The results of 2 experiments investigating the early stages of the formation
of vesicular- arbuscular (VA) mycorrhizas in response to both soil temperature
and the timing of autumn rains are reported for a Mediterranean environment in
the south-west of Western Australia.
In Expt 1, treatments including an early break, a late break, and a false
break followed by a late break were applied to a mixed and sieved field soil
collected dry in the summer and placed in pots in a glasshouse. In each break,
pots were watered to field capacity and planted with subterranean clover
(Trifolium subterraneum) or capeweed
(Arctotheca calendula). In early and false breaks, both
initiated on the same day in early autumn, the soil temperature was maintained
at 30°C, and in the late break, initiated 50 days later in autumn, the
soil temperature was maintained at 18°C. In both early and late breaks,
pots were watered to field capacity for either 21 or 42 days when plant and
mycorrhizal variables were assessed. In a false break, pots were watered to
field capacity for 7 days after which the soil was allowed to dry and newly
emerged plants died. These pots were then rewatered and replanted at the same
time as pots receiving a late break, and subjected to the same soil
temperature (18°C). In Expt 2 performed the following year, soil
temperature was maintained at 31 or 18°C in both early and late breaks.
Pots were planted with clover and watered to field capacity for 21 or 42 days,
when plant and mycorrhizal variables were assessed.
In Expt 1, VA mycorrhizal colonisation of both clover and capeweed was
initially low in an early break compared with levels observed in a late break.
Only mycorrhizas formed by Glomus spp. were observed in
the early break, whereas mycorrhizas of Glomus,
Acaulospora, and Scutellospora
spp. and fine endophytes were observed in the late break. Colonisation was
decreased by a false break, predominantly because of a decrease in formation
of mycorrhizas of Glomus spp. In Expt 2, mycorrhizas of
Glomus spp. predominated in warm soil in both early and
late breaks and mycorrhizas of Acaulospora spp.,
Scutellospora spp., and fine endophytes were observed in
greater abundance in cool soil in early and late breaks.
These experiments indicate that soil temperature at the time of the break will
have a large impact on both the overall levels of VA mycorrhizal colonisation
of pasture plants and colonisation by different fungi. In addition, fungi that
remain quiescent in warm soil may avoid damage in a false break.
Genetic Diversity and Association Characters of Bacteria Isolated from Arbuscular Mycorrhizal Fungal Spore Walls
