VA mycorrhizal fungi and VA mycorrhizae in citrus roots
1: VA mycorrhizae in citrus roots |
According to a fossil record, VAM fungi were infecting in the roots of Aglaophyton major, an Early Devonian land plant, at least 400 million years ago (Remy, et al., 1994). The Early Devonian has been known to be the period when plants invaded the land, and it will be of great interest to imagine how the plants invaded the land where there was a severe situation for them with the help of VAM fungi.
The known functions of VAM fungi on plants are that 1) the absorption of minerals, especially P, is promoted, 2) growth is stimulated, 3) high quality of fruits is produced every year, 4) resistance to environmental stresses is enhanced, and 5) resistance to soil disease is enhanced.
At present, most countries have some projects about sustainable agriculture. I believe that VAM fungi can play a major role in the quest for sustained plant productivity in all segments of agriculture. Soil degradation involved disturbance of the balance by cultivation, monoculture, excessive inputs of agrochemicals and fertilizers, removal of organic matter from the soil by crop removal and accelerated decomposition without replacement, and erosion. To maintain economic levels of productivity, farmers have been forced to increase inputs. Our goal is to reduce chemical inputs for environmental and health reasons. Thus, I am convinced that VAM fungi need to be reestablished to a high level of effectiveness to offset the reduced inputs. On the utilization of VAM fungi, we must consider as follows;
1. Cut down the amount of agrochemicals and chemical fertilizers applied.
Several kinds of fungicides, such as copper fungicides, iprodione, iprodione-like compounds, benomyl and benomyl-like compounds, severely inhibit VAM growth. The application of herbicides may also inhibit the intensity of VAM infection by decreasing the population of some weeds which strengthen the activity of VAM fungi. Thus it is necessary to cut down the amounts of these agrochemicals to be applied.
Although VAM fungi stimulate P uptake of plants from the soil, a great deal of P in the soil inhibits VAM growth. A concentration of over 50 ppm PO43- in the soil has been reported to inhibit severely VAM colonization. However, the application of insoluble phosphate, such as rock phosphate (Graham and Timmer, 1985), calcium phosphate and bone powder, is effective to maintain VAM fungi. An additional advantage of insoluble phosphate is long- term availability of P, compared to soluble phosphate.
2. Use fermented organic matter.
The application of unfermented organic matter severely inhibited VAM development. Fermented organic matter application, however, stimulated VAM formation and the growth of plants. One of the VAM stimulators, such as lower concentrations of ethylene which evolved from fermented organic matter, may be effective for VAM growth. Ethylene is a plant hormone. About 0.05 ppm ethylene stimulates VAM hyphal growth and VAM development of plants. But ethylene at concentrations above 0.5 ppm depress the hyphal growth and the mycorrhizal formation. In soils where unfermented organic matter has been applied, ethylene is often detected at concentrations high to inhibit VAM growth.
3. Use soil conditioners, such as charcoal and zeolite.
The application of charcoal is very effective for VAM development. But an excess of charcoal inhibits plant growth. This inhibition by an excessive application of charcoal might be concerned with an increment of soil pH value. Therefore, an appropriate amount of charcoal to be applied is less than 20 ton per hectare. Zeolite is one of the soil materilas. It absorbs chemicals which inhibit VAM growth, and cleans the soil as charcoal does. But the application of zeolite does not change soil pH value.
4. Consider the effectiveness of grasses.
VA mycorrhizae develop well in citrus orchards where Bahia grass is used for sod. We have also indicated that the intensity of VAM formation on some weeds grown in citrus orchards was high. However, sod culture in commercial citrus orchards has been unsuccessful in Japan; most citrus growers believe that a clean culture is best for the production of high-quality fruits. Thus our soil management system must be re-evaluated. On the introduction of the system, a small amount of fertilizers is needed for grasses at the bigining. After that, the erosion of top soil and minerals is prevented by grasses, and then the minerals will circulate in the plant-soil system without running off.
In general, the population of VAM spores in poor soils, such as the soils damaged by severe dryness or erosion, is very low. Planting grasses is very effective for the revival of the soil with low density of VAM fungi. Therefore, we propose to cover the soil with grasses which are familiar with VAM fungi, as plants invaded the sterile lands with the help of VAM fungi at least 400 million years ago.
5. Improve soil pH and aeration.
Although several kinds of VAM fungi have been known to have the resistance to adverse pH conditions, most of the VAM fungi are vulnerable to such soil pH conditions. We reported that a suitable pH on hyphal growth of Gigaspora ramisporophora was from 5.1 to 7.1.
The growth of VAM fungi also is poor in anaerobic soils. Although VAM fungi infect the root of aerenchima-developing plants such as mangrove (Ishii et al, 1996) and paddy rice plants (Soliman and Hirata, 1996), the percentage of VAM infection is generally low.