Mycorrhizal dependence of modern wheat varieties, landraces, and ancestors

Using mycorrhizal fungi known to colonize wheat, the mycorrhizal dependence of various small grains including modem wheat varieties, primitive wheat lines, and wheat ancestors was studied. With the exception of the United States cultivar Newton and the German cultivars Apollo, Kanzler, and Sperber, dry weight of eight other modern wheats from the United States and Great Britain were increased by 29–100% following inoculation with mycorrhizal fungi. All landraces from Asian collections or early introduced American cultivars were also dependent on the symbiosis, with dry weight increases averaging 169 and 55%, respectively. All wheat ancestors of the AA and BB genomes (except Aegilops speltoides) benefitted significantly from the symbiosis, whereas no benefit was observed for ancestors of the DD genome, tetraploid wheats of the AABB or AAGG genomes, or in the hexaploid ancestor Triticum zhukovskyi (AAAAGG genome). These differences in mycorrhizal response of the ancestors, lines, and cultivars were highly correlated with root fibrousness ratings. When the fungi used as a combined inoculum in the previous experiment were inoculated individually onto selected plant species or cultivars, 6 of the 10 isolates stimulated growth of Andropogon gerardii, a highly dependent grass species, and 8 of the 10 stimulated the growth of 'Turkey' wheat. In contrast, none of the isolates positively affected growth of 'Newton' or 'Kanzler' wheat cultivars, and in fact several fungi decreased the biomass produced by these two cultivars. These studies have demonstrated a strong genetic basis for differences in mycorrhizal dependence among cultivars. A trend for greater reliance on the symbiosis in older cultivated wheats than iin wheat ancestors or modern wheats was also observed. The depression in growth associated with certain mycorrhizal fungi and wheat cultivars demonstrates that colonization of roots does not guarantee benefit from the symbiosis. Key words: root fibrousness, growth response, vesicular–arbuscular mycorrhizae.