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Environmental heterogeneity can create differential opportunities for seedling recruitment among plant species. We collected soil seed banks from alvar habitats in southern Ontario and exposed them to three soil moisture treatments. Density and species richness of germinants were greatest in treatments kept moist compared with treatments where soil was either saturated with standing water at the soil surface or where drought was imposed. Contrary to previous studies, the drought treatment did not stimulate the germination of species that remained ungerminated in other treatments, but did increase the germination of five species that also germinated in wetter soils. Although 12 of the 40 species germinated in only one of the three treatments, overall community composition among watering treatments was relatively consistent; few species showed evidence of differential responses to soil moisture conditions. Variability in soil moisture in this system can alter population and community properties by rarefaction effects, as opposed to niche differences among species.

Biomass allocation patterns in plants are known to be affected by soil nitrogen availability. Since nitrogen availability can depress symbiotic nitrogen fixation, and nitrogen fixation can make plant growth independent of soil nitrogen availability but is energetically costly, it is unclear how allocation patterns in nitrogen-fixing species should respond to variation in soil nitrogen availability. We examined the effect of nitrogen source and concentration on the growth and allocation patterns in the nitrogen-fixing shrub Alnus viridis subsp. crispa (Aiton) Turrill. Plants were grown with either NH4+-N or NO3–-N at a range of low N concentrations, from 0 to 2 mmol·L–1, and either inoculated with Frankia or not. Plants without nodules had 25.l% lower biomass and had double the allocation to roots at all but the 2 mmol·L–1 nitrogen concentration. Even though nodulated plants increased growth with nitrogen concentration, allocation to roots as a fraction of total biomass did not vary in these plants, suggesting increased growth resulted from more efficient nitrogen acquisition. Allocation to roots was a significant predictor of plant growth in non-nodulated plants (r2 = 0.318, for linear least squares fit with log mass) but not for nodulated plants (r2 = 0.108). As nitrogen concentrations increased, allocation to nodules, specific nodule numbers, and the proportion of nitrogen fixed by the plants decreased, demonstrating a shift to soil nitrogen use.

Hybridization between the herbicide-resistant transgenic crop Brassica napus L. (canola) and its weedy relative Brassica rapa L. (bird rape) has been documented in Quebec. Our goal was to evaluate the actual hybridization potential based on range overlap and actual in situ hybridization rates. This was done by mapping B. napus canola fields, comparing them with the sampling locations of B. rapa herbarium specimens from Quebec, gathering information on the presence of B. rapa in certified canola seed production fields, and surveying for B. rapa populations located in, or close to B. napus field margins. Progeny from these populations were screened for herbicide resistance (HR) and for the presence of the HR transgene. Two fields were also selected to evaluate B. rapa density effects on hybridization rates. Significant sympatry was observed in several areas of the province; hybridization occurred in all eight populations (1.1% to 17.5% hybrid seed) located in field margins and in one (1.1%) out of three populations located less than 10 m from a B. napus field. Hybridization rates decreased exponentially as B. rapa density increased, but interplant rates (0% to 68%) were highly variable. Environmental problems could be generated by the release of B. napus crops with traits conferring fitness benefits in nonmanaged areas.

Global warming is widely thought to promote the dominance of grasslands over woody vegetation, and shift the location of ecotones. In contrast, forest vegetation along the northern edge of the North American Great Plains has migrated southward over the past century into areas dominated by native grassland. Because climate change is also predicted to increase storm frequency and intensity, we quantified the impacts of an intense hailstorm on woody and herbaceous species in native grassland in the northern Great Plains of North America. The hailstorm disturbance killed or damaged mature stems of the dominant tree, aspen ( Populus tremuloides Michx.), but damaged aspen stands subsequently invaded ca. 10 m into neighbouring grassland. Damaged aspen stands also produced up to 20-fold more new stems having 67-fold higher total biomass compared with relatively undisturbed stands. Grasses and lichens suffered much higher rates of biomass removal (60%–76%) than did shrubs (6%–8%) immediately following the storm. The disturbance-mediated recruitment of clonal woody plants, and the unexpected sensitivity of grasses and lichens to this disturbance, may contribute to the counterintuitive expansion of trees into grasslands under a regime of increased storm frequency that is not predicted by simple ecosystem responses to warming.

A methodology is presented for the hydroponic cultivation of Arabidopsis thaliana (L.) Heynh. that is simple and inexpensive to construct from easily obtainable and reusable materials. High-quality and uniform plant material, free of algae and disease, are consistently produced through all life-stages. Minor modifications to previous protocols that reduce cost and labour include the use of small food-storage containers with snap-on lids, aluminum foil wrap to cover the containers to occlude light, commercial hydroponic fertilizer as nutrient solution, and clipped 1.5 mL microfuge tubes as plug holders. Seed is germinated on nutrient agar plates, instead of directly on rockwool plugs; after the seedlings have grown to the six-leaf stage, they are removed from the plates and the roots are enclosed within rockwool plugs that have been cut longitudinally. Subsequent seedling growth is very uniform and the elimination of oversowing and subsequent thinning results in a dramatic reduction in seed and seedling wastage.

Wild rice ( Zizania palustris L.) litter accumulation may inhibit plant growth and production both by physically suppressing seedling emergence and by nitrogen immobilization in fresh litter. This latter mechanism could reduce nitrogen availability to plants early in the growing season at a period when more than half of the annual uptake occurs. To test the importance of these mechanisms, we planted wild rice in mesocosms. Half the tanks were planted with seeds sown below the litter and half were planted with seedlings grown to a height taller than litter thickness. One-third of the tanks were treated with fresh (nitrogen immobilizing) litter, one-third were treated with litter that had been incubated for 26 d and was mineralizing nitrogen, and one-third did not receive litter. These treatments resulted in a fully crossed factorial design, with nine replicates for each treatment combination, totaling 54 tanks. We measured plant growth, vegetative, root, and seed biomass, total plant N, and available N at 2.5 cm sediment depth. The presence of litter and its stage of decay caused plant, root, and seed biomass, and seed and total plant nitrogen content to increase. We found no physical inhibition of litter on the potential growth of plants started as seeds. Therefore, the timing of litter nitrogen immobilization or mineralization affects the potential growth of wild rice.

Size-dependent reproductive effort is an important component of plant fitness. The responses of reproductive effort to environmental factors in Amaranthus retroflexus L. were measured in two experiments. A wide range of selection pressures were generated by manipulating the sowing date (29 April, 23 May, 18 June, and 14 July) and planting density (13.4, 36, 121, and 441 plants·m–2). Allometric analysis between reproductive biomass and vegetative biomass across treatments showed that reproductive effort increased with size in response to different planting densities but decreased with size in sowing dates experiment. The allometric exponent between treatments was not influenced by planting densities but had significant variation with sowing date. Total branch length could explain most of the variation of reproductive accumulation and allocation in planting density experiments. For the plants with different sowing dates, total branch length was the main determinant of reproductive biomass, while reproductive effort mainly depended on the number of primary branches per unit stem mass. Architectural constraints with size result in size-dependent reproduction. Size-dependent reproduction in A. retroflexus was influenced by available resources and environmental conditions through the mechanisms of self-regulation of architectural traits.

