Low Temperature Germination Research Articles

Seed size variation in cold and freezing tolerance during seed germination of winterfat (Krascheninnikovia lanata) (Chenopodiaceae)

Native plants have adaptations to their local environments and elucidation of these traits has implications in both agronomy and restoration ecology. Winterfat ( Krascheninnikovia lanata (Pursh) A.D.J. Meeuse & Smit) is a native perennial shrub in North America capable of germinating at low temperatures. The effect of seed size on germination ability at low and subzero temperatures and the physiological mechanisms were investigated. Winterfat seeds achieved 50%–72% germination at –3 °C, a temperature slightly above the base temperature estimated using thermal time models. Small seeds required a longer time to reach 50% germination at subzero temperatures than large seeds. Large seeds maintained stable water uptake rate for both the seed and the embryo when temperatures decreased from 5 to –1 °C. In contrast, faster water uptake and greater relative K+leakage in small seeds indicated possible damage to membrane integrity at subzero temperatures. Carbohydrate conversion efficiency (Rq/RCO2) of large seeds was significantly higher than that of small seeds at 10 °C but not at 20 °C. Higher cold resistance in large seeds was also correlated with higher concentrations of glucose, raffinose, and sucrose. This study revealed the potential basis of the low-temperature germination advantage of large seeds and provided the first direct evidence of germination under freezing temperatures in winterfat.

Inclusion of Acetyl Salicylic Acid and Methyl Jasmonate into the Priming Solution Improves Low-temperature Germination and Emergence of Sweet Pepper

The effects of incorporating plant growth regulators into the priming solution on low temperature germination and emergence percentage performance of sweet pepper (Capsicum annuum `Demre') seeds before and after seed storage were investigated. Seeds were primed in 3% KNO3 solution for 6 days at 25 °C in darkness containing one of the following: 1, 3, 5, or 10 μm methyl jasmonate (MeJA) or 0.05, 0.1, 0.5, or 1 mm acetyl salicylic acid (ASA). Following priming, seeds were either immediately subjected to germination and emergence tests at 15 °C or stored at 4 °C for 1 month after which they were subjected to germination test at 15 °C. Priming pepper seeds in the presence or absence of plant growth regulators in general improved final germination percentage (FGP), germination rate (G50) and germination synchrony (G10-90) at 15 °C compared to nonprimed seeds which had an FGP of 44%, G50 of 7.3 days and G10-90 of 7.3 days. Priming seeds in KNO3 solution containing 0.1 mm of ASA resulted in the highest germination percentage (91%), fastest germination rate (G50 = 2.2 days) and the most synchronous germination (G10-90 = 6.1 days). Emergence percentages were the highest for the seeds primed in the presence of 0.1 mm ASA (85%) and 3 μm MeJA (84%) while nonprimed seeds had an emergence percentage of 40%. Fastest emergence rates (E50) were also obtained from seeds primed in KNO3 supplemented with 3 μm MeJA (E50 = 15.2 days) and 0.1 mm ASA (E50 = 15.2 days). Shoot fresh and dry weights of pepper seedlings were significantly affected by priming treatments and priming in the presence of 0.1 mm ASA resulted in highest seedling shoot fresh and dry weights. Although all priming treatments improved germination performance of pepper seeds at 15 °C following 1 month of storage, inclusion of 0.1 mm ASA into the priming solution resulted in the highest germination percentage (84%) and germination rate (G50 = 3.8 days). These results indicate that priming seeds in 0.1 mm of ASA or 3 μm MeJA incorporated into the KNO3 solution can be used as an effective method to improve low temperature performance of sweet pepper seeds and that these seeds can be stored for 1 month at 4 °C and still exhibit improved germination performance at 15 °C.

