We conducted several experiments on linuron-resistant and -susceptible
Portulaca oleraceaand on atrazine-resistant and -susceptible Chenopodium albumto determine their immediate and long-term responses to photosynthesis-inhibiting herbicides. Several photosynthesis-inhibiting herbicides were used, and O2evolution was measured with a Clark-type O2electrode. Resistance ratios (RRs) for P. oleracea, based on O2evolution inhibition, were 8 and > 6 for linuron and diuron, respectively; > 800 for atrazine; and > 20 for terbacil. Linuron-resistant P. oleraceawas negatively cross-resistant to bentazon and pyridate (RR = 0.5 and 0.75, respectively). Time-course measurements of fresh weight, photosynthetic CO2assimilation, and photochemical efficiency indicated that linuron and atrazine inhibited electron transport in susceptible (S) P. oleraceaand C. album, ultimately resulting in death. Measurements of photochemical efficiency and CO2assimilation of linuron-resistant P. oleraceatreated with linuron indicated a transient injury from which plants recovered within 14 d. Recovery of linuron-resistant P. oleraceafrom atrazine injury was more rapid than from linuron injury for all measured variables. Atrazine-resistant C. albumhad no cross-resistance to linuron. Sequence analysis of the D1 protein revealed that linuron-resistant P. oleraceahad a serine-to-threonine substitution at position 264.