Relationships between cold- and salt-tolerance during seed germination in tomato: Analysis of response and correlated response to selection

Seeds of F₂ progeny of a cross between a slow-germinating (UCT5) and a fast-germinating tomato line (PI120256) were evaluated for germination under non-stress (control), cold-stress and salt-stress conditions, and in each treatment the most rapidly (first 5%) germinating seeds were selected, grown to maturity and self-pollinated to produce F₃ progeny. The selected F₃ progeny from each experiment were evaluated for germination in each of the three treatments, and compared with germination rate of unselected F₃ progeny. Selection for rapid seed germination was effective under cold stress and salt stress, but not effective under non-stress conditions. Furthermore, selection in either cold-stress or salt-stress treatment significantly improved progeny germination rate under both cold-stress and salt-stress treatments, as well as the non-stress treatment. The results support the suggestion that the same genes contribute to rapid seed germination under cold-, salt- and non-stress conditions. In practice therefore, selection for rapid seed germination under a single-stress environment may result in progeny with improved seed germination under a wide range of environmental conditions. Furthermore, to improve germination rate under non-stress conditions, it may be more effective to make selections under a stress treatment.     

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.     

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.     

Simultaneous improvement in cold tolerance and yield of temperate japonica rice (Oryza sativa L.) by introgression breeding

To combine high yield and improved cold tolerance (CT) in a japonica rice variety, ‘Chaoyou1’ (CY1), 324 BC₂F₅ introgression lines (ILs) selected for CT from 11 CY1 BC₂F₄ populations were evaluated in replicated experiments for their CT at the reproductive and seedling stages. A mean realized heritability of 0.747 was achieved in this study for CT. Evaluation of 116 ILs from five BC populations in replicated experiments under stress and normal conditions identified 18 promising ILs that had greatly improved CT and yield compared with CY1. Detailed comparisons between the ILs and CY1 for CT and yield‐related traits under stress and non‐stress conditions provided useful information and better understanding of important issues such as donor selection, selection efficiency and associated changes in non‐target traits in the BC breeding process. The large numbers of CT ILs developed provide useful materials for genetic, physiological and molecular dissection of CT and yield traits using DNA markers and ‐omic tools, and as parents for further improving these traits by designed QTL pyramiding.     

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.     

Comparison of QTLs for seed germination under non-stress, cold stress and salt stress in tomato

The purpose of this study was to examine whether rate of tomato seed germination under non-stress, cold-stress and salt-stress conditions was under similar genetic control by identifying and comparing quantitative trait loci (QTLs) which affect germination rate under these conditions. A fast-germinating accession (LA722) of the wild tomato species Lycopersicon pimpinellifolium Jusl. and a slow-germinating cultivar (NC84173, maternal and recurrent parent) of tomato (Lycopersicon esculentum Mill.) were hybridized and BC₁ and BC₁S₁ progeny produced. The BC₁ population was used to construct a linkage map with 151 restriction fragment length polymorphism (RFLP) markers. The BC₁S₁ population (consisting of 119 BC₁S₁ families) was evaluated for germination under non-stress (control), cold-stress and salt-stress conditions and the mean time to 50% germination (T50) in each treatment was determined. Germination analyses indicated the presence of significant (P < 0.01) phenotypic correlations between T50 under control and cold stress (r = 0.71), control and salt stress (r = 0.58) and cold stress and salt stress (r = 0.67). The QTL analysis indicated the presence of genetic relationships between germination under these three conditions: a few QTLs were identified which commonly affected germination under both stress- (cold-, salt- or both) and non-stress conditions, and thus were called stress-nonspecific QTLs. A few QTLs were also identified which affected germination only under cold or salt stress and thus were called stress-specific QTLs. However, the stress-nonspecific QTLs generally exhibited larger individual effects and together accounted for a greater portion of the total phenotypic variation under each condition than the stress-specific QTLs. Whether the effects of stress-nonspecific QTLs were due to pleiotropic effects of the same genes, physical linkage of different genes, or a combination of both could not be determined in this study. The results, however, indicate that the rate of tomato seed germination under different stress and nonstress conditions is partly under the same genetic control.     

