Growth Medium and Phosphorus Supply Affect Cluster Root Formation and Citrate Exudation by Lupinus albus Grown in a Sand/Solution Split-Root System

We examined cluster root formation and root exudation by white lupin (Lupinus albus L. cv. Kiev Mutant) in response to growth medium and phosphorus supply in a sand/solution split-root system. The split-root system consisted of a nutrient solution compartment and a siliceous sand compartment. Phosphorus was applied at 1 (low-P plants) or 50 (high-P plants) μM as KH₂PO₄ to the solution compartment and at 10, 50 or 250 mg P kg⁻¹ as hydroxyapatite (Ca-P) to the sand compartment. In contrast to the high-P plants, P concentration and P uptake in the low-P plants increased with increasing P supply to the sand compartment. The NaHCO₃-extractable P was lower in the rhizosphere of the low-P plants than the high-P ones. The proton extrusion rate by the solution-grown roots of the low-P plants was higher than that of the high-P plants at the early growth stage. For the low-P plants, the proportion of dry root biomass allocated to cluster roots was higher in the solution compartment than that in the sand compartment. The citrate exudation increased in the sand compartment and decreased in the solution compartment with time, showing a lack of synchronization in citrate exudation by two root halves grown in different media. The cluster root proportion and citrate exudation in both compartments decreased with increasing shoot P concentration. An additional experiment with no P added to either root compartment showed that the proportion of cluster roots was about 9% lower in the sand than solution compartments. The results suggest that cluster root formation and citrate exudation can be significantly affected by the root growth medium in addition to being regulated by shoot P status. More P can be exploited from sparingly available Ca-P by the low-P plants than the high-P ones due to greater citrate exudation under P deficiency.     

Characterization of phosphorus uptake in slow- and fast-growing southern pine seedlings grown in solution culture

The effects of low-P growth conditions on growth and net P acquisition were examined in two species of pine that are indigenous to P-deficient soils of the Atlantic Coastal Plain: pond pine (Pinus serotina Michx.), a moderately-fast growing pine, and a slow-growing seed source of loblolly pine P. taeda L.) from Texas. Short-term ³²P uptake experiments were conducted using intact nonmycorrhizal seedlings that had been grown for 7 weeks in continuously-flowing solution culture at 5 or 100 µM P. Growth and P uptake of pond pine were more responsive to a higher P supply than the slow-growing loblolly pine. Pond pine seedlings in the 100 µM P treatment were twice the size of those grown in 5 µM P and accumulated almost five times as much seedling P. In contrast, seedling biomass of loblolly pine increased by only 8% under high-P growth conditions, and seedlings accumulated twice as much P, reflecting the higher P concentrations in shoot and root tissues. Although rates of unidirectional influx of ³²P were 22 and 61% higher under low-P growth conditions in pond and loblolly pine, respectively, net uptake rates in seedlings from the 5 µM P treatment were over three times those of seedlings grown in 100 µM P. These results suggest that unidirectional efflux out of the root was controlling net uptake of P as much, if not more, than unidirectional influx. Efflux of³² P out of root tissue, particularly older tissue, decreased in seedlings grown under low-P conditions, possibly due to a reduction in the size of the phosphorus pool available for efflux, i.e. the soluble P_i pool. Over 75% of the total root P in both loblolly and pond pine seedlings grown in 100 µM P treatment was present as organic P, suggesting that organic P, particularly phytate, may represent important storage pools in roots of woody species. Within each species, higher rates of influx and net uptake in seedlings from the low-P treatment were associated with lower P concentrations in shoot and root tissue, and shoot FW:root FW ratios. Efflux may represent a short-term means of regulating net P uptake, while the demand for P created by growth and storage may represent a long-term regulation.     

