Variation in pollen viability in the onion (Allium cepa L.)

Summary The pollen viability of onions in a glasshouse was recorded from May to October 1975, using the fluorescein test. The average viability was 60–95% for most of this period but fell to less than 1% during the last two weeks of August. There was great variation in pollen viability between anthers within a flower and between flowers within a head. Attempts to induce pollen inviability by low temperature treatments at various stages of inflorescence development were unsuccessful. Low levels of pollen inviability appear to be a characteristic feature of onions, but the high level of inviability which was found both in this and in a previous season was associated particularly with the August period.     

Heritability estimates and response to selection for the pungency and single center traits in onion

Summary Heritabilities of the pungency and single center traits were estimated in onion breeding populations using selection response and half-sib family analyses. Pungency was determined indirectly by measuring enzymatically produced pyruvic acid in individual bulbs. After one generation of selection, pungency was lowered by 8.1% and 8.9% in the populations 90-61-1 and 89-69-8, respectively, and realized heritabilities of 0.21 and 0.51 were estimated.Selection had no effect in lowering the pungency of population 90–62. Heritability estimates calculated through half-sib progeny analysis were 0.53, 0.48, and 0.25 for pungency in the populations 90-61-1, 90–62 and 89-69-8, respectively. The number of single centered onions was increased by 19% and 22% in populations 90–62 and 89-69-8, respectively, after one generation of selection, and the realized heritability estimates were 0.37 and 0.34, respectively.     

Linkage of downy mildew resistance genes Pd 1 and Pd 2from Allium roylei Stearn in progeny of its interspecific hybrid with onion (A. cepa L.)

Summary Male meiosis of the interspecific hybrid between Allium roylei and A. cepa is undisturbed relative to its parents. Based on meiotic data, A. roylei is concluded to be a closer relative of A. cepa than A. fistulosum. Segregation ratios for downy mildew resistance among BC₁ and F₂ progenies from the F₁ between A. roylei and A. cepa indicate the presence of two dominantly inherited, weakly linked nuclear resistance genes, Pd ₁ and Pd ₂, in A. roylei (recombination frequency 0.32±0.03). Presumably Pd ₁ and Pd ₂are the first genes described in Allium residing in one linkage group. The prospects of exploiting A. roylei in onion breeding seem very promising.     

Gibberellic acid as a gametocide for the common onion (Allium cepa L.). II. The effect of GA 4/7

Summary The gametocidal effect of Berelex GA 4/7 on onions was investigated. Frequent spraying during the bolting period with relatively high concentrations (0.1 and 0.3%) resulted in a very high percentage of completely male sterile plants in the beginning of the flowering period. The gametocidal effect was accompanied by a considerable reduction in seed production.Differences in reaction to GA 4/7 between the three varieties involved were small.     

Measuring onion cultivars with image analysis using inflection points

Summary The suitability of image analysis was studied to measure bulb characteristics for varietal testing of onions (Allium cepa L.). Eighteen genotypes were used, which covered a whole range of onion shapes, including some quite identical ones. The characteristic height and diameter were measured both by image analysis and by hand. The methods gave comparable results with regard to precision and discriminating power. The discriminating power of the image analysis method could be increased by adding a new characteristic: the relative position of the diameter.For determining the height of the bulb with image analysis two methods are described to locate the top of a bulb. One method is to use the location of the inflection point as the top of the bulb. For most bulb shapes, this point coincides with the top of the bulb as indicated by a crop expert. For ovate shaped onions, however, the inflection point is below the top of the bulb as indicated by the crop expert. The other method is to use the location of the intercept of the tangent at the inflection point with the length axis of the bulb as the top of the bulb. This point is always above the top as indicated by the crop expert. Moreover it seems to perform slightly worse for the discrimination of cultivars. Therefore the location of the inflection point as the top of the bulb is preferred.     

