Diversity and colonization levels of naturally occurring arbuscular mycorrhizal fungi (AMF) in onion roots were studied to compare organic and conventional farming systems in the Netherlands. In 2004, 20 onion fields were sampled in a balanced survey between farming systems and between two regions, namely, Zeeland and Flevoland. In 2005, nine conventional and ten organic fields were additionally surveyed in Flevoland. AMF phylotypes were identified by rDNA sequencing. All plants were colonized, with 60% for arbuscular colonization and 84% for hyphal colonization as grand means. In Zeeland, onion roots from organic fields had higher fractional colonization levels than those from conventional fields. Onion yields in conventional farming were positively correlated with colonization level. Overall, 14 AMF phylotypes were identified. The number of phylotypes per field ranged from one to six. Two phylotypes associated with the Glomus mosseae–coronatum and the G. caledonium–geosporum species complexes were the most abundant, whereas other phylotypes were infrequently found. Organic and conventional farming systems had similar number of phylotypes per field and Shannon diversity indices. A few organic and conventional fields had larger number of phylotypes, including phylotypes associated with the genera Glomus-B, Archaeospora, and Paraglomus. This suggests that farming systems as such did not influence AMF diversity, but rather specific environmental conditions or agricultural practices.

The response of Allium cepa, A. roylei, A. fistulosum, and the hybrid A. fistulosum × A. roylei to the arbuscular mycorrhizal fungus (AMF) Glomus intraradices was studied. The genetic basis for response to AMF was analyzed in a tri-hybrid A. cepa × (A. roylei × A. fistulosum) population. Plant response to mycorrhizal symbiosis was expressed as relative mycorrhizal responsiveness (R′) and absolute responsiveness (R). In addition, the average performance (AP) of genotypes under mycorrhizal and non-mycorrhizal conditions was determined. Experiments were executed in 2 years, and comprised clonally propagated plants of each genotype grown in sterile soil, inoculated with G. intraradices or non-inoculated. Results were significantly correlated between both years. Biomass of non-mycorrhizal and mycorrhizal plants was significantly positively correlated. R′ was negatively correlated with biomass of non-mycorrhizal plants and hence unsuitable as a breeding criterion. R and AP were positively correlated with biomass of mycorrhizal and non-mycorrhizal plants. QTLs contributing to mycorrhizal response were located on a linkage map of the A. roylei × A. fistulosum parental genotype. Two QTLs from A. roylei were detected on chromosomes 2 and 3 for R, AP, and biomass of mycorrhizal plants. A QTL from A. fistulosum was detected on linkage group 9 for AP (but not R), biomass of mycorrhizal and non-mycorrhizal plants, and the number of stem-borne roots. Co-segregating QTLs for plant biomass, R and AP indicate that selection for plant biomass also selects for enhanced R and AP. Moreover, our findings suggest that modern onion breeding did not select against the response to AMF, as was suggested before for other cultivated species. Positive correlation between high number of roots, biomass and large response to AMF in close relatives of onion opens prospects to combine these traits for the development of more robust onion cultivars.

In Uruguay, onion (Allium cepaL.) germplasm is mainly derived from the genetic material introduced by several waves of European immigrants and subsequently multiplied by household farmers, resulting in a wealth of locally adapted populations. This study examined the genetic diversity in a collection of 27 local onion populations and two cultivars derived from them. A total of 843 onion plants were fingerprinted, and 83 inter-simple sequence repeat polymorphic bands were generated. Analysis of molecular variance showed high diversity within the populations (66% of the total variation). Some short-day populations from different geographical origins were grouped together by the unweighted pair-group method using arithmetic averages, principal coordinate analysis and cluster analysis, while the more extensively sampled long- and intermediate-day populations showed a widespread distribution, with no significant subgrouping among them. This weakly structured gene pool is probably the consequence of seed and bulb exchange between farmers and natural inter-pollination. Nevertheless, a Bayesian analysis allowed the distinction of four genetic backgrounds of alleles in the whole collection, and populations were predominately assigned to each genetic background. In addition, mitochondrial variants determining normal (N) pollen fertility or the sterile S or T types were analysed for the same set of plants using specific primers. Most accessions showed a proportion of male-sterile individuals. Cytoplasm type T was the most represented (26 out of 29 accessions), and cytoplasm type S was found in a low proportion of individuals in seven populations. Uruguayan onion germplasm maintains a low degree of genetic differentiation despite the small cultivated area and intense seed exchange, probably due in part to different market purposes based on the growing cycle.