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.     

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 cold tolerance during vegetative growth in tomato, Lycopersicon esculentum Mill.

The genetic basis of cold tolerance (CT) during vegetative growth in tomato was investigated by evaluating plants of a cold-tolerant primitive cultivar (PI120256) and a cold-sensitive breeding line (UCT5) and their reciprocal F₁, F₂, F₃, BC₁P₁ and BC₁P₂ progeny under two temperature regimes of 15/10 ^°C (cold stress) and 25/15 ^°C (control). Plants were evaluated for shoot dry weight (DW) under cold stress and by a tolerance index (TI) measured as the ratio of DW under cold stress (DW_s) to DW under control (DW_c) conditions. Shoot DW was reduced in all genotypes in response to cold stress. However, PI120256 exhibited the highest CT (TI = 90.5%) and UCT5 the lowest (TI = 38.9%). The TIs of the filial and backcross progeny were intermediate to the parents. Across generations, there was a positive correlation (r = 0.76, p < 0.01) between DW_s and DW_c indicating that growth under cold stress was influenced by plant vigor. However, the absence of a significant correlation (r=0.47, p >0.05) between DW_c and the TI and, in contrast, the presence of a significant correlation (r =0.92, p <0.01) between DW_s and the TI suggest that plant vigor was not a determining factor in the expression of CT in PI120256 and its progeny. Generation means analyses of DW_s and TI indicated that the variation among generations was genetically controlled, with additive effects accounting for most of the variation. There were no significant dominance effects, and epistatic effects were minor and involved only additive × additive interactions. The results suggest that the inherent CT of PI120256 should be useful for improving CT of commercial cultivars of tomato. 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: 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.     

Quantitative and molecular characterization of heat tolerance in hexaploid wheat

Understanding the genetic basis of tolerance to high temperature is important for improving the productivity of wheat (Triticum aestivum L.) in regions where the stress occurs. The objective of this study was to estimate inheritance of heat tolerance and the minimum number of genes for the trait in bread wheat by combining quantitative genetic estimates and molecular marker analyses. Two cultivars, Ventnor (heat-tolerant) and Karl92 (heat-susceptible), were crossed to produce F₁, F₂, and F₃populations, and their grain-filling duration (GFD) at 30/25 ^°C 16/8 h day/night was determined as a measure of heat tolerance. Distribution of GFD in the F₁ and F₂ populations followed the normal model (χ², p > 0.10). A minimum of 1.4 genes with both additive and dominance effects, broad-sense heritability of 80%, and realized heritability of 96%for GFD were determined from F₂ and F₃ populations. Products from 59primer pairs among 232 simple sequence repeat (SSR) pairs were polymorphic between the parents. Two markers, Xgwm11 andXgwm293, were linked to GFD by quantitative trait loci (QTL) analysis of the F₂ population. The Xgwm11-linked QTL had only additive gene action and contributed 11% to the total phenotypic variation in GFD in the F₂population, whereas the Xgwm293-linked QTL had both additive and dominance action and contributed 12% to the total variation in GFD. The results demonstrated that heat tolerance of common wheat is controlled by multiple genes and suggested that marker-assisted selection with microsatellite primers might be useful for developing improved cultivars. This revised version was published online in August 2006 with corrections to the Cover Date.     

Combining ability of soybean (Glycine max) for low phosphorus tolerance on acidic soils of Western Ethiopia

Determining the gene actions governing the inheritance of traits of interest has paramount importance in designing a breeding approach to improve the progeny populations. This study was undertaken to determine the combining ability of nine selected parental lines in the F₂/F₃ segregating populations for low P tolerance. The experiment was laid out in an alpha lattice design in two locations, that is Mettu and Assossa of Western Ethiopia that are characterized by soils with low P and pH. General combining ability (GCA) effects were highly significant for grain yield, pod length, days to maturity and plant height, while specific combining ability (SCA) effects were highly significant for grain yield, 100-seed weight, pod length and plant height. The parent Hardee-1 was identified as the best general combiner for yield, number of seeds per pod, pod length, plant height and pod number. The results suggest that additive gene action was important for several of the studied traits, implied by significant GCA effect, which might indicate selection for these traits could be effective in later segregating generations.     

Low-temperature-related growth and photosynthetic performance of alloplasmic tomato (Lycopersicon esculentum Mill.) with chloroplasts from L. hirsutum Humb. & Bonpl.

Growth and photosynthetic performance were analyzed in alloplasmic tomato at a high- (25/17 ^°C; HTR) and low-temperature regime (12/6 ^°C; LTR) in order to establish the role of cytoplasmic variation on low-temperature tolerance of tomato (Lycopersicon esculentum Mill.). Four alloplasmic tomato lines, containing the nuclear genome of tomato and the plastome of L. hirsutum LA 1777 Humb. & Bonpl., an accession collected at high-altitude in Peru, were reciprocally crossed with 11 tomato entries with a high inbreeding level and a wide genetic variation, resulting in a set of 44 reciprocal crosses. Irrespective of growth temperature, alloplasmic families with alien chloroplasts of L. hirsutum (h) were on average characterized by a high shoot biomass, a large leaf area, and a low specific leaf area in comparison with their euplasmic counterparts. These results do not directly point to an advantageous effect of h-chloroplasts on biomass accumulation at low temperature but rather towards a small general beneficial effect on growth and/or distribution of assimilates. Significant chloroplast-related differences in photosynthetic performance, however, were not detected at both temperature regimes, indicating that h-chloroplasts can properly function in a variable nuclear background of L. esculentum. It is concluded that chloroplast substitution is not an effective method for breeding tomato plants with improved low-temperature tolerance. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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.     