Genetic mapping of AFLP markers in Japanese bunching onion (<Emphasis Type="Italic">Allium fistulosum</Emphasis>)

Summary The first genetic linkage map of Japanese bunching onion (Allium fistulosum) based primarily on AFLP markers was constructed using reciprocally backcrossed progenies. They were 120 plants each of (P₁)BC₁ and (P₂)BC₁ populations derived from a cross between single plants of two inbred lines: D1s-15s-22 (P₁) and J1s-14s-20 (P₂). Based on the (P₂)BC₁ population, a linkage map of P₁ was constructed. It comprises 164 markers – 149 amplified fragment length polymorphisms (AFLPs), 2 cleaved amplified polymorphic sequences (CAPSs), and 12 simple sequence repeats (SSRs) from Japanese bunching onion, and 1 SSR from bulb onion (A. cepa) – on 15 linkage groups covering 947 centiMorgans (cM). The linkage map of P₂ was constructed with the (P₁)BC₁ population and composed of 120 loci – 105 AFLPs, 1 CAPS, and 13 SSRs developed from Japanese bunching onion and 1 SSR from bulb onion – on 14 linkage groups covering 775 cM. Both maps were not saturated but were considered to cover the majority of the genome. Nine linkage groups in P₂ map were connected with their counterparts in P₁ map using co-dominant anchor markers, 13 SSRs and 1 CAPS.     

Breeding and genetics of Fusarium basal rot resistance in onion

Fusarium basal plate rot (FBR), caused by Fusarium oxysporum f. sp. cepae, is an important soil-borne disease of onions worldwide. The causal organism infects the basal stem plate of the bulb and eventually kills the entire plant through degradation of the basal plate. F. o. f. sp.cepae infections in dormant bulbs during storage allow secondary infections to occur. The primary method of infection by F. o. f. sp. cepaeis through direct penetration of the basal stem plate. Infection can also occur through wounded tissue particularly roots and basal portions of bulb scales. The most cost-effective methods of control are crop rotation and host plant resistance. Current research suggests that a single gene, two genes, or multiple genes govern resistance to FBR. Breeding programs have successfully used screening procedures to develop intermediate- and long-day, FBR-resistant cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isolation of 1,796 SSR clones from SSR-enriched DNA libraries of bunching onion (Allium fistulosum)

Bunching onion (Allium fistulosum L.) is one of the most important vegetables in Japan. To establish a genetic basis for molecular breeding of bunching onion, we isolated 1,796 simple sequence repeat (SSR) clones by large-scale sequencing of SSR-enriched genomic DNA libraries. Of these, 1,331 (74.1%) contained (GT) _n repeats (n > 5), while 314 (17.5%) were (GA) _n -containing clones. The average number of SSR repeats was 10.5 and 10.4 in the (GT) _n - and (GA) _n -containing clones, respectively. In a sample of five bunching onion inbred lines, an average of 3.2 alleles were detected in the 100 SSR loci investigated, with the polymorphic information content averaging 0.55. These results indicate that bunching onion SSRs are very rich sources of highly informative genetic markers.     

Haploid induction from F1 hybrids between CMS shallot with Allium galanthum cytoplasm and common onion by unpollinated flower culture

The induction of haploid plants from F₁ hybrids between CMS shallot with Allium galanthum cytoplasm and common onion was examined. Starting with 535 unpollinated flowers cultured in B5 medium 25 seedlings from part henogenetic embryos were obtained of which 13 seedlings survived. Eleven seedlings were determined as haploid plants (2n = x = 8) and 2 seedlings were doubled haploid plants (2n = 2x = 16). All haploid and doubled haploid plants preserved chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) from A. galanthum. Segregation in different characters was observed among the haploid plants. The haploid and doubled haploid plants exhibited the different combinations of genes from shallot and common onion. Crossing of the doubled haploid plants with other shallot strains, common onion cultivars or related species may produce excellent F₁ hybrids for bulb production. This revised version was published online in July 2006 with corrections to the Cover Date.     

SSR-tagged breeding scheme for allogamous crops: a trial in bunching onion (<Emphasis Type="Italic">Allium fistulosum</Emphasis>)

In several autogamous and vegetatively propagated crops, DNA markers have been used for cultivar identification. However, allogamous crops such as bunching onion (Allium fistulosum L.) are recalcitrant to marker-aided cultivar identification, as well as hybrid seed purity tests, due to the high degree of genetic heterogeneity within each cultivar. To aid cultivar identification and ensure its accuracy in bunching onion, we proposed the “SSR-tagged breeding” scheme in our previous study. The feasibility of this scheme was investigated here using a landrace of bunching onion with two populations tagged with two or four selected SSR markers. Compared with a control population, no significant differences were detected in the agronomic traits of the SSR-tagged populations. The targeted SSR loci were genetically uniform within each population whereas other loci maintained high heterogeneity. These results demonstrate that the SSR-tagged breeding scheme, even with a very small number of markers, is efficient for the identification of newly bred cultivars, and consequently for F₁ purity tests, in allogamous crops in which inbreeding depression is as severe as in bunching onion.     

