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64 RESEARCH REPORTS PNL Volume 12 1980
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THE IMPORTANCE OF MYCORRHIZAL INFECTION FOR THE GROWTH AND DEVELOPMENT
OF THE PEA PLANT
Snoad, B. John Innes Institute, Norwich, United Kingdom
In the autumn of 1978 an area of land on the John Innes Farm was treated
with a nematicide, Telone, and a sterilant, Basamid, in order to kill the
Trichodorus nematode and the weeds which were maintaining Pea Early Browning
Virus (PEBV) and thus inhibiting the planned development of a dried pea
breeding program.
It was realized that these treatments would be likely to disturb the
microflora and fauna of the soil and so, before drilling any pea seed, the
soil was tested for the presence of effective strains of Rhizobium. These
tests proved positive and drilling went ahead as planned in April, 1979,
the resulting plants exhibiting an encouraging vigor. Examination of the
root systems of young plants showed that infection with Rhizobium had taken
place and that the nodules which had developed were effective. However,
by the time flowering began, the plants showed signs of wilting and became
pale green. Growth slowed and the production of flowering nodes was
reduced. The root systems proved to be very atypical at this stage in being
hair-like, shallow, with very few main penetrating "tap roots", and almost
devoid of nodules.
Drs. Mosse and (layman of Rothamsted Experimental Station agreed to
examine the root systems of these pea plants for mycorrhizal infection and
quickly confirmed that there was none. In contrast, pea plants growing
on unsterilized land nearby were heavily infected. The absence of a mycorr-
hizal relationship with a plant is likely to result in a deficiency of
phosphates since the fungal hyphae are particularly efficient in the uptake
of phosphates and, in effect, increase the size of the plant root system.
This deficiency was confirmed when the amount of phosphorous in dry matter
was shown, by the Agricultural Development and Advisory Service in Cambridge,
to be 0.21% in plants from normal soil and 0.09% in plants from sterilized
soil.
It was important to confirm the relationship between soil treatment,
mycorrhizal infection, and pea plant growth and so, in collaboration with
Drs. Mosse and Dayman, a small trial was initiated in August, 1979. Peas
were sown on sterilized soil in microplots some of which had added nitrogen,
some had added phosphate, some were inoculated with mycorrhiza, and some
were left as controls. With the advance of autumn it was impossible to
grow the plants through to maturity and they were therefore harvested at
the early flowering stage which was nine weeks after sowing. Examination
of the roots showed that only plants from plots inoculated with mycorrhiza
had been infected, the remainder being devoid of any fungal relationship.
Obvious differences in plant growth according to treatment were to be seen
and these arc summarized as a series of dry-weight values in Table 1.
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PNL Volume 12
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1980
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RESEARCH REPORTS 65
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Table 1. Mean dry weights in gms and their standard errors based on
samples of ten pea plants from eight treatment plots sown on
_sterilized soil in mid-August and harvested nine weeks later.
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The addition of phosphate to the sterilized soil, which already contained
30-40 mg/liter, did lead to improvement in plant growth but none of the treat-
ments was as effective as inoculation with mycorrhiza. Even in the absence
of additional phosphate the mycorrhizal relationship led to the development
of larger plants. The addition of nitrogen was not beneficial to plant devel-
opment.
Soil sterilization therefore did not solve the problem of using virus-
infected land in a breeding program since the resulting plants were atypical.
Neither was the phosphate deficiency overcome by adding extra phosphate since
the plants did not take it up. An alternative approach is now being applied
which relies upon the continuous use of herbicides for two seasons to kill
all hosts of PEBV but hopefully leaving sufficient of the mycorrhizal popula-
tion unharmed.
Soil sterilization has, however, demonstrated the importance of the
three-way symbiotic relationship between the bacteria, the fungi, and the
pea plants and is likely to lead to a series of more detailed studies which,
as well as being of academic interest and value, could also be of importance
in the agronomy of the crop. It is a relationship which seems to have been
somewhat neglected in Pisum and one about which readers of this Newsletter
should be reminded.
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