Inclusion of plant growth regulators into priming solution improves low-temperature germination and emergence of watermelon seeds

The effects of incorporating plant growth regulators into the priming solution on low temperature germination and emergence performance of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai. cv. Crimson Sweet] seeds were investigated. Seeds were primed in 2.5% (0.25 M) KNO3 solution for 6 d at 25°C in darkness containing one of the following: 1, 3 or 5 µM methyl jasmonate (MeJA), or 1, 3 or 5 mM spermine. Following priming, seeds were subjected to germination and emergence tests at 15°C. Priming watermelon seeds in the presence or absence of plant growth regulators significantly improved germination percentage and rate at 15°C compared to untreated seeds, which failed to germinate. Seeds primed in KNO3 solution containing 1 or 3 µM of MeJA had significantly higher germination percentages, 96 and 85%, respectively, compared to seeds primed in KNO3 only (69%). Germination rate and synchrony were improved by 1 and 3 µM of MeJA added to the priming solution. Emergence was enhanced by priming seeds in the presence of 1 µM (75%) and 3 µM (63%) MeJA compared to seeds primed in KNO3 solution, (35%) while non-primed seeds were unable to emerge at 15°C. Inclusion of spermine at all three concentrations into the priming solution did not significantly improve germination and emergence characteristics of watermelon seeds compared to seeds that were primed in KNO3 solution only. Therefore, priming watermelon seeds in 1 or 3 µM of MeJA incorporated into the KNO3 solution can be used as an effective method to improve low temperature performance of watermelon seeds. Key words: Watermelon, methyl jasmonate, polyamines, germination, emergence.

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.

Genetical Studies on Germination of Seed and Seedling Establishment for Breeding of Improved Rice Varieties Suitable for Direct Seeding Culture

For the introduction of low temperature germinability (LTG), seed longevity and accelerated coleoptile growth derived from foreign varieties to Japanese elite varieties to breed rice varieties for direct seeding culture in Japan, we tried to establish backcross breeding to introduce these traits with marker-assisted selection (MAS). The objectives of this study were to identify quantitative trait loci (QTLs) for LTG and seed longevity using backcross inbred lines derived from a cross between a japonica variety Nipponbare and an indica variety Kasalath in order to facilitate MAS of these traits. Five putative QTLs controlling LTG were detected on chromosomes 2, 4 and 11. A putative QTL with a large seed longevity effect was detected on chromosome 9. We bred a near isogenic line for coleoptile growth by a backcross between Kitaibuki as the recurrent parent and Arroz da Terra as the donor parent to have more rapid coleoptile growth and higher seedling establishment rate than Kitaibuki. From the above experimental data, we conclude that backcross breeding with MAS is effective to introduce genes for adaptability to direct seeding from foreign varieties in Japanese rice breeding programs.

Mapping of quantitative trait loci controlling low-temperature germinability in rice (Oryza sativa L.).

Low-temperature germination is one of the major determinants for stable stand establishment in the direct seeding method in temperate regions, and at high altitudes of tropical regions. Quantitative trait loci (QTLs) controlling low-temperature germinability in rice were identified using 122 backcross inbred lines (BILs) derived from a cross between temperate japonica varieties, Italica Livorno and Hayamasari. The germination rate at 15 degrees C was measured to represent low-temperature germination and used for QTL analysis. The germination rate at 15 degrees C for 7 days of Italica Livorno and Hayamasari was 98.7 and 26.8%, respectively, and that of BILs ranged from 0 to 83.3%. Using restriction fragment length polymorphism (RFLP) and simple sequence repeat (SSR) markers, we constructed a linkage map which corresponded to about 90% of the rice genome. Three putative QTLs associated with low-temperature germination were detected. The most effective QTL, qLTG-3-1 on chromosome 3, accounted for 35.0% of the total phenotypic variation for low-temperature germinability. Two additional QTLs, qLTG-3-2 on chromosome 3 and qLTG-4 on chromosome 4, were detected and accounted for 17.4 and 5.5% of the total phenotypic variation, respectively. The Italica Livorno alleles in all detected QTLs increased the low-temperature germination rate.