Geographical distribution of allozyme patterns in shallot (Allium cepa var. ascalonicum Backer) and wakegi onion (A. × wakegi Araki)

Summary Shallot and wakegi were collected in the main islands of Indonesia, and in Japan, Korea, Taiwan, Malaysia, Thailand and Bangladesh. Five isozyme resolutions, phosphoglucomutase (PGM), glutamate oxaloacetate (GOT), glutamate dehydrogenase (GDH), esterase (EST) and peroxidase (POX) were employed for demonstrating inter-and intraspecific differences. A dendrogram separated 189 collected accessions into 25 types of wakegi onion and 18 types of shallot. All accessions of Japan, Korea and Taiwan were determined to be wakegi onion, whereas those of Bangladesh, Malaysia and Thailand were shallot. Twenty-six out of 165 Indonesian accessions indicated wakegi onion distribution in Sumatra, West Java province and in Sulawesi Island. This confirmed that there is mixed-cultivation of the two Allium species with no distinction made between them. Japan and Indonesia had respectively 12 and eight unique types of wakegi onion, while Korea had only one type. West Java showed the most various type of wakegi onion, whereas East Java had many types of shallot. Shallots collected from Bangladesh were distinetly different from those of South East Asian types.     

Introgression of leaf blight resistance from Allium roylei Stearn into onion (A. cepa L.)

Summary Segregation analysis in BC₁ and F₂ progenies from interspecific hybrids between Allium roylei and A. cepa demonstrated that leaf blight resistance is conditioned by one single dominant gene from A. roylei. This gene, designated Bs ₁ , was unlinked to the genes Pd ₁ and Pd ₂ determining downy mildew resistance. The prospects of exploiting A. roylei as a source for disease resistances in onion breeding are very promising.     

Pre- and post-fertilization barriers to backcrossing the interspecific hybrid between Allium fistulosum L. and A. cepa L. with A. cepa

Summary To facilitate the introgression of desirable traits of Allium fistulosum into the genome of A. cepa, several accessions of the hybrid between these species were pollinated with A. cepa as the recurrent parent, and in vitro ovary and ovule culture were performed to obtain an increase in the recovery of backcross progeny. Compared to the results obtained from seed development in planta, the increase in the number of backcross progeny was generally very limited, and in some cases even a decrease was found. Raising the sucrose concentration in the ovary culture medium resulted in a higher frequency of ovules developing back seed coats but this was not followed by an increase in the number of backcross progeny obtained. Pollen tube growth of A. cepa was disturbed in the styles of the interspecific hybrids. Per ovule, frequencies of micropylar penetration exceeded frequencies of backcross progeny only to a limited extent. Hence, it was concluded that in the tested interspecific hybrid accessions the attainable gain in viable backcross progeny by the application of in vitro culture techniques is limited by strong pre-fertilization barriers acting at the level of stylar incongruity.     

The long and winding road leading to the successful introgression of downy mildew resistance into onion

Downy mildew resistance originating from Allium roylei Stearn provides a complete resistance to onions and is based on one, dominant gene. Since A. roylei can successfully be hybridized with onion (A. cepa L.), a breeding scheme aimed at the introgression of this gene was initiated ca. 20 years ago. Several setbacks in this programme were encountered, firstly the identified molecular marker linked to the downy mildew resistance locus became increasingly difficult to use and finally lost its discriminating power and secondly the final step, making homozygous introgression lines (ILs), turned out to be more difficult then was hoped. GISH analysis showed that the chromosomal region harbouring the resistance locus was the only remaining piece of A. roylei in the nuclear background of onion and it also confirmed that this region was located on the distal end of chromosome 3. It was hypothesized that some factor present in the remaining A. roylei region was lethal when homozygously present in an onion genetic background. The identification of an individual with a smaller and more distally located introgression fragment and homozygous ILs in its progeny validated this hypothesis. With the help of these nearly isogenic lines four AFLP^® markers closely linked to the resistance gene were identified, which can be used for marker-aided selection. The introduction of downy mildew resistance caused by Peronospora destructor into onion is a significant step forward in the development of environmentally-friendly onion cultivars.     

