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Skewed segregations are frequent events in segregating populations derived from different interspecific crosses in tomato. To determine a basis for skewed segregations in the progeny of the cross between Lycopersicon esculentum and L. pennellii, monogenic segregations of 16 isozyme loci were analyzed in an F₂ and two backcross populations of this cross. In the F₂, nine loci mapping to chromosomes 1, 2, 4, 9, 10, and 12 exhibited skewed segregations and in all cases there was an excess of L. pennellii homozygotes. The genotypic frequencies at all but one locus were at Hardy–Weinberg equilibria. In the backcross populations, all except two loci exhibited normal Mendelian segregations. No postzygotic selection model could statistically or biologically explain the observed segregation patterns. A prezygotic selection model, assuming selective elimination of the male gametophytes during pollen function (i.e., from pollination to karyogamy) adequately explained the observed segregations in all three populations. The direction of the skewed segregations in the F₂ was consistent with that expected based on the effects of unilateral incompatibility reactions between the two species. In addition, the chromosomal locations of five of the nine markers that exhibited skewed segregations coincided with the locations of several known compatibility-related genes in tomato. Multigenic unilateral incompatibility reactions between L. esculentum pollen and the stigma or style of L. pennellii (or its hybrid derivatives) are suggested to be the major cause of the skewed segregations in the F₂ progeny of this cross.

Genetic relationships between salt tolerance and expression of various physiological traits during vegetative growth in tomato were investigated. Parental, F₁, F₂, and backcross progeny of a cross between a salt tolerant (PI174263) and a salt sensitive cultivar (UCT5) were evaluated in saline solutions with electrical conductivity of 0.5 (non-stress) and 20 dS·m^–1 (salt-stress). Absolute growth, relative growth, tissue ion content, leaf solute potential and the rate of ethylene evolution by leaf petioles were measured. Growth of both parents were reduced under stress, however, the reduction was significantly less in PI174263 than UCT5 suggesting greater salt tolerance of the former. Under salt-stress, PI174263 accumulated in the leaf significantly less Na⁺ and Cl^– and more Ca²⁺ than UCT5. The F₁ hybrid performed intermediate relative to parents and the backcross populations approached recurrent parents in both growth response and ion accumulation. In all generations, leaf solute potential decreased and the rate of ethylene evolution increased under salt-stress, however, there were little or no differences among generations under either treatment. Across generations growth under salt-stress was positively correlated with Ca²⁺ and negatively correlated with Na⁺ accumulation in the leaf. In contrast, growth was not correlated with either leaf solute potential or the rate of ethylene evolution. Generation means analysis indicated that Na⁺ and Ca²⁺ accumulations were genetically controlled with additivity being the major genetic components. The results indicated that the inherent genetic capabilities of PI174263 to maintain high tissue Ca²⁺ levels and to exclude Na⁺ from shoot were essential features underlying its adaptation to salinity. Thus, tissue ion concentration may be a useful selection criterion when breeding for improved salt tolerance of tomato using progeny derived from PI174263.

Breeding for salt tolerance in tomato (Lycopersicon esculentum Mill.) has been restricted 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^–1 (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 vigor 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 were due to additive genetic effects. A moderate estimate of narrow sense heritability (0.49 ± 0.09) was obtained for shoot dry weight under salt-stress treatment. The results indicate that tomato salt tolerance during vegetative growth can be improved by breeding and selection.

The effectiveness of directional phenotypic selection to improve tomato seed germination under salt-stress was investigated. Seed of F₂ and F₃ progeny of F₁ hybrids between a salt-tolerant and a salt-sensitive tomato cultivar were evaluated for germination response at three stress levels of 100 (low), 150 (intermediate), and 200 mm (high) synthetic sea salt (SSS). At each salt-stress level, the most tolerant individuals were selected. Selected individuals (F₂s or F₃s) were grown to maturity and self-pollinated to produce F₃ and F₄ progeny families. The selected progeny from each experiment were evaluated for germination at four treatment levels of 0 (nonstress), 100, 150, and 200 mm SSS and compared with unselected populations. The results indicated that selections were equally effective at all three stress levels and in both F₂ and F₃ generations and significantly improved progeny seed germination under both salt-stress and nonstress treatments. Estimates of realized heritability for rapid germination under the various salt-stress levels ranged from 0.67 to 0.76. Analysis of response and correlated response to selection indicated a genetic correspondence of up to 100% between germination at different salt-stress levels. Genotypic family correlations between germination at the low, intermediate, and high salt-stress levels ranged from 0.67 to 0.89 and those between nonstress and salt-stress conditions ranged from 0.25 (between 0 and 200 mm) to 0.71 (between 0 and 100 mm salt). The results indicated that similar or identical genes contributed to rapid germination response of tomato seeds at different salt-stress levels. Thus, selection at one stress level resulted in progeny with improved germination at diverse salt-stress levels.

Seed of 42 wild accessions (Plant Introductions) of Lycopersicon pimpinellifolium Jusl., 11 cultigens (cultivated accessions) of L. esculentum Mill., and three control genotypes [LA716 (a salt-tolerant wild accession of L. pennellii Corr.), PI 174263 (a salt-tolerant cultigen), and UCT5 (a salt-sensitive breeding line)] were evaluated for germination in either 0 mm (control) or 100 mm synthetic sea salt (SSS, Na⁺/Ca²⁺ molar ratio equal to 5). Germination time increased in response to salt-stress in all genotypes, however, genotypic variation was observed. One accession of L. pimpinellifolium, LA1578, germinated as rapidly as LA716, and both germinated more rapidly than any other genotype under salt-stress. Ten accessions of L. pimpinellifolium germinated more rapidly than PI 174263 and 35 accessions germinated more rapidly than UCT5 under salt-stress. The results indicate a strong genetic potential for salt tolerance during germination within L. pimpinellifolium. Across genotypes, germination under salt-stress was positively correlated (r = 0.62, P < 0.01) with germination in the control treatment. The stability of germination response at diverse salt-stress levels was determined by evaluating germination of a subset of wild, cultivated accessions and the three control genotypes at 75, 150, and 200 mm SSS. Seeds that germinated rapidly at 75 mm also germinated rapidly at 150 mm salt. A strong correlation (r = 0.90, P < 0.01) existed between the speed of germination at these two salt-stress levels. At 200 mm salt, most accessions (76%) did not reach 50% germination by 38 days, demonstrating limited genetic potential within Lycopersicon for salt tolerance during germination at this high salinity.

A genetic linkage map of Prunus has been constructed using an interspecific F2 population generated from self-pollinating a single F1 plant of a cross between a dwarf peach selection (54P455) and an almond cultivar (Padre). This map consists of approximately 80 markers including 10 isozymes. 12 plum genomic, 19 almond genomic and 40 peach mesocarp specific cDNA clones. The backbone map will be used for identifying the genomic locations and characterization of genes governing important economic traits in the genus Prunus. Of particular interests are those genes associated with fruit ripening and mesocarp development in peach and almond.