Environment Affects Cotton and Velvetleaf Response to Pyrithiobac

Growth chamber experiments evaluated the influence of ambient temperature and soil moisture on cotton and velvetleaf response to pyrithiobac. Additional studies determined the basis for observed plant responses to14C pyrithiobac. Cotton injury from six times the normal dosage was < 20% at 2 wk for all temperatures and soil moistures. Pyrithiobac injured velvetleaf less at lower soil moistures. Both species absorbed more14C-pyrithiobac at 30/28 or 35/33 C than at 25/23 C. Cotton absorbed more herbicide than velvetleaf at all temperatures and soil moistures. Velvetleaf translocated < 16% of absorbed14C out of the treated leaf while cotton translocated < 3% of absorbed material. At warmer temperatures, velvetleaf translocated less14C when soil was dry (–1.0 MPa) than when plants were watered to field capacity (–0.03 MPa). This decreased absorption and translocation may affect pyrithiobac activity on velvetleaf growing in dry soil. Translocation differences did not fully explain whole plant effects. The metabolism difference may account for cotton tolerance.