Network Establishment of Vesicular-Arbuscular Mycorrhizal Hyphae in the Rhizospheres between Trifoliate Orange and Some Plants.

Abstract

The distributions of vesicular-arbuscular mycorrhizal (VAM) hyphae in the rhizospheres of trifoliate orange (Poncirus trifoliata Raf.) and some plants grown under greenhouse conditions were explored. Furthermore, the effect of the 25% MeOH eluates of bahiagrass (Paspalum notatum Flugge.) and millet (Pennisetum glaucum L. R. Br.) roots fractionated by flash chromatography on the network establishment of VAM hyphae was examined. Special acrylic root boxes with three compartments separated by nylon screens were used for the experiment. Seedlings of trifoliate orange were transplanted into one outer compartment in all boxes, and seedlings of bahiagrass, millet, cockscomb (Celosia cristata L. cv. Parade), radish (Raphanus sativus L. cv. Natsumino), and tomato (Lycopersicon esculentum Mill. cv. Fukuju) were transplanted into the other outer compartment. A VAM fungus, Gigaspora margarita Becker and Hall, was inoculated in the center compartment. In plots with only trifoliate orange seedlings, 25% MeOH eluates of bahiagrass root extract (BRE) and millet root extract (MRE) were applied into the other outer compartment once a week. A plot with only trifoliate orange seedlings and without BRE and MRE applications was also prepared as a control. The density of hyphae, percentage of infection, and number of spores in the bahiagrass and millet compartments were higher than those in the compartments with trifoliate orange. These parameters in the cockscomb, radish and tomato compartments, however, were markedly low as compared with the trifoliate orange compartments. Although few VAM hyphae and spores were observed in the control (no plant compartment), the density of hyphae in the compartments treated with BRE and MRE increased to around 18%, and a few spores were found in these compartments. Root exudates of trifoliate orange, bahiagrass, and millet stimulated the hyphal growth of G. margarita in vitro. Particularly, the stimulation by bahiagrass and millet was significantly greater than that of the control (agar only). But the hyphal length of cockscomb, radish, and tomato treatments was shorter than that of the control. These results suggest that under field conditions, root exudates will affect the behaviour of VAM fungi in the soil. Moreover, some compounds in BRE and MRE may act as chemotropic signals for attracting VAM hyphae.