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.     

大豆导入系群体芽期耐低温位点的基因型分析及QTL定位

利用美国大豆品种Clark (供体亲本)与主栽品种红丰11 (轮回亲本)所构建的回交导入系,经过严格的芽期耐低温筛选鉴定,得到46个在芽期耐低温性状上明显超过轮回亲本的导入系个体。利用这套选择群体结合随机对照群体和基因型分析,通过基于遗传搭车原理的卡方分析和单向方差分析方法,检测到分布于大豆10个连锁群的14个与大豆芽期耐低温相关的QTL。其中卡方分析检测到12个供体片段的超导入位点,对芽期耐低温性状表现为正效应。方差分析检测到5个位点,也表现为正效应,且其中Satt237、SOYPRP1和Satt540 3个位点是两种方法共同检测到的,应视为与耐低温直接相关的QTL。本研究旨在创建大豆芽期耐低温分子育种的检测方法平台,为大豆芽期耐低温研究提供有用的分子标记。     

The Genetic Basis of Variation for Salt Tolerance in Sorghum bicolor (L.) Moench Seedlings

The genetic basis of salt (NaCll tolerance at the seedling stage was examined in eight accessions of Sorghum bicolor (L.) Moench which differed in relative NaCl tolerance. The eight accessions were crossed in all possible combinations. The Nad tolerances of the resulting 56 F: hybrids and their eight parents were assessed after two weeks growth in solution cultures containing 0, 103, and 150mM NaCl. Indices of salt tolerance (relative tolerance) were used for analysis using the diallel method. Both additive and dominance effects appeared to be important for the expression of variation under low and high salinity levels, but the effect of genes with dominance properties was more pronounced. Estimates of narrow sense hentabilities for sail tolerance were appreciable, suggesting that rapid improvement in salinity tolerance is possible in this species using high selection pressures in the F₂ population.     

Generation mean analysis of spring wheat (Triticum aestivum L.) seedlings tolerant to high levels of manganese

Effective utilisation of wheat (Triticum aestivum L.) germplasm tolerant to high manganese (Mn) in a breeding program requires an understanding of the genetics of Mn tolerance. Five wheat cultivars differing in their response to high Mn were crossed in a half diallel design with no reciprocals. Seedlings of parents and progeny generations were phenotyped in high Mn nutrient solutions. Total chlorophyll concentration, which was positively correlated with leaf elongation rate during recovery from Mn stress, was used for phenotyping Mn tolerance. Means of chlorophyll concentration of P₁, P₂, F₁, F₂, BC₁, and BC₂ generations, in all crosses which showed significant variation among generations, were subjected to line crosses analysis to estimate gene effects. The continuous frequency distribution of seedlings with differential Mn tolerance, the similarity of the F₁ and F₂ means, and the high and significant levels of additive gene action indicated quantitative inheritance for Mn tolerance at seedling stage. A preponderance of additive effects indicated that selection for Mn tolerance in early generations should be effective.     

甘蓝型油菜发芽种子耐湿性的主基因+多基因遗传分析

油菜湿害是我国特有的自然灾害,对油菜种子发芽、出苗和幼苗生长造成严重影响,导致单产显著下降。为研究油菜种子发芽耐湿性的遗传规律,本文利用耐湿性遗传差异较大的2个甘蓝型油菜纯系中双9号和GH01的杂交后代衍生的世代家系群体构建油菜耐湿性遗传体系,并以主基因+多基因家系世代联合分析方法对油菜耐湿性的遗传规律进行分析。结果表明,中双9号×GH01组合的耐湿性的遗传受2对完全显性主基因+加性-显性多基因控制,表现为耐湿对不耐湿完全显性。该组合的第1对主基因加性效应与显性效应相等,为0.0696;第2对主基因的加性效应与显性效应相等,为0.0530。多基因加性效应为0.3275,显性效应为负值([h]= –0.2137)。主基因存在显性效应,该组合的耐湿性存在杂种优势,多基因显性效应为负,多基因显性效应使F₁代耐湿性降低。F₂家系的主基因遗传力为73.57%,表现出较高的遗传力,建议育种工作者对耐湿性在早期选择效率较高。     

Inheritance of Bolting in Three Sugar Beet Crosses after Different Periods of Vernalization

The induction of bolting after different periods of vernalization was studied in the parental, F₁, F₂ and backcross generations (F₂× P₁ and F₂× P₂) of three crosses between five sugar beet lines. The parental lines represented different levels of bolting resistance. The populations were evaluated in replicated field or greenhouse trials. Generation mean analysis was used to quantify the genetic effects of bolting. The additive (d) parameter was predominant for those levels of cold treatment that stimulated the bolting to occur. The dominance (h) parameter was also shown to be important in most cases. In one cross, the additive × additive (i) type of epistasis was significant. The chances of detecting genetic effects are increased by exposing the plants to vernalization conditions which maximize the difference between two parental lines of each cross. Hence, it would be sensible to exploit both additive and non-additive genetic effects in any selection programme.     

