Detailed genetic analysis of faba bean (Vicia faba L.) winter-hardiness and related traits

Abstract

Among grain legume species, faba bean (Vicia faba L.) could reduce the dependency of the European Union on international supply of vegetable protein. In order to promote and to genetically improve the crop, large European project was initiated. The present study, as part of the project, studied faba bean tolerance to North Europe winters, major abiotic constraint in these agro-ecological regions. This research aimed to study faba bean winter-hardiness and auxiliary traits, to determine the effect of hardening on plants, to detect QTL for frost tolerance, and to study heterosis for frost tolerance. Based on a representative sample of frost tolerant and frost susceptible faba bean genotypes, extensive field experiments enabled to identify European winter genotypes with high winter-hardiness and grain yield. GxE interaction analyses using additive main effect and multiplicative interaction (AMMI) showed additionally that these genotypes were stable for both traits; displaying a general adaptation to most environments. Artificial and so called provocative frost tests indicated that frost tolerance was a significant, but not an exhaustive component of winter-hardiness (0.021 < ÐrÐ < 0.737**). These tests enabled to identify highly frost tolerant experimental lines such as F7-95, but also European genotypes known as winterhardy either with low frost tolerance e.g. Bulldog/1 or with high frost tolerance e.g. Karl. For environments with intermediate winter strength, these artificial and provocative frost tests could be used to indirectly screen for winter-hardiness. Fatty acid content in leaves and their changes due to hardening, proline content, and membrane stability index were significantly correlated with frost tolerance, corroborating their physiological importance for the frost tolerance. Moreover, the analyses of leaves and stems revealed that unhardened oleic acid content and changes in oleic acid and in linoleic plus linolenic acid content in leaves due to hardening partly explained their frost tolerance (0.347+ < ÐrÐ < 0.543**). In stems, although significant changes due to hardening were observed on oleic acid (-1.77%) and linolenic acid (+9.06%), they were unrelated with frost tolerance. In order to molecularly assist selection for frost tolerance and ultimately for winterhardiness, QTL analyses were performed on a population of 101 recombinant inbred lines. For all studied traits, a LOD threshold of 3.25 was used to declare putative QTL. For frost tolerance, four putative QTL were detected; two for unhardened frosttolerance that explained 28.3% (3.44% after cross-validation) of its genotypic variance and two for hardened frost tolerance that explained 12.5% (5.22% after cross-validation). Referring to physiological traits correlated to frost tolerance, three QTL were detected for oleic acid content in unhardened leaves that explained 57.7% (37.2% after CV) of its genotypic variance. No QTL were detected for proline and soluble sugar content with LOD threshold of 3.25. The unbiased proportion of the genotypic variance that was explained after CV enabled to assess realistic prospects of MAS. Thus, combined MAS was more efficient than CPS and significant gains of selection for frost tolerance could therefore be expected on large populations at first generations of selection. Additionally, favourable alleles inherited from BPL 4628 could be used to further improve frost tolerance of European winter beans. Since most released faba bean cultivars are synthetics or populations, heterosis for frost tolerance represent an important aspect. In a diallel mating scheme with four parental lines, significant heterosis was observed. However, a cross between two European lines showed no heterosis. Unless hybrids cultivars are produced, heterosis could not be fully exploited for frost tolerance.