Inaba Shoko, Watanabe Toshihiro, Osaki Mitsuru, Ezawa Tatsuhiro
Hokkaido University Graduate School of Agriculture W9, N9, Kita-ku 060-8589 Sapporo Japan
Plant growth is generally inhibited in acidic soil due to Al toxicity and P deficiency. It has been known that arbuscular mycorrhizal (AM) fungi enhance plant growth even under low-pH conditions, although the mechanism underlying has not been elucidated. We designed the experiments according to the following hypothesis: mycorrhizal formation provides an alternative pathway of P uptake for the host plant of which P uptake through the root pathway is restricted due to Al toxicity. An Al-sensitive wheat cultivar ES8 was grown on autoclaved acidic (pH 4.4) or neutralized (pH 6.6) low-P soil (31.9 mg available-P kg-1 soil) in a greenhouse with the following four treatments: soil P, the media were amended with 100 mg P kg-1 soil; foliar P, the liquid fertilizer was applied to the above-ground part twice a week; +AMF, Glomus sp. HR1 isolated from acidic soil was inoculated at 1000 spores pot-1; control, the plants were grown without either P fertilizer or AM fungi. The plants were harvested 10 weeks after sowing. Shoot and root biomass were measured, and the levels of AM colonization was assessed. The root tips were collected and stained with fluorescein diacetate-propidium iodide (FDA-PI) to assess the damage of plasma membrane. P contents of the plant materials were determined colorimetrically after digestion. Mycorrhizal colonization was observed only in the +AMF treatments. In acidic soil, the growth and P uptake of the plants were significantly improved by the inoculation of AM fungi but not by (either foliar or soil) P fertilization. In contrast, plant P uptake in the neutralized soil was much greater, and no response to AM fungi was observed. The FDA-PI staining showed that the most of root tips were viable in the neutralized soil but 100% of the tips were damaged in acidic soil in the absence of AM fungi. Whereas about 20% of the root tips sustained the functional integrity in the presence of AM fungi even in acidic soil. These results ensure the validity of the present hypothesis but raise new questions how AM fungi alleviate Al stress on the plant roots and whether P uptake through the root pathway contribute to the P acquisition of the mycorrhizal plants significantly.