It is well known that lupin forms cluster roots, which help in dissolving insoluble P in soils. In nonleguminous species, cluster roots also appear to contribute to the utilization of organic N in soils. In white lupin ( Lupinus albus L.), however, the characteristics of its organic N utilization have not been studied. Therefore, we examined whether white lupin can utilize organic N in soils. Soybean ( Glycine max (L.) Merr.), which does not form cluster roots, was used as a control plant. Seedlings of lupin and soybean were cultivated in soils with different N sources (non-N, ammonium sulphate, ammonium sulphate plus cattle farmyard manure, or cattle farmyard manure). The rate of glycine uptake by excised roots was determined in a hydroponic experiment to investigate the ability of lupin and soybean to directly utilize amino acids. Nitrogen accumulation in soybean corresponded to the decrease in inorganic N in the soils. In contrast, N accumulation in lupin was higher than the decrease in inorganic N in the soil, especially with the cattle farmyard manure treatment, indicating that lupin derived more N from an organic N source. Wheat ( Triticum aestivum L.) cultivated with lupin in a pot accessed more available N than wheat with soybean or wheat in monoculture, suggesting that lupin roots themselves or the lupin rhizosphere microorganisms were able to decompose organic N in soils. Excised roots of lupin, especially cluster roots, exhibited higher rates of glycine uptake than roots of soybean. In conclusion, lupin decomposed organic N in the rhizosphere and was able to absorb amino acids from decomposition in addition to any inorganic N produced by further microbial decomposition.

The capacity of the spring ephemeral Erythronium americanum L. to grow and absorb nutrient either as nonmycorrhizal (NM) or mycorrhizal (M) plants under the low temperature regime characteristic of its growth period was investigated. Specimens of E. americanum were collected in the field as either NM (early September) or as M plants (late October). Both groups of plants were submitted to different nutrient regimes during the hypogeous growth period at 5 °C, and during the subsequent epigeous growth period conducted at temperature regimes of either 12 °C day : 10 °C night or 17 °C day : 15 °C night. Nutrient regime influenced bulb nutrient content only during the epigeous growth period. The presence of mycorrhizas did not influence nutrient content, but favoured a greater bulb biomass at the final harvest (epigeous growth period), as did the lower temperature regime. Net nutrient uptake was not reduced at lower temperatures and appeared to follow plant demand. These findings confirm that E. americanum is adapted to perform better under a low temperature regime and that mineral nutrition in this species occurs mainly in spring in response to active growth. Arbuscular mycorrhizal fungi benefit E. americanum maybe through less expensive nutrient uptake or sustained carbon sink demand.

The impact on the performance of dwarf shrub Vaccinium myrtillus L. (bilberry) subjected to differing natural intensities of grazing by Cervus elaphus L. (red deer) was examined in a mature Scandinavian pine forest, on Svanøy, an island on the western coast of Norway. All the study sites were in forest where bilberry dominated the forest floor and no forestry activities had been carried out for several decades. Pellet group counts were used as measure of grazing intensity. Bilberry size, abundance, and fruit set, and invertebrate activity on bilberry were negatively related to grazing intensity. The responses varied between early and late summer (negative or neutral), but impact on plant size was negative throughout the season. Because of the large variation in grazing intensity represented in our study, we can, in contrast to many experimental studies, show that grazing affects the performance of bilberry even at low grazing intensities. The effect on fruit production and invertebrate activity on bilberry indicates that red deer grazing has a negative impact on the population dynamics of the plant and invertebrate herbivores.

In the greenhouse, we investigated the growth and biomass allocation of three juvenile wetland species ( Carex tribuloides Wahl., Phalaris arundinacea L., and Rumex orbiculatus Gray) under three different water depths (–4, 0, and +2 cm relative to the soil surface), three N:P supply ratios (1:30, 1:1, 30:1), and two mycorrhizal inoculant treatments (arbuscular mycorrhizal (AM) fungi present, absent). After 6 weeks, the plants were harvested, separated to above- and below-ground parts, oven-dried, and weighed. The mycorrhizal inoculant significantly increased plant growth and reduced root:shoot ratios. At an N:P supply ratio of 30:1, plants with AM fungi had significantly greater biomass than those plants without AM fungi. However, at 1:1 N:P supply ratio, plants without AM fungi had greater biomass. Plants without AM fungi had higher root:shoot ratios at 0 and –4 cm water depth than plants with AM fungi. In general, C. tribuloides had the lowest growth, and unlike P. arundinacea and R. orbiculatus, was not affected by the water depth treatment. Growth of the wetland plants was limited more by nitrogen than by phosphorus. Our results suggest that at high N:P nutrient supply ratios and non-flooded conditions the growth of wetland seedlings can benefit by being inoculated with AM fungi.

The relationships among lodging, stem anatomy, degree of stem lignification, and resistance to mycosphaerella blight (Mycosphaerella pinodes (Berk. & Blox.) Vestergr.) in field pea (Pisum sativum L.) were investigated in field experiments in Saskatchewan from 2000 to 2002. Disease severity, lodging resistance, plant height, internode length and diameter, and the cellulose, lignin, and fibre contents of stems were measured from 9 (2000) and 20 (2001, 2002) cultivars of field pea. Significant differences were observed in resistance to mycosphaerella blight and lodging among cultivars. In all years, ‘AC Tamor’ was among those cultivars with the highest disease ratings and lodging scores, whereas ‘CDC Striker’, ‘Carneval’, and ‘Integra’ were among those with the lowest ratings. Disease severity was positively correlated with lodging scores of plants. Lodging was negatively correlated with the proportions of supportive tissue (formerly parenchyma cells in the pith periphery, which have differentiated to sclerenchyma) and xylem in stem sections, whereas disease ratings were negatively correlated with xylem. Lignin and fibre contents in pea stems were negatively correlated with both lodging and mycosphaerella blight severity. Cellulose content was also negatively correlated with lodging scores. This study establishes an anatomical and biochemical basis for further research into pea stem lodging and disease resistance. Key words: cellulose, fibre, lignin, sclerenchyma, supportive tissue, xylem.

We examined whether fungicide and the subsequent reduction of soilborne pathogenic fungi would differentially enhance the productivity and foliar nutrient content of two coexisting species, Calamagrostis canadensis (Michx.) Beauv. and Carex stricta Lam. This was tested under hydrologic regimes that simulated those in prairie wetlands and included a 32-d cycle (flooded 16 d, dry 16 d), 6-d cycle (flooded 3 d, dry 3 d), flooded, well-watered, and dry hydroperiods. Calamagrostis canadensis biomass increased 26%–45% in the wet hydroperiods (6-d cycle, well watered, and flooded) when fungicide was applied but remained fairly constant over all hydroperiods in nonfungicide treatments. Calamagrostis canadensis grown in the wet hydroperiods without fungicide produced the same biomass and growth rates as plants treated with fungicide in the dry hydrologic regime, suggesting that pathogenic fungi in wet hydrologic regimes have the same effect as major environmental stresses such as drought. In contrast, the biomass and growth rate of Carex stricta generally did not vary significantly with fungicide treatment. The only exception was in the rapidly alternating hydroperiod (6-d cycle), where Carex stricta treated with fungicide produced 48% more biomass and grew 46% faster than plants not treated with fungicide. Mean concentrations of foliar phosphorus generally were not significantly different between the fungicide and nonfungicide treatments for either plant species, while foliar nitrogen concentrations were higher in both species when treated with fungicide in the 32-d cycle, dry, and well-watered hydroperiods. The effect of fungicide on the biomass and foliar nutrients of these two co-occurring plant species depended on the species and the hydrologic regime, and our results suggest that seasonal and interannual changes in hydrologic regimes may confer a temporary advantage to one species or the other that, over the long term, allow them to coexist.Key words: benomyl fungicide, foliar nitrogen, foliar phosphorus, hydroperiod, marsh reed grass, wetland.

