PHOSPHORUS EFFECTS ON THE MYCELIUM AND STORAGE STRUCTURES OF AN ARBUSCULAR MYCORRHIZAL FUNGUS AS STUDIED IN THE SOIL AND ROOTS BY ANALYSIS OF FATTY-ACID SIGNATURES

The distribution of an arbuscular mycorrhizal (AM) fungus between soil and roots, and between mycelial and storage structures, was studied b y use of the fatty acid signature 16:1 omega 5. Increasing the soil ph osphorus level resulted in a decrease in the level of the fatty acid 1 6:1 omega 5 in the soil and roots. A similar decrease was detected by microscopic measurements of root colonization and of the length of AM fungal hyphae in the soil. The fatty acid 16:1 omega 5 was estimated f rom two types of lipids, phospholipids and neutral lipids, which mainl y represent membrane lipids and storage lipids, respectively. The numb ers of spores of the AM fungus formed in the soil correlated most clos ely, with neutral lipid fatty acid 16:1 omega 5, whereas the hyphal le ngth in the soil correlated most closely with phospholipid fatty acid 16:1 omega 5. The fungal neutral lipid/phospholipid ratio in the extra radical mycelium was positively correlated with the level of root infe ction and thus decreased with increasing applications of P. The neutra l lipid/phospholipid ratio indicated that at high P levels, less carbo n was allocated to storage structures. At all levels of P applied, the major part of the AM fungus was found to he present outside the roots , as estimated from phospholipid fatty acid 16:1 omega 5. The ratio of extraradical biomass/intraradical biomass was not affected by the app lication of P, except for a decrease at the highest level of P applied .