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Fred, Edwin Broun; Baldwin, Ira Lawrence; McCoy, Elizabeth / Root nodule bacteria and leguminous plants
(1932)

Chapter 10: Relationship between leguminous plants and bacteria,   pp. 160-191


Page 168


UNIVERSITY OF WISCONSIN STUDIES
gar, 1929, agrees approximately with this idea. Although the high nitrogen
con-
tent may account in part for the temporary immunity of the seedling, Thornton,
1929d, showed that seedlings of the first leaf stage possess an extractable
ma-
terial which both encourages growth of the rhizobia and stimulates nodule
pro-
duction on younger seedlings.
     Because of the experimental difficulties involved, little work has been
done
on the harmful effects of the rhizobia. Wunschik's, 1925, work on the effects
of repeated passage of the rhizobia through the host plant convinced him
that the
"vegetative energy" of the rhizobia is so enhanced by the process
that the benefit
derived by the plant is lessened. By a comparison of the activities of effective
and ineffective strains of the rhizobia under varying conditions, Allen and
Bald-
win, 1931a, and Dunham and Baldwin, 1931, have concluded that there are
at least two phases to the action of the bacteria within the host plant;
that is, con-
tribution to the process of nitrogen fixation and effect upon the growth
of the
plant. This effect may be either stimulatory or inhibitory to growth, and
from
the evidence in hand the latter seems more probable.
     It is interesting to note that several of the earlier writers observed
a less
perfect accord between the bean plant (Phaseolus) and its associated species
of rhizobia. Frank, 1890b, and Schneider, 1892, claimed that the bean bacteria
are purely parasitic. Dawson, 1900b, and Dangeard, 1926, have also observed
that the association seems to be different from that of other leguminous
plants and
their rhizobia. A recent cytological study (McCoy, 1929) suggests that the
cause
of the failure of the bean bacteria to aid their host is a reflection of
their malnutri-
tion resulting from the abnormally large deposit of starch in the bean nodule.
A
deficiency of available carbohydrate then reacts to hold in check the synthetic
activities of the bacteria and may account for the functional deficiency
of the
nodules.
     In view of this supposed parasitic existence of the bean bacteria, it
is of
interest that in 1911 the Michigan Agricultural Experiment Station, 1914,
dis-
continued the issue of cultures for inoculation of bean crops because of
an
accumulation of unfavorable reports in previous years.  However, the culture
was again distributed the following year. Recently there have been two reports
of definite utilization of atmospheric nitrogen and enhanced plant growth
on the
part of nodulated bean plants (Wilson and Leland, 1929; and Sears and Clark,
1930). It seems possible that the effective or ineffective state of the particular
bacteria used may account for the success or failure of the nodulation. It
is
conceivable that a large proportion of the strains of Rh. phaseoli exist
in the
ineffective state, whereas the reverse is true of most rhizobia.
     It is at least possible that rhizobia and leguminous plants may enter
into
a partnership without actual nodule formation.' Such a case has been reported
by
Friesner, 1926, and Feher and Bokor, 1926, who reported that the roots of
Gleditsia triacanthos bear peculiar cylindrical swellings caused by an organism
which is probably a species of Rhizobium. The plant appears to benefit from
the association, as other Leguminosae benefit from the rhizobia in their
true
nodules. Earp-Thomas, 1907; Hiltuer, 1907; and Joshi, 1920, have reported
   1The observations of Leonard and Reed, 1930, on Cassia and of Gutschy,
1931, on a soybean,
growing with a non-nodule-forming strain of rbizobia, are suggestive of tbis.
168


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