A comparison of mineral nutrient and phytic acid-phosphorus (PA-P) distribution in the grains of wild-type (WT) and low phytic acid1-1 (lpa1-1) corn (Zea mays L.) was conducted to determine how the lpa1-1 mutation influences mineral element concentrations in different grain parts and impacts the structure of phosphorus-rich inclusions (globoids) in the grain cells. This is the first report regarding total phosphorus (P) and PA-P concentrations in scutellum and root-shoot axis portions of cereal embryos of WT in comparison to its matching lpa1-1 genotype. In WT, 95% of the grain PA-P was located in the embryo, mostly in the scutellum. The lpa1-1 mutation reduced whole-grain PA-P by 62% but influenced the scutella more than the root-shoot axes and rest-of-grain fractions. In spite of the lpa1-1 mutants containing greatly reduced PA-P, whole-grain amounts of Mg, Fe, and Mn were higher in lpa1-1 than in WT, K and Zn were similar, and Ca was lower. Iron was 1/3 higher in lpa1-1 grains than WT while Ca was 18% lower. Decreased phytic acid in lpa1-1 grains resulted in reduction in globoid size in both scutellum and aleurone layer cells. Most lpa1-1 aleurone globoids were non-spherical and scutellum globoids were clusters of small spheres while WT globoids were large discrete spheres. X-ray analyses of globoids in both grain types revealed major amounts of P, K, and Mg and traces of Ca, Fe, and Zn. Both grain types contained almost no mineral nutrient stores in the starchy endosperm.Key words: corn (Zea mays L.), phytic acid-phosphorus, low phytic acid1-1 (lpa1-1) grains, mineral nutrients, globoids, electron microscopy.

Aflatoxin contamination in corn caused by Aspergillus flavus Link is a serious constraint on economical corn (Zea mays L.) production in the Mississippi Delta. The ecology of A. flavus was evaluated in a 3-year study assessing the spatial variability of soil populations of A. flavus in a Mississippi Delta field under different crops. A 1.07-ha section of the field was laid out in 126 9.2-m2plots, and soil was sampled in May 2000, March 2001, and April 2002. Aspergillus flavus populations were determined by plating on selective media, and A. flavus colonization was assessed in corn during 2000. Aspergillus flavus populations in soil were significantly (P < 0.01 level) influenced by previous crop. The highest propagule density (794 cfu·g–1) was found following the corn crop in 2001 versus 251 cfu·g–1soil in 2000 following cotton and 457 cfu·g–1following wheat in 2002. Aspergillus flavus populations in 2001 and 2002 exhibited a moderate degree of spatial structure, described by spherical and exponential models, respectively, but populations in 2000 exhibited little spatial structure. Colonization of corn kernels by A. flavus in 2000 ranged from 0% to 100% (mean = 15% colonized kernels), and aflatoxin levels ranged from 0 to 1590 ppb (mean = 57 ppb). Aflatoxin levels were randomly distributed in the field and not correlated with A. flavus colonization. Aflatoxin production was found in 43% to 59% of A. flavus soil isolates with the highest incidence in soil populations following corn in 2001. However, 84% of A. flavus isolated from corn kernels produced aflatoxin. Results indicate that within a single field there was a wide range of A. flavus soil propagule densities varying in potential to produce aflatoxin.Key words: Aspergillus flavus, aflatoxins, soil, corn (Zea mays), cotton, wheat, spatial variability.

This paper compares seed and vegetative banks, and the emergent understory in unburned, lightly burned, and intensely burned patches within an aspen-dominated boreal forest in northeastern Alberta, Canada. Propagule banks were measured immediately after the fire, while the understory was surveyed 2 years later. Seedling and shoot emergence techniques were used to assess the abundance and assemblage of species within seed and vegetative banks. Median seed density was ordered unburned > lightly burned = intensely burned patches. A cumulative index of vegetative bank abundance was ordered unburned > lightly burned > intensely burned patches. Species assemblages were significantly different amongst burn intensities for seed banks and emergent understory. Vegetative bank assemblages were significantly different between unburned and burned patches but not between lightly and intensely burned patches. Furthermore, seed and vegetative bank assemblages within each burn intensity were also significantly different. Indicator species analysis suggested that all significant differences were due largely to broad assemblage differences rather than a few unique species. Ordination with nonmetric multidimensional scaling correspondence analysis separated seed and vegetative banks, and emergent understory along two axes (88.8% of the total variation). The first axis (50.3% of the total variation) indicated that the unburned and lightly burned species assemblages were more similar to the vegetative bank, while the intensely burned patches were more similar to the seed bank. The second axis (38.5% of the total variation) placed vegetative banks closer to emergent vegetation than seed banks.Key words: seed bank, bud bank, vegetative bank, aspen, boreal, fire.

The clonal shrub Cornus drummondii C.A. Mey. is rapidly increasing in cover and displacing mesic grassland species in the central USA as a consequence of fire suppression. We assessed the impact of C. drummondii on carbon (C) and nitrogen (N) pools and C fluxes in a tallgrass prairie in eastern Kansas, USA, through a comparison of both burned and unburned C. drummondii islands with open grassland areas. Allometric equations relating C. drum mondii foliage and wood biomass to basal stem diameter were developed to estimate aboveground biomass and net primary productivity (ANPP) of C. drummondii. Within C. drummondii islands, ANPP was 496 ± 45 g C·m–2·year–1, nearly three times that within open grassland (167 ± 13 g C·m–2·year–1). As a result of greater aboveground biomass, aboveground C and N storage within shrub islands (3270 ± 466 g C·m–2, 37.9 ± 5.3 g N·m–2) was substantially greater than that within open grassland (241 ± 33 g C·m–2, 6.1 ± 0.8 g N·m–2). No change in soil organic C or total N to 10-cm depth was evident; however, soil CO2 flux was significantly reduced in C. drummondii islands relative to the open grassland. The storage of C in aboveground biomass of C. drummondii represents a significant short-term increase in C storage relative to open grassland. However, potential alterations in belowground processes must be quantified before the long-term net effect of shrub encroachment on C and N pools within this mesic grassland can be determined.Key words: aboveground biomass, Cornus drummondii, net primary productivity, shrub encroachment, tallgrass prairie.

