Plants in the genera Astragalus and Oxytropis, collectively referred to as “locoweeds,” contain swainsonine, a toxic alkaloid synthesized by their fungal endophyte Alternaria sect. Undifilum. The ecological role of this endophyte across the mutualism–commensalism–parasitism continuum is unknown. We examined the fitness traits of Astragalus and Oxytropis species growing with and without the endophyte, in a 9-yr, common-garden experiment. Silky crazyweed (Oxytropis sericea Nutt.) and woolly loco (Astragalus mollissimus Torr.) plants germinated from seeds that naturally host the endophyte (E+) and with it mechanically removed (E−) were established in a common garden in southwest Montana. We measured mortality, gas exchange, flower and seed production, seed germination, and final biomass. Astragalus mollissimus plants grew as annuals under common-garden conditions regardless of endophyte status. Oxytropis sericea plants grew as perennials with survival unaffected by endophyte; however, E+ O. sericea plants produced slightly more reproductive stems, flowers per stem, and crown and stem biomass. Maternal effects detected in the parental generation disappeared in subsequent generations. Gas exchange, fecundity, and seed germination were unaffected by endophyte. Contrary to our initial hypothesis of mutualism, the endophyte did not improve host survival or fecundity, nor did we detect transgenerational effects. However, the endophyte did slightly increase the number of reproductive stems and flowers per stem and crown and stem mass in O. sericea, suggesting endophytic effects on carbohydrate biochemistry and pollination parameters should be examined. Lack of selection for or against endophyte-containing plants allows both nontoxic and toxic swainsonine-producing plants to persist in Astragalus and Oxytropis populations, posing a continued threat to grazing livestock.

Absorption, translocation, root release, and metabolism of imazethapyr by leafy spurge were determined under growth chamber conditions. 14C-imazethapyr was applied to vegetatively propagated leafy spurge plants in a 1% solution of 28% urea ammonium nitrate containing 0.25% by vol nonionic surfactant Plants were harvested 2 and 8 d after herbicide application. Imazethapyr absorption increased from 9% at 2d to 20% at 8d. Acropetal and basipetal translocation out of the treated leaf was observed, with 3.4 to 4.2% of the applied radioactivity accumulating in the root by the end of the 8-d time course. Eight days after herbicide application, radioactivity in dormant and elongated adventitious shoot buds was twofold higher than in root tissue (compared on a dry wt basis). Two days after herbicide application, 93% of the radioactivity remained as intact imazethapyr in the treated leaf, crown, root, and shoot buds. Eight days after application, crown, roots, and adventitious shoot buds had metabolized an average of 61, 36, and 47% of the imazethapyr, respectively, while only 14% was metabolized in the treated leaf. The primary metabolite cochromatographed with 5-hydroxyethyl-imazethapyr standard.

Absorption, translocation, and metabolism of AC 263,222 by leafy spurge were studied over 8 d. Based on the amount of herbicide applied and recovered from the leaf surfaces, 40% of applied AC 263,222 was absorbed by leafy spurge 2 d after treatment (DAT), with no further absorption observed by 8 DAT. Eight DAT, 19% of applied [14C]-AC 263,222 had translocated to below-ground plant parts while 4% was exuded from the roots into the sand media. AC 263,222 was not metabolized 2 DAT in the crown, root, and root buds, but 42% was metabolized in the treated leaves. Only 17% of recovered [14C] was AC 263,222 in treated leaves 8 DAT (83% metabolized), while AC 263,222 accounted for 70% of recovered [14C] in the root and root buds. HPLC analysis indicated that the balance of [14C] was associated with a single, polar metabolite. Total recovery of [14C] was 88% at 8 DAT.

African rue is an exotic, herbaceous perennial established in several western states that tolerates harsh, water-stressed conditions. The influence of water-deficit stress on herbicide response and subsequent herbicide fate within the plant were compared. African rue seedlings were deprived of water for 0 to 7 d to establish a gradient of water-deficit levels before treatment with hexazinone, imazapyr, or metsulfuron. At herbicide application, water-deficit treatments reduced plant water potential values from −1.0 MPa to −4.7 MPa, causing concomitant reductions in photosynthesis. Thirty-five days after treatment, dry weight of imazapyr- and metsulfuron-treated plants was reduced in plants exposed to more than 4 d water-deficit stress before herbicide application. In contrast, hexazinone-treated plants had less dry weight than water-stressed, nonsprayed control plants regardless of water-deficit stress. Seventy-two hours after herbicide application, African rue leaves absorbed from 5 to 42% of herbicide applied; however, herbicide absorption did not correlate to efficacy. Less than 12% of absorbed herbicide translocated out of the treated leaf, regardless of herbicide. Radiolabel translocated from the treated leaf to acropetal or root tissue did not differ among herbicide treatments, regardless of water deficit before herbicide application. However, compared to other herbicides, translocation to basipetal shoot tissue was greatest in imazapyr-treated seedlings with the largest water deficit at herbicide application. Increased translocation occurred at higher levels of water stress than were necessary to increase herbicide efficacy, suggesting differential translocation was not involved in enhanced efficacy. In summary, African rue seedlings absorbed and mobilized three different herbicides at all levels of water-deficit stress. In addition, the efficacy of metsulfuron and imazapyr increased as water-deficit stress increased, but efficacy of hexazinone was not influenced by plant water status. This unusual relationship between water-deficit stress and herbicide performance may enable improved African rue management under stressful environments.

African rue is an invasive herbaceous perennial that occurs in several states in the western United States. The ability of African rue seedlings to tolerate and recover from progressive drought was examined in greenhouse experiments. Water was withheld for 15 d, and a subset of plants were rewatered after 12 d of water deficit to examine recovery. Conductance rate decreased to 0.1 mol H2O m−2 s−1 and photosynthesis rate decreased to 2 µmol CO2 m−2 s−1 within 6 and 12 d, respectively. Leaf water potential decreased more slowly than gas exchange rates; after 15 d of water deficit plants maintained net carbon gain at −4.8 MPa. Photosynthesis and conductance rates of rewatered plants recovered to levels similar to well-watered controls within 9 and 12 d, respectively. After 9 d of water deficit, seedlings needed only 4 d to recover physiological function similar to well-watered controls. Reduced seedling biomass was observed after 6 d of water deficit, and biomass remained smaller than controls after 15 d of recovery. The rapid change in conductance rate and slower response in leaf water potential indicates that stomatal control is an important component of seedling response to water deficit. The success of African rue in arid environments is due in part to the ability of seedlings to tolerate and recover from water deficit.

Hound’s-tongue is an invasive, biennial weed that thrives in dry rangelands of British Columbia. Rosette formation in the first year of growth and a deep root system offer this weed a competitive advantage against associated grasses under dry conditions. To study effects of water stress on seedling growth and mycorrhizal colonization in hound’s-tongue, seedlings of this weed were grown in pots in a greenhouse and subjected to four (100, 80, 60, and 40% of field capacity) soil moisture treatments. Effects of soil moisture stress (SMS) on several growth parameters as well as mycorrhizal colonization of roots were studied. The total biomass, shoot and root fresh and dry weights, leaf number, petiole length, leaf area, and specific leaf weight (leaf dry weight per unit leaf area) decreased with increasing SMS; shoot and root water content was not affected. Because of a greater effect of SMS on root compared with the shoot biomass, shoot:root ratio increased as the moisture stress increased. Water stress decreased mycorrhizal colonization and arbuscule and vesicle abundance. A reduction in total biomass, leaf number and leaf area per plant, petiole length, and mycorrhizal colonization may reduce the competitive advantage of hound’s-tongue over its neighbors under drought conditions. The effect on plant size may also influence herbivory, by biocontrol agents and other herbivores, and fecundity of this weed.