A biotype of Amaranthus retro¯exus L. is the ®rst weed in Israel to develop resistance to acetolactate synthase (ALS)-inhibiting herbicides. The resistant biotype (Su-R) was collected from Ganot, a site that had been treated for more than 3 consecutive years with sulfometuronmethyl + simazine. On the whole-plant basis, the resistance ratio (ED 50 Su-R)/(ED 50 Su-S) was 6±127 for sulfonylureas, 4±63 for imidazolinones, 20±35 for triazolopyrimidines and 11 for pyrithiobac-sodium. Similar levels of resistance were found also when the herbicides were applied before emergence. Based on a root elongation bioassay, Su-R was 3240-fold more resistant to sulfometuron-methyl than Su-S. In vitro studies have shown that the Su-R biotype was resistant at the enzyme level to all ALS inhibitors tested. The nucleotide sequences of two ampli®ed regions between the Su-S and the Su-R diered in only one nucleotide. One substitution has occurred in domain A, cytosine by thymine (CCC to CTC) at position 248, that confers an exchange of the amino acid proline in the susceptible to leucine in the Su-R 1. The proline to leucine change in domain A is the only dierence in the amino acid primary structure of the regions sequenced, indicating that it is responsible for the ALS-inhibitor resistance observed.

Phalaris paradoxa (awned canary-grass) is an aggressive annual winter weed in wheat and other arable crops that is controlled mainly by ACCase-inhibiting herbicides: cyclohexanediones (DIMs), aryloxyphenoxypropionates (FOPs) and phenylpyrazolines (DENs, e.g. pinoxaden). The selection pressure imposed on the weed populations by repeated use of these herbicides has resulted in the evolution of increased numbers of ACCase-resistant populations of P. paradoxa in Israel and other countries. Two populations, Revadim (RV) and Mishmar HaÕemek (MH) that were exposed to differing weed and crop management tactics were investigated. Both populations were highly resistant to all FOPs, pinoxaden and cycloxydim, but responded differently to some DIMs. RV plants exhibited much higher resistance to tralkoxydim than MH plants, while showing similar low levels of resistance to tepraloxydim and clethodim. Both populations were equally susceptible to graminicides with other modes of action. The mutations responsible for the observed resistance were identified using PCR-RFLP and by sequencing the carboxyl transferase domain of the chloroplastic ACCase gene. RV plants possess a substitution of Asp 2078 to Gly, whereas in MH population a mixture of Ile 2041 to Asn or Asp 2078 to Gly was found. Our study demonstrates that lack of herbicide and crop rotation may result in the evolution of diverse target site mutations and differential response of the whole plant to ACCase inhibitors.

Field studies were conducted to evaluate the ecological fitness of Amaranthus spp. biotypes that evolved resistance to either acetolactate synthase (ALS) inhibitors (A. retroflexus, SuR), to triazine herbicides (A. blitoides, SuS ⁄ TR), or to both (A. blitoides, SuR ⁄ TR), and estimate their ecological fitness under competitive conditions. The plants were grown in monoculture and in replacement series experiments. The examined mixtures were 100%S, 75%S ⁄ 25%R, 50%S ⁄ 50%R, 25%S ⁄ 75%R and 100%R, at a constant stand of 400 plants m )2 . The SuR and SuS A. retroflexus biotypes attained similar shoot dry biomass per plant, biomass per plot and relative yield total (RYT) ¼ 1. In monoculture, the final shoot biomass of A. blitoides biotypes SuS ⁄ TS plants was higher than that of SuR ⁄ TR and SuS ⁄ TR. A negative effect of association was observed, amensalism, when SuS ⁄ TS was grown in mixture with SuR ⁄ TR, in favour of the wild type. However, SuR ⁄ TR and SuS ⁄ TR biomass was not influenced by the presence of the competitor. These data support the hypothesis that the ALS-resistance trait in A. retroflexus and A. blitoides is not associated with growth penalty and did not incur ecological cost in the field. We suggest that the cause of the observed reduction in growth rendering the SuS ⁄ TR and SuR ⁄ TR less fit than the wild type is due to the triazine resistance, and may facilitate their dissipation.

Amaranthus blitoides S. Watson (prostrate pigweed) populations resistant to acetolactate synthase (ALS; EC 4.1.3.18)-inhibiting herbicides and triazines (SuR/TR) were found in Israel. The Ganot population was 6-to 790-fold more resistant to ALS inhibitors than the wild type due to an altered target site. Molecular analyses showed that the Ganot population was a mixture of two biotypes: (i) SuRA/TR in which domain A of the als gene differed in one nucleotide, resulting in substitution of Pro by Ser 188; (ii) SuRB/TR in which a mutation in domain B led to a substitution of Trp by Leu 569. The mutation in domain A resulted in resistance to all ALS inhibitors except imidazolinones, whereas the mutation in domain B led to resistance to all ALS inhibitors tested. SuRA/TR and SuRB/TR are multiple-resistant with an additional single mutation in the plastidic psbA gene that changes Ser 264 to Gly in the D1 protein, leading to triazine resistance. It is evident that plants within a population exposed to a similar selection pressure may show different patterns of crossresistance due to three different point mutations. This unique phenomenon renders planning of rational weed management difficult or even impossible.

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