Abundant data are available on some aspects of the arbuscular mycorrhizal symbiosis, for example, plant nutrition, but because of difficulties immanent to arbuscular mycorrhizal fungi, such as the inability to culture them axenically, the relatively long time it takes to achieve root colonization, and the simultaneous presence of different morphologic stages of the fungus in the root, less information is accumulated on other aspects such as the regulation of mycorrhization. Regulatory processes in the plant – arbuscular mycorrhizal fungus interaction start before root colonization by the fungus and even before a direct physical contact between the host and the fungal symbiont. Some of the signals exchanged are still a matter of debate and will be discussed further on. After the penetration of the root by the fungus, depending on the developmental stage of the arbuscular mycorrhizal association (e.g., early or mature), a range of plant responses is activated. The possible function of several plant responses in the regulation of mycorrhization is discussed.Key words: arbuscular mycorrhiza, Glomales, autoregulation, flavonoid, recognition, root exudates.

Extraradical mycelia of mycorrhizal fungi are normally the “hidden half” of the symbiosis, but they are powerful underground influences upon biogeochemical cycling, the composition of plant communities, and agroecosystem functioning. Mycorrhizal mycelial networks are the most dynamic and functionally diverse components of the symbiosis, and recent estimates suggest they are empowered by receiving as much as 10% or more of the net photosynthate of their host plants. They often constitute 20%–30% of total soil microbial biomass yet are undetected by standard measures of biomass used by soil scientists and agromomists. Mycorrhizal mycelia provide extensive pathways for carbon and nutrient fluxes through soil, often exceeding tens of metres per gram of soil. We consider the amounts of photosynthate “power” allocated to these mycelial networks and how this is used in fungal respiration, biomass, and growth and in influencing soil, plant, and ecosystem processes. The costs and functional “benefits” to plants linking to these networks are fungal specific and, because of variations in physiology and host specificity, are not shared equally; some plants even depend exclusively on these networks for carbon. We briefly assess the potential contribution of extraradical mycorrhizal mycelium to sustainable agriculture and maintenance of biodiversity and highlight technologies that promise new vistas and improved fine-scale resolution of the dynamic spatial and temporal functioning of these networks in soil.Key words: arbuscular mycorrhiza, ectomycorrhiza, extraradical mycelium, hyphal networks.

Herbivory and competition are known to influence plant performance. Recent investigations showed that tolerance is an important strategy for plant survival under herbivory, but few have examined its interaction with plant competition. We evaluated in a greenhouse experiment if plant tolerance to defoliation is related to species presence in a grazed habitat and how plant tolerance to herbivory changes in a competitive environment. Regrowth capacity of four grassland species, Trifolium repens L., Rumex acetosella L., Vicia sativa L. subsp. nigra (L.) Ehrh., and Senecio jacobaea L., was evaluated as the capacity to compensate for 75% of aboveground biomass removed. Target plants were also grown in competition with Festuca rubra L. subsp. rubra, and different clipping treatments were applied. Plant biomass (above- and below-ground) was measured after 5 months. Rumex acetosella, T. repens, and S. jacobaea were able to compensate fully when grown alone. In competition, only R. acetosella was still able to regrow. In no case did clipping the competitor improve target plant performance (i.e., no beneficial effect from competitor release). These results highlight the importance of considering plant competition when studying plant responses to herbivory.Key words: herbivory, regrowth, competitor release, biomass compensation.

Arbuscular mycorrhizae from four sites in semiarid parts of Namibia were studied. In addition to areas with different rainfall, different land management systems are compared. High numbers of morphological species with a patchy distribution were recorded. Arbuscular mycorrhizal fungal (AMF) sequences derived from grass roots are less diverse than the community of spores found in adjacent soil, indicating different AMF communities in grass roots during the vegetation period or different AMF communities in the roots of trees and herbs. The AMF community does not appear to be influenced by land management systems, but rather by vegetation cover or rainfall regime.Key words: Namibia, arbuscular mycorrhizae, molecular identification.

We examined the role of interspecific competition in the regulation of abundance and coexistence of the dominant grasses in tallgrass prairie using a removal experiment with Andropogon gerardii Vitman and Sorghastrum nutans L. Nash, two of the most abundant grasses in tallgrass prairie. Plant removal treatments (using foliar herbicide), applied to 0.3-m2 plots at the Konza Prairie Biological Station (northeast Kansas, USA), included removal of all A. gerardii, removal of all S. nutans, and no removal. To determine whether soil fertility altered the outcome, we included a fertilizer addition treatment (10 g N·m–2) fully crossed with the removal treatments. Andropogon gerardii removal resulted in significantly increased net photosynthesis, stomatal conductance, and tiller mass in S. nutans. Sorghastrum nutans removal had little effect on A. gerardii, suggesting asymmetric competition. Fertilizer significantly increased tiller mass and flowering stalk production in S. nutans, but had little effect on A. gerardii. The ability of A. gerardii to suppress the performance of S. nutans is consistent with the greater abundance of A. gerardii over much of the tallgrass prairie, while the ability of S. nutans to take advantage of increased resources may be one mechanism by which it avoids competitive exclusion. Because of the greater variability in the performance of S. nutans than in that of A. gerardii, any natural or anthropogenic alterations to this grassland that lead to shifts in dominance between these species may affect ecosystem productivity and stability.Key words: Andropogon gerardii, competition, grassland, neighbour removal, photosynthesis, Sorghastrum nutans, tallgrass prairie.

Strawberries (Fragaria ×ananassa Duch.) grown in alfisols of semiarid areas in India results in suboptimal yields. Low organic carbon and low phosphorus (P) availability, in addition to high P fixation, affects P availability drastically, even when it is applied externally. The benefit to the yield of micropropagated strawberry Fragaria ×ana nassa 'Pajaro' through inoculation with arbuscular mycorrhizal (AM) fungi and P application was examined in a field experiment. AM response was evaluated at four doses of P (50, 100, 150, and 200 kg P·ha–1) in a high P-fixing capacity and P-deficient alfisol. At harvest, all inoculated plants, except those at the highest level of applied P (200 kg·ha–1), had larger fruit yields per plant, unit mass, number of runners, higher shoot dry matter, and shoot P content. However, the AM response as measured by yield varied with P concentration. Inoculated plants had a significantly greater fruit yield when grown at 150 kg P·ha–1, and the yield was comparable with uninoculated plants grown at 200 kg P·ha–1. The external P requirements were 71 kg·ha–1 for mycorrhizal and 106 kg·ha–1 for nonmycorrhizal strawberry plants to obtain 90% of the maximum fruit yield. In terms of P fertilization, this corresponds to a savings of 35 kg·ha–1. The percent mycorrhizal root length colonization, both in inoculated and noninoculated plants, was not found to differ significantly with P application.Key words: strawberry, P fertilization, mycorrhizal dependency.

Leaves of Spartina alterniflora Loisel, a halophytic salt marsh grass, show rapid changes in photosynthetic performance and allocation of carbon and nitrogen resources to key macromolecules during the growing season. Photo system II (PSII) electron transport correlated primarily with the developmental state of the plants and less with the ambient temperature. The ratio of gross CO2 uptake to PSII electron transport showed high interleaf variation but rose steeply to about 0.22 mol CO2·mol PSII electrons–1 through June to late July, with subsequent decline. Seasonal variation in CO2 per PSII electrons correlated with the pattern of maximum capacity for CO2 uptake. RuBisCO (ribulose-1,5-bisphosphate carboxylase-oxygenase) and chlorophyll (Chl) (µmol·m–2) peaked in early June, before the period of maximum leaf elongation. RuBisCO then declined through net dilution as the leaves elongated to maximal length in mid-July. RuBisCO and Chl levels stabilized once leaf elongation stopped in mid-July, but from mid-August onward the leaves showed net loss of RuBisCO and Chl. The plants thus show a developmental program of early remobili zation of nitrogenous macromolecules from leaves even though the plants thereby forego maximal photo synthetic performance during 75 d in late summer and early autumn, when temperature and light are near optimal. Key words: electron transport, photosynthesis, PSII, RuBisCO, Spartina.

