Interaction of Light Intensity and Soil Temperature With Phosphorus Inhibition of Vesicular‐arbuscular Mycorrhiza Formation

Abstract

SUMMARYThe interaction between light intensity and soil temperature with phosphorus inhibition of vesicular-arbuscular mycorrhiza (VAM) formation in sudangrass (Sorghum vulgare, Pers.) was investigated in P-deficient (0-5 mg P kg~^) sandy soil amended with 0 (OP), 15 (15P) and 30 (30P) mg P kg"' soil as superphosphate. For seedlings inoculated with Glomus fasciculatus and grown at 100, 75, and 50"0 of glasshouse light intensity, the reduction in VAM formation by added soil P (15 P and 30P) was greatest at the lower light levels, whereas in the OP treatment, VAM fornaation was unaffected by a decrease in light intensity. Increases in root P concentration resulted from both soil P amendment and, in the 15P treatment, from a decrease in light intensity, and were associated with decreases in root membrane permeability and corresponding decreases in root exudation of amino acids and sugars. The reduction in sugar exudation in the 15P treatment at lower light intensity was paralleled by a decrease in root extract levels of sugars, but decreases in amino acid exudation could not be explained by changes in amino acid levels in root extracts. These results suggest that P inhibition of VAM formation is more severe at lower light intensities because of a decrease in membrane-mediated exudation, although a reduction in sugar levels in root cells may also contribute to tbe observed decrease in exudation.When inoculated seedlings were grown at 25, 30, and 35°C soil temperatures, the reduction in VAM formation in the 1 5P and 30P treatments was overcome by increase in soil temperature. The increase in VAM formation by increased temperature in the 15P plants was associated with significant increases in root membrane permeability and exudation without a corresponding change in root P concentration. In the OP treatment, an increase in soil temperature increased VAM formation with little effect on root exudation, which suggested an effect of temperature on Glomus fasciculatus. Higher soil temperature may increase VAM formation through either a direct effect of temperature on tbe fungus or an indirect effect via an increase in leakage of root metabolites necessary for fungal activity, or both.

Phosphate Treatment Strongly Inhibits New Arbuscule Development But Not the Maintenance of Arbuscule in Mycorrhizal Rice Roots

“…In contrast with our observations, early studies showed that P inhibition decreases the numbers of vesicles Stribley et al, 1980;Abbott et al, 1984;Amijee et al, 1989: Bruce et al, 1994 and lengths of colonized regions (Sanders and Tinker, 1973;Jasper et al, 1979;Buwalda et al, 1982;Graham and Leonard, 1982;Smith, 1982;Amijee et al, 1989;Miranda et al, 1989;Smith and Gianinazzi-Pearson, 1990;Braunberger et al, 1991;Thomson et al, 1991;Baon et al, 1992;Bruce et al, 1994;Tawaraya et al, 1994). However, reduced intraradical fungal Figure 6.…”

Mycorrhizas formed by Gigaspora calospora and Glomus fasciculatum on subterranean clover in relation to soluble carbohydrate concentrations in roots

“…In the past, several treatments have been used to separate efiects of the concentrations of soluble carbohydrates in roots and root exudates and the concentrations of P in roots on the formation of VA mycorrhizas: photon flux, soil temperature (Graham, Leonard & Menge, 1982;Same et al, 1983), defoliation (Same et al, 1983), photoperiod (Johnson et al, 19826), ozone stress (McCool & Menge, 1983), simazine application (Schwab, Johnson & Menge, 1982), flower bud initiation (Johnson et al, 1982 a) and sulphur supply (Thomson, Robson & Abbott, 1985). In our study, subterranean clover was grown at two trradiances and two concentrations of soil P in an attempt to vary the concentrations of soluble carbohydrates in roots independently of the concentrations of P in roots.…”

Defense-Related Transcript Accumulation in Phaseolus vulgaris L. Colonized by the Arbuscular Mycorrhizal Fungus Glomus intraradices Schenck & Smith

“…Light and developmental status of the plant may also affect colonization. Altered carbon flow to the root system may result from low-light-intensity photosynthesis (Ferguson and Menge, 1982;Graham et al, 1982) or redirection to support the energy demands of flowering (Johnson et al, 1982). Other compounds from plant roots may also affect colonization.…”