Íslenskar landbúnaðarrannsóknir - 01.09.1977, Síða 42

40 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
component of pastures and no prolonged
attempt has been made to improve the effici-
ency of the white clover-Rhizobium symbios-
is. Iceland has a cold climate and a short
growing season and these sub-arctic conditions
impose the most important limitations on
symbiotic nitrogen fixation. Soil temperature
at sowing time can be wéll under 10° C and
only increase to 12—13°C during the warm-
est period in July.
Symbiotic nitrogen-fixation by leguminous
plants depends upon both the host plant
genotype and the associated Rbizobmm strain
(Jones and Harðarson, unpublished data)
but each symbiotic association can be greatly
influenced by the environment. Vincent
(1965) and Lie (1974) have reviewed the
symbiosis-environment interaction and it has
been shown that temperature affects all stages
of the symbiosis from the occurrence of the
bacteria (by affecting growth and survival)
to nodule formation and function.
It might be expected that nodulated plants
are better buffered to withstand environment-
ai stress than non-nodulated plants. It is most
important, therefore, that the Rhizobium
strain which is to nodulate the clover plant
infects the root hair and thus stimulates
nodule formation as quickly as possible and
that the symbiotic partnership is capable of
initiating the nitrogen fixation process at low
temperamres.
The present investigation reports on ex-
periments in which strains of Rhizobium
trifolii were screened for their effectiveness
at low temperamre and to report on a white
clover-Rhizobium inoculation field trial
conducted in Iceland using bacterial strains
selected after the screening trials.
MATERIALS AND METHODS
Experiment 1.
40 isolates of Rhizobium bacteria from Ice-
land were tested in this investigation, 10 from
Wales, 4 from England (Rothamsted Collect-
ion of Rhizobium) and 6 from Sweden
(kindly supplied by Prof. G. Fahraeus, Upp-
sala).
The 40 Icelandic strains were collected
from the same number of areas in South
(Ámes and Rangárvallasýsld), North (Eyja-
fjarðarsýsla) and East of Iceland (N-Þing-
eyjarsýsla and Múlasýslur). The areas sampled
varied in altimde from 700 m down to sea
level. For comparison samples were taken
from Wales, five from highland soils and
five from lowland soil. All the isolates from
Iceland and Wáles were isolated from nodules
on white clover plants either growing wild or
from sown pasmres. From each site clover
plants were removed and effective looking
nodules were cut off the roots and placed in
small plastic boxes with silica gel as desiccant
(NORRIS, 1963) for later isolation following
standard procedures (VlNCENT, 1970). After
isolation, one strain from each site was
sélected and tested for symbiotic effectiveness.
Strains from England and Sweden were in-
cluded in this investigation for comparison.
Symbiotic effectiveness and rate of nodu-
lation were investigated on seedlings of
Aberystwyth SI84 white clover plants which
were grown aseptically in 150x19 mm mbes
containing 10 ml slopes of nitrogen-free agar
(Jensen, 1942). Ten plants were used to
test each isolate and each test was carried out
at two different temperatures, 12°C and
20°C, in two controlled enviroment rooms
giving 16 hours day. Data were collected for
the time to form the first nodule and the
rate of nodulation. After 8 weeks the experi-
ment was harvested and the symbiotic eff-