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.     

Pollen storage at low temperature as a procedure for the improvement of cold tolerance in spring rape, Brassica napus L.

The possibility of selecting spring rape for cold tolerance at the mature pollen grain stage was studied by investigating the effects of pollen storage at low temperatures on the quality of pollen grains and on the cold tolerance of the plants generated from them. Pollen treatments of F¹ hybrids affected fertilization ability much more than viability and even after 10 days storage at 3 or 10°C the pollen germination percentage was reasonably high. Pollen storage for 7 or 10 days at 3 or 10°C significantly increased the cold tolerance of F² seed germination, with 3°C being more effective. Pollen storage for a shorter time had no effect upon the number of resulting genotypes tolerant to low temperature. This approach may be successfully applied in plant breeding to enrich segregating plant populations with cold-tolerant genotypes.     

Inheritance of salt tolerance in rice

Summary The genetic behavior of salt tolerance was studied in artificially salinized conditions at the International Rice Research Institute.Divergent selection, carried out at a salinity level where the ECe was 15.2 mmhos/cm at 25 C in F₃ lines from two crosses confirmed the effects of salt tolerance on F₄ progeny with realized heritability values of 0.39 and 0.62, respectively.In a cross between two tolerant cultivars there was clear over-dominance for tolerance, despite the high environmental fluctuation which resulted in a low genetic response as indicated by a low but significant repeatability of 0.20–0.25, and many progeny lines more tolerant than the parents were recovered. The superior tolerance of these progenies compared to the parents was confirmed subsequently at 3 different salt levels. In the same experiment a cross between tolerant and susceptible cultivars produced some progeny of comparable tolerance with tolerant sources.In a 6×6 diallel cross experiment with two tolerant, moderate, and susceptible varieties each, both general and specific combining ability were significant.The findings indicate the possibility of breeding rices more tolerant than existing tolerant cultivars through cumulative crosses of tolerant cultivars. Further improvement can be attained by crossing highly tolerant lines with donors of good agronomic traits and pest and disease resistance.     

Genetic effects on Fe, Zn, Mn and P contents in Indica black pericarp rice and their genetic correlations with grain characteristics

Complete diallel crosses with 6 varieties of black pericarp rice and 1 variety of aromatic white rice were conducted to analyze the seed, maternal and cytoplasmic genetic effects on Fe, Zn, Mn and P contents in kernels by using a genetic model for quantitative traits of seeds in parents and their F₁s and F₂s. Seed, maternal as well as cytoplasmic genetic effects controlled the contents of all the mineral elements studied. The seed genetic effects were found to be more influential than the maternal genetic effects on Fe, Zn, Mn contents. Seed additive effects constituted a major component of the genetic effects whereas the seed additive along with maternal additive and dominant effects formed the main part in the inheritance of P content. The heritabilities of seed effects on all the mineral contents were highly significant (p< 0.01). The estimated values of narrow-sense heritabilites of seed genetic effects on Fe, Zn and Mn contents were high, while those of seed and maternal effects on P content were intermediate. Single plant selection and single grain selection based on the seed mineral element contents were advocated to improve the hybrid progeny. Genetic correlations showed that there existed significant genetic correlations of seed additive, seed dominance, cytoplasm, maternal additive and maternal dominance between grain characteristics such as 100-grain weight,grain length, grain width, grain shape and mineral elements Fe, Zn, Mn and P contents. Indirect selection of grain characteristics may be one of the breeding methods to select for higher contents of Fe, Zn, Mn and P in black pericarp indica rice. This revised version was published online in July 2006 with corrections to the Cover Date.     

Breeding tomato for pollen tolerance to low temperatures by gametophytic selection

Haploid selection for traits related to pollen cold tolerance in tomato was performed in segregating populations derived from a Lycopersicon esculentum × L. pennellii hybrid. BC₁ populations were obtained by combining normal and low temperature treatments on two stages of pollen development: pollen formation, and germination and pollen tube growth. F₁ hybrids were cultivated under low and normal temperatures and their pollen was used to pollinate L. esculentum plants at low and normal temperatures. The four BC₁ populations obtained were tested for the quality and quantity of pollen produced at low temperatures. The population obtained by cold treatment at both stages had a significantly improved pollen germination ability at low temperatures. The two other coldselected BC₁ populations showed no differences compared with the unselected BC population. A second cycle of pollen selection, corresponding to BC₂, was applied in order to test its persistence in the subsequent generations and the possibility to further improve the character. This second cycle showed no improvement although some plants retained the high pollen germination ability at low temperatures that was observed in the first cycle. Hence, gametophytic selection of some characters related with tomato pollen performance may be feasible, at least for the first cycle of selection.