Breeding of Mutant Lines with Low Temperature Germinability Induced by Mutagen 5-azaC in Akitakomachi. 2. Variation of Germinability and Stand Establishment

Breeding of Mutant Lines with Low Temperature Germinability Induced by Mutagen 5-azaC in Akitakomachi. 2. Variation of Germinability and Stand Establishment

QTL analysis of rice low temperature germinability

A double haploid population, derived from anther culture of F1 hybrid between a typicalindica and ajaponica (ZYQ8/JX17), has been used to investigate the low temperature germinability (LTG) at 15°C. The low temperature germinability of two parents was significantly different. In 6–11 d, the germination percentage of ZYQ8 was higher than that of JX17. In 12–16 d, the germination percentage of JX17 was higher than that of ZYQ8. The quantitative trait loci (QTLs) of every day for low temperature germinability have been mapped based on a molecular linkage map constructed from this population. In 8–11 d, qLTG-9 was identified in C397B-RZ617B on chromosome 9, the additive effect was positive, showing that the allele from JX17 could increase low temperature germinability. In 12–16 d, qLTG-4 was mapped between RG908 and CT563 on chromosome 4, the additive effect was negative, showing that the allele from ZYQ8 could increase low temperature germinability. These two QTLs were detected at different stages, showing the complexity of the mechanism of low temperature germinability.

Mapping Quantitative Trait Loci Controlling Low Temperature Germinability in Rice (Oryza sativa L.).

Quantitative trait loci (QTLs) controlling low temperature germinability (LTG) in rice were identified using 98 backcross inbred lines (BILs) derived from a cross between a japonica variety Nipponbare and an indica variety Kasalath. Seeds of each BIL were stored at 30°C in a drying machine for 7 months to exclude the effects of seed dormancy. Then the germination rate at 15°C for 4 days was scored to represent the LTG. The LTG of Nipponbare and Kasalath was 17 and 61%, respectively, and that of the BILs ranged from 0 to 100%. Five putative QTLs, qLTG-2, qLTG-4-1, qLTG-4-2, qLTG-5 and qLTG-11, were detected on chromosome 2 (G1327: nearest marker locus), 4 (two regions, C946 and C513), 5 (R830) and chromosome 11 (G1465). In the case of qLTG-2, qLTG-4-1 and qLTG-11, Kasalath alleles increased the LTG, while Nipponbare alleles increased it in the case of qLTG-4-2 and qLTG-5. These QTLs explained 40.7% of the total phenotypic variation in the BC1F9 lines. Using chromosome segment substitution lines (SLs), the existence of qLTG-4-1 and qLTG-11 was confirmed and the effect of the Kasalath allele on qLTG-11 was more pronounced than that on qLTG-4-1. The effect of the Kasalath allele on qLTG-4-2, which suppressed the germination, was temporary and disappeared after 7 to 8 month’s storage, suggesting that the disappearance was due to the breaking of seed dormancy and that qLTG-4-2 might control it. Kasalath allele in the case of qLTG-5 decreased the germination rate when the effect of dormancy was completely removed. Therefore, we concluded that qLTG-5 was involved in LTG but not in seed dormancy.

Genetic Regulation of Seed Dormancy in Purshia tridentata(Rosaceae)

Seeds of Purshia tridentata, a shrub of semi-arid North America, require chilling to become non-dormant. Using seeds produced from controlled crosses in a common garden, we examined effects of ovule parent, pollen parent, and year of production on germination percentage in response to chilling for 2 weeks at 2°C. Differences among ovule parents accounted for most of the variance in chilling response, and these differences (2 to 83% germination) were consistent across years. Differences among pollen parents were also significant, producing a two- to five-fold difference in mean germination percentage. Differences among years were significant but small. Ovule parent by year interactions showed that among-year variation in ripening environment did not affect ovule parents equally. There was no significant pollen parent by year interaction, suggesting that the effect of maturation environment was mediated through maternal tissues. In reciprocal crosses, two plants that showed contrasting dormancy levels as ovule parents produced seeds with similar dormancy as pollen parents, indicating that the genetic difference between them was at the testa level. Two plants that produced seeds with contrasting dormancy as pollen parents showed a similar but stronger pattern of contrast as ovule parents, showing that the genetic difference between them was at both embryo and endosperm or testa level. Testa or endosperm genotype was primarily responsible for chilling response of intact seeds, while embryo genotype affected chilling response and also exercised primary control over low temperature germination rate, whether of excised embryos or of intact seeds.

