In vitro pollen germination at 10°C, 14°C and 22°C of four groups of two pure line tomato varieties was compared with their plant growth at 19°C D/10°C N under controlled environmental conditions. Generally, pollen germination was slow at 10°C but after 6 h the percentage of germination was similar to that at 22°C. Maximum germination was obtained at 14°C already after 4 h. The longest pollen tubes occurred at 22°C. The two varieties within each group differed significantly from each other for percentage of pollen germination. For one group, this difference was greater at 10°C than at 22°C. The varieties in two groups also differed significantly for pollen tube growth. These differences in pollen tube growth rate were greater at 22°C than at 10°C. There was no clear relationship between pollen germination, pollen tube growth and plant growth in any of the four pairs of varieties. The results are discussed with regard to the possibility of pollen selection at low temperature in order to improve the efficiency of breeding for growth at low temperature.     

Maternal and Genetic Effects on Seed Weight of Tomato, and Effects of Seed Weight on Growth of Genotypes of Tomato (Lycopersicon esculentum Mill.)

A study was made on the inheritance of seed weight of tomato, and on the effect of seed weight on growth of tomato plants. Use was made of 15 parental genotypes and 105 F₁s produced by diallel crossing (F₁-seeds). From combining ability analysis of variance carried out for weight of F₁-seeds it appeared that the weight of seeds harvested from tomato plants was mainly determined by the maternal genotype and largely independent of the genotype of the male parent. A combining ability analysis of variance for weight of seeds produced by seilfing F₁'s (F₂-seeds) showed, however, that female and male parents contributed equally to the inheritance of seed weight and that the inheritance of seed weight is determined mainly by chromosomal additiveiy acting factors, but that also non-additive gene action occurred. Genotypes with large seeds produced heavier seedlings than genotypes with small seeds, In later growth stages, the correlation between seed weight and plant weight became smaller, presumably, due to a lower relative growth rate of the seedlings from large seeds. For this reason, it is doubtful whether breeding of large seeded tomato cultivars opens up prospects of improving growth and yield of tomato plants.     

Studies on resistance to bacterial speck (Pseudomonas syringae pv. tomato) in tomato cv. Ontario 7710

Plants of 17 tomato cultivars and four wild Lycopersicon accessions were evaluated for their reaction to Pseudomonas syringae pv. tomato (Pst) in a greenhouse following a leaf‐spray inoculation. The genotypes exhibited a large amount of variation in response to Pst infection, with disease severity index (DSI) ratings from 0.2 to 3.9. The cultivar ‘Ontario 7710’ and two accessions of Lycopersicon hirsutum (LA 1773 and LA 1775) were the most resistant, with DSI values of 0.2, 0.4 and 0.6, respectively. Three varieties, M 1812, Kujawski and Warszawski, also showed a high level of tolerance. The most susceptible was ‘A 100’(DSI = 3.9). The inheritance of resistance to bacterial speck was investigated by disease tests in segregated populations obtained by hybridizing the tomato cv. Ontario 7710 with the susceptible variety ‘A 100′. Plants were rated for disease severity by inspecting each plant and were then evaluated according to phenotypic similarity to ‘Ontario 7710’ or ‘A 100’ in respect of the number and size of the spots. Genetic analysis in F1, F2 and backcross segregations indicated that resistance of'Ontario 7710’ to Pst is conferred by one incompletely dominant gene, Pto.