Genetic relationships among cold, salt and drought tolerance during seed germination in an interspecific cross of tomato

Seed of BC₁ progeny of an interspecific cross between a slow germinating Lycopersicon esculentum breeding line(NC84173; maternal and recurrent parent) and a fast germinating L.pimpinellifolium accession (LA722) were evaluated for germination under cold stress, salt stress and drought stress, and in each treatment the most rapidly germinating seeds (first 2%) were selected. Selected individuals were grown to maturity and self-pollinated to produce BC₁S₁ progeny families. The selected BC₁S₁ progeny from each experiment were evaluated for germination rate in each of a non stress (control),cold-, salt- and drought-stress treatment, and their performances were compared with those of a non selected BC₁S₁population in the same treatments. Results indicated that selection for rapid seed germination in each of the three stress treatments was effective and significantly improved progeny germination rate under all three stress conditions. The results support the suggestion that same genes might control the rate of seed germination under cold, salt and drought stress. Furthermore, selection in each of the three stress treatments resulted in improved progeny seed germination rate under nonstress conditions, suggesting that genetic mechanisms that facilitate rapid seed germination under stress conditions might also contribute to rapid germination under nonstress conditions. In practice, therefore, selection for rapid seed germination under a single stress environment may result in progeny with improved seed germination under a wide range of environmental conditions. Furthermore, to improve germination rate under nonstress conditions, it might be more efficient to make selections under stress conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Generation means analysis of right-censored response-time traits: Low temperature seed germination in tomato

Summary In studies to determine the inheritance of response-time traits, such as time to seed germination, some viable individuals may fail to respond during an experiment. If these right-censored observations are ignored, sample means and variances will be underestimated. This is illustrated using data from time to seed germination at 9°C for Lycopersicon esculentum (Mill.) fast germinating PI 120256, slow-germinating T3 and their reciprocal F₁, F₂ and backcross progeny. This paper presents methods to detect and to accommodate right-censored data in generation means analysis. Genetic interpretations derived from corrected and uncorrected estimates of generation means and variances are compared. Correction for right-censoring increased estimates of environmental and phenotypic variances, and decreased heritability estimates.     

Competition Affects the Production of First Backcross Offspring on F1-hybrids, Brassica Napus × B. Rapa

Summary Interspecific F₁-hybrids may arise in fields with transplastomic oilseed rape where B. rapa occurs as a weed. Spilled seeds, including transplastomic F₁-hybrids with B. rapa, may germinate, which creates an opportunity for production of transplastomic BC₁ with B. rapa as father (BC_1r). Field trials were made with three different proportions of B. napus, B. rapa and F₁-hybrids and three different densities. Contrary to most studies on how plant competition affects introgression between oilseed rape and B. rapa, this study focused on offspring produced on F₁-hybrids, where the F₁-hybrids had oilseed rape as maternal parent. We estimated the BC_1r production in all combinations of proportion and density, and found that B. rapa sired from 0.6–7.8% of the offspring. At the proportion with the highest abundance of F₁-hybrids the entire paternity was assessed. There was a significant density effect on the production of BC_1r but the effect differed among proportions. Both the highest and lowest frequencies of BC_1r were obtained at high plant density. Neither the proportion nor density affected the number of BC_1r per square-meter significantly. Biomass components decreased significantly from low to intermediate density, whereas a further increase in density only affected the thousand-seed weight significantly. On the basis of the results from the present experiment we conclude that introgression of transgenes from transplastomic oilseed rape to B. rapa seems most likely at current field densities of B. napus, and when B. rapa is an abundant weed.     

