Weed suppression in green pea was evaluated when green pea was planted after a fall planting of rapeseed, white mustard, rye, or wheat had been incorporated into the soil in spring. Tests were conducted at Mount Vernon, WA, in 1994 and 1995. Weed suppression in peas varied between different green manure crops. One month after planting, the highest weed population was in green pea following wheat, whereas the lowest was in green pea following rapeseed. Rye and white mustard suppressed early weeds relative to wheat by 25 and 30%, respectively. However, at harvest, weed density was similar in green pea planted after all green manure crops. Weed suppression improved when cultivation or metribuzin at 0.14 kg ha−1was used in combination with white mustard, rapeseed, or rye. Pea population was not affected by green manure crops, except for rapeseed, which reduced pea population. In greenhouse experiments, white mustard added to the soil at 20 g per 400 g air dry soil reduced emergence of shepherd's-purse, kochia, and green foxtail by 97, 54, and 49%, respectively. Rapeseed suppressed emergence of shepherd's-purse, kochia, and green foxtail by 76, 25, and 25%, respectively.

Fall-planted rapeseed and sudangrass were evaluated for weed control in potato during a two-year study. Rapeseed incorporated in the spring in a loamy sand soil reduced weed density 85 and 73% in 1992 and 1993, respectively, and reduced weed biomass 96 and 50% in 1992 and 1993, respectively, in following potato crops compared to potato after fallow. Potato following rapeseed yielded 25% and 17% more total tuber weight than potato following sudangrass in 1992 and fallow in 1993, respectively. In greenhouse trials, rapeseed tissue added to a loamy sand soil at 20 g fresh weight per 400 g dry soil reduced biomass of hairy nightshade and longspine sandbur by 90 and 83%, respectively. Similarly, white mustard tissue added at 20 g fresh weight per 400 g dry soil reduced biomass of hairy nightshade and green foxtail by 83 and 70%, respectively.

Peppermint (Mentha × piperita L.) was transformed with various gene constructs to evaluate the utility of metabolic engineering for improving essential oil yield and composition. Oil yield increases were achieved by overexpressing genes involved in the supply of precursors through the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway. Two-gene combinations to enhance both oil yield and composition in a single transgenic line were assessed as well. The most promising results were obtained by transforming plants expressing an antisense version of (+)-menthofuran synthase, which is critical for adjusting the levels of specific undesirable oil constituents, with a construct for the overexpression of the MEP pathway gene 1-deoxy-D-xylulose 5-phosphate reductoisomerase (up to 61% oil yield increase over wild-type controls with low levels of the undesirable side-product (+)-menthofuran and its intermediate (+)-pulegone). Elite transgenic lines were advanced to multiyear field trials, which demonstrated consistent oil yield increases of up to 78% over wild-type controls and desirable effects on oil composition under commercial growth conditions. The transgenic expression of a gene encoding (+)-limonene synthase was used to accumulate elevated levels of (+)-limonene, which allows oil derived from transgenic plants to be recognized during the processing of commercial formulations containing peppermint oil. Our study illustrates the utility of metabolic engineering for the sustainable agricultural production of high quality essential oils at a competitive cost.