The effects of parental CO2 environment on seed quality and subsequent seedling performance in Bromusrubens

Seeds were collected and compared from parent plants of Bromusrubens L. (Poaceae), an exotic Mojave Desert annual grass, grown in ambient (360 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂ to determine if parental CO₂ growth conditions affected seed quality. Performance of seeds developed on the above plants was evaluated to determine the influence of parental CO₂ growth conditions on germination, growth rate, and leaf production. Seeds of B. rubens developed on parents grown in elevated CO₂ had a larger pericarp surface area, higher C:N ratio, and less total mass than ambient-developed seeds. Parental CO₂ environment did not have an effect on germination percentage or mean germination time, as determined by radicle emergence. Seedlings from elevated-CO₂-developed seeds had a reduced relative growth rate and achieved smaller final mass over the same growth period. Elevated-CO₂-developed seeds had smaller seed reserves than ambient seeds, as determined by growing seedlings in sterile media and monitoring senescence. It appears that increased seed C:N ratios associated with plants grown under elevated CO₂ may have a major effect on seed quality (morphology, nutrition) and seedling performance (e.g., growth rate and leaf production). Since the invasive success of B. rubens is primarily due to its ability to rapidly germinate, increase leaf area and maintain a relatively high growth rate compared to native annuals and perennial grasses, reductions in seed quality and seedling performance in elevated CO₂ may have significant impacts on future community composition in the Mojave Desert. Received: 11 April 1997 / Accepted: 20 November 1997     

Buckwheat accumulates aluminum in leaves but not in seeds

Buckwheat (Fagopyrum esculentum Moench. cv Jianxi) is highly resistant to Al stress and is known to be an Al-accumulator. Pot experiments were carried out in a greenhouse to investigate the accumulation of Al in leaves and seeds of buckwheat. Plants were grown for 12 weeks in a strong acid soil amended with or without CaCO₃ at a rate of 1 g kg⁻¹ soil. Old leaves accumulated as much as 10 g kg⁻¹ Al of dry weight when the plants were grown in the acid soil, while the Al concentrations in leaves immediately adjacent to seeds, seed coats, and embryos were, on average, 4516, 41.2 and 7.7 mg kg⁻¹, respectively. The Al concentration significantly decreased in leaves when the plants were grown in the limed soil, and the Al concentrations in leaves immediately adjacent to seeds, seed coats, and embryos were, on average, 1586, 21.3 and 3.1 mg kg⁻¹, respectively. These results show that seeds accumulate much less Al than buckwheat leaves. The underlying mechanisms are discussed. Section Editor: H. Lambers     

Seed germination and seedling physiology of Larix kaempferi and Pinus densiflora in seedbeds with charcoal and elevated CO2

We investigated the effect of ectomycorrhizal colonization, charcoal and CO₂ levels on the germination of seeds of Larix kaempferi and Pinus densiflora, and also their subsequent physiological activity and growth. The seeds were sown in brown forest soil or brown forest soil mixed with charcoal, at ambient CO₂ (360 μmol mol⁻¹) or elevated CO₂ (720 μmol mol⁻¹), with or without ectomycorrhiza. The proportions of both conifer seeds that germinated in forest soil mixed with charcoal were significantly greater than for seeds sown in forest soil grown at each CO₂ level (P < 0.05; t-test). However, the ectomycorrhizal colonization rate of each species grown in brown forest soil mixed with charcoal was significantly lower than in forest soil at each CO₂ treatment [CO₂] (P < 0.01; t-test). The phosphorus concentrations in needles of each seedling colonized with ectomycorrhiza and grown in forest soil were greater than in nonectomycorrhizal seedlings at each CO₂ level, especially for L. kaempferi seedlings (P < 0.05; t-test), but the concentrations in seedlings grown in brown forest soil mixed with charcoal were not increased at any CO₂ level. Moreover, the maximum net photosynthetic rate of each seedling for light and CO₂ saturation (P _max) increased when the seedlings were grown with ectomycorrhiza at 720 μmol mol⁻¹ [CO₂]. Ectomycorrhizal colonization led to an increase in the stem diameter of each species grown in each soil treatment at each CO₂ level. However, charcoal slowed the initial growth of both species of seedling, constraining ectomycorrhizal development. These results indicate that charcoal strongly assists seed germination and physiological activity.     

