Functional diversity in arbuscular mycorrhizal (AM) symbioses: the contribution of the mycorrhizal P uptake pathway is not correlated with mycorrhizal responses in growth or total P uptake

We investigated structural and functional diversity in arbuscular mycorrhizal (AM) symbioses involving three plant species and three AAA fungi and measured contributions of the fungi to P uptake using compartmented pots and P-33. The plant/ fungus combinations varied in growth and P responses. Flax (Linum usitatissimum) responded positively to all fungi, and medic (Medicago truncatula) to Glomus caledonium and G. intraradices, but not Gigaspora rosea. Tomato (Lycopersicon esculentum) showed no positive responses. Hyphal growth in soil was very low for Gi. rosea and high for both Glomus spp. Hyphal lengths in root + hyphal compartment (RHC) and hyphal compartment (HQ were similar for G. intraradices, but much higher in HC for G. caledonium. Specific activities of P-33 in plants and soil indicated that fungal P uptake made substantial contributions to five plant/fungus combinations and significant contributions to a further two. G. intraradices delivered close to 100% of the P in all three plants. G. caledonium and Gi. rosea delivered less P. The amount was not related to colonisation or to growth or P responses. We conclude that: AM colonisation can result in complete inactivation of the direct P uptake pathway via root hairs and epidermis; calculations of AM contributions to P uptake from total plant P will often be highly inaccurate; and lack of plant responsiveness does not mean that an AM fungus makes no contribution to P uptake.