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THE INFLUENCE OF MYCORRHIZA ON VINES
Stephan Joubert1 & Eben Archer2
INTRODUCTION In the South African vineyard industry the new approach
to production practices is more environment friendly. Biological
production practices are increasingly replacing chemical production
practices so as to comply with guidelines for integrated production. This
new approach is not only valuable above ground – the subterranean
environment benefits equally from a bio-friendly approach. With this in
mind the mycorrhiza group of fungi is receiving more and more attention,
since it allows vine roots to function in greater harmony with their
environment. These fungi live symbiotically with the roots of the plant
and they take up water and nutrients from the soil, making them available
to the plant in exchange for carbohydrates. By so doing the nutritional
requirements (phosphorus in particular) of the plant, in this instance the
vine, as well as water are more efficiently provided and the performance
of the plant is considerably enhanced.
Two main groups of mycorrhiza make a contribution to
plantlife, namely ecto- and endomycorrhiza. Within the endomycorrhiza
group the so-called vesicular arbuscular mycorrhiza (VAM) species are most
commonly encountered in vine roots. The various species occurring in vine
soils are Glomus mosseae, G. microcarpum, G. monosporum, G. occultum,
G. deserticola and G. fasciculatum. These species form arbuscular and
vesicular structures inside the cortex cells of vine roots and outwardly,
in the soil, they form long, stringy structures (hyphae) (Fig. 1). With
the hyphae thus constituting an intensive network in the surrounding soil,
the contact surface between roots and soil is considerably increased.
Consequently the same volume of soil is used far more efficiently in that
available water and nutrients are taken up more thoroughly. The enhanced
performance of vines colonised with mycorrhiza was first observed early in
the 1950's. Since then various researchers in field and pot plant trials
have noticed significant increases in vigour and production of vines
colonised with mycorrhiza. This article will briefly eludicate the
potential advantages of mycorrhiza. 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.
FIG. 2: 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. Other advantages 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. ADDITIONAL
LITERATURE Fig. 1 Hypha; Spore;
Soil; Epidermis; Cortex; Appressorium BAVARESCO, L. &
FOGHER, C., 1996. Lime induced chlorosis of grapevine as affected by
rootstock and root infection with arbuscular mycorrhiza and Pseudomonas
fluorescens. Vitis 35(3), 119-123. BAVARESCO, L. &
FOGHER, C., 1992. Effect of root infection with Pseudomonas fluorescens
and Glomus mosseae in improving Fe-efficiency of grapevine ungrafted
rootstocks. Vitis 31, 163-168. BETHLENFALVAY, G.J.,
1989. Relationships between soil aggregation and mycorrhizae as influenced
by soil biota and nitrogen nutrition. Biol. Fertil. Soils 28(4), 356-363.
BETHLENFALVAY, G.J.
& SCHÜEPP, H., 1994. Arbuscular mycorrhizas and agrosystem stability.
In, Impact of arbuscular mycorrhizal on sustainable agriculture and
natural ecosystems; 191-200; Reds; Gianinazzi, S. & Schüepp, H.
Berlin, Birkhauser. BIRICOLTI, S., FERRINI,
F., RINALDELLI, E., TAMANTINI, I. & VIGNOZZI, N., 1997. VAM fungi and
soil lime content influence rootstock growth and nutrient content. AM. J.
Enol. Vitic. 48(1), 93-99. CARLISLE, M.J. &
WATKINSON, S.C., 1994. The fungi. Academic Press Ltd. DOUDS, D.D., BéCARD,
G., PFEIFFER, P.E., DONER, L.W., DYMANT, J. & KAYSER, W.M., 1995.
Effect of vesicular arbuscular mycorrhizae fungi on Resting of Sciodopitys
verticillate. Hort Science 30(1), 133 – 134. HOOKER, J.E.,
JAIZME-VEGA, M. & ATKINSON, D., 1994. Biocontrol of plant pathogens
using arbuscular mycorrihizal fungi. In, Impact of arbuscular mycorrhizal
on sustainable agriculture and natural ecosystems; 191-200; Reds;
Gianinazzi, S. & Schüepp, H., Berlin, Birkhauser. KARAGIANNIDIS, N.,
NIKOLAOU, N. & MATTHEOU, A., 1995. [ Influence of three VA-mycorrhiza
species on the growth and nutrient uptake of three grapevine rootstocks
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MENGE, J.A., RASKI,
D.J., LIDER, L.A., JOHNSON, L.V., JONES, N.O., KISSLER, J.J. &
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HONRUBIA, M., 1994. Water relations and alleviation of drought stress
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SMITH, S.E. & READ,
D.J., 1997. Mycorrhizal symbiosis. 2nd ed. San Diego & Londen:
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