Unharvested rice (Oryza sativa L.) straw gets incorporated into soil and interferes with the growth of the next season's crop. Water-soluble phenolics leached from straw into soil may suppress the growth of the next crop. A study was carried out to investigate (i) the effect of soil treated with rice straw (ashes of burned and unburned) leachates on seedling growth and foliar protein content of mustard (Brassica napus var. toria L.), (ii) the modification of rice straw phytotoxicity with abiotic soil, activated charcoal, and nitrogen solution, and (iii) any change in soil inorganic ions and phenolics after treatment with rice straw leachate. Maximum inhibition in root growth of mustard was observed when it was grown in soil treated with leachate prepared by using 100 g of unburned (71.1%, expt. 1; 60.2%, expt. 2) and ashes of burned straw (53.4%, expt. 1; 31.5%, expt. 2). Compared with the untreated control, an increase was observed in the total phenolic content of soil treated with straw leachate, prepared by taking 100, 80, 60, 40, and 20 g unburned straw. When soils were treated with leachate prepared by taking 100, 80, and 60 g straw, a lower level of inhibition was observed in abiotic soil compared with biotic soil. An opposite trend was observed when soil was treated with leachate prepared by taking 40 and 20 g straw. The addition of charcoal eliminated the inhibitory effects of rice straw leachate when leachates were prepared using 40 and 20 g straw. Inhibitory effects of soil treated with leachate prepared from 100 g straw on root growth of mustard were not eliminated after the addition of nitrogen solution. The present study showed that rice straw leachate interferes with seedling growth of mustard and that water-soluble phenolics play an important role in mustard seedling growth inhibition.Key words: allelopathy, rice straw, rice, mustard, phenolics.

The effect of reduced arbuscular mycorrhiza (AM) colonization on nitrogen and phosphorus concentration in grass tissue, species diversity, cover, and productivity was investigated after 4 years of benomyl application in two boreal grasslands, Storvordlia (a high plant diversity field) and Kalvsvangen (a low plant diversity field), in Eastern Norway. In addition, AM colonization, spore numbers, and mycorrhizal inoculum potential was studied during one season. A split-plot design was used, with grazing as the main treatment and the application of benomyl as a subtreatment. AM colonization was significantly reduced because of benomyl application, whereas spore numbers were not significantly affected by the treatment. Mycorrhizal inoculum potential was reduced, but not consistently. In both sites nitrogen and phosphorus concentration of grass tissue was significantly higher in grazed plots than in ungrazed ones, and significantly lower in benomyl subplots than in controls. Plant diversity, composition, and cover had changed significantly after 4 years in grazed controls in Storvordlia, but this was not seen in Kalvsvangen. In both sites, total productivity increased significantly after 4 years of benomyl application. This experiment indicates that AM associations can influence grass nutrient contents and species responses, plant diversity, and productivity in boreal grasslands. These differences were larger in the higher plant diversity site, which is situated at a higher elevation and is under more marginal growth conditions.Key words: arbuscular mycorrhiza, benomyl, productivity, biodiversity, boreal grasslands, ecosystem processes.

Rhizobia from Canadian soils were selected for cold adaptation with the aim of improving productivity of legumes that are subjected to cool temperatures during the growing season. One approach was to use rhizobia associated with legume species indigenous to arctic and subarctic regions: (i) Mesorhizobium sp. isolated from Astragalus and Oxytropis spp. and (ii) Rhizobium leguminosarum from Lathryrus spp. The majority of these rhizobia are considered psychrotrophs because they can grow at 0 °C. The advantages of cold adaptation of arctic Mesorhizobium to improve legume symbiosis were demonstrated with the temperate forage legume sainfoin (Onobrychis viciifolia). In laboratory and field studies, arctic rhizobia were more efficient than temperate (commercial) rhizobia in improving growth of sainfoin and were more competitive in forming nodules. Biochemical studies on cold adaptation showed higher synthesis of cold shock proteins in cold-adapted than in nonadapted arctic rhizobia. Since arctic Mesorhizobium cannot nodulate agronomically important legumes, the nodulation genes and the bacterial signals (Nod factors) were characterized as a first step to modifying the host specificity of nodulation. Another valuable approach was to screen for cold adaptation, that is, rhizobia naturally associated with agronomic legumes cultivated in temperate areas. A superior strain of Sinorhizobium meliloti adapted for nodulation of alfalfa at low temperatures was selected and was the most efficient for improving growth of alfalfa in laboratory and field studies. This strain also performed well in improving regrowth of alfalfa after overwintering under cold and anaerobic (ice encasement) stresses, indicating a possible cross-adaptation of selected rhizobia for various abiotic stresses inherent to temperate climates.Key words: cold adaptation, legumes, symbiotic efficiency, cold shock protein, nodulation genes, anaerobiosis.

To examine intraspecific variability, we studied the biomass production and physiological responses of clonal replicates of the widespread perennial herb Commelina erecta L. grown under different phosphorus concentrations. Five replicates per genotype collected from a small population were exposed to 0.01 or 1.0 mmol·L–1 phosphate in a hydroponic solution and grown under natural radiation. High intraspecific variability occurred for ecologically important traits such as root, leaf, and total plant biomass, with significantly higher responses in the high phosphorus treatment. The norms of reaction for high versus low phosphorus supply often crossed, indicating that selection for different genotypes may be enhanced in some environments and reduced in others. The root phosphorus concentration and phosphorus absorption rate varied among clones. Phosphorus supply increased photosynthetic rates in all clones 15 days after the start of the experiment, but treatment differences were lost with development when significant clone variability was observed. Maximum photosynthetic rates and carboxylation efficiencies were different between clones and treatments; relative stomatal and mesophyll limitations to photosynthesis also varied among clones. Predawn photochemical efficiency remained relatively constant among treatments and clones. Commelina erecta showed a very high intraspecific variability, which is an important factor in plant survival and success.Key words: growth, norms of reaction, photosynthesis, phosphorus absorption.

The developmental patterns of seed, seed coat, and hardseededness were studied in naturally growing crop plants of beach pea (Lathyrus maritimus (L.) Bigel.) at six reproductive growth stages (S1–S6). Grass pea (Lathyrus sativus L.) seeds were used for comparison in some experiments. The accumulation of fresh and dry weight in pod shell and seed of beach pea and pod shell of grass pea followed an almost sigmoidal pattern. However, grass pea seed showed a linear pattern of weight accumulation. During maturation, moisture content of pod shells and seeds decreased because of dehydration. Beach pea seeds were able to germinate precociously at S4. Seeds collected between S1 and S3 failed to germinate because of immaturity, whereas the development of hard seed coats prevented germination in seeds gathered at S5 and S6. An imbibition test revealed that hardseededness completely prevented water absorption of S5 and S6 seeds even after 24 days of soaking. In grass pea, precocious seed germination was observed at S3. However, speed of germination, germination percentage, seedling length and dry weight increased as seeds approached maturity. Lipid and protein accumulation in seeds of both species increased progressively with maturity and showed a positive correlation with seed weight accumulation. In both beach pea and grass pea seeds, S6 was identified as a physiological maturity stage.Key words: beach pea, grass pea, hard seed, imbibition, Lathyrus, seed coat, seed development, water impermeability.