The Low Oxygen Germinability and the Low Temperature Germinability of Rice Varieties

The Low Oxygen Germinability and the Low Temperature Germinability of Rice Varieties

Storage reserve mobilization during low temperature germination and early seedling growth in Brassica napus

In western Canada, oilseed rape ( Brassica napus L. var. oleifera cv. Westar) is seeded during the early months of spring, when ambient temperatures are well below the optimum. This can result in poor seedling emergence. The objectives of the present study were to determine which developmental stages are sensitive to low temperature and whether the effects are thermal or developmental in nature. Seed was germinated at 22, 10 and 6 °C. Fresh weight changes and seedling growth were assessed on the basis of equal accumulated heat units, and the mobilization of storage reserves was assessed by employing antibodies against isocitrate lyase (ICL; EC 4.1.3.1), oleosin and cruciferin. Additionally, de novo protein synthesis was determined by quantifying the incorporation of methionine via in vivo labelling. Low temperature resulted in poor germination and early seedling growth with phase II of germination being most sensitive. At 10 °C, there was a temporal delay in germination that did not affect the overall success of germination. This was a thermal effect as seed at the lower temperatures required the equivalent of 16–24 degree days before germination occurred. Also, seedling growth at 10 °C was lower in comparison to seedlings grown at 22 °C. Seed at 6 °C displayed slow and incomplete germination and poor seedling growth as a result of both thermal and developmental effects.

Introduction of Low Temperature Germinability into Highly Palatable Cultivars of Rice.

In order to breed highly palatable cultivars suitable for direct sowing in rice, we tried to incorporate the genes for low-temperature-germinability into highly palatable Japanese cultivars and selected excellent backcrossed lines with low-temperature-germinability. First, we examined ten Japanese cultivars and ten foreign cultivars for their low-temperature-germinability (the proportion of the germination rate at 15°C to that at 25°C after 7 days of incubation) and found that most of the foreign cultivars which were directly sown in their own countries were extremely useful as donors of low-temperature-germinability. Secondly, we developed five backcrossed BC2F2 populations (Koshihikari 3/M202, Hinohikari 3/M202, Koshihikari 3/Italica Livorno, Koshihikari 3/Mutashali and Kinuhikari 3/Mutashali) and selected them for low-temperature-germinability. From all populations, we obtained the lines superior to each recurrent parents. In particular, from Koshihikari 3/M202 (66 lines), Hinohikari 3/M202 (51 lines) and Koshihikari 3/Mutashali (51 lines), we obtained eight, eleven and one line, respectively, which showed excellent (over 81%) low-temperature-germinability. Then we picked out these excellent lines (Hinohikari 3/M202) as donors of futher back-crossing. Finally, we developed backcrossed BC3F2 lines (Hinohikari 4/M202, 35lines) and selected five excellent lines with high low-temperature-germinability. In conclusion, it would be possible to breed highly palatable cultivars suitable for direct sowing by introducing low-temperature-germinability derived from foreign cultivars such as M202 into good eating-quality cultivars.

Canola seed germination and seedling emergence from pre‐hydrated and re‐dried seeds subjected to salt and water stresses at low temperatures

SummaryLow soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre‐hydration and re‐drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na2SO4 was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na2SO4, suggesting that NaCl was more toxic than Na2SO4 during seedling development. Pre‐hydration and re‐drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg‐1 of NaCl or 50 mmol kg‐1 of Na2SO4. These results suggest that the pre‐hydration and re‐drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.

