Bromus Diandrus Research Articles

Differences in the distribution and seed germination behaviour of populations of Bromus rigidus and Bromus diandrus in South Australia: adaptations to habitat and implications for weed management

A field survey was undertaken in South Australia to determine the relative distribution of the brome grass species B. diandrus and B. rigidus. Seeds of brome grass plants were collected from locations across the Yorke (n = 10) and Eyre Peninsulas (n = 25). B. rigidus was found more frequently and at higher densities in South Australian crops than B. diandrus, which showed a distinct preference for undisturbed fence-line margins. Species identity of brome plants in each sample was initially determined by assessing morphology of the callus-scar of the caryopsis as well as the structure of the panicle. Species identity was later confirmed by counting somatic chromosome number. There was consistent agreement between the 2 approaches to identification, indicating that these morphological features can be used with confidence when identifying B. diandrus and B. rigidus in the field. Although B. diandrus and B. rigidus are morphologically very similar, they showed large differences in germination behaviour. B. diandrus seeds collected from fence-line margins were more germinable than B. rigidus from neighbouring cropped areas. Populations of B. rigidus also showed strong inhibition of seed germination when exposed to light. This inhibitory effect of light on seed germination was not seen in the B. diandrus collections. Two populations of B. rigidus from Yorke Peninsula showed little germination (<15% germination in complete darkness) until well after the start of the next growing season. These 2 populations did, however, show a large response to treatment with gibberellic acid (1 mm), indicating high seed viability but presence of deep dormancy. From a practical point of view, the germination behaviour (longer dormancy and light inhibition) exhibited by B. rigidus would allow this species to proliferate under conservation tillage systems such as no-till, where seeds only experience complete darkness after burial following the sowing operation. Germination behaviour of B. rigidus observed in this study is expected to contribute to greater seed carry-over from one season to the next, and favour its colonisation in crops, as seen in the current field survey.

New Zealand grasslands revisited: identification ofCocksfoot mild mosaic virus

In the springs of 2003 and 2004, Bromus diandrus plants showing conspicuous mosaic symptoms were observed at Waitati, East Otago, New Zealand. Electron microscopic examination of sap samples in 2% phosphotungstate (pH 6.5) revealed large numbers of isometric particles 25 nm in diameter. The virus was mechanically transmitted to Setaria italica and Secale cereale cvv. Amilo and Rahu but not to Avena sativa, Chenopodium spp., Hordeum vulgare, Nicotiana benthamiana, Triticum aestivum or Zea spp. In Ouchterlony double diffusion tests, sap from infected plants and purified particles reacted strongly with antisera to Cocksfoot mild mosaic virus (CMMV and CMMV-Phleum mottle).Weaker reactions were observed with an antiserum to Panicum mosaic virus and there were no reactions with antisera to eight other sobemoviruses and ten other viruses known to infect the Poaceae. ELISA was used to test archived and fresh samples for CMMV and other viruses known to infect the Poaceae. CMMV was detected in B. diandrus and Ryegrass mosaic virus was detected in Holcus lanatus and Lolium perenne. However, Brome mosaic virus, Brome streak virus, Soilborne wheat mosaic virus, Wheat dwarf virus and Wheat streak mosaic virus were not detected in samples collected between 1995–2005 from Auckland and the South Island of New Zealand. This is the first report of CMMV in the Southern Hemisphere.

