POTENTIAL ADVANTAGES OF MYCORRHIZA
Influence on nutrient uptake
The best known and most advantageous effect of mycorrhiza is the improved uptake of nutrients, phosphorus in particular. Phosphorus is essential for cell division and meristematic growth, while being an important building block of phospholipids and nucleic acid. Furthermore it is the core building block of adenocindiphosphate (ADP) and adenocintriphosphate (ATP), the most important energy vehicles in the vine. Phosphorus is therefore of cardinal importance for the plant to function optimally.
Mycorrhiza inoculated roots have been found to be more efficient with regard to nutrient uptake, for not only does the performance of the plant improve, but such plants also have a higher concentration of phosphate than plants not inoculated with mycorrhiza. The uptake of mycorrhiza by VAM-colonised roots, just like VAM-free roots, is dependent on the amount and kind of phosphate in the soil. Phosphate is absorbed as PO4 ions (H2PO4) from the soil solution where it is present in very low concentrations. Anorganic phosphate in soil occurs in so-called labile and non-labile pools. A non-labile pool of phosphate is chemically bound to the surfaces of clay minerals or may also occur as insoluble crystals (with Ca, Fe and Al). This non-labile pool of phosphate is considered inaccesible to plants. The labile pool consists of weaker bounded phosphate which exchanges relatively quickly with the soil solution and is in an isotopical balance to the soil solution. This labile pool of phosphate is accessible to plants.
Research has already shown the mass flow of the solution in the soil to be such that it cannot satisfy the rate of phosphate uptake through the roots. Seeing that the uptake is faster than the diffusion rate, zones are created around each root in which phosphates do not occur at all. This means that the phosphate requirements of vines are often not satisfied, even if there is sufficient phosphate present in the soil. Since mycorrhiza has the ability to take up phosphate from the non-labile pool as well, it efficiently bridges the above-mentioned “phosphate insufficiencies”. It is possible to obtain this advantage because the hyphae of mycorrhiza are able to colonise soil volumes which cannot be reached by vine roots.
Mycorrhiza also improves the uptake of other nutritional elements such as nitrogen (N), potassium (K), copper (Cu), zinc (Zn) and other micro-elements. It is still uncertain, however, whether other nutrients enjoy an improved uptake. There are various explanations for the improved nutrient uptake: The distribution of hyphae in soil zones where roots are absent (the micro-sized cross-section of the hyphae facilitates the penetration of much smaller soil pores where roots cannot penetrate), as well as the bigger contact surface of the hyphae with the soil, contributes largely to the increased nutrient uptake. What is more, mycorrhiza competes much better than plant roots with other soil micro-organisms for the nutrients that are available to plants, mostly because they are more effectively distributed. A lot of the nutrients in the soil are also in a chemical form that is either unavailable to plants or cannot be taken up by them. However, mycorrhiza has the ability to secrete enzymes that catabolise these compounds so that they may be taken up by the hyphae. The improved nutrient uptake caused by mycorrhiza has huge potential benefits for the wine farmer in that it may reduce the fertilisation requirements and therefore bring about savings.
Influence on drought resistance
Plant roots colonised by VAM show a lower resistance to water movement from the soil to the roots than uncolonised roots. VAM therefore facilitates water uptake. Furthermore, plants with VAM colonisation have a higher transpiration rate, as well as stoma conduction, than uncolonised plants. Of all the species Glomus deserticola appears to adjust best to dry conditions. The improved water uptake caused by this specie is ascribed to improved soil colonisation since the hyphae penetrate soil particles where roots are absent. The improved water consumption caused by this specie is ascribed to phosphate uptake which improves stoma conduction, as well as improved hormone balances in the plant which regulate stoma closure.
In the manner described above, VAM improves the drought resistance of plants and in a country like South Africa, where water is scarce, it has huge potential advantages for grape vine producers. A considerable amount of research is still necessary, however, to prove these benefits and to determine the exact mechanism by which it works.
Influence on lime resistance
Trace element shortages often occur in vines on soils with high pH. Research indicates, however, that VAM colonised rootstocks have a better lime tolerance than uncolonised vines. In pot trials, rootstock cultivars such as 101-14 Mgt, 3309 Couderc and 140 Ruggeri that were inoculated with VAM, showed significantly fewer symptoms of lime chlorosis than the control plants. The iron and chlorophyl concentrations in the leaves of VAM colonised plants were significantly higher than that of the control. From this one may deduct that trace element uptake in calcareous soil is improved by VAM colonisation. On the other hand, VAM had no significant influence on the lime tolerance of SO4 and it is clear that rootstocks differ from one another in this regard. There are also differences in the sensitivity to lime in different VAM species. Glomus deserticola is more sensitive to high lime concentrations than G. mosseae and G. constrictus. It is clear that a lot of research in this regard is still required to be able to benefit from the potential advantages of mycorrhiza.
Protection against plant pathogens
Mycorrhiza also has the characteristic of suppressing plant pathogens by competing with the pathogens for infection points on the root. The bigger the VAM colonisation, the more difficult for the pathogens to acquire infection points. The increasing lignification of the endodermal root cells of VAM plants also offer more resistance to infection by other plant pathogens.
Effect on soil compaction
Mycorrhiza makes an important contribution to the ecological stability of soil by improving the aggregate stability of soil. The improved soil aggregation counteracts soil compaction and is considered very advantageous for vine roots since these penetrate compacted soil zones only with the greatest difficulty. The improvement in the soil structure may be explained by the high frequency of fungal hyphae which become entangled in one another as well as in primary soil particles, thus forming a skeleton-like structure. The plant roots and fungal hyphae then cause physical and chemical conditions which result in the formation of organic and amorphous material for the binding of particles. These micro-aggregates then become so entangled that they start forming macro-aggregates, which improves the storage of nutrients and carbon in the soil. Effectively, therefore, a better micro-habitat is created for soil micro-organisms.
An increase in the rate of photosynthesis in VAM plants has also been observed and the potential advantages for vines are manifold. Researchers have also proven that VAM colonisation can improve the saline resistance of plants and that it can be beneficial, particularly in drier areas, for vines planted in alkaline soil, or where brackish irrigation water is used.
Another advantage which may be useful to nurserymen, is the possibility of improved callus formation. All these advantages of VAM still have to be investigated and ascertained in vines.
CONCLUSIONS AND RECOMMENDATIONS
Mycorrhiza, in particular VAM, has huge potential advantages for viticulture. Although one can accept that it occurs fairly commonly in soils, it is not clear what the population composition in South African vine soils looks like. It is also uncertain what effect continued chemical treatment and mechanical cultivation of the soil over many years has on the VAM populations in vine soils. To grapevine producers VAM is a natural and environmentally friendly partner that can make a big contribution to ensure the success of Integrated Production in the South African wine industry. At present two research projects, both of them funded by Winetech, are attempting to gain a better understanding of the symbiotic relationship between VAM and vines. These projects, one of which is a pot trial by the University of Stellenbosch and the other a field trial by ARC-Infruitec/Nietvoorbij, are closely linked and hopefully practical recommendations may be made to the grapevine industry in the foreseeable future.
Fig. 1 Hypha; Spore; Soil; Epidermis; Cortex; Appressorium
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Stephan Joubert & Eben Archer