Dihydroquercetin protects barley seeds against mold and increases seedling adaptive potential under soil flooding

Barley (Hordeum vulgare L.) seeds were soaked in aqueous 10⁻⁴ M dihydroquercetin (DHQ) to examine its influence on seed germination and further growth of seedlings under optimal soil watering and flooding conditions. The adaptive potential of the plants was estimated by the content of thiobarbituric acid reactive substances (TBARs) and the activity of ascorbate peroxidase (AsP). High-grade seeds were germinated evenly under (−DHQ)- and (+DHQ)-treatments. Low-grade seeds soaked in DHQ, showed no mold and twofold germination rate in comparison with the same seeds soaked in water. The seedlings grown from the similarly germinated seeds did not differ from each other in the shoot growth, independent of the DHQ-pretreatment. The root growth was higher in DHQ-pretreated plants. Soil flooding suppressed the shoot and root growth rates in non-pretreated and DHQ-pretreated plants, however TBARs content was lower in the roots and leaves of (+DHQ)-seedlings as compared to the (−DHQ)-ones. The activity of AsP increased more significantly in the (+DHQ)-plants. The ratio between TBARs content and the AsP activity was lower in the leaves of (+DHQ)-plants both under optimal soil conditions and flooding. Thus, the treatment of low-grade barley seeds with DHQ protects the seeds against mold and increases adaptive potential of the seedlings.     

The influence of ultraviolet-B radiation on the growth, pigment production and chlorophyll fluorescence of Norway spruce seedlings

Norway spruce (Picea abies (L.)Karst.) from seven seed sources was grown in a greenhouse with 8.3 and 14.7 kJ·m⁻²·d⁻¹ m UV-B_BE (biologically effective UV-B: 280–320 nm) irradiation, and with no supplemental irradiation as control. The seedlings total biomass (dry weight) and shoot growth decreased with high UV-B treatment but spruce from low elevation seed sources were more affected. The seedlings grown at the highest UV-B irradiance (14.7 kJ·m⁻²·d⁻¹) showed from 5 to 38% inhibition of total biomass and 15 to 70 % shoot growth inhibition. Norway spruce populations from higher altitude seed sources manifested greater tolerance to UV-B radiation compared to plants from low altitudes. Changes in phospholipids and protective pigments were also determined. The plants grown at the lower UV-B irradiance (8.3 kJ·m⁻²·d⁻¹) showed greater ability to concentrations UV-B-absorbing pigments then control plants. Chlorophyll a fluorescence parameter R_fd, (R_fd=(F_m-F_s)/F_s) showed a significant decrease in needles of UV-B treated plants and this correlated with the altitude of seed source. Exposure to UV-B affect levels of the ratio of variable to maximum fluorescence (F_v/F_m). Results from this study suggest that the response to increased levels of UV-B radiation is depended upon the ecotypic differentiation of Norway spruce and involved changes in metabolites in plant tissues.     

Chemical,physical and biological control of carrot seedling diseases

In naturally infested soil containingPythium ultimum, P. acanthicum andPhytophthora megasperma, onlyP. ultimum was associated with root rot and damped-off seedlings. Damping-off was promoted by low soil temperatures and by flooding. Seedling stands were markedly reduced when seed was pre-incubated in soil at 12°C but not at 25°C or 35°C. Dusting carrot seed with metalaxyl significantly increased seedling stands in the field at rates from 1.5–6 g kg⁻¹ seed and in both flooded and unflooded, naturally infested soil at 3.15 g kg⁻¹. In greenhouse experiments using artifically infested soil,P. ultimum andP. paroecandrum caused damping-off of carrot seedlings andRhizoctonia solani reduced root and shoot weights.R. solani caused damping-off in nutrient-enriched soil.P. acanthicum andP. megasperma were not pathogenic to seedlings, although both fungi colonized roots. Soil populations of allPythium spp., particularlyP. ultimum, increased during growth of seedlings and population growth ofP. megasperma was promoted by periodic flooding. Infestation of soil withP. acanthicum did not reduce damping-off of carrot seedlings byP. ultimum orP. paroecandrum, but significantly increased root and shoot weights and decreased root colonization byR. solani P. acanthicum has potential as a biocontrol agent againstR. solani.     

