2-Plants: Some more agroecological problems with RR soybeans?
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TITLE: Effects of glypohsate on Bradyrhizobium japonicum interactions in
SOURCE: Weed Science Society of America Abstracts 1999
Meeting of the Weed Science Society of America, Vol 39
by R. E. Hoagland et al., sent by NLP Wessex, UK
DATE: November 2000
------------------ archive: http://www.gene.ch/genet.html ------------------
Dear GENET-news readers,
the effect of glyphosate on soil microbes has been studied by several
authors because glyphosate Ð unlike most other herbicides Ð kills the
by blocking a biochemical pathway which is also essential for most of the
bacteria and fungi. Special attention has been paid to the symbotic
relationship between nitrogen-fixing rhizobia and legumes. The below cited
studies use laboratory and green house systems and once again illustrate
that discrepancies between MonsantoÕs and independent science has a long
Soon after Monsanto published its own experiments proving the harmlessness
of glyphosate on soil microbes in a 33-day-study (RUEPPEL et al. 1977),
ROSLYCKY (1982) stated in an independent publication: ÓConcluding, in
contrast to Rueppel et al. (1977) this work shows a profound influence of
glyphosate in concentrations of 1 to 1000 microgram on soil microbiota. The
discrepancies in our population, respiration and replication studies showed
that limited, short-term inquiries lead to confusing or misleading
EBERBACH und DOUGLAS published in 1983 that glyphosate could inhibit the
root growth of subterranean clover and Rhizobium trifolii. Contrary to the
expectations glyphosate had been deactivated very slowly in the sandy soils
of the experiment thus leading to damages even 120 days after its
A DECADE LATER MRTENSSON (1992) could prove negative effects of glyphosate
on the symbiosis between rhizobia and clover at concentrations below the
recommended field dosis. He underlined that the glyphosate does not
necessarily kills the microbes but influences their function. To
substantiate these effects more sophisticated methods have to be applied
than just measuring the microbial biomass.
Various reserach projects lead to a highly complex picture concering the
impact of glyphosate on the interactions between bacteria, fungi and plants
(LVESQUE & RAHE 1992). Scientists could show negative and positive effects
as well on plant pathogens as on beneficial microbes. The overall
conclusion is that the effects of glyphosate on soil microbes are poorly
understood and that there is no scientific clarity about most of the
These studies have been performed with soil planted with conventional
crops, below you will find a new work focussing on RR soy and the symbiosis
Eberbach, P.L.; L.A.Douglas. 1983. Persistence of glyphosate in a sandy
loam. Soil Biology and Biochemistry 15: 485-487.
Lvesque, C. A.; J. E. Rahe. 1992. Herbicide interactions with fungal root
pathogens, with special reference to glyphosate. Annual Review of
Phytopathology 30: 579-602.
Mrtensson, A.M.. 1992. Effects of agrochemicals and heavy metals on fast-
growing rhizobia and their symbiosis with small-seeded legumes. Soil
Biology and Biochemistry 24: 435-445.
Roslycky, E. B., 1982. Glyphosate and the response of the soil microbiota.
Soil Biology and Biochemistry 14: 87-92.
Rueppel, M. L.; B. B. Brightwell; J. Schaefer; J. T. Marvel. 1977.
Metabolism and degradation of glyphosate in soil and water. Agriculture and
Food Chemistry 25: 517-528.
COMMENT by NLP:
Very little work has been done on the impact of the introduction of
transgenic crops on soil micro-organism activity, a fundamental aspect of
soil fertility maintenance. The study below indicates that leghemoglobin
content in root nodules of Roundup Ready soy managed under a glyphosate
herbicide regime may fall. Leghemoglobin is associated with nitrogen-fixing
legumes and is believed to be an essential component for nitrogen fixation
by nodulated legumes. The role of leghemoglobin is to facilitate the
diffusion of O2 through the plant cell to nitrogen-fixing rhizobia (http://
www.pigment.unl.edu/dept/faculty1.htm). Biological nitrogen fixation is
essential for life in the biosphere. It is the microbial process in which
inert atmospheric dinitrogen is converted to ammonia, a form of nitrogen
that is used by biological systems. In legumes, nitrogen fixation occurs in
root nodules which are complex structures composed of plant cells
containing leghemoglobin and rhizobia. The study below also found that
reductions in nitrogen fixing root nodule mass and number in RR soy may
also occur in some circumstances under a glyphosate herbicide regime.
NATURAL LAW PARTY WESSEX email@example.com www.btinternet.com/~nlpwessex
A) Effects of glypohsate on Bradyrhizobium japonicum interactions in
R. E. Hoagland*, K. N. Reddy, and R. M. Zablotowicz
USDA-ARS, SWSRU, Stoneville, MS
Transgenic crops resistant to herbicides [e.g., glyphosate-tolerant
(Roundup-Ready, RR) soybeans], are a promising weed control strategy.
Because of a lack of published information on the B. japonicum - soybean
symbiosis in glyphosate-treated RR soybeans, we examined conventional (DP
3588) and RR soybeans (DP 5806 RR) for glyphosate effects on plant growth,
chlorophyll content, nodulation, and nodule leghemoglobin content, two
weeks after application in the greenhouse. In DP 3588, a single application
of 0.28 kg/ha two weeks after planting reduced chlorophyll content (49%)
and shoot and root dry weight (50 & 57%, respectively). In DP 5806 RR,
there were no significant effects on these parameters by single
applications up to 1.12 kg/ha, but 2.24 kg/ha reduced shoot and root dry
weight by 25 to 30%. Glyphosate at 0.28 kg/ha reduced nodule mass and
number by 75% and 33%, respectively, in DP 3588 compared to untreated
plants. Two tests assessed effects of 1.12 or 2.28 kg/ha glyphosate on DP
5806 RR applied two weeks after planting. A significant reduction in nodule
mass and number occurred in only 1 test, and only at 1.12 kg/ha.
Leghemoglobin content was reduced by glyphosate in both tests, but effects
were inconsistent with rate. Application of 1.12 kg/ha three weeks after
planting did not affect nodule number or mass, but 2.24 kg/ha reduced these
parameters 30 and 39%, respectively, compared to untreated. Leghemoglobin
content was reduced (10 to 18%) by both glyphosate rates. Subtle reductions
of nodulation in RR soybeans using label rates of glyphosate can occur, but
these effects may be of minimal consequence due to the potential of
soybeans to compensate for short durations of stress.
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