Relationships Between Host and Endophyte Development in Mycorrhizal Soybeans

Abstract

SUMMARYSymbiotic associations of soybeans and the vesicular-arbuscular mycorrhizal fungus Glomus fasciculatus were grown to maturity in a sand-perlite rooting medium watered with a nutrient solution containing growth-limiting amounts of soluble phosphorus. Development of fungal mycelia external and internal to the host plant's root system was measured by determining chitin in isolated fungal mycelium, in the rooting medium, and in tbe mycorrhizae. The biomasses of the extra-and intraradical mycelia were calculated from the values of chitin obtained spectrophotometrically. The amount of total fungal biomass relative to that of the host plant varied throughout the lifespan of the association and reached a maximum of 2*3 % 10 weeks after planting. The amount of intraradical mycelium increased throughout the host plant's life span. Extraradical fungal structures attained a maximum weight at the onset of logarithmic growth of the soybean pods and decreased thereafter. Cessation of the rapid growth phase of G. fasciculatus lagged behind that of the host plant's vegetative structures, but appeared to be related to pod development. The fungus to root dry wt ratio was 12-3% at senescence. The ratio of extra-to intraradical mycelium decreased throughout the association's lifespan. Since the extraradical hyphae are the organs responsible for enhanced nutrient uptake this ratio is proposed as an index of the endophyte's usefulness to the host. High values for this usefulness index coincided with significant growth enhancement of the host plant. Source-sink relationships in the host appear to be a determining factor in the growth of the fungal endophyte.

Grassroots Ecology: Plant–microbe–soil Interactions as Drivers of Plant Community Structure and Dynamics

“…For example, the benefits that plants receive from nutritional mutualisms with soil microbes (e.g., nitrogen-fixing bacteria and mycorrhizal fungi) depend upon the availability of essential soil minerals. Available phosphorus (P) is often the primary determinant of plant dependence on arbuscular mycorrhizal (AM) fungi, such that the effect of the interaction can be negative under high P conditions (Bethlenfalvay et al 1982). The benefit that plants receive from protective mutualisms is also environment dependent, as the benefit depends upon the presence of enemies.…”

THE EFFECT OF HIGH NUTRIENT ADDITION UPON SEASONAL PATTERNS OF MYCORRHIZAL DEVELOPMENT, HOST GROWTH, AND ROOT PHOSPHORUS AND CARBOHYDRATE CONTENT IN FRAXINUS PENNSYLVANICA MARSH

“…This is near the level of 75 /ig P g-i soil which was found by Kormanik (1984) to enable non-mycorrhizal hardwood tree seedlings to grow a well as mycorrhizal seedlings, but much higher than the 10/^g"^ proposed by Bethlenfalvay et al (1982d) as the level at which non-mycotrophy occurs. No measurements were made of the ratio of extra-to intraradicle fungal biomass, so it is unknown whether plants of the high nutrient treatment were supporting a disproportionately large internal infection relative to external nutrient absorbing hyphae (Bethlenfalvay et al, 1982b). Carbohydrate demands by the fungus in excess of benefits derived by the host may cause suppression (Bethlenfalvay et al, 1982a;Buwalda & Goh, 1982).…”

DEVELOPMENT OF TOTAL AND VIABLE EXTRARADICAL MYCELIUM IN THE VESICULAR–ARBUSCULAR MYCORRHIZAL FUNGUS GLOMUS CLARUM NICOL. & SCHENCK

“…The importance of evaluating the amount of extraradical mycelium produced by the fungal symbionts has only recently been fully realized. Methods based on chitin assay (Bethlenfalvay, Brown & Pacovsky, 1982), on measurement of the weight of soil particles attached to mycelium (Graham, Linderman & Menge, 1982) and on optical observation (Abbott, Robson & De Boer, 1984) have been developed for this purpose.…”