Silique dehiscence (shattering) in Brassica species has a pronounced effect on agricultural yields. Shattering is highly variable and difficult to quantify, and consequently there have been few studies that explore interspecies variation in shattering in relation to silique development. In this paper, a rapid and simple method has been developed for quantifying silique dehiscence. The variable-speed pod splitter is a mechanical device that provides a measure of the impact force required to trigger shattering of individual siliques. We have used the variable-speed pod splitter to show that siliques of Brassica rapa cv. Parkland were significantly more resistant to shattering than those of Brassica napus cv. Quantum. Siliques of both species became prone to shattering following a short period of rapid dehydration during which their water content fell from approx. 70% to approx. 10% (based on weight). Magnetic resonance imaging of individual siliques of varying ages revealed that water loss occurred from the inside the pericarp in B. napus cv. Quantum and from the outside of the pericarp in B. rapa cv. Parkland. We suggest a mechanism for how this difference in the pattern of water loss contributed to the difference in susceptibility to shatter between the two cultivars.Key words: shattering, magnetic resonance imaging, canola, valve.

The influence of nutrients and intraspecific variations in growth form were examined for their effect on plant density in stands of northern wild rice, Zizania palustris L. In a field situation, densities of a size-restricted wild rice population increased as nutrient levels in the sediment increased. No self-thinning occurred until the population density exceeded 350 plants/m2. A series of controlled experiments examined whether these wild rice densities were determined by resource depletion and (or) intraspecific competition. As nutrient levels increased under constant plant densities, tillering, dry weight, and seed production increased more for populations with the capacity for higher vegetative and reproductive potential. As plant densities increased under constant nutrient levels, height, weight, and seed production declined but inequality of individual plants increased. When both nutrient levels and population densities were increased simultaneously, seed production per panicle declined at higher densities under unfertilized conditions but was unaffected in the treatment with the highest fertilizer level. It was hypothesized that plant densities under field conditions were the result of an integrated mechanism that was influenced by nutrients and the degree of asymmetric competition characteristic of the population. As nutrient levels increase, plant densities would be expected to decrease for populations with high levels of asymmetric competition and increase for populations with low levels of asymmetric competition.Key words: wild rice, density effects, nutrients, intraspecific variation.

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis – outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants.Key words: AM, epidermis, evolution, pea, rhizobia, sym mutant.

The aim was to investigate the impacts of trampling on water loss and partitioning in vegetation with contrasting structure and species composition. A new design of weighing lysimeter was used in glasshouse experiments to compare evapotranspiration from intact and trampled blocks of vegetation. The lysimeter system was able to detect differences between treatments after only a few hours. Evapotranspiration was recorded for six communities, representative of cryptogam - vascular plant communities found in the Cairngorm Mountains of Scotland. Vegetation blocks of Racomitrium and Vaccinium/Hylocomium heath communities had the greatest cumulative evapotranspiration and lichen heath the least over 48 h. Blocks from three of the communities (Agrostis/Festuca grassland, Calluna wet heath, and lichen heath) were used in a trampling experiment with five levels of damage. Trampling progressively destroyed the structure of the vegetation of all communities and increased the rates of water loss from the blocks. The grassland community vegetation was the most resilient. These results help to link the massive changes in vegetation structure resulting from trampling to effects on water loss and microclimate.Key words: weighing lysimeter, evapotranspiration, recreation, trampling, cryptogams, montane vegetation.

The relative susceptibilities of major cereal species to mycotoxin contamination have rarely been studied in eastern Canada or elsewhere. The concentration of 13 mycotoxins in 673 corn (Zea mays L.), 99 wheat (Triticum aestivum L.), 116 barley (Hordeum vulgare L.), and 73 oat (Avena sativa L.) samples collected from eastern Canada from 1991 to 1998 crops were compared. Deoxynivalenol (DON) was found to be the most common mycotoxin in all four species. DON contamination was more frequent but less severe in corn than in wheat and barley, and it was least frequent and least severe in oats. Wheat and barley were equally susceptible to DON contamination. The DON content of 8.9% of the corn, 31.3% of the wheat, 22.4% of the barley, and 1.4% of the oat samples exceeded 1 mg·kg-1, the maximum tolerance level recommended for swine feed. Contamination with zearalenone, T-2, HT-2, diacetoxyscirpenol, ochratoxin A, nivalenol, fumonisins, 3-acetyl DON, or 15-acetyl DON was minor in eastern Canada and varied from species to species. Fusarenon X, 15-monoacetoxyscirpenol, and neosolaniol were not detected. Equally significant, approximately one third of the corn and barley samples were contaminated with two to seven mycotoxins. The presence of two or more mycotoxins could have additive or synergistic effects on the toxicity. Measures to reduce DON contamination are needed for corn, wheat, and barley.Key words: Fusarium, mycotoxins, corn, wheat, barley, oats.

We examined 90 dry tropical tree species growing on contrasting soil types (relatively infertile Ultisol and more fertile Inceptisol) for leaf traits such as leaf habit, specific leaf mass (SLM, leaf dry mass per leaf area), leaf chemistry (nutrient concentrations and C/N ratios), and nutrient resorption. Across the species, SLM ranged from 4.06 to 15.74 mg·cm–2 in mature leaves and from 2.60 to 15.12 mg·cm–2 in senesced leaves. Mature leaf N and P concentrations varied from 0.86% to 4.11% and 0.13% to 0.21%, respectively. Senesced leaf N concentrations varied from 0.49% to 1.90% and P from 0.04% to 0.47%. Resorption efficiencies varied from 26% to 83% (mean = 58.32% ± 1.20%) for N and from 16% to 80% (mean = 49.57% ± 1.48%) for P indicating that the woody species of dry tropical environments resorbed different nutrients in substantial amounts to support new growth. Deciduous species had greater resorbed nutrient pools and resorption efficiencies than evergreen species. Compared with the nutrient-rich environment, species from the nutrient-poor environment had a lower resorbed P pool and lower resorption efficiencies for N and P, but had similar N and P concentrations in mature leaves. Resorption efficiencies for C, N, and P were generally correlated, suggesting that the resorbed C pool acted as a vehicle for mobilizing nutrients, especially N. Species with a low or high C/N ratio in senesced leaf and a low or high N resorption efficiency occurred in both nutrient-poor and nutrient-rich environments, as well as among deciduous and evergreen leaf habits, indicating individualistic adaptations to optimize the efficiency of nutrient resource use and conservation of the dry tropical woody vegetation.Key words: leaf chemistry, leaf traits, resorption efficiency, resorbed nutrient pools, substrate-quality stability.

Fine roots (diameter

Restored prairie pothole wetlands in North America are often enriched by nitrate-N (NO3-N) that has been lost from surrounding agricultural systems. In addition, these wetlands are increasingly col...

