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2-Plants: Some more agroecological problems with RR soybeans?



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TITLE:  Effects of glypohsate on Bradyrhizobium japonicum interactions in
        Roundup-Ready soybeans
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 
plant 
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 
history.
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 
conclusions.Ó
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 
application.
A DECADE LATER MRTENSSON (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 
(LƒVESQUE & 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 
questions.
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 
with rhizobia.

Eberbach, P.L.; L.A.Douglas. 1983. Persistence of glyphosate in a sandy 
loam. Soil Biology and Biochemistry 15: 485-487.
LŽvesque, C. A.; J. E. Rahe. 1992. Herbicide interactions with fungal root 
pathogens, with special reference to glyphosate. Annual Review of 
Phytopathology 30: 579-602.
MŒrtensson, 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.

Yours,

Hartmut Meyer

                             *****


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 nlpwessex@bigfoot.com www.btinternet.com/~nlpwessex

                             *****


A) Effects of glypohsate on Bradyrhizobium japonicum interactions in 
Roundup-Ready soybeans

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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