Calcium-Dependent Lamina Production of Nymphoides peltata (Gmel.) O. Kuntze (Menyanthaceae): Implications for Distribution

Nymphoides peltata (Gmel.) O. Kuntze, a nymphaeid macrophyte,occurs commonly in polder and fluviatile areas in large partsof Europe and Asia. In contrast to the nymphaeid macrophytesNymphaea alba L. and Nuphar lutea (L.) Sm., Nymphoides peltatais almost completely absent from poorly-buffered waters andis never found in acid water bodies. Transplantation experimentsin water bodies of varying alkalinity demonstrated that, irrespectiveof the sediment type, leaf production of Nymphoides did occurin poorly-buffered waters, but not in acid waters. Cultivation experiments showed that floating leaf developmentof Nymphoides peltata could only take place if sufficient calciumwas available in the water layer or in twice-demineralized water.Addition of calcium to an acid cultivation medium or to watercollected from an acid moorland pool resulted in leaf production.Growth of Nymphoides in acid waters is impossible due to insufficientcalcium concentrations in the water layer of such waters. Itis suggested that the absence of Nymphoides peltata in somepoorly-buffered water bodies is partly due to the spatial isolationfrom rivers and canals and the high frequence of desiccation.The restricted occurrence of Nymphoides peltata to well-bufferedalkaline waters is functionally more related to the calciumavailability than to the bicarbonate content. Key words: Aquatic macrophytes, distribution, Nymphoides peltata, leaf production, calcium, acid, poorly-buffered and alkaline water     

Phenolic content of daylight-exposed and shaded floating leaves of water lilies (Nymphaeaceae) in relation to infection by fungi

Under suboptimal growing conditions (e.g. a lack of sunshine), floating leaves of Nymphaea alba and Nuphar lutea can become heavily infected with the fungi Colletotrichum nymphaeae and Pythium F, respectively. These fungi normally act as decomposers of senescent leaves. Mature leaves of Nymphaea alba and Nuphar lutea contain high concentrations of phenolics, secondary substances known for their fungistatic properties. The production of these compounds requires energy and primary metabolites. The hypothesis that suboptimal growing conditions reduce the ability of nymphaeids to maintain a sufficiently high level of phenolics, thereby making them more vulnerable to infection by fungi, was tested. Outdoor mesocosm experiments were used to examine the response of floating leaves of Nymphaea alba and Nuphar lutea to reduced light availability. Shading significantly reduced the phenolic content of the leaves. This was accompanied by higher disease severity. The outcome of this experiment is also discussed in relation to the higher nitrogen content measured in shaded leaves. Received: 8 February 1997 / Accepted: 12 July 1997     

Successional changes in the soft-water macrophyte vegetation of (sub)atlantic, sandy, lowland regions during this century

SUMMARY.

• 1 Considerable changes in macrophyte vegetation can be noticed in 146 originally soft waters, when data on the recent aquatic vegetation are compared with historical information from the period 1900–60. Changes in nutrient status (N, P and C) and accumulation of organic material can be regarded as the operative factors.
• 2 The processes observed in soft waters are acidification, eutrophication and water hardening. Which process dominates depends on the type of soft water.
• 3 Acidification as well as eutrophication of water bodies may ultimately result in the total disappearance of all aquatic macrophytes, with the exception of the floating-leaved nymphaeids Nymphaea alba L. and Nuphar lutea (L.) Sm. Observed successional stages are described and summarized.

     