Plant factors influencing phosphorus uptake by white clover from solution culture

Summary The phosphorus (P) uptake rate of several white clover populations was determined in two solution culture experiments. Populations and cultivars differed in P uptake per plant and per unit root length in both experiments. Correlation and multiple regression analysis showed that differences between populations for P uptake per plant were largely related (r²>80%) to differences in leaf area and absolute growth rate, when plants had been grown at high-P levels, and by differences in root size and absolute growth rate when plants had been grown at low-P levels. Differences between populations for P uptake per unit root length were related (r²≈50%) to leaf area and relative growth rate in experiment 1 and to transpiration rate and water influx in experiment 2, when plants were pretreated at high-P levels. Differences between populations for P uptake per unit root length were negatively related to root size when plants had been grown at low-P levels. On the basis of these and other results it is suggested that P uptake per plant is determined largely by shoot factors. However, P uptake per unit root length is negatively related to root size, because demand for P is largely determined by shoot factors, and so differences in root size lead to an apparent difference in uptake per unit of root size.     

Nitrogen fixation in the rhizosphere and rhizoplane of barley

Summary Aerobic and anaerobic N₂-fixing bacteria developed in the rhizosphere of barley seedlings and exhibited N₂ase activity when seedlings were grown in sterilized sand-nutrient cultures containing low levels of combined nitrogen. The source of the N₂-fixing bacteria appeared to be the seed. Average daily rates up to 0.9 μmoles C₂H₄ h⁻¹ g⁻¹ dry root tissue were measured, but the intensity of the activity was affected by moisture levels and concentration of combined N in the rhizosphere. Removal and washing of the roots did not remove the activity, and roots remained active even after surface-sterilization. An unidentified aerobic N₂-fixing bacterium was isolated from the rhizoplane of active barley roots. Inoculation of barley seedlings with the aerobic N₂-fixing bacterium enhanced N₂ase activity of excised roots 10-fold, with average rates of 0.9, 1.1 and 1.3 μmoles h⁻¹ g⁻¹ dry root assayed under pO₂ of 0.01, 0.02 and 0.04 atm respectively. The aerobic N₂-fixing bacterium also exhibited N₂ase activity when inoculated into the rhizosphere of oat, rice and wheat seedlings. Microscopic observations of sterilized live and stained barley roots suggest that the aerobic N₂-fixing bacterium is an endophyte which infects root tissue and metamorphoses into vesicle-like structures.     

Canola seed germination and seedling emergence from pre-hydrated and re-dried seeds subjected to salt and water stresses at low temperatures

Low soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre-hydration and re-drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na₂SO₄ was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na₂SO₄, suggesting that NaCl was more toxic than Na₂SO₄ during seedling development. Pre-hydration and re-drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg^-1 of NaCl or 50 mmol kg^-1 of Na₂SO₄. These results suggest that the pre-hydration and re-drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.     

A smoke-derived butenolide improves early growth of tomato seedlings

3-Methyl-2H-furo[2,3-c]pyran-2-one is an active compound isolated from plant-derived smoke water. It has a stimulatory role during seed germination similar to that of smoke or aqueous extracts of smoke. The present study was undertaken to gain insight into the physiological events involved in seed germination and seedling development and which are affected by butenolide using tomato (Lycopersicon esculentum Mill.) cultivar “Heinz 1370” seeds. No stimulatory role on the seed germination of tomato was recorded following the use of the butenolide, however, post-germinative growth of tomato seedlings was significantly improved over the control (P ≤ 0.05). The emergence of the radicle and elongation of the hypocotyls and radicles were accelerated in seeds imbibed with butenolide at 10⁻⁷ M. Flow cytometry studies showed that in butenolide-treated seeds the ratio of cells with replicated DNA was increased. Seedling vigour and weight were significantly increased by the butenolide (P ≤ 0.05). An inverse correlation was observed between the weight of cotyledons and the weight of the hypocotyls and radicle during seedling development. This is an indication that the butenolide is implicated in mobilization and utilization of stored reserve materials in developing tomato seedlings.     