Seeds of bull thistle, Cirsium vulgare (Savi) Ten., were exposed to varying numbers of cycles of wetting and drying, in both Petri dishes and pots of soil, to investigate the effects of exposure to such cycles on subsequent germination of the seeds or emergence of the seedlings. Following exposure to the cycles, seeds in Petri dishes were set to germinate in one of four diurnal environments: 20:10°C alternating light and darkness, 20:10°C constant darkness, 30:15°C alternating light and darkness, or 30:15°C constant darkness. Total percent germination was reduced after exposure to eight cycles of wetting and drying, and germination rate was reduced after exposure to two or more cycles. Percent germination was reduced at the higher temperature but light availability had little effect. Reduction in seedling emergence in pots of soil after exposure to an intermediate number of cycles was greater than in Petri dishes but not as great with exposure to eight cycles. Seedling emergence patterns in pots that experienced any wetting-drying treatment were bimodal, with a second pulse of emergence several weeks after the termination of the cycles. This suggests that some seeds were induced into a dormant state through exposure to the cycles of wetting and drying. Such induced dormancy may serve to prevent seed germination in the autumn, promoting an attenuated and intermittent pattern of germination.Key words: Cirsium vulgare, bull thistle, wetting and drying, seed germination, induced dormancy, intermittent germination.

According to the vernal dam hypothesis, spring ephemeral herbs temporarily sequester large nutrient pools in deciduous forests prior to canopy closure and return the nutrients to the soil following senescence of aboveground tissues. However, many species resorb nutrients from their leaves back to belowground tissues during senescence, and the degree of resorption is often associated with soil nutrient availability. Species that store large proportions of their absorbed nutrients between years are not participating in the temporary sequestering and rapid recycling of nutrients implied by the vernal dam. We investigated the extent to which Claytonia virginica L. sequestered and returned nutrients to the soil in response to nitrogen (N) and phosphorus (P) availability. We tested the effect of nutrient availability on nutrient use efficiency, resorption efficiency, and resorption proficiency (% nutrient in senescent leaves) of Claytonia. Nutrient additions significantly decreased N but not P use efficiency of Claytonia, particularly as the growing season progressed. Nutrient additions also significantly reduced N resorption efficiency from 80 to 47% and decreased P resorption efficiency from 86 to 56%. N and P resorption proficiencies were also significantly lower in senesced leaves of fertilized plants: N concentrations were 2.33% when unfertilized and 4.13% when fertilized, while P concentrations were 0.43% when unfertilized versus 0.57% when fertilized. When unfertilized, Claytonia was more efficient at resorption compared with other spring herbs, but similar to other species when fertilized. However, Claytonia was much less proficient in resorbing nutrients than other reported plants, because senescent tissues maintained substantially higher concentrations of N and P, particularly when fertilized. In conclusion, Claytonia, an important spring ephemeral species, exhibits physiological responses that emphasize its role in the vernal dam by its temporary sequestration and substantial, rapid return of nutrients in deciduous forests. Adding nutrients to the site increases the total mass and the relative proportion of nutrients that Claytonia returns to the soil rather than sequestering between seasons, which ultimately increases nutrient recycling rates within the entire system.Key words: Claytonia virginica, nutrient response, resorption efficiency, nutrient cycling, spring ephemerals, vernal dam.

Stomatal conductance and leaf water potential of aspen (Populus tremuloides Michx.) were measured in response to defoliation intensity, both in the field and under controlled conditions. There was evidence of increased stomatal conductance in trees with 50 and 98% defoliation, but no change in leaf water potential. Under controlled conditions, stomatal conductance and rate of photosynthesis were measured under high and low vapour pressure deficits (VPD). Under high VPD, overall stomatal conductance and rates of photosynthesis were greatly reduced. However, in both VPD treatments, there was evidence of increased stomatal conductance and compensatory photosynthesis following defoliation. These findings may be due to increases in leaf specific hydraulic conductance following defoliation.Key words: defoliation, stomatal conductance, leaf water potential, compensatory photosynthesis.

The grass Amphibromus scabrivalvis Trin. Swallen produces new ramets from the base of established ramets (to produce a phalanx growth form) and along rhizomes (to produce a guerrilla growth form). It stores resources in stem bases. This research examined genotypic variation in clonal growth and storage and its relation to the performance of replicated genets under competition. It was hypothesized that (i) genets would vary significantly in traits important to clonal growth and storage during competition, (ii) competition would reduce clonal growth capability, (iii) guerrilla-like spacing would enhance clonal growth under competition at low competitor density, and (iv) allocation to storage organs would increase under competition. Fourteen genets of A. scabrivalvis were divided into ramets and planted into pots in the glasshouse alone or in competition with four equally distributed seedlings of Lolium perenne L., a phalanx competitor. Leaf area and mass, as well as the distance of new ramets from the parental ramet, were measured at 9 and 18 weeks. Dry mass of leaves, stem bases, rhizomes, and roots was determined at 18 weeks. Relative competition intensity, based on the difference in total mass per genet between control and competition treatments relative to the control, varied between 0.69 and 0.83. Ramet production, the proportion of the total leaf area in new ramets, and ramet distances varied significantly among genets and were reduced by competition. Mean ramet distance was positively correlated with total mass. Allocation of dry mass to storage organs was significantly increased under competition and may allow genet persistence within an occupied area. The relative ranking of genets in the control and competition treatment was not different, and there were no differences in the relative degree to which the total mass of genets was reduced by competition, despite significant variation in total mass accumulated in the absence of competition.Key words: Amphibromus scabrivalvis, clonal grass, competition, genotypic variation, guerrilla strategies, phalanx strategies, rhizomatous perennial.

Two cultivars of durum wheat (Triticum turgidum L.) with known, and different, grain-Cd accumulation were used to compare root tissue accumulation of Cd with root morphology. Six-day-old 'Kyle' and 'Arcola' seedlings were exposed to a range of Cd2 + concentrations (3.91 × 10-8 - 3.91 × 10-7 M) for 0-200 min, and root Cd contents for the two cultivars were compared with root morphological characteristics. 'Kyle' roots contained 35% less Cd per root system after 200 min of exposure and had less root surface area and fewer root tips than 'Arcola'. 'Kyle' roots also contained 30% less Cd per gram of dry weight than 'Arcola' roots after 200 min of exposure. 'Kyle' roots also had fewer root tips per unit of root dry weight and less surface area per unit of dry weight (DW) than 'Arcola'. When cadmium concentration data (µg·g-1 DW) were expressed per unit of root surface area (µg·cm-2) and per number of root tips (µg·tip-1), the difference in root Cd content between the two cultivars was smaller. These results suggest that greater root Cd content of 'Arcola' than 'Kyle' can be explained by differences in morphology, specifically that a greater surface area and more root tips in 'Arcola' leads to greater Cd accumulation in root tissue.Key words: cadmium accumulation, durum wheat, root morphology.