Absence of a genetic relationship between salt tolerance during seed germination and vegetative growth in tomato

Two approaches were used to determine the relationship between salt tolerance during seed germination and vegetative growth in tomato. First, F₄ progeny families of a cross between a breeding line, ‘UCT5’ (salt sensitive at all developmental stages), and a primitive cultivar, ‘PI 174263’ (salt tolerant during germination and vegetative growth), were evaluated in separate experiments for salt tolerance during germination and vegetative growth. There were significant differences among the F₄ families in both the rate of seed germination and the plant growth (dry matter production) under salt stress. There was, however, no significant correlation between the ability of seeds to germinate rapidly and the ability of plants to grow under salt stress. In the second approach, selection was made for rapid germination under salt stress in an F₂ population of the same cross and the selected progeny was evaluated for salt tolerance during both germination and vegetative growth. The results indicated that selection for salt tolerance during germination significantly improved germination performance under salt stress; a realized heritability estimate of 0.73 was obtained. Selection for salt tolerance during germination, however, did not affect plant salt tolerance during vegetative growth; there was no significant difference between the selected and unselected progeny based on either absolute or relative growth under salt stress. Obviously, in these genetic materials, salt tolerance during germination and vegetative growth are controlled by different mechanisms. Thus, to develop tomato cultivars with improved salt tolerance, selection protocols that include all critical developmental stages would be desirable.     

Marker-assisted introgression of QTLs for yield under moisture stress into elite varieties of rice (Oryza sativa)

Rice production is severely constrained by the moisture stress. The present study was undertaken to transfer two important QTLs viz., (qDTY2.2 and qDTY4.1), which controls yield under moisture stress (DTY) into two elite varieties, that is, MTU1010 and NLR34449 through marker-assisted breeding. Foreground and background selections of backcross generations, that is, BC₁F₁, BC₂F₁ and BC₂F₂ identified several promising introgression lines (ILs) with two QTLs and single QTLs with an appreciable level of recovery of recipient parent genome. In ILs, the recovery of MTU1010 genome content was estimated to be 83%–93% while the recovery of NLR34449 was 84%–92%. The two-QTL ILs of MTU1010 and NLR34449 backgrounds (qDTY2.2 and qDTY4.1) have shown substantial yield advantage (32 to 84%) over the single-QTL ILs (either qDTY2.2 or qDTY4.1) under moisture stress conditions. Among all, two ILs, MBC-124 and NBC-127 are the best high yielding lines under moisture stress conditions. These outyielded ILs have the potential to be released as varieties in rainfed ecosystem and also can be used as donors in the existing breeding programme in rice.     

Introgression of resistance to Columbia and Northern root-knot nematodes fromSolanum bulbocastanum into cultivated potato

Summary Resistance toMeliodogyne chitwoodi races 1 (MC1) and 2 (MC2) andM. hapla (MH) derived fromSolanum bulbocastanum was introduced into the cultivated potato gene pool through somatic fusion. The initial F₁ hybrids showed resistance to the three nematodes. Resistance to reproduction on roots by MC1 was accompanied by resistance to tuber damage in F₁ clones. Tuber damage sometimes occurred, however, in hybrids of BC₁ progeny resistant to reproduction on roots when MC2 and MH were the challenging nematodes. Resistance to reproduction was transferred into BC₁ individuals, but a greater proportion of BC₁ progeny was resistant to MC1 than to MC2 or MH. Resistance to MC1 appears to be dominant and discretely inherited. F₁ and BC₁ progeny were pollen sterile, but seed were produced from crosses using cultivated tetraploid pollen sources. Approximately 11 and 33 per cent of pollinations produced berries on F₁ and BC₁ pistillate parents, respectively. Seed yield increased fourfold overall in crosses with F₁ compared to BC₁ individuals.Abbreviations MC1 Meloidogyne chitwoodi race 1 - MC2 Meloidogyne chitwoodi race 2 - MH Meloidogyne hapla - Rf Reproductive factor     