Restoration of Native Perennials in a California Annual Grassland after Prescribed Spring Burning and Solarization

Abstract Grasslands dominated by exotic annual grasses have replaced native perennial vegetation types in vast areas of California. Prescribed spring fires can cause a temporary replacement of exotic annual grasses by native and non‐native forbs, but generally do not lead to recovery of native perennials, especially where these have been entirely displaced for many years. Successful reintroduction of perennial species after fire depends on establishment in the postfire environment. We studied the effects of vegetation changes after an April fire on competition for soil moisture, a key factor in exotic annual grass dominance. As an alternative to fire, solarization effectively kills seeds of most plant species but with a high labor investment per area. We compared the burn to solarization in a study of establishment and growth of seeds and transplants of the native perennial grass Purple needlegrass (Nassella pulchra) and coastal sage species California sagebrush (Artemisia californica). After the fire, initial seed bank and seedling densities and regular percent cover and soil moisture (0–20 cm) data were collected in burned and unburned areas. Burned areas had 96% fewer viable seeds of the dominant annual grass, Ripgut brome (Bromus diandrus), leading to replacement by forbs from the seed bank, especially non‐native Black mustard (Brassica nigra). In the early growing season, B. diandrus dominating unburned areas consistently depleted soil moisture to a greater extent between rains than forbs in burned areas. However, B. diandrus senesced early, leaving more moisture available in unburned areas after late‐season rains. Nassella pulchra and A. californica established better on plots treated with fire and/or solarization than on untreated plots. We conclude that both spring burns and solarization can produce conditions where native perennials can establish in annual grasslands. However, the relative contribution of these treatments to restoration appears to depend on the native species being reintroduced, and the long‐term success of these initial restoration experiments remains to be determined.

Competition of artichoke thistle (Cynara cardunculus) with native and exotic grassland species

Determining the nature of root and shoot competition can elucidate the competitive ability of an invasive species and direct management strategies. In a set of competition experiments, artichoke thistle (Cynara cardunculus), an exotic invasive perennial forb, was subjected to full or shoot competition with four species: black mustard (Brassica nigra), an exotic annual forb; ripgut grass (Bromus diandrus), an exotic annual grass; purple needle-grass (Nassella pulchra), a native perennial grass; and itself. For shoot competition, a smaller pot nested in a larger experimental pot sequestered the target plant root system. A bare ground invasion experiment, in which all plants were transplanted on the same date, and a community invasion experiment, in which competitor species were planted 1 mo before targets, were conducted. In the bare ground invasion experiment, target plant size was reduced (P ≤ 0.05) when exposed to full competition with the exotic species, but not purple needle-grass. Effects on target plants included reductions in height, number of leaves, rosette diameter, and shoot and leaf dry weight. In the community invasion experiment, full competition with all species reduced target plant growth (P ≤ 0.05). Shoot competition was more important when all species were planted synchronously, whereas root competition was more important when target plant establishment was delayed. In a separate experiment, artichoke thistle was grown under four light levels simulating field conditions under canopies of the same competitors. Midday carbon assimilation decreased linearly with increased shade, indicating the likely effects of shoot competition on artichoke thistle. Results indicated that exotic species are more competitive than native purple needle-grass against artichoke thistle and that restoration directly to native grassland after artichoke thistle removal might be difficult. However, artichoke thistle seedling growth is reduced by root competition from grasses that emerge earlier, indicating that early season management of grasslands to delay artichoke thistle establishment might provide effective control.

Evaluation of the susceptibility of various grass species to <i>Gaeumannomyces graminis</i> var <i>tritici</i>

Takeall caused by the soilborne pathogen Gaeumannomyces graminis var tritici (Ggt) is a devastating root disease of wheat As well as infected host residues from previous wheat crops grass crop or weed species also play an important role in the carryover of inoculum to the next wheat crop However the survival and spread of inoculum on different grasses differs considerably depending on their susceptibility to the pathogen Using Triticum aestivum (wheat) and Avena sativa (oat) as susceptible and resistant standards the susceptibility to Ggt of 24 grass species commonly found within wheat crops in New Zealand was examined in a simple laboratory assay Of all the grass species evaluated 83 were susceptible to Ggt with Bromus diandrus Bromus willdenowii Bromus inermis and Pennisetum clandestinum being highly susceptible while Cynosurus cristatus Cynodon dactylon and Paspalum dilatatum were highly resistant to the pathogen

Isoenzyme Data on the Genetic Divergence and Allopolyploidy in the Section Genea of the Grass Genus Bromus (Poaceae)

