INHERITANCE OF GLYPHOSATE RESISTANCE IN RIGID RYEGRASS (Lolium rigidum) FROM CALIFORNIA

Glyphosate resistance was found in a rigid ryegrass population in northern California.
A sample of the resistant plants were collected and grown under greenhouse
conditions. The objective of this study was to evaluate glyphosate resistance in the
progeny of the collected plants by recurrent selection, obtain the homozygous resistant
and sensitive lines to establish dose-response curves, and to determine the inheritance
of glyphosate resistance in rigid ryegrass. Diverse levels of resistance were
observed in the first generation with survival of 89, 59, 45, and 9% from glyphosate
at 1x, 2x, 4x, and 8x respectively, where x 5 1.12 kg ha21 isopropylamine salt of
glyphosate. Clones of plants that died from 1x were allowed to produce seed and
were further subjected to recurrent selection to generate the most sensitive plants (S
lines), which died from 0.125x glyphosate. The most resistant plants (R lines) were
generated from the survivors receiving 8x glyphosate. The ratio between I50 rates for
the glyphosate resistant and the glyphosate sensitive plants was . 100-fold. The R
and S lines were crossed reciprocally and F1 progeny of both (R 3 S) and (S 3 R)
showed intermediate resistance. These survived up to 2x glyphosate. The F2 progeny
were generated by intercrossing of F1 plants. The ratio of sensitive, intermediate,
and resistant plants in the F2 population before the treatment of glyphosate at 0.125x
followed by 8x was 1 : 16, 14 : 16, and 1 : 16 respectively, which corresponded to
the Mendelian segregation ratio of two genes. The results indicated that the inheritance
of glyphosate resistance in rigid ryegrass from California appeared to be nuclear,
incompletely dominant, multigenic, and pollen-transmitted with no indication
of maternal inheritance.