Reproductive Behavior of Hexaploid/Diploid Wheat Hybrids 1

The bottleneck restricting introgression of useful genes directly from diploid into hexaploid wheats is the low number of BC₁F₁ seeds obtained. In crosses between hexaploid wheat (Triticum aestivum L.; AABBDD) and Aegilops squarrosa L. (DD) or T. urartu Thum. (AA), this bottleneck may be overcome simply by pollinating a sufficient number of F₁ spikes. However, hybrids between hexaploid wheat cultivars (T. aestivum) and T. monococcum L. (AA) generally are highly female-sterile, often having no pistils. One T. monococcum accession, PI 355520, when crossed with T. aestivum, produced hybrids with female fertility in the same range as that of T. aestivum/A. squarrosa or T. aestivum/T. urartu hybrids, ca. 0.5 to 1.0 backcross seed per spike. We found that female fertility was controlled by two duplicate genes in PI 355520, and that this accession can be used as a bridging parent to introgress genes from other T. monococcum accessions into hexaploid wheat. Pairing of homologous chromosomes was less frequent and weaker in such crosses than in T. aestivum/A. squarrosa crosses, but homoeologous bivalents occurred at a rate of almost 0.5 II per cell. Restitution division was detected in crosses involving all three diploid species and was confirmed cytologically in crosses with PI 355520. Chromosome numbers of BC₁F₁ plants ranged from 35 to 67; plants with 49 or more chromosomes occurred at frequencies of 0.09 to 0.21 among progeny of A. squarrosa and T. urartu and 0.29 in progeny of T. aestivum/T. monococcum crosses involving PI 355520. These results are consistent with those of previous studies, demonstrating the potential of direct Hexaploid/diploid crosses for rapidly introgressing useful genes into Hexaploid wheat with minimum disturbance of the background genotype.     

Inheritance of black leaf mold resistance in tomato

Summary Inheritance of black leaf mold (BLM) (caused by Pseudocercospora fuligena) resistance was studied in four crosses involving two resistant Lycopersicon accessions (PI134417, L. hirsutum and PI254655, L. esculentum) and four susceptible Asian Vegetable Research and Development Center tomato lines (CLN657BC₁F₂-267-0-3-12-7, CL143-0-10-3-0-1-10, CLN698BC₁F₂-358-4-13 and CL5915-93D₄-1-0-3). For each cross, six generations, i.e. P₁, P₂, F₁, F₂, BC₁F₁ and BC₁F₂ were evaluated following inoculations with isolate Pf-2 of P. fuligena. Chi-square analyses of the data based on the ratio of resistant to susceptible plants in the F₂ in three of four crosses gave a good fit to a segregation ratio of 1 R : 15 S, and BC₁F₂ data in three of four crosses gave an acceptable fit to the segregation ratio of 1 R : 63 S. The results indicate that resistance to BLM may be conditioned by two recessive genes acting epistatically in both PI134417 and PI254655.     

Production of interspecific F1 hybrids, BC1, BC2 and BC3 populations between Lycopersicon esculentum and two accessions of Lycopersicon peruvianum carrying new root-knot nematode resistance genes

Two accessions of Lycopersicon peruvianum Mill. (PI270435, PI126443) carrying Mi-2 and Mi-3, respectively, new root-knot nematode resistance genes, were selected as the male parents for crosses with L. esculentum Mill. in order to produce interspecific hybrids. Crossability barriers between these two distantly related species were circumvented by ovule culture. A total of ten interspecific F₁ hybrid plants were produced. The hybrid nature of the putative F₁ plants was confirmed by a comparison of several morphological characteristics and a PCR-based assay. Eight of ten hybrid plants were backcrossed with L. esculentum to generate a total of 98 BC₁ progeny. Two lines were advanced to the BC₂ and BC₃ levels. Based on these results, ovule culture was found to be an effective method for the production of novel interspecific F₁ and BC₁ plants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Intergeneric hybridization of Diplotaxis erucoides with Brassica napus. I. cytogenetic analysis of F1 and BC1 progeny

Summary Intergeneric hybrids (F₁) Diplotaxis erucoides (DeDe) x Brassica napus (AACC) and the first backcross to B. napus (BC₁) have been obtained through in vitro culture of excised ovaries. The chromosome numbers of F₁ and BC₁ plants proved the occurrence of unreduced gametes. The study of metaphase I chromosome pairing showed that autosyndesis in De genome and allosyndesis between De and A/C genomes might exist. The male fertility of the F₁ plants was low. Some male-sterile plants were found in F₁ and BC₁ progeny. The possibilities of creating addition lines B. napus-D. erucoides and of obtaining a new cytoplasmic male sterility in B. napus are discussed.     

