Occurrence and Significance of Vesicular-Arbuscular Mycorrhizae in Saskatchewan Soils and Field Crops
Talukdar, Narayan Chandra
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Vesicular-arbuscular mycorrhizal (VAM) fungi form an association with most crop roots. Knowledge on the occurrence of VAM in soils and their symbiosis with crops is important to understand the impact of VAM on crops in agroecosystems. This information may be used to manage VAM so as to maximize their contribution to plant growth and development My research assessed the occurrence and significance of VAM in Saskatchewan soils and field crops. This was accomplished through a series of field and growth chamber experiments designed to: (i) study the occurrence of VAM in wheat and lentil and to isolate different VAM from wheat field soils. (ii) assess the effect of crop rotations on VAM infection in field grown crops. (ii) develop a method to propagate monospecific cultures of VAM strains isolated from Saskatchewan soils. (iv) study the effect of selected VAM inocula on growth of wheat and lentil. Soil and root samples collected from wheat and lentil fields at 11 sites during 198a and 1989 were analyzed for spore numbers, level of VAM infection, and VAM species. The number of VAM spores detected in field soils ranged from 78-272 per 100 g soil in the 1988 study and 410-2113 per 100 g soil in the 1989 study. Vesicular-arbuscular mycorrhizae infected wheat and lentil at all the field study sites, but levels of infection in the two crops varied from site to site and the differences were more pronounced in wheat than in lentil. Differences in VAM infection in wheat at different sites were not related to the moisture and temperature gradient of the four soil zones or soil properties. Generally, lentil exhibited both a higher percentage of VAM infected roots, and contained more G. mosseae and G. verslforme, and the level of spore production in soil was: G. geosporum > G. versiforme > G. mosseae. The ability of G. geosporum to produce a large number of spores reflected either an inherent ability, or a more favorable response to maize as host crop or the conditions in the growth chamber. The ability of G. geosporum, G. mosseae and G. verstforme to infect wheat and lentil and the response of the two crops to VAM infection were assessed in sterile soil under growth chamber conditions. Seedlings were inoculated with 100 VAM spores of a species, or an equal mixture of two species. The two crops responded differently depending on the VAM species. In the case of lentil, G. geosporum was more effective than G. versiforme and G. mosseae, and increased the shoot dry weight (37%) and grain yield (56%) compared to the uninoculated control. There was a significant positive correlation between the percentage of VAM infected roots and shoot dry weight of lentil at ? 1 and 56 days after planting, but not at harvest. Inoculation also increased the P content of lentil shoots, following a trend similar to that for infection. Glomus geosporum increased the shoot weight, grain yield (12%) and the shoot and grain P content of wheat. Although G. geosporum and G. mosseae both produced similar levels of infection in wheat, the only response of wheat to G. mosseae was an increase in plant height at harvest. The efficacy of G. geosporum on both crops appeared to be related to its ability to produce a higher percentage of arbuscular infection compared to G. mosseae. Co-inoculation of seedlings with G. geosporum and G. mosseae resulted in competition between the two VAM. This was evident from a comparison of shoot dry weight and spore production on the two crops inoculated with either G. geosporum or G. geosporum plus G. mosseae. Glomus mosseae reduced the efficacy of G. geosporum by 16% when co-inoculated on lentil, but 'had no effect when the host was wheat. Based on spore production it was found that G. geosporum was more competitive than G. mosseae when co-inoculated to lentil and wheat. Glomus geosporum produced ca. 2000 and 500 spores per 100 g substrate, respectively in lentil and wheat soil, which was approximately 2 to 3 fold more spores than produced by G. mosseae. When co-inoculated, there was a 15 to 19% reduction in spore production by G. geosporum versus a 50-70% decrease in spore production by G. mosseae. These results indicated that these VAM species exhibit a host specificity in terms of their ability to infect and increase the growth and yield of lentil and wheat. Glomus geosporum was more competitive and effective than G. mosseae or G. versiforme as reflected by its positive impact on growth of lentil and wheat. The difference in the ability of wheat and lentil to support spore production by G. geosporum, G. mosseae and G. versiforme isolates from Saskatchewan soils under growth chamber conditions suggested that these crops may have a similar effect in agricultural fields. Thus, depending upon cropping practices the dynamics of VAM spore populations in soil may change. In addition, production of canola or summerfallow influenced the population dynamics of indigenous VAM (i.e, spore numbers), resulting in reduced infection of subsequent wheat and barley. The number of spores recovered from the canola and summerfa1low field soils was 11 to 39% and 18 to 54% lower, respectively than that recovered from the adjacent wheat field soils. The low number of spores resulted in low levels of infection in the subsequent wheat and barley crops grown in these canola and summerfallow fields. Thus my results suggest that cropping practices influenced the population dynamics and activity of indigenous VAM and their potential benefits to field grown crops in Saskatchewan.