SEASONAL PATTERNS OF HOST GROWTH, FUNGAL MORPHOLOGY, AND ROOT CARBOHYDRATE AND PHOSPHORUS CONTENT IN A GREEN ASH MYCORRHIZA

Two long-term experiments with vesicular-arbuscular {VA} mycorrhizal green ash seedlings (Fraxinus pennsylvanica Marsh.) were conducted to determine seasonal changes in mycorrhizal morphology, spore production, host plant growth, and root carbohydrate and phosphorus levels. Seedlings, with or without the VA mycorrhizal fungus Glomus macrocarpum var. macrocarpum Tul. & Tul., were grown in a greenhouse under natural photoperiods and low and high nutrient regimes. The seedlings were moved to a lathhouse to overwinter and were returned to the greenhouse in the spring. Inoculation with G. macrocarpum significantly increased the growth and P content of green ash seedlings receiving a low nutrient addition over similarly treated noninoculated seedlings. At the end of their first growing season, mycorrhizal plants were 25cm tall, weighed 4.2g, and contained 50 nmoles P mg(\u27-1) root. The non-mycorrhizal plants were 14.5cm tall, weighed 0.9g, and contained 25 nmoles P mg(\u27-1) root. Non-mycorrhizal seedlings receiving a high nutrient solution at frequent intervals grew as well and contained similar amounts of P as mycorrhizal seedlings. Concentration of nutrient solution had little effect on the growth of mycorrhizal seedlings. Content of soluble sugars and starch in roots of green ash followed predictable seasonal and phenological patterns, but showed little clear, consistent trend with respect to mycorrhizal or nutrient treatments. Patterns of growth and development of the host and morphology of the VA fungus in the first growing season suggest a yearly cycle with three phases. Phase I (late spring) is a period of fast shoot growth, slow root growth, rapid colonization of the root system by the fungus, and greater numbers of fungal arbuscules than vesicles. Phase II (summer) is a period of fast root growth and decline in percentage root length infected. Phase III (late summer and fall) is characterized by senescing leaves, infection percentages increasing to a static level, greater numbers of fungal vesicles than arbuscules, and increased numbers of mycorrhizal spores in the rhizosphere. This cycle continued in the second growing season except that the percentage root length infected by the fungus remained fairly constant