A serine-to-threonine mutation in linuron-resistant<i>Portulaca oleracea</i>

Abstract

We conducted several experiments on linuron-resistant and -susceptible Portulaca oleracea and on atrazine-resistant and -susceptible Chenopodium album to determine their immediate and long-term responses to photosynthesis-inhibiting herbicides. Several photosynthesis-inhibiting herbicides were used, and O 2 evolution was measured with a Clark-type O 2 electrode. Resistance ratios (RRs) for P. oleracea , based on O 2 evolution inhibition, were 8 and &gt; 6 for linuron and diuron, respectively; &gt; 800 for atrazine; and &gt; 20 for terbacil. Linuron-resistant P. oleracea was negatively cross-resistant to bentazon and pyridate (RR = 0.5 and 0.75, respectively). Time-course measurements of fresh weight, photosynthetic CO 2 assimilation, and photochemical efficiency indicated that linuron and atrazine inhibited electron transport in susceptible (S) P. oleracea and C. album , ultimately resulting in death. Measurements of photochemical efficiency and CO 2 assimilation of linuron-resistant P. oleracea treated with linuron indicated a transient injury from which plants recovered within 14 d. Recovery of linuron-resistant P. oleracea from atrazine injury was more rapid than from linuron injury for all measured variables. Atrazine-resistant C. album had no cross-resistance to linuron. Sequence analysis of the D1 protein revealed that linuron-resistant P. oleracea had a serine-to-threonine substitution at position 264.