The root systems of onion and Allium fistulosum in the context of organic farming: a breeding approach

Keywords: Allium cepa , Allium roylei , Japanese bunching onion, introgression breeding, organic agriculture, linkage map, AFLP, QTL analysis, Arbuscular Mycorrhizal Fungi, in vitro multiplication

A massive amount of synthetic fertilizers is needed to grow onions ( Allium cepa L.) due to their meager and inefficient root system. While the sustainability of such high-input systems is being questioned, low-input systems, such as organic agriculture, are gaining ground. For organic agriculture, plants have to be good nutrient scavengers. Therefore, productivity and stability of onion production in organic systems can be problematic. Plant breeding can improve the efficiency of onion roots, but breeding relies on available variation and there was no information about that in onions or in Allium. The aim of this thesis was to search for variation in root morphology in onion and in its allied species A. fistulosum L., to understand the role this variation could play in organic agriculture and to perform a genetic analysis of root traits. The variation found in root traits in onion was limited, although old onion cultivars had a higher root length density than modern ones. Huge variation was observed betweenonionand A. fistulosum . A. fistulosum developed substantially more stem-borne and lateral roots and, consequently, a much denser root system. Experiments carried out in an organic farm revealed that onion explored a smaller volume of soil and had a lower root density than A. fistulosum . In addition, onion, contrary to A. fistulosum , showed a reduction in total and fine root density when cultivated in a soil with low nitrogen content. It was also demonstrated that A. fistulosum was very responsive to indigenous and inoculated AMF (50 to 60% increase in both shoot biomass and root length). A Quantitative Trait Locus (QTL) analysis was done on the genetic linkage map of the progeny from the cross A. cepa x ( A. roylei x A. fistulosum ) to locate some of the genes responsible for the better performance of the A. fistulosum root system. All traits were evaluated in a replicated trial using in vitro cloned plants. QTLs were found for number of bulbs (1) and stem-borne roots (2) and, more interesting for breeding, for the number of lateral roots (1) and for the relative root length of fine and thick roots (1). The results showed the feasibility of breeding for onions with improved root systems using the interspecific hybrid between A. roylei and A. fistulosum as a genetic source. Some perspectives on the use of cultivars carrying such roots traits in onion organic production are highlighted.