Agriculture Reference
In-Depth Information
1.
Ectomycorrhizae
. The fungal mycelium forms a sheath—the
Hartig net
—
around the root cylinder and also grows through the intercellular spaces in
the root. Ectomycorrhizae are primarily associated with woody trees such as
pines, oak, beech, birch, alder, poplar, and
Eucalyptus
.
2.
Endomycorrhizae
. This is the form that infects grapevine roots. The fungus
does not develop an external sheath, but grows mainly within the root,
forming two characteristic structures: the
vesicles
, which are temporary
storage bodies in the intercellular spaces, and the
arbuscules
, which are
branched hyphae that invade the host cells (fig. 4.12). Hence the common
name is
vesicular-arbuscular mycorrhiza
(
VAM
). The fungus supplies
nutrients to the host and receives a supply of organic substrates for growth
from the host.
VAM
s are widely distributed on
Vitis vinifera
roots in Australia and have also
been recorded in France, Germany, Poland, and Russia (Possingham and Groot
Obbink 1971). Although the fungal spores may be present in the soil, they ger-
minate only when a suitable host root is near. If the young mycelium does not
penetrate a host root, it dies. Generally, mycorrhizal infection improves the growth
of plants, including grapevines, especially on infertile soils, where the mycorrhizal
infection enhances the uptake of P. The fungus may mineralize organic P com-
pounds, and the external hyphae provide a pathway for rapid transport of P into
the plant. The incidence of infection declines when P fertilizer is applied to the
soil, although the extent of this decline varies for cultivated crops. There is evi-
dence for improved water uptake by mycorrhizal plants, probably because of the
external hyphal network.
VAM
s can also enhance the uptake of Cu and Zn from
soil.
Diagram of a vesicular-arbuscular mycorrhiza associated with a plant root (White 1997).
Reproduced with permission of Blackwell Science Ltd.
Figure 4.12
Large
spores
External
hyphae
External
vesicles
Root hair
Vesicles
Arbuscles
