CELLULAR MECHANISMS INFLUENCE DIFFERENTIAL GLYPHOSATE SENSITIVITY IN-FIELD BINDWEED (CONVOLVULUS-ARVENSIS) BIOTYPES

Biotypes of field bindweed that vary in sensitivity to glyphosate were studied to determine the physiological or biochemical bases for these differences. Studies conducted using whole plants and in vitro cultur ed shoots identified several potentially important differences between the most tolerant (biotype 4) and most susceptible (biotype 1) bio ty pes. Bio type 4 plants had greater 3-deoxy-D-arabino-heptulosonate-7-p hosphate synthase (DAHPS) activity and higher concentrations of phenol ic compounds, indicating greater shikimate pathway activity than bioty pe 1. This may reflect a higher growth ability of biotype 4, as observ ed in shoot cultures. 5-Enolpyruvylshikimate-3-phosphate synthase (EPS PS) activity in whole plants increased in all parts of biotype 4 by 7 d after a glyphosate treatment: of 1.1 kg ae ha(-1), whereas activitie s in biotype 1 plants did not increase at any time. However, this may not be the only mechanism of glyphosate tolerance because EPSPS activi ty in cultured shoots of both biotypes increased equally in response t o glyphosate, even though biotype 4 shoots were able to survive and gr ow on a glyphosate-containing medium that inhibited growth of biotype 1 shoots. We propose that multiple mechanisms operating at a cellular/ metabolic level combine to enable biotype 4 to tolerate higher glyphos ate rates than biotype 1.