Cytogenetic study of Helianthus laevigatus and its F1 and BC1F1 hybrids with cultivated sunflower, Helianthus annuus

Although the wild sunflower species Helianthus laevigatus has not been extensively studied it may be considered for sunflower breeding as a potential source of desirable genes for Sclerotinia stalk rot resistance and high contents of proteins and linoleic acid in the seed. A set of six H. laevigatus populations was crossed to cultivated sun~ower lines and produced nine F₁ (2-14 plants) and 66 BC₁F₁ hybrid combinations (1-13 plants). Male sterility occurred in F₁ and BC₁F₁ hybrid combinations and pollen viability was lower in the progenies than in the parents (51.6-77.2%in F₁ and in F₁ and 4.8-34.0% in BC₁F₁). Meiosis was normal in the H. laevigatus populations It was found that this tetraploid species also occurred in a hexaploid form Numerous irregularities were observed in the meiosis of the F₁ interspecific hybrids During diakinesis, quadrivalents and hexavalents were recorded in addition to bivalents Dislocated chromosomes and chromosome bridges were present in the other phases The chromosome number in F₁ was 68 (tetraploid). Irregularities in chromosome pairing were observed in the interspecific hybrids at BC₁F₁. There were many univalents, and trivalents quadrivalents and hexavalents were also present The chromosome number in the BC₁F₁ generation ranged from 34 to 60. The occurrence of meiotic irregularities in the F₁ and BC₁F₁ interspecific hybrids indicates that H. laevigatus and the cultivated sunflower differ in genome constitution.     

Low temperature seed germination of Lycopersicon species evaluated by survival analysis

Summary Low temperature germination responses were evaluated for 18 high altitude accessions representing five wild Lycopersicon species and 19 accessions of L. esculentum which have reputed ability to germinate in the cold. Survival analysis indicated that one accession of L. chilense germinates better at 10°C than PI 120256, the fastest-germinating L. esculentum genotype, and that PI 120256 germinates as well as PI 126435 (L. peruvianum). Additional wild ecotypes exhibiting rapid germination at 10°C were identified from L. peruvianum and L. hirsutum. These ecotypes may possess genetic potential for introgressing cold germination ability into L. esculentum cultivars.     

A major gene for low temperature germinability in rice (Oryza sativa L.)

Low temperature-induced retardation of seedling growth is a common problem in temperate rice-growing areas, at high altitudes of tropical and sub-tropical areas, and in areas with a cold irrigation water supply. Studies on low temperature germinability have revealed complex inheritance of the trait. The purpose of this study was to identify the gene(s) for low temperature germinability using Italica Livorno as a donor parent. The frequency distributions for the germination rate at 15 °C in the F₂ and BC₁F₁ populations showed continuous segregation, suggesting that low temperature germinability was under polygenic control. Since some lines in the BC₁F₁ population showed vigorous low temperature germinability similar to that of Italica Livorno, backcrosses until the BC₅F₁ generation was carried out using Hayamasari as the recurrent parent. Clear segregations of low temperature germinability were observed in the BC₅F₁ and BC₅F₂ populations. The distribution of low temperature germinability fitted a single-gene segregation, indicating that a single dominant gene with a large effect was transferred to Hayamasari. This gene is tentatively symbolized as Ltg(t). Low temperature germinability of near isogenic lines for Ltg(t) was similar to that of Italica Livorno. Ltg(t) should greatly contribute to the improvement and manipulation of low temperature germinability in rice breeding programs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Relationships between cold- and salt-tolerance during seed germination in tomato: Germplasm evaluation

Thirty tomato accessions representing six Lycopersicon species were evaluated for the rate of seed germination under no stress, cold-stress and salt-stress conditions. Most accessions responded similarly to both cold- and salt-stress conditions (i.e. they were equally sensitive or tolerant to both stresses), however, a few accessions exhibited more sensitivity (or tolerance) to one stress than the other. In addition, some accessions that germinated relatively rapidly under non-stress conditions exhibited great sensitivity to both cold stress and salt stress. Across accessions, significant (P < 0.01) positive phenotypic correlations were observed between germination rate under control and cold stress (r_P= 0.89), control and salt stress (r_P= 0.63) and cold stress and salt stress (r_P= 0.77). The results indicate that the rate of tomato seed germination under non-stress, cold- and salt-stress conditions may be controlled by the same genes (or physiological mechanisms), but additional components may be involved which affect germination rate under specific stress conditions.