~Electrophoretic variation of isoenzyme characters of ten enzymes (ADH, AAD, MDH, SKD, AAT, SOD, PRX, PGD, PGI, LAP) has been studied in five brome species, comprising the section Genea of the genus Bromus. The three polyploids, B. madritensis, B. rubens and B. diandrus (incl. B. rigidus), exhibited different fixed heterozygosities of several heterozymes, suggesting their alloploid nature. The morphologically similar polyploids B. madritensis and B. rubens differed in seven heterozymes out of fourteen studied, supporting their specific recognition. The two diploids, B. sterilis and B. rectorurn, differed in four heterozymes. One genome of the polyploid species corresponded closely to that of B. sterilis. All species of the section could be distinguished from each other by diagnositic isoenzyme phenotypes. Bromus diandrus and B. rigidus revealed identical zymograms, providing new support for their conspecific recognition.

The effect of temperature on the germination of six selected weed species

Field-collected seeds were incubated in a germination cabinet at three temperature regimes and germination percentage was determined after incubation. Species such as Arctotheca calendula, Avena fatua, Bromus diandrus and Raphanus raphanistrum germinated best when subjected to a diurnal temperature regime of 12h/12h at 10/15 °C, which correlates with late autumn to early winter temperatures in the Western Cape. Among the germinated weed species, A. fatua germinated most with a cumulative value of 90% as compared to R. raphanistrum which germinated least with a cumulative value of 12%.

Chemical control of brome grasses (<i>Bromus spp</i>) in cereals

Ripgut brome (Bromus diandrus) prairie grass (B willdenowii) and soft brome (B hordeaceus) are becoming serious weed problems in arable farms of New Zealand This paper reports results from three years of field experiments evaluating chemical treatments for brome control in cereal crops Ripgut brome was shown to be very competitive with moderate infestations reducing grain yields by 2530 A strong relationship (R2096) was found between seedling numbers of ripgut brome and final grain yield The study found several promising chemical treatments that offer a good degree of control These include preemergence applications of cyanazine terbuthylazine chlorsulfuron terbuthylazine and metribuzin The same herbicides applied postemergence were not as effective for brome grass control

Phytotoxicity of metabolites produced by Pyrenophora semeniperda in liquid culture

Pyrenophora semeniperda was shown to produce heat stable, biologically active metabolites under agitated liquid culture conditions. Using a seedling bioassay it was shown that filtrates harvested from P.�semeniperda cultures had a significant impact on coleoptile length of both wheat and Bromus diandrus, but had no effect on seed germination. The relative toxicity of filtrates derived from several isolates of P. semeniperda and infiltrated into wheat leaves was highly correlated with the virulence of these isolates. A comparison of metabolites harvested from P. semeniperda and Pyrenophora teres grown under the same cultural conditions revealed that P.�teres did not affect wheat coleoptile growth, but affected the coleoptile elongation of B. diandrus, although less than filtrates produced by P. semeniperda. Culture filtrates harvested after 6 days were toxic to wheat and B.�diandrus and toxicity was maximal in filtrates derived from cultures that were 12 days old. Culture filtrates diluted to 1 in 20 produced symptoms in wheat seedlings, but only undiluted or 5 × concentrated filtrates produced symptoms on B.�diandrus seedling leaves. Plants older than first node stage (Z 31) were significantly less sensitive to filtrate than younger plants. A degree of host selectivity to the metabolites was observed since leaves of Gossypium hirsutum, Helianthus annuus, Lablab purpureus and Xanthium occidentale were unaffected by infiltrates.

