POTENTIAL BASIS OF GLYPHOSATE RESISTANCE IN CALIFORNIA RIGID RYEGRASS (Lolium rigidum)

Glyphosate-resistant rigid ryegrass has been identified in California, but research has
yet to elucidate the resistance mechanism. The objectives of this study were to
examine the differences between sensitive and resistant rigid ryegrass in absorption
and distribution of glyphosate, in vivo and in vitro absorption by chloroplasts, and
shikimic acid accumulation after glyphosate treatment. Foliar absorption and distribution
of 14C-glyphosate did not differ 1 to 3 d after treatment (DAT) between the
susceptible (S) and resistant (R) biotypes. Absorption of 14C-glyphosate by isolated
chloroplasts also did not differ between the S and R biotypes. After foliar application
of 14C-glyphosate, chloroplasts were isolated from treated leaves from both biotypes.
Accumulation of 14C-glyphosate in the chloroplasts did not differ between the two
biotypes. Shikimic acid level increased significantly in the S biotype after treatment
with glyphosate at 2.24 kg ai ha21 to levels 10-fold greater than in the R biotype
11 DAT. Shikimic acid in the germination media at 2 to 5 mM did not affect seed
germination of S and R biotypes but drastically decreased the length of coleoptiles
of both at 5 DAT. Thus, biotype differences in sensitivity or metabolism of shikimic
acid do not explain differences in sensitivity to glyphosate.