An imidazolinone-tolerant wheat (Triticum aestivum L. em Thell) mutant in the winter wheat cultivar Fidel has been identified and characterized. The mutant was isolated from a population derived through seed mutagenesis of the variety with an aqueous solution containing sodium azide. Imidazolinone-tolerant wheat seedlings were selected from the M2 generation of the population in the presence of imazethapyr herbicide and identified as herbicideinsensitive individuals. The trait is inherited as a single semidominant gene and confers high levels of tolerance to imazethapyr. Acetohydroxyacid synthase activity in extracts from imidazolinonetolerant plants was less inhibited by imazethapyr than the enzyme from the wild type. The herbicide-tolerant plants have a completely normal phenotype and display no negative effects on growth and yield in either the absence or presence of imazethapyr.find imidazolinone-tolerant wheat (Triticum aestivum L. em Thell) through seed mutagenesis and selection with an M2 seedling screen. MATERIALS AND METHODS Seed MutagenesisFive thousand seeds of the French winter wheat (Triticum aestivum L. em Thell cv Fidel) were mutagenized by the procedure of Kueh and Bright (9). Wheat seeds were soaked in water for 18 h at 50C. Air was then bubbled through the seeds for 6 h at 200C. Following imbibition, seeds were soaked in 1 mm sodium azide (pH 3) for 2 h. The seeds were rinsed for 30 min, dried on paper towels, and planted in the field. The plants arising from these seeds were Ml generation plants. M2 seeds were harvested from these plants at maturity.The imidazolinone class of herbicides has been demonstrated to have a broad spectrum of weed control activity, flexibility in timing of application, low usage rates, and low mammalian toxicity. These herbicides inhibit the enzymic activity of AHAS2 (EC 4.1.3.18) (15), the first enzyme in the pathway for the synthesis of the branched chain amino acids valine, leucine, and isoleucine. This same enzyme has been shown to be the site of action for the sulfonylurea and triazolopyrimidine herbicides (4,19). The basis for crop selectivity of the imidazolinones has been shown to be the result of a difference in the nature or rate of metabolism of the herbicide (2,3,16 882Screening Procedure M2 seeds were surface disinfected in 70% ethanol for 30 s. This was followed by a disinfection in a solution of 2.6% sodium hypochlorite for 30 min under vacuum with gentle agitation. Two drops of the nonionic surfactant Tween 20 were used per 100 mL of the sodium hypochlorite solution to aid the disinfestation procedure. Subsequently, seeds were rinsed three times in sterile distilled water and placed into sterile plastic 10-x 1.5-cm Petri dishes at 250 seeds/dish. Twenty-five milliliters of 1 M imazethapyr was placed in each dish. After 3 d in the imazethapyr solution, the seeds were drained and blotted dry. The seeds were planted at a 1-to 2-cm depth in 15-x 20-cm peat flats containing Metromix 350 (W.R. Grace Co.) at the rate of 1000 seeds/flat. These flats...

Three corn (Zea mays L.) lines resistant to imidazolinone herbicides were developed by in vitro selection and plant regeneration. For all three lines, resistance is inherited as a single semidominant allele. The resistance alleles from resistant lines XA17, XI12, and QJ22 have been crossed into the inbred line B73, and in each case homozygotes are tolerant of commercial use rates of imidazolinone herbicides. All resistant selections have herbicide-resistant forms of acetohydroxyacid synthase (AHAS), the known site of action of imidazolinone herbicides. The herbicide-resistant phenotypes displayed at the whole plant level correlate directly with herbicide insensitivity of the AHAS activities of the selections. The AHAS activities from all three selections have normal feedback regulation by valine and leucine, and plants containing the mutations display a normal phenotype.

The use of conservation tillage for commercial production of maize (Zea mays L.) has expanded substantially in the past 10 yr. Few previous studies have compared the relative performance of a large number of hybrids in no-tillage and conventional tillage cropping systems. In this study, 60 commercial hybrids were grown at two locations in Iowa in 1981 and 1982 to examine the interaction of commercial maize hybrids with no-tillage and conventional tillage systems. Hybrid X tillage interactions were not significant for any of the seven traits examined in a combined analysis of variance over tillage systems. Hybrid X tillage X environment interactions were significant for percentage emergence and mature plant height. Rank correlations of hybrid means from one tillage system to the other were significant and large, ranging from 0.66 to 0.94 for the agronomic traits measured, which was consistent with the lack of significant hybrid X tillage interactions. No specific traits were of more importance for performance in no-till than in conventional tillage. Also, no differential hybrid response to tillage systems, which was repeatable across environments, was observed. The study suggested that evaluation of performance in conventional tillage environments provided sufficient information to select among contemporary hybrids for use in conservation tillage environments.

Hybrids presently being used for commercial production of maize (Zea mays L.) were developed and evaluated primarily in fields tilled with moldboard plows. In contrast, 37% of the commercial maize hectar age in the USA in 1984 was planted using some type of reduced tillage practice. The effects of using genotypes selected with conventional tillage in reduced tillage field environments are not known. This study evaluated 100 S1 lines from each of two different maize synthetics to examine the consequences of using different tillage systems for field evaluations. The lines were grown in five environments, over a 2‐yr period using no‐till and conventional tillage. The S1 lines from the BS22(R)C1 synthetic had few significant interactions of genotypes with tillage systems and expressed levels of genotypic variance in no‐till equal to or greater than the levels expressed in conventional tillage. The S1 lines from the BS13(SCT)C6 synthetic had significant interactions between genotypes and tillage systems for six of the eight traits reported, and expressed similar levels of genetic variance in each tillage system. Correlations between traits were similar in both tillage systems. Selection efficiency was better in no‐till than in conventional tillage for the BS22(R)C1 synthetic. No obstacles to the use of no‐till systems for evaluation and selection of maize genotypes were observed.

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