Photoregulation of the Greening Process of Wheat Seedlings Grown in Red Light*

Wheat seedling grown with their shoot bottom exposed to red light (400 μmol m⁻² s⁻¹) either with constant illumination or light-dark cycles did not accumulate chlorophyll. This near-etiolation response was manifested by a critical threshold intensity of red light and did not need continuous illumination. The inhibition of the greening process resulted from reduced synthesis of glutamate-1-semialdehyde and consequent reduction in tetrapyrrole precursor 5-aminolevulinic acid. Red light perceived by the shoot bottom down regulated the protein and/or gene expression of enzymes involved in the biosynthesis of tetrapyrroles. The contents of endogenous cytokinins, i.e., isopentenyl-adenosine and dihydrozeatinriboside, were reduced in seedlings grown in red light having their shoot bottom exposed. Application of exogenous cytokinin and its analogue to roots of seedlings grown in red light reversed the down regulation of the greening process. The reversal of red-light-induced near-etiolation morphogenesis by far-red (200 μmol m⁻² s⁻¹) or blue (25 μmol m⁻² s⁻¹) light suggests that it could be a very high red-irradiance response of phytochrome, in the meristematic layers of the shoot bottom, that works in concert with blue light receptor(s). This work was supported by a competitive grant from the Department of Science and Technology, Govt. of India (DST/SP/SO/A-49/95) to BCT. Suchi Sood Varsha Gupta: Equal contributors     

The Uptake of Phosphate by Plants from Flowing Nutrient Solution: III. EFFECT OF CHANGED PHOSPHATE CONCENTRATIONS ON THE GROWTH AND DISTRIBUTION OF PHOSPHATE WITHIN PLANTS OF LOLIUM PERENNE L.

Perennial ryegrass (Lolium perenne L.) was grown from seed for29 d in flowing solution culture containing 0.1, 0.4 or 6.4mmol m^–3 P before the concentrations were changed (0.1and 0.4 raised to 6.4; 6.4 lowered to 0.4; controls unchanged)for an experimental period of two weeks to test the hypothesisthat after the seedling stage, the maximum rate of plant growthcould be sustained by a lower concentration of phosphate atthe root/solution interface than was necessary for the maximumrate of seedling growth. During the 29 d seedling period growthwas greatest on 6.4 mmol m^–3 P achieving 179 mg per plantdry weight compared with 122 and 26 mg on 0.4 and 0.1 mmol m^–3P respectively. During the experimental period growth on thetreatment 6.4 lowered to 0.4 mmol m^–3 P continued at thesame rate as the 6.4 control achieving 981 and 983 mg per plantdry weight respectively. Similarly growth of the treatment 0.4raised to 6.4 mol m^–3 P was unaffected by the change inconcentration and was comparable with the 0.4 control. Bothresults support the hypothesis for seedlings exceeding about100 mg per plant dry weight. In contrast the small plants ofthe treatment 0.1 raised to 6.4 mmol m^–3 P behaved similarlyto seedlings and responded rapidly to the increased concentrationof phosphate in solution, achieving high rates of phosphateuptake and increasing the growth of shoot more than the growthof root so that the ratio of root: shoot declined from 065 to0.34, a value similar to that for the seedlings grown on 6.4mmol m^–3 P. Key words: Lolium perenne L, Phosphate concentration, Seedling growth     

Plant cation-anion balance as affected by the ionic composition of the growing medium

Plants that remove an excess of cations over anions may cause soil acidification. The acidification potential of plants has been evaluated using solution culture techniques, but the influence of ionic composition of the medium on the plant cation-anion balance remains unclear. Our objective was to determine how electrolyte concentration and salt type affect the cation- anion balance of two test plants [barley (Hordeum vulgare L.) and kochia (Kochia scoparia L. Schrad.)]. Seedlings were grown in sand culture and irrigated with nutrient solution (Hoagland’s solution), which was adjusted to a range of electrolyte concentrations (target electrical conductivity of 7.5, 17.5 and 27.5 dS m⁻¹) using either chloride or sulphate salts. Increase in electrolyte concentration reduced yield of kochia, a salt-tolerant plant, by up to 38%. Total cation (Ca + Mg + K + Na) equivalents in kochia exceeded those of anions (Cl + S + P + NO₃) by 250 to 280 cmol_c kg⁻¹ of dry matter. Electrolyte concentration had no effect on the cation-anion balance of kochia, but excess cation values were significantly greater in the sulphate than in the chloride system. Kochia had a large content of water-soluble oxalate (194 to 226 cmol_c kg⁻¹), which was linearly related to the excess cation content. Growth of barley was severely restricted at the intermediate and high electrolyte concentrations. Cations exceeded anions by 21 to 59 cmol_c kg⁻¹ of barley dry matter. Excess cation content was greater in the sulphate than in the chloride medium, but electrolyte concentration did not have a consistent effect on the cation-anion balance. The small amounts of oxalate found in barley (0.9 to 2.6 cmol_c kg⁻¹) were insufficient to balance the cation excess.     