First report of apothecia of Tapesia yallundae occurring on the wild grass Holcus lanatus (Yorkshire Fog) in New Zealand

Tapesia yallundae is a causal agent of eyespot, a damaging stem base disease of cereal crops. The pathogen has also been detected on many wild and cultivated grasses even if characteristic eyespot symptoms are not visible (Lucas et al ., 2000). The pathogen exhibits a heterothallic mating system and apothecia of the sexual stage have been detected on straw stubble in many European countries, Australasia, southern Africa and North America (Dyer et al ., 2001). During field studies in New Zealand in 1995, apothecia resembling those of T. yallundae were discovered on decaying stem bases of Holcus lanatus (Yorkshire Fog) at the margin of one wheat field near Carterton, Wairarapa, North Island. To verify that apothecia were indeed T. yallundae , ascospore discharge was induced and single ascospore cultures established. Resultant colonies were confirmed as T. yallundae on the basis of conidia and colony morphology, and growth rate (2·3 ± 0·1 mm day − 1 ) on potato dextrose agar (Dyer et al ., 1996). Isolates from H. lanatus also produced a 1·05-kb PCR product characteristic of T. yallundae following amplification with the speciesspecific primers Ty 16F and Ty 16R (Nicholson et al ., 1997). No product was obtained with primers specific to the closely related species T. acuformis . Furthermore, amplification with RAPD primer OPA-10 resulted in the production of 0·68and 1·32-kb bands, again characteristic of T. yallundae (Dyer et al ., 1996). Finally, inoculation of wheat seedlings (cv. Avalon) with conidial suspensions resulted in production of characteristic eyespot lesions. This represents the first report of apothecia of T. yallundae occurring on H. lanatus and the first evidence of infection of H. lanatus by T. yallundae . The sexual stage of T. yallundae has also been detected on the wild grasses Bromus diandrus and Hordeum leporinum (barley grass) (Wallwork, 1987). Should the sexual cycle occur regularly on wild grasses then these species may provide an important reservoir of eyespot disease able to infect nearby cereal crops as a result of the dispersal of airborne ascospores. This problem may be exacerbated by the recent establishment of grassy islands (‘beetle banks’) as wildlife refuges in UK crops.

EDAPHIC FACTORS AND COMPETITION AFFECT PATTERN FORMATION AND INVASION IN A CALIFORNIA GRASSLAND

We used soil analyses, a greenhouse experiment, and a field experiment to examine the roles of competition and abiotic soil factors in plant community pattern formation in a California grassland. We collected soils from five microhabitats: one dominated by the native perennial grass Sporobolus airoides Torrey (alkali sacaton), two dominated by the nonnative annual grass species Hordeum marinum (Hudson) ssp. gussoneanum (Parl.) Thell (foxtail) and Bromus diandrus Roth (ripgut grass), and bare soil created either by gopher mounds or by high salt concentrations (sodic soils). We used principal components analysis (PCA) to determine whether soils from these five microhabitats differed in any discernible pattern. In the greenhouse we examined the emergence, growth, and survival of the native perennial in soil from the five habitat types. In the field we then examined a combined survival and reproduction measure to assess how competition with the two nonnative annual grasses affected the fitness of S. airoides....

Ability of weeds to host the root lesion nematodes Pratylenchus neglectus and P. thornei I. Grass weeds

Nine grass weed species common to cropping rotations in southern Australia were assessed as hosts of the root lesion nematodes, Pratylenchus neglectus (Rensch) Filipjev Schuurmans & Stekhoven and P. thornei Sher & Allen. Weeds were grown at 20°C and individual plants inoculated with 1000 P. neglectus or P. thornei. Weed species were classified as non-hosts, poor hosts or good hosts, based on comparison of the reproductive factor (final population/initial population, or Pf/Pi) with the cereal controls, triticale cv. Abacus (moderately resistant to P. neglectus and moderately susceptible to P. thornei) and wheat cv. Machete (susceptible to both nematodes). Hordeum glaucum Steudel, H. leporinum Link, Lolium rigidum Guadin and Bromus diandrus Roth were not hosts of P. neglectus (Pf/Pi = 0.35-0.77). B. rubens Roth, Vulpia bromoides (L.) Gray and V. fasciculata (Forssk.) Gray (Pf/Pi = 0.94-1.19) were poor hosts, and Avena sterilis L. and A. fatua L. (Pf/Pi = 4.10-5.93) were good hosts of P. neglectus. The Avena spp. were more susceptible (P < 0.05) to P. neglectus than triticale cv. Abacus, and were as susceptible as wheat cv. Machete. V. bromoides, V. fasciculata, L. rigidum and A. fatua were resistant to P. thornei (Pf/Pi = 0.15-0.54). H. glaucum, B. diandrus and B. rubens were poor hosts of P. thornei (Pf/Pi = 1.06-1.46). A. sterilis and H. leporinum (Pf/Pi = 1.58-1.92) were good hosts of P. thornei, and both were as susceptible as wheat cv. Machete. Growth of susceptible weeds will allow increase and persistence of nematodes in cropping soils, compromising the use of resistant crops in rotations for management of P. neglectus and P. thornei.

