Effects of humidity and moisture stress on glyphosate control of Cyperus rotundus L.*

Glyphosate at 2 kg/ha was more effective in reducing regrowth of purple nutsedge (Cyperus rotundus L.) scapes at 90% than at 50% relative humidity (r.h.), and more effective at ?2 bars than at ?11 bars of plant water potential. Regrowth of treated plants subjected to water potentials of ?1 to ?8 bars was reduced 54–60% while at ?11 bars growth inhibition was only 34%. A time interval of as little as 8 h between application and excision was sufficient to give 47% reduction in regrowth at 90% r.h. None of the treated plants, except those clipped immediately after application, produced new shoots from the basal bulb, while all the untreated control plants produced one or more new shoots. Experiments using ¹⁴C-glyphosate substantiated these results. Three times more ¹⁴C-label was translocated into the underground parts of nutsedge at 90% than at 50% r.h. Twice as much translocated at ?2 bars than at ?11 bars of water potential.