The importance of seed reserves for seedling performance: an integrated approach using morphological,physiological, and stable isotope techniques

To investigate how seed reserves affect early seedling performance, we conducted a factorial greenhouse experiment using Lithocarpus densiflora (Tanoak). Seedlings were grown from large (5.8±0.7 g) and small (3.2±0.4 g) seeds and, following shoot emergence, seeds were either removed or left attached. Seedlings were harvested for quantification of biomass and ¹³C at seven time periods following seed removal (2, 4, 8, 16, 32, 64, 128 days) and seedling photosynthesis was measured three separate time periods (2–4, 49–82, 95–128 days after seed removal). Biomass increased for all seedlings, but the increase was significantly larger for seedlings with attached seeds than with removed seeds. Seed removal just after shoot emergence significantly decreased seedling biomass, but seed removal 64 days after shoot emergence had no effect on seedling biomass. Seedling photosynthesis per unit leaf area varied by time and seed presence, but not by seed size. At the first period, seedlings with attached seeds had significantly higher photosynthetic rates than seedlings with removed seeds, at the second period there was no effect of seed removal, and at the third time period seedlings with attached seeds had significantly lower photosynthetic rates than seedlings with removed seeds. Despite temporal variation in photosynthesis per unit leaf area, seedlings with attached seeds always had significantly greater leaf area than seedlings with removed seeds, resulting in significantly higher total plant photosynthesis at all three time periods. The ¹³C values of both the leaves and roots were more similar to that of the seed for seedlings with attached seeds than for seedlings with removed seeds, however, seed removal and seed size strongly affected root ¹³C. This study demonstrates that seed reserves have important effects on the early growth, physiology, and ¹³C of L. densiflora seedlings.     

Effects of plant competition,seed predation,and nutrient limitation on seedling survivorship of spotted knapweed (<Emphasis Type="Italic">Centaurea stoebe</Emphasis>)

We measured seed germination and seedling survivorship of spotted knapweed, Centaurea stoebe, in a series of laboratory and field experiments to evaluate the efficacy of seed limitation as a management focus. This work was initiated 6 years after introduction of several biological control agents. The soil seed bank of the site used in this study contained a mean density of 5,848 seeds/m² (ranging from 0 to 16,364 seeds/m²), and 92% of the seeds isolated from soils were shriveled, discolored, and/or partially decayed. Additionally, none of the intact seeds germinated, suggesting that the viable seed bank at our field study site has been exhausted. Centaurea stoebe seeds were planted into pots under a range of soil nitrogen (N) availability, with half of the pots containing a single density of previously established seedlings of a native cool-season grass, slender wheatgrass (Elymus trachycaulus). A watering regime mimicking local precipitation was applied. Spotted knapweed exhibited large biomass responses to N addition, but the presence of grasses suppressed the ability to exploit this N. Surprisingly, low soil N conditions improved knapweed survivorship in the presence of grasses. Nevertheless, recruitment and biomass were still far below the levels reached in the absence of competition. To evaluate the effect of density on successful recruitment, Centaurea stoebe seed was introduced into a meadow at three densities matching reduced levels of seed production under the constraints of seed predators. These densities were sown with or without a seed mixture of native species, into an existing plant community lacking C. stoebe, and seedling recruitment was recorded over 2.5 years. Across all plots and densities sown (568–2,272 seeds m⁻² year⁻¹), seedling recruitment was less than 1%. The invasion potential of spotted knapweed was greatly diminished when realistic levels of plant competition and biological control limit seed production. We therefore conclude that a combination of seed limitation and shortage of ‘safe sites’ within undisturbed vegetation can limit densities of C. stoebe.     

