TITLE:
Susceptible and Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Response to Glyphosate Using C14 as a Tracer: Retention, Uptake, and Translocation
AUTHORS:
Timothy L. Grey, Donn Shilling
KEYWORDS:
Amaranthus palmeri S. Wats, Absorption, Becquerel’s, Glyphosate-Resistance, Herbicide Resistance, Translocation
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.9 No.12,
November
9,
2018
ABSTRACT: The foliar retention, absorption, translocation, and
diffusion of glyphosate in glyphosate resistant-(R) and susceptible (S)-Palmer
amaranth populations from seed collected in Georgia in 2007 were examined. The
R population of Palmer amaranth had an elevated copy number of the EPSPS gene
conferring the mechanism of resistance. When applications of 14C-glyphosate to a single leaf
followed entire plant treatment with glyphosate, the distribution percentages
were similar for R and S for the above and below treated leaves when harvested
at 1, 6, 12, 24, and 48 hours after treatment (HAT). There were initially no differences
between R and S at 1 HAT with an average of
8% absorption for both biotypes. However, data indicated that glyphosate
absorption increased for R-Palmer amaranth reaching 41% within 6 HAT and was
significantly different (P = 0.01) from the 28% absorbed by S-Palmer amaranth.
Glyphosate resistant and susceptible Palmer amaranth averaged 44% 14C-glyphosate
absorption by 24 HAT. There were no differences for 14C-glyphosate
Bq/mg of plant tissue between R and S for the above the treated leaf and below
the treated leaf portions of plants at 1, 6, 12, 24, or 48 HAT. However, root
accumulation of 14C-glyphosate in plant tissue was significantly
greater by 12 HAT for the roots of R (1.21 Bq/mg) than for S (0.51 Bq/mg). The
treated leaf of the R-Palmer amaranth plants exhibited greater translocation of 14C-glyphosate in Bq/mg of tissue than the susceptible over time,
indicating no detrimental effect or cost of fitness due to EPSPS gene
amplification. Additionally, there were no differences in glyphosate retention
in leaf discs assays between R and S biotypes. In spite of an average of 6.5 Bq
efflux out of R and S leaf discs after 15 minute, only 0.4 Bq was retained
after 150 minutes. Glyphosate was not retained over time in the leaf discs for
R and S, and there were no biotype differences within bathing times. However,
the rate of efflux (the slope of the curves) was greater for the R biotype.
These data support the reported gene amplification non-target site glyphosate
resistance mechanism in Palmer amaranth.