The importance of competition as a major influence on the composition and structure of plant communities has recently been questioned, because other types of interactions can cause significant compositional changes. The goal of this research was to broaden our understanding of disease as a process structuring plant communities under a variety of competitive scenarios. Two cultivars of spring wheat (Triticum aestivum L., cv. Twin and cv. Penawawa) and wild oats (Avena fatua L.) were planted at three densities and at five proportions. One-half of the experimental material was inoculated with uredospores of Puccinia recondita. Increasing the proportion of wheat or oats in mixtures led to significant increases in the amount of aboveground biomass and total seed weight for that species. The seed weight and aboveground biomass per culm or per planted seed decreased for wheat and wild oats as the proportion of wild oats increased in mixtures, indicating a competitive advantage for wild oats when grown with wheat. Wild oats generally did not respond significantly to the effects of leaf rust on wheat, while wheat performance declined. Lowered wheat performance in inoculated stands was the main reason for lower relative biomass ratios of wheat to wild oats. Puccinia recondita infections occurred late in the life cycle of wheat, thereby decreasing the potential impact on wild oats' adults through competitive interactions.

Leafy spurge (Euphorbia esula L.) represents a non-native, invasive weed that dominates many regions across North America. While many research efforts are aimed at controlling the growth and spread of this plant, relatively little is known about its breeding system. This study provides evidence that leafy spurge is self-compatible, with selfed plants producing roughly half as many seeds as outcrossed plants. Unpollinated flowers failed to set seed and thus preliminary tests for apomixis were negative. However, microscopic examination of pollinated flowers revealed that pollen tubes did not appear to enter ovules in either selfed or outcrossed flowers. Therefore, leafy spurge exhibits structural evidence that suggests pseudogamy may play a role in the reproductive strategies of this invasive weed.

We investigated the response of the important forage plant Luzula arcuata Swartz ssp. confusa (Lindeb.) Blytt to simulated grazing during two growing seasons in a phytotron. Plants were clipped at five levels of intensity and fertilized at two levels of nitrogen. There was no reduction in net primary production of clipped compared with unclipped plants after either of the two growing seasons. Such results indicate that L. arcuata ssp. confusa can compensate for defoliation. Plants overcompensated in net aboveground primary production (NAPP) in the first season and compensated in NAPP in the second season. Plant traits contributing to the compensation in NAPP were stimulation of tillering in clipped plants, increase in specific leaf area of the most heavily clipped plants, and a higher proportion of NAPP occurring below clipping height in frequently clipped plants. Sexual reproduction (number of flowering shoots) was enhanced in the second season in plants clipped at 6 cm above the soil and reduced in plan...

Dormancy-breaking requirements and type of dormancy were determined for seeds of Diervilla lonicera Mill. Seeds have an underdeveloped linear embryo that is about 35% of the length of the seed at maturity. Embryos (in intact seeds) grew at 25:15°C but not at 5°C. Up to 85% of the freshly matured seeds had morphological dormancy (MD), and thus, they germinated within about 30 days on a moist substrate in light at 30:15°C; a maximum of 3% of the seeds germinated in constant darkness. The other portion of fresh seeds had nondeep simple morphophysiological dormancy (MPD) and required a period of warm stratification or treatment with GA3 to break dormancy. These seeds also required light to germinate. In contrast, cold stratification induced dormancy, and dry storage for up to 1 year did not effectively break dormancy. Seeds with MD germinated to significantly higher percentages on soil than on filter paper or on sand. Seeds sown on soil in a non-temperature-controlled greenhouse in mid-November germinated mos...

We examined the effect of prolonged (up to 450 days) soil burial in the field on seed morphological traits (seed coat structure, permeability) to identify their potential roles in seed dormancy and release. Such traits were examined in species with seeds that demonstrated an obligate requirement for soil storage before germination: the dormant seeds of Anigozanthos manglesii D. Don, Conostylis neocymosa Hopper, Stylidium affine Sonder, and Stylidium crossocephalum F. Muell., and the deeply dormant fruits of Leucopogon conostephioides D.C. We detected species-specific and environmentally induced variation in seed morphology following soil burial. In A. manglesii and L. conostephioides, a significant deterioration of the seed coat or fruit wall and an increased permeability of the seed coat to water and solutes were correlated with germination responses. In these species, the seed coat and (or) fruit wall delayed germination until (morpho) physiological dormancy was broken. In C. neocymosa, S. affine, and S. crossocephalum, weathering of the seed coat, permeability, and germination were not correlated traits. These species appeared to possess physiological dormancy mechanisms and required environmental cues for dormancy release.Key words: physiological dormancy, soil burial, seed coat, morphology.

We examined the development of ovules and components of reproductive yield as they related to plant size and fruit position within plants of Alliaria petiolata (M. Bieb.) Cavara and Grande from three sites. We found that both factors affected fruit and seed production, and patterns of ovule fate. Larger plants were more fecund than their smaller counterparts, while proximally positioned flowers within an inflorescence produced more seeds. Maternal investment was regulated according to plant mass at two levels: among fruits (small plants had significantly lower fruit-set than did large plants), and within fruits (small plants had significantly fewer ovules and significantly higher proportions of aborted ovules per fruit than did large plants). All absolute measures of reproduction had positive linear relationships with plant size, but a threshold size for flowering was not found. Most proportional measures of reproduction were independent of plant size. Small plants produced the same relative proportions o...

The competitive ability for nodulation of Alnus glutinosa (L.) Gaertn. plants by Frankia strains inoculated into soil with indigenous Frankia populations was studied at two matric potentials representing "dry" (-0.016 MPa) and "wet" (-0.001 MPa) conditions. In pots kept at a matric potential of -0.001 MPa, nitrate concentrations decreased within 3 weeks more than 10-fold to an average of approx. 200 µmol·(g soil dry wt.)-1. After 4 months, nitrate concentrations in these pots were 16 ± 16 and 277 ± 328 µmol·(g soil dry wt.)-1 (mean ± SD) for non-inoculated and inoculated soils, respectively. At a matric potential of -0.016 MPa, nitrate concentrations for non-inoculated and inoculated soils were 687 ± 491 and 1796 ± 1746 µmol·(g soil dry wt.)-1, respectively. Inoculated plants always grew better than their non-inoculated counterparts. The largest plants were found on inoculated soil at a matric potential of -0.001 MPa, whereas the smallest plants were found on non-inoculated soil at the same matric potential. At a matric potential of -0.016 MPa, plants grown on non-inoculated soil were not as tall as those grown on inoculated soil and were slightly chlorotic, indicating that the high level of nitrate in the soil was not providing optimal plant growth conditions. The number of nodule lobes formed on plants was not significantly different among treatments, though size and weight of lobes differed. Nodules from plants grown on inoculated soils always harbored vesicle-producing Frankia populations, while nodules from plants grown on non-inoculated soils harbored only Frankia with distorted vesicles or no Frankia at all. All strains in nodules from plants grown on non-inoculated soil were of Alnus host infection group IIIa. Nodules from plants grown on soil inoculated with strains ArI3 (group IIIa), Ag45/Mut15 (group IV), and AgB1.9 (group I) were also infected by Frankia strain Ag45/Mut15. These results indicate that by inoculation, Frankia populations could be established under conditions that did not favour vesicle formation in root nodules formed by the indigenous Frankia population. Inoculation even in soils with high nitrogen content might therefore be an appropriate strategy to enhance plant growth.Key words: competition, fluorescent oligonucleotide probes, inoculation, in situ hybridization, matric potential, nitrate, rRNA.