Mechanism of Glyphosate Tolerance in Birdsfoot Trefoil (Lotus corniculatus)

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Abstract
The mechanism of glyphosate tolerance was investigated in nine birdsfoot trefoil selections that exhibited a threefold difference in glyphosate tolerance. Single-stemmed ramets (vegetative clones) established from the nine selections were used to evaluate tolerance to glyphosate, spray retention, and 14C-glyphosate absorption and translocation in growth chamber experiments. The tolerance of greenhouse-grown ramets correlated with the tolerance of field-grown plants, indicating that tolerance was not a function of plant size or affected by environment. The nine selections differed in spray retention and 14C-glyphosate translocation but not in glyphosate absorption. The differences in retention and translocation were not correlated with the level of glyphosate tolerance but could contribute to the tolerance of an individual plant. The specific activity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) (EC 2.5.1.19) ranged from 1.3 to 3.5 nmol min−1 mg−1 protein among the nine selections assayed and was positively correlated with plant tolerance level. These results indicate that the primary mechanism of glyphosate tolerance in birdsfoot trefoil is based on the level of EPSPS activity.