Mycorrhizal dependence of modern wheat cultivars and ancestors: a synthesis

To supplement previous studies, wheat ancestors with the A, S(B), D, and AB genomes and modern spring and winter wheat cultivars were vernalized for 42 days in a growth chamber prior to transplanting and growth in a greenhouse for 14 weeks. One-half of the seedlings were inoculated with a mixture of equal numbers of spores of six vesicular–arbuscular mycorrhizal fungi at transplanting. Dry weights of plants and component parts were determined. Percent colonization of roots was determined microscopically. Growth response and mycorrhizal dependence were calculated. However, no mycorrhizal dependence was observed in Triticum monococcum PI 266844 and PI 355520 (A genome), Aegilops speltoides 1773 (S(B) genome), or in the AB genome ancestors Triticum carthlicum 2825, Triticum polonicum 2808, Triticum dicoccum 1165, Triticum pyramidale 2809, Triticum orientale 2805, Triticum paleocolchicum 2807, and Triticum persidum 2811 and 2812. Mycorrhizal dependence in the D genome ancestors was more variable. Triticum tauschii var. typica accessions 1649, 1691, 2378, 2448, 2492, 2495, 2528, 2541, and 2567 lacked dependence, whereas T. tauschii var. meyeri 2529 and var. strangulata 2377 and 2452 were dependent on mycorrhizal symbiosis. The spring wheat cultivars Chinese Spring 3008, Spelta 2603, Pavon 76 2980, and Norin 29 3025 and the winter wheat cultivars TAM 200, Wrangler, Saluda, and Karl were not dependent on mycorrhizae, whereas winter wheat cultivars TAM 107 and Century were dependent. When these data are synthesized with previously tested ancestors, landraces, and cultivars, it appears that both dependent and nondependent diploid ancestors with the A, S(B), and D genomes existed, but dependence was lost in tetraploid ancestors. Since no dependence on mycorrhizae has been demonstrated in any AB genome ancestors, the presence of dependence in modern cultivars of the hexaploid ABD genome is probably derived from the D genome. The consistent dependence of wheat cultivars released before 1950 suggests that modern breeding practices have reduced dependence on mycorrhizal symbiosis. The implications of having mycorrhizal colonization in the absence of mycorrhizal dependence (benefit) are discussed. Key words: pathogenesis, growth response, vesicular – arbuscular mycorrhizae.