Rodent seed predation and seedling recruitment in mesic grassland

Seedling recruitment of two grasses (Arrhenatherum elatius and Festuca rubra) and two herbs (Centaurea nigra and Rumex acetosa) was measured in areas with and without rodents to which seeds of each species were sown at three seed densities (1000, 10,000 and 50,000 seeds m⁻²) in two seasons (spring and autumn 1995). Seed removal was measured for 10-day periods and the fate of seedlings was followed for 15 months after sowing. The proportion of seed removed ranged from 6 to 85% and increased with increasing seed density for each species. Rodents had no effect on seedling emergence or survival in the spring sowing. In the autumn sowing, rodents reduced seedling emergence of all four species sown at 1000 and 10,000 seeds m⁻² but had no impact at 50,000 seeds m⁻², presumably because of microsite limitation. We suggest the difference between spring and autumn arose because emergence was seed limited in autumn but microsite limited in spring; microsite availability was higher in autumn because a summer drought killed plants, reduced plant biomass and opened up the sward. Fifteen months after the autumn sowing, fewer A. elatius and C. nigra seedlings survived on plots exposed to rodents. This result reflected not only the reduced seedling emergence but also increased seedling mortality (seedling herbivory) in sites exposed to rodents. In contrast, F. rubra and R.acteosa showed density-dependent seedling survival which compensated for initial differences in seedling emergence, so that no effect of rodents remained after 15 months. The results suggest that rodent seed predation and seedling herbivory exert strong effects on seedling recruitment of A.elatius and C. nigra when recruitment conditions are favourable (conditions that lead to high microsite availability) and may contribute to both species being maintained at low densities in the grassland. The results also demonstrate that highly significant impacts of rodent seed predation at the seedling emergence stage can disappear by the time of plant maturation. Received: 2 March 1998 / Accepted: 28 September 1998     

Interactive effects of elevated CO2, phosphorus deficiency, and soil drought on nodulation and nitrogenase activity in Alnus hirsuta and Alnus maximowiczii

We examined the interactive effects of elevated CO₂, soil phosphorus (P) availability, and soil drought on nodulation, nitrogenase activity, and biomass allocation in Alnus hirsuta and Alnus maximowiczii. Potted seedlings were grown in either ambient or elevated CO₂ (36 Pa and 72 Pa CO₂), with different levels of P (7.7 and 0.77 mgP pot^?1 week^?1 for high-P and low-P, respectively) and water supply in a natural daylight phytotron. Measurements of nitrogenase activity by an acetylene reduction assay failed to reveal significant effects of the treatments in any species. In high-P, nodule biomass increased under elevated CO₂ and decreased under drought. In low-P, nodule biomass decreased substantially compared to high-P, but the effect of elevated CO₂ on nodule biomass was unclear. Soil drought increased the partitioning of biomass into nodules, especially in A. hirsuta. These results suggest that with high P availability, elevated CO₂ could promote N₂ fixation by increasing nodule biomass even under drought. On the other hand, if soil P is limiting, elevated CO₂ may not enhance N₂ fixation because of the suppression of growth.     

Natural Occurrence of Mycotoxins in Staple Cereals from Ethiopia

The occurrence of mycotoxins in barley, sorghum, teff (Eragrostis tef) and wheat from Ethiopia has been studied. Samples were analyzed for aflatoxin B₁ (AFB1), ochratoxin A (OTA), deoxynivalenol (DON), nivalenol (NIV) and zearalenone (ZEN) using high performance liquid chromatography (HPLC) and for fumonisins (FUM) using enzyme linked immunosorbent assay (ELISA). AFB1 and OTA were detected in samples of all the four crops. AFB1 was detected in 8.8% of the 352 samples analyzed at concentrations ranging from trace to 26 μg kg⁻¹. OTA occurred in 24.3% of 321 samples at a mean concentration of 54.1 μg kg⁻¹ and a maximum of 2106 μg kg⁻¹. DON occurred in barley, sorghum and wheat at 40–2340 μg kg⁻¹ with an overall incidence of 48.8% among the 84 mainly ‘suspect’ samples analyzed; NIV was co-analyzed with DON and was detected at 40 μg kg⁻¹ in a wheat sample and at 50, 380, and 490 μg kg⁻¹ in three sorghum samples. FUM and ZEN occurred only in sorghum samples with low frequencies at concentrations reaching 2117 and 32 μg kg⁻¹, respectively. The analytical results indicate higher mycotoxin contamination in sorghum, which could be related to the widespread storage of sorghum grain in underground pits leading to elevated seed moisture contents. This is the first report on the occurrence of OTA in teff.