RFLP mapping of QTLs conferring salt tolerance during germination in an interspecific cross of tomato

Most cultivars of tomato, Lycopersicon esculentum, are sensitive to low (chilling) temperatures (0–15 °C) during seed germination; however, genetic sources of cold (chilling) tolerance have been identified within the related wild species. The purpose of this study was to identify quantitative trait loci (QTLs) that contribute to cold tolerance during germination in tomato using a backcross population of an interspecific cross between a cold-sensitive tomato line (NC84173, recurrent parent) and a L. pimpinellifolium accession (LA722) that germinates rapidly under low temperatures. A total of 119 BC1 individuals were genotyped for 151 restriction fragment length polymorphism (RFLP) markers and a genetic linkage map was constructed. The parental lines and 119 BC1S1 families (self-pollinated progeny of the BC1 individuals) were evaluated for germination at a low temperature (11±0.5 °C). Germination was scored visually as radicle protrusion at 8 h intervals for 28 consecutive days. Germination response was analyzed by the survival analysis and the times to 25, 50 and 75% germination were calculated. In addition, a germination index (GI) was calculated as the weighted mean of the time from imbibition to germination for each family/line. Two QTL mapping techniques, interval mapping (using MAPMAKER/QTL) and single-point analysis (using QGENE), were used to identify QTLs. The results of both methods were similar and two chromosomal locations (3–5 putative QTLs) with significant effects on low temperature germination were identified. The L. pimpinellifolium accession had favorable QTL alleles on chromosomes 1 and NC84173 had favorable QTL alleles on chromosome 4. The percentage of phenotypic variation explained (PVE) by individual QTLs ranged from 11.9% to 33.4%. Multilocus analysis indicated that the cumulative action of all significant QTLs accounted for 43.8% of the total phenotypic variance. Digenic epistatic interactions were evident between two of the QTL-linked markers and two unlinked markers. Transgressive phenotypes were observed in the direction of cold sensitivity. The results indicate that low temperature germination of tomato seed can be improved by marker-assisted selection.

Combining Osmopriming and Biopriming

Osmopriming has been shown to enhance seed performance by increasing germination rates and uniformity. Furthermore, these enhancements persist under less-than-optimum conditions, such as salinity, reduced water availability, and excessively high or low temperatures. Additional benefits include resistance to soil pathogens due to lower leachate levels and more rapid emergence. To augment these existing qualities, it would be advantageous to incorporate beneficial organisms that antagonize soil-borne diseases, combining the benefits of both systems into a single procedure. To accomplish this, processing tomato seeds (Lycopersicon esculentum Mill. OH8245) were bioprimed in aerated –0.8 Mpa NaNO3 at 20°C for 4 days, at which time a mixture of nutrient broth, a defoaming agent, and beneficial bacteria that has been adjusted to the same osmotic potential is added. The bacteria used, Pseudomonas aureofaciens AB254, has been proved to control Pythium ultimum on a variety of crop seeds. After 7 days the seeds are removed having been primed and colonized with 105 colony forming units (cfu)/seed. In the absence of pathogen pressure, osmoprimed and bio-osmoprimed seeds performed similarly improving overall germination by 40% after 3 days, as well as low temperature (10–15°C) germination. However, when these seeds were sown in soilless media inoculated with P. ultimum, osmoprimed and bio-osmoprimed emergence was 57% and 74%, respectively, showing the improvements that these biologicals can provide. Thermogradient table results, storage tests, cfu/seed, and pathogen control will be discussed.

Bio-osmopriming Tomato and Cucumber Seeds for Better Stand Establishment

Improved germination under unfavorable soil conditions is an important safeguard against yield losses in direct-seeded crops. Osmoprimed seed has been shown to provide earlier and more uniform germination as well as improve low temperature germination. These attributes combined with the reduced rates of damping-off associated with Pseudomonas aureofaciens AB254 creates a bioosmopriming seed treatment that provides rapid germination under a wider range of soil temperatures while exhibiting the disease resistance and improved growth associated with bacterial coatings. The objective of this work is to combine biopriming and osmopriming into one procedure, thus creating an environment for adequate seed hydration and rapid multiplication of beneficial bacteria which will thoroughly colonize the seed surface. Processing tomato seeds (Lycopersicon esculentum Mill. `OH8245') were bio-osmoprimed in aerated –0.8 MPa NaNO3 at 20°C for 4 days. On the fourth day, a mixture of nutrient broth, a defoaming agent, and bacteria that have been adjusted to the same osmotic potential is added. This is done so that the removal of seeds from the tank at the end of the 7-day treatment coincides with peak populations of bacteria. Pseudomonas aureofaciens AB254 multiplies very rapidly in this environment, with colony forming units for tomato averaging 4 × 105/seed. Results will also be reported for cucumber seed (Cucumis sativus L. `Score'), which were treated using a similar procedure. Bacterial populations per seed, germination characteristics and pathogen control will be discussed.