BUSH LUPINE MORTALITY, ALTERED RESOURCE AVAILABILITY, AND ALTERNATIVE VEGETATION STATES

Nitrogen-fixing plants, by altering the availability of soil N, potentially facilitate plant invasion. Here we describe how herbivore-driven mortality of a native N-fixing shrub, bush lupine (Lupinus arboreus), increases soil N and light availability, which promotes invasion by introduced grasses to the detriment of a native plant community. Soils under live and dead lupine stands contained large amounts of total N, averaging 3.14 mg N/g dry mass of soil (398 g/m2) and 3.45 mg N/g dry mass of soil (438 g/m2), respectively, over four years. In contrast, similar lupine-free soil was low in N and averaged only 1.66 mg N/g dry mass of soil (211 g/m2) over three years. The addition of N fertilizer to lupine-free soil produced an 81% increase in aboveground plant biomass compared to plots unamended with N. Mean rates of net N mineralization were higher under live lupine and where mass die-off of lupine had occurred compared to soils free of bush lupine. At all sites, only 2.5–4.2% of the total soil N pool was mineralized annually. Soil enriched by lupine is not available to colonists while lupines are alive. The dense canopy of lupine shades soil under shrubs, reducing average photon-flux density in late spring from 1725 μmol·m−2·s−1 (full sunlight) to 13 μmol·m−2·s−1 (underneath shrubs). Stand die-off due to insect herbivory exposed this bare, enriched soil. In January, when annual plants are establishing, average photon-flux density under dead lupines killed by insect herbivores was 370 μmol·m−2·s−1, compared to the photon-flux density under live lupines of the same age, which averaged 83 μmol·m−2·s−1. The availability of bare, N-rich patches of soil enabled nonnative annuals (primarily Lolium multiflorum and Bromus diandrus) to colonize sites, grow rapidly, and dominate the plant assemblage until lupines reestablished after several years. The N content of these grasses was significantly greater than the N content of the mostly native plants that occupied adjacent coastal prairie devoid of bush lupine. Between 57 and 70% of the net amount of N mineralized annually was taken up by introduced grasses and subsequently returned to the soil upon the death of these annuals. Even in the absence of further N inputs, we estimate that it would take at least 25 yr to reduce the soil N pool by 50%, indicating that the reestablishment of the native prairie flora is likely to be long term.

Bromus diandrus Roth. control with MON 37532 in spring wheat (Triticum aestivum L.). 1. Time of application and dosage rate

MON 37532 (sulfosulfuron) is a new herbicide for the selective control of grasses and annual broadleaf weeds in wheat (Triticum aestivum L.). In this glasshouse study done in 2-litre pots filled with coarse sand, the product was evaluated for the selective control of Bromus diandrus Roth, in spring wheat. Dosage rates of MON 37532 varying between 0 and 45 g a.i ha-1 plus 0.25% of the surfactant, Agral 90, were applied at the 1-, 2-, 3- and 4-leaf stage of the weed plants. Although efficient control (> 85%) was only achieved when weeds were sprayed at the 1 -leaf stage, leaf growth of all B. diandrus plants ceased within 5 days after spraying, irrespective of time of spraying. Leaf area and dry mass determinations done at 6 weeks after spraying in this study confirmed that even if sprayed at the 4-leaf stage, MON 37532 stops B. diandrus from further growth and competition with the wheat plants. Higher dosage rates may be needed when sprayed at later growth stages. Growth of the spring wheat cultivar, SST 55, used in this study was not affected by the herbicide.