Breeding for resistance to neck rot (Botrytis allii) in onions (Allium cepa)

Summary For 10 years the method developed by Van der Meer et al, (1970) to test neck rot resistance in onions has been applied in a breeding programme. From experiments with market varieties, inbred lines and half-sib families it is concluded that neck rot resistance as determined by this method is a quantitative character, which shows continuous variation and is in part fixable by selection. The practical importance of this type of neck rot resistance is not yet clear.Report no. 68     

Evaluation of a simple technique for characterizing rice root systems in relation to drought resistance

Summary Drought is a constraint to rice yields in much of Asia's rainfed rice crop. Although genotypic variation for root system characteristics is well documented in this semiaquatic species, little improvement of rice for drought resistance has been attempted. No simple method to evaluate root systems exists. In this paper we report the development and testing of a simple device to measure the pulling force required to uproot rice seedlings. The force required to pull rice seedlings from the paddy soil was correlated with root weight, root branching and thick root number. The capability to separate rice cultivars by this technique and relationship of the root pulling force to other drought screening methods are illustrated.     

Seed productivity test of CMS lines of Japanese bunching onion (Allium fistulosum L.) possessing the cytoplasm of a wild species, A. galanthum Kar. et Kir.

In a previous study, we developed male sterile lines of Japanese bunching onion (Allium fistulosum L.) possessing the cytoplasm of a wild species, A. galanthum Kar. et Kir., by backcrossing. To evaluate seed productivity of the male sterile lines in practise, they were crossed with the male fertile line, cultivar 'Kujyo', using honeybees as pollinators under field conditions. The number of florets and seeds per inflorescence, seed set and seed germination of the material were investigated. Although variation was observed among the male sterile lines, there were several lines having seed productivity equal to cultivar 'Kujyo'. Our data demonstrate that the male sterile lines of A. fistulosum possessing the cytoplasm of A. galanthum are useful as seed parents for the commercial F seed production of A. fistulosum.     

Inheritance of resistance to Pyrenochaeta lycopersici in tomato

Summary Tomato accessions (Lycopersicon sp.), along with commercial cultivars and breeding lines were grown in a field infested with the brown root rot (BRR) organism, Pyrenochaeta lycopersici and evaluated for resistance. Three L. esculentum Mill. accessions, P.I. 260397, P.I. 262906 and P.I. 203231, were resistant and were used as male parents in crosses designed to transfer resistance to tomatoes of fresh market type. Through analysis of parental generations and F₁ and F₂ progenies from three crosses the heritability of resistance in the broad sense was estimated to range from 25 to 43 percent. The minimum number of genes influencing resistance was estimated to be from 4 to 8.Florida Agricultural Experiment Stations Journal Series Paper no. 317.     

Onion chromosome nomenclature and homoeology relationships — workshop report

Summary Participants in a workshop on onion chromosome nomenclature held during the Eucarpia 4th Allium Symposium, agreed to consider the nomenclature for the chromosomes of Allium cepa L. proposed by E.R. Kalkman (1984) as the standard. Recommendations were formulated for the chromosome nomenclature of species which are cross compatible with onion.     

Pollen competition in onion (Allium cepa L.)

Summary Pollen mixtures with two components, one of which carried a dominant marker gene for red or white bulb skin colour, were used to pollinate flowers on onion umbels from several cultivars. Scoring progenies for the marker revealed that pollen components differed in their ability to effect fertilization, suggesting that gametophytic competition can occur in onions. In many cases, self-pollen appeared to have a competitive advantage over cross-pollen. Both of the male components and the female parent played a part in determining the final ratio obtained from a mixed pollination.Crossed seeds were slightly but significantly heavier than selfed seeds in nine out of ten umbels studied.