Introduction
Glyphosate was discovered by J.E. Franz in Monsanto Corp. in 1971 and released
commercially in 1974. In the early 1980's it became the first individual pesticide
with world-wide sales of over $1 billion.
Glyphosate (Roundup) is a non-selective herbicide used primarily in Iowa for perennial
weed control and in no-till grain production systems. Beginning in the growing season of
1997 its importance was dramatically increased with the introduction of glyphosate
resistant soybean cultivars (RoundupReady soybeans). This importance was again increased
in 1998 with the introduction of glyphosate resistant corn. Roundup Ready crops of
many types have subsequently been commercialized.
Due to its behavior in soils, it is used strictly as a postemergence herbicide.
Chemistry
Structurally, glyphosate resembles that of an amino acid. Specifically, it is reminiscent of phosphorylated glycine. Glyphosate (Phosphono-methyl-glycine):
H2PO3-CH2-NH-CH2-C=O-OH
Physiology and Metabolism of Glyphosate
Mode of Glyphosate Action
Glyphosate inhibits the shikimate pathway enzyme EPSPSase, and enzyme that acts late in
that pathway. The pathway is responsible for, among other things, the biosynthesis of
aromatic amino acids: phenylalanine, tyrosine and tryptophane. This pathway is also
responsible for biosynthesis of such diverse plant compounds as phytoalexins,
plastoquinone, alkaloids, cinnamate, coumarin and flavonoids
Mode of Glyphosate Lethality
Glyphosate rapidly moves to apical areas of the plant and inhibits protein synthesis.
Cessation of growth happens almost immediately after the herbicide reaches the apical
areas. Plants stop growing and many plant tissues and parts slowly degrade due to lack of
proteins. Symptomology on plants usually develops very slowly, with gradually increasing
chlorosis, yellowing, and necrosis. Death ultimately results from dehydration and
desiccation.
Uptake and Movement of Glyphosate in plants
Glyphosate is readily absorbed by foliage and other shoot portions of plants. Once
absorbed into the plant, glyphosate readily and extensively translocates symplastically.
Once in the phloem, it generally follows the source-sink photosynthate movement pattern in
the plant and accumulates in areas of active growth (meristems).
Basis of Selectivity between Plant Species
Glyphosate is a non-selective herbicide and is not metabolized, detoxified, in plants.
Some selectivity in crop production has been achieved with placement selectivity by
the use of specialized application equipment:
Selectivity has also been gained by appling glyphosate to dormant conifers in forestry production. Selectivity in that case is due to lack of herbicide uptake and translocation.
Resistance: Weeds
Because glyphosate is non-selective, Monsanto Corp. has insisted that glyphosate
resistant weeds will not appear with the use of this herbicide. With the release of
glyphosate resistant corn and soybeans, this hypothesis will soon be tested on millions of
acres. Resistant weed populations have been reported already in locations outside the USA.
Glyphosate resistance has been found in field bindweed (Convolvulvus arvensis ) by
Steve Weller at Purdue University. In that research, variable levels of resistance to
glyphosate was found in several biotypes at the whole plant and cell culture level.
Ongoing studies have indicated the mechanism of resistance is gene amplification: greater
copy numbers of the gene coding for EPSPSase. The result of this is greater EPSPSase
activity in the presence of glyphosate, enough for both inhibition and continued enzymatic
activity.
Resistance: Crops
This same resistance mechanism of gene amplification has been exploited in the development
of glyphosate resistant crops. Glyphosate resistant crops have been aggressively sought
due to this herbicides desirable environmental qualities (low toxicity, rapid
unavailability in the soil) and excellent weed control properties
Fate of Glyphosate in the Environment
Soil
Glyphosate is strongly adsorbed to soil particles, the colloidal fraction upon first
contact with the soil. Once it is strongly adsorbed, it is unavailable for plant uptake.
As such, seeding and other activities bringing plants in contact with glyphosate do not
cause injury to those plants. Some isolated instances of glyphosate injuring adjacent
plants subsequent to application are believed to be caused by root exudation directly to
neighbor plants.
Microbial breakdown is the primary decomposition mechanism of glyphosate in the soil,
although chemical hydrolysis, oxidation and photodegradation also play a role. These
long-term degradative process are not apparent in the field in terms of bioavailability
due to its strong adsorption to soil particles.
Water
Glyphosate is a polar, water soluable, molecule (solubility 12 PPT[housand] in water at
25oC). It may reach surface waters by movement with soil erosion, but is unlikely to be a
ground water contaminate due to its strong soil adsorptive qualities.
Air
Glyphosate vapor pressure is negligible, but movement to off-target plantss due to drift
often occurs. This potential for drift injury may be a major problem with the introduction
of glyphosate resistant crops and high use of glyphosate in them.
Animal Toxicity
Glyphosate has a very low acute toxicity.
Plant Injury Symptomology of Glyphosate in Plants
Treated plants
stop growing very rapidly, but symptom development after that often occurs very slowly.
Chlorosis on leaves occurs gradually, often in a mottled or interveinal pattern (below).
Stunting
(left) becomes more obvious as uninjured adjacent plants continue to grow, but glyphosate
injured plants still appear somewhat normal, despite cessation of growth.
These symptoms usually are evident on new growth, meristem areas (below).
Necrosis and
tissue destruction follow more slowly after cessation of growth and slowly increasing
chlorosis. These symptoms develop gradually (1-4 weeks) in perennial plants, but in small
annual plants they can occur very rapidly (days), depending on rate of herbicide and
environmental conditions
Underground perennial plant parts (rhizome buds, rootstock buds) become necrotic, brown (below).
Death
ultimately results from dehydration and desiccation.
