Phosphatase activities of arbuscular mycorrhizal intraradical and extraradical mycelium, and their relation to phosphorus availability

We investigated the influence of changes in external phosphorus (P) concentration on the proportion of phosphatase-active structures of the arbuscular mycorrhizal fungus Gigaspora margarita associated with Allium cepa. The P treatment was started when mycorrhizal colonisation had been established, and plant systems were harvested three times after the start of the P treatment. Higher shoot dry weights and P contents were observed in the high-P treated plants at the last harvest. We did not find any change in the proportion of phosphatase-active extraradical mycelium following P treatment. However, the proportion of alkaline phosphatase-active mycelium was positively correlated for extraradical and intraradical mycelium. Also, the proportion of alkaline phosphatase-active arbuscules seemed to increase with the shoot fresh weight, whereas the proportion of acid phosphatase-active arbuscules decreased with higher shoot P concentration and dry weight. We have shown experimentally that the intraradical mycelium of G. margarita, but not the extraradical mycelium, responds metabolically to plant P concentration, and possibly also to external P availability.