SEASONAL AND TEMPERATURE EFFECTS ON MYCORRHIZAL ACTIVITY AND DEPENDENCE OF COOL-SEASON AND WARM-SEASON TALLGRASS PRAIRIE GRASSES

Previous research on North American tallgrass prairie grasses has shown that warm-season grasses rely heavily on vesicular-arbuscular mycorrhizal symbiosis, while cool-season grasses are less dependent on the symbiosis (i.e., receive less benefit). This led to the hypothesis that cool-season grasses are less dependent on the symbiosis, because the growth of these plants occurs when mycorrhizal fungi are inactive. Field studies were performed to assess the effect of phenology of cool- and warm-season grasses on mycorrhizal fungal activity and fungal species composition. Mycorrhizal fungal activity in field samples was assessed using the vital stain nitro blue tetrazolium in addition to traditional staining techniques. Mycorrhizal activity was greater in cool-season grasses than in warm-season grasses early (April and May) and late (December) in the growing season, while mycorrhizal activity in roots of the warm-season grasses was greater (compared with cool-season grasses) in midseason (July and August). Active mycorrhizal colonization was relatively high in both groups of grasses late in the growing season, suggesting that mycorrhizal fungi may proliferate internally or may be parasitic at this time. Total Glomales sporulation was generally greater in the rhizosphere of cool-season grasses in June and in the rhizosphere of the warm-season grasses in October. A growth chamber experiment was conducted to examine the effect of temperature on mycorrhizal dependence of cool- and warm-season grasses. For both groups of grasses, mycorrhizal dependence was greatest at the temperature that favored growth of the host. The results suggest that mycorrhizal fungi are active in roots when cool-season grasses are growing and that cool-season grasses may receive benefit from the symbiosis under relatively cool temperature regimes.