Evaluation of chilling sensitivity in different rice varieties. Relationship between screening procedures applied during germination and vegetative growth

Rice varieties were screened for chilling tolerance during germination and vegetative growth using different techniques. The effects of temperature during germination were investigated from 10 to 25 degrees C. The screening was most effective at 10 degrees C. Time of data collection has to be considered in order to discriminate slow germinating from chilling sensitive varieties. Chilling survival tests (10 degrees C) applied at the 2-leaf stage revealed that different chilling durations brought complementary results: the longer treatments allowed to identify the most tolerant varieties, while shorter treatments allowed to distinguish intermediate from most sensitive rices. Results of chilling tests applied at the 2- and the 8-leaf stages showed a high correlation. Low-temperature electrolyte leakage and low-temperature fluorescence were also highly correlated to each other and to survival tests applied at the same development stage. By contrast, weak correlations were observed between low temperature germination and plantlet chilling survival. It is concluded that low-temperature electrolyte leakage and low-temperature fluorescence are good indicators of chilling tolerance during the vegetative growth. The possibility to use a chilling screening procedure applied at this stage in order to predict the chilling tolerance at other development stages is discussed.

Optimization of Seed Priming Treatments to Increase Low-Temperature Germination Rate

Seeds of 4 range grass species were evaluated to determine the effects of priming-temperature, priming-water potential, and treatment-duration on subsequent germination response at 10 degrees C. Seeds of bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Love], thickspike wheatgrass [Elymus lanceolatus (Scribn. and J.G. Smith) Gould; syn. Agropyron dasystachyum (Hook.) Scribn.], sandberg bluegrass (Poa sandbergii Vasey.), and bottlebrush squirreltail [Sitanion hystrix (Nutt.) J.G. Smith] were primed over the temperature range of 5 to 35 degrees C and the water potential range of 0 to -2.5 MPa for up to 10 days to determine optimal priming conditions among all treatment combinations that did not result in premature radicle emergence from the seed coat. Most rapid germination of treated seeds was obtained at priming temperatures considered optimal for germination of untreated seeds. Optimal priming conditions were found to be at water potentials equal to, or less negative than, the threshold water potential at which radicle emergence was prevented for untreated seeds. Germination response data for untreated seeds can be used to simplify the estimation of optimal temperature and water potential conditions for seed priming.

Selection of somaclonal variants with low-temperature germinability in melon (Cucumis melo L.)

Plants were regenerated from adventitious buds and somatic embryos (R0) of melon (Cucumis melo L.), the cultivar Andes. Somaclonal variants of melon with low temperature germinability were selected from the progenies (R1) of R0 plants. Among 5,618 R1 seeds harvested from 23 R0 plants that were regenerated from adventitious buds 4 seeds germinated after 5 days of culture at 15 °C (selection rate; 0.07%). However, among 374 R2 seeds harvested from 2 R1 plants no seed germinated after 7 days of culture at 14 °C. Among 9,181 R1 seeds harvested from 50 R0 plants regenerated from somatic embryos 110 seeds germinated after 5 days of culture at 15 °C (selection rate; 1.20%). Among 3,717 R2 seeds harvested from 17 R1 plants 113 seeds germinated after 7 days of culture at 14 °C (selection rate; 3.04%). R3 seeds were collected from these R2 plants following self-pollination. Forty-five of the 47 lines (R3) originated from 10 R0 plants showed higher germination rates than that of the original cultivar. Selected lines with low-temperature germinability showed greater fruit growth rate than the original cultivar during the middle stage when they were cultivated in a greenhouse under low-temperature conditions. Of fruits harvested from 31 lines, 15 lines showed greater fruit volume than the original cultivar.