Agriculture Reference
In-Depth Information
MYCORRHIZAL FUNGI
The roots of alliums are frequently colonized by fungi of the family
Endogonaceae. The hyphae of these fungi penetrate between the cells of the
root cortex and extend into the soil. Within the root the hyphae form branched
appendages that penetrate the host cell walls, but remain outside the
cytoplasmic membrane of root cells. Globules of dense cytoplasm accumulate
around these invaginations, forming characteristic tree-shaped microscopic
structures termed 'arbuscules' (see Fig 5.20b). Hence, these fungi are termed
arbuscular mycorrhiza (AM). Arbuscules are thought to be where the fungus
and plant exchange nutrients. Dense, oil-rich globules or vesicles are also
formed between root cells by the fungus (see Fig 5.20c).
AM fungi infect the roots of most plant species, with no marked specificity,
and occur naturally in soils worldwide. The review by Stribley (1990) of the
significance of AM for allium crops is the basis for much of this brief account.
More recently de Melo (2003) reviewed research on AM and allium crops, with
particular emphasis on their potential role in organic production. These two
reviews have extensive bibliographies of the original research on the subject.
Alliums have been used in many studies on AM because of their simple
roots and responsiveness to infection. The association can be of mutual benefit
to both fungus and plant - in other words, symbiotic. The most important
benefit to the plant is increased phosphate (P) uptake, whereas the fungus
gains a supply of carbon metabolites from the host. Alliums have thick, little-
branched roots, and hence a low ratio of root length to shoot weight, and they
also lack root hairs. Both these features make them poorly adapted to absorb
nutrients present in low concentration in soil. Phosphate exists in low
concentration in the soil solution and is strongly adsorbed to the solid phase of
soils. As a consequence, the transfer of P from soil to plant is frequently limited
by the rate of diffusion to the root surface, especially in soils of low available P
concentration. The P uptake per unit length of mycorrhizal onion root can be
increased fourfold over non-infected roots because the external hyphae extend
the absorbing surface. In low-P soils this can cause big increases in plant
growth rate: in one trial it was estimated that infection by mycorrhiza was
equivalent to 250 kg/ha of phosphate fertilizer.
Increases in photosynthetically fixed carbon exported to roots of about 7%
in mycorrhizal, as opposed to non-mycorrhizal, leek plants have been
measured, and this gives an indication of the 'cost' to the host plant in terms of
carbon assimilates of the association (Snellgrove et al. , 1982). In soils of low P
status the benefits to the plant in terms of the extra growth promoted by
greater P uptake outweighs this 'cost' of AM association, but in high-P soils it
can slow growth rate and lower maximum yields (Morgan et al. , 2005).
However, root colonization by AM fungi is slow on plants of high P status and
this may reduce the 'cost' to the plant in these conditions. AM infection may
also increase the uptake rate of micronutrients present in limiting quantities in
 
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