Bromus diandrus Roth. control with MON 37532 in spring wheat (Triticum aestivum L.). 2. Effect of relative weed density and time of spraying

Bromus diandrus Roth, is one of the most troublesome weeds in spring wheat crops of the western Cape wheat producing area of the Republic of South Africa and Australia. The selective control of this weed in wheat crops may, in future, be possible with MON 37532 (sulfosulfuron) a new herbicide developed by Monsanto SA. The effect of the relative density of B. diandrus in a wheat crop on the efficacy of this herbicide sprayed at either the 2- or6-leaf stage of the weed was studied in a pot experiment in a temperature-controlled glasshouse. A density of 3 plants pot-1 (170 plants rrr-2) was used for the wheat crop, while that of B. diandrus varied between 0 and 10 plants pot-1 (565 plants -2). At early applications (2-leaf stage), efficacy of MON 37532 improved with higher densities of B. diandrus. This was possibly due to stress conditions created by higher weed densities. This tendency was less evident when MON 37532 was applied at the 6-leaf stage. Reduced susceptibility in the older plants and less efficient wetting of weeds in pots with high weed densities were probably the reason for reduced percentage control when plants were sprayed at this stage.

Seed dormancy release in three common pasture grasses from a Mediterranean-type environment under contrasting conditions

Pastures in the Mediterranean region of Australia are typically comprised of a mixture of winter annuals, including grasses and legumes, which utilise a seed bank to survive the long, dry summer. The risk of out-of-season summer rain requires species from such environments to adopt appropriate strategies to protect their seed banks. The seed bank strategies of the grasses from these environments have, so far, received little attention. We conducted an experiment to observe the dormancy release characteristics of 3 grasses common to these environments when stored under contrasting thermal regimes. The grasses studied were great brome grass (Bromus diandrus Roth), annual ryegrass (Lolium rigidum Gaud.), and barley grass (Hordeum leporinum Link). The different species displayed contrasting dormancy release characteristics over the summer. Two of the species examined (great brome grass and annual ryegrass) displayed relatively late release from dormancy which gave high levels of protection from false breaks. The other (barley grass) displayed earlier release from dormancy which would enable it to better exploit earlier true breaks but make it more sensitive to false breaks. Unlike pasture legumes, high and fluctuating temperatures were not necessary for the seeds of these grasses to become germinable. In fact, exposure to high temperatures appeared more likely to suppress release from dormancy.

Influence of rotation and time of germinating rains on the productivity and composition of annual pastures in Western Australia

A long-term rotation experiment located in south-western Australia was used to measure the effect of rotation and time of germinating rains on the productivity and botanical composition of grazed annual pastures in 2 contrasting seasons in an environment with an average annual rainfall of 325 mm. The density of self-regenerating seedlings of subterranean clover (Trifolium subterraneum), capeweed (Arctotheca calendula), and grasses (Lolium rigidum, Hordeum leporinum, Bromus diandrus) was greatly increased (approx. 3 times the density) when there was a second year of pasture after crop compared with the first year after crop. The lower plant density resulted in first-year pastures having only about 33% of the autumn biomass accumulation of second-year pastures. This difference in early pasture growth had no effect on total pasture production in 1992, but in 1993 total pasture production was 30% greater in second-year pastures compared with first-year pastures. Botanical composition varied between and within seasons with the percentage of subterranean clover increasing throughout the season and the percentage of capeweed decreasing throughout the season. Grasses comprised &lt;20% of the biomass in all seasons and treatments. Production of subterranean clover seed in 1993 was higher in a 1 : 2 crop-pasture rotation than in a 1 : 1 crop-pasture rotation and direct drilling in the cropping phase increased seed set compared with conventional tillage in both 1 : 1 and 1 : 2 crop-pasture rotations. Capeweed seedlings emerged in large numbers after rainfall between February and May and subsequently showed a relative growth rate twice that of subterranean clover and the grasses, but exclusion of rainfall until June resulted in a significant reduction in the emergence of capeweed seedlings. Additionally, capeweed had a lower rate of seedling survival compared with other pasture species, and this is contrary to observations by other researchers that capeweed is highly resistant to moisture stress during early growth.

Invasibility of Experimental Habitat Islands in a California Winter Annual Grassland

In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica), at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 m2 to 32 m2, had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species—rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species—rich plots were more invasible.

Selectivity of SMY 1500 (4‐amino‐6‐tert‐butyl‐3‐ethylthio‐l,2,4,triazin‐5(4H)‐one) in Triticum durum

AbstractSusceptibility to SMY 1500 (4‐amino‐6‐tert‐butyl‐3‐ethylthio‐1.2.4‐triazin‐5(4H)‐one) was studied in durum wheat (Triticum durum Desf.) cultivars in growth‐chamber assays. Weight reduction, photosynthesis inhibition and herbicide levels were determined at various times after a 24‐h herbicide treatment in two cultivars of durum wheat, one cultivar of spring wheat (Triticum aestivum L). and in the weed Bromus diandrus (Roth). Measurements of weight reduction distinguished between plants tolerant and plants susceptible to SMY 1500. Tolerant cultivars showed a lower herbicide content and photosynthesis inhibition than susceptible plants after the 24‐h treatment. Thereafter, herbicide levels and photosystem II inhibition decreased faster in tolerant plants. Comparison with previous work showed that plants tolerated doses of SMY 1500 about four times higher than of its analogue, metribuzin. Herbicide content in leaves just after treatment, and rate of detoxification may explain the selectivity of SMY 1500 in these cereals.

Phenotypic variation in seedlings of a "keystone" tree species (Quercus douglasii): the interactive effects of acorn source and competitive environment.

Blue oak (Quercus douglasii) is a deciduous tree species endemic to California that currently exhibits poor seedling survival to sapling age classes. We used common garden techniques to examine how genetic variation at regional and local scales affected phenotypic expression in traits affecting oak seedling growth and survival. Between-population variation was examined for seedlings grown from acorns collected from a northern, mesic population and a southern, xeric population. Within-population variation was examined by comparing seedlings from different maternal families within the mesic population. Acorns were planted into neighborhoods of an annual dicot (Erodium botrys), an annual grass (Bromus diandrus), and a perennial bunchgrass (Nassella pulchra). By varying the species composition of herbaceous neighborhoods into which acorns were planted, the interactive effects of competition and acorn germplasm source on phenotypic expression could also be examined. Potential maternal effects, expressed as variation in acorn size, were assessed by weighing each acorn before planting. Probability of seedling emergence increased significantly with acorn size in the xeric population but not in the mesic population. Similarly, the effect of acorn size on seedling leaf area, stem weight, and root weight was also population-dependent. At a within-population level, acorn size effects on seedling traits varied significantly among maternal families. In addition to acorn size effects, rates of oak seedling emergence were also dependent on an interaction of population source and competitive environment. Interactions between maternal family and competitive environment in the expression of seedling leaf characters suggest the possibility of genetic variation for plasticity in traits such as specific leaf area. Using carbon isotope discrimination (Δ) as an index of relative water-use efficiency (WUE), higher water use efficiency was indicated for oak seedlings grown in the annual plant neighborhoods compared to seedlings grown in the bunchgrass neighborhood. This trend may represent an adaptive plastic response because, compared to the bunchgrass neighborhood, soil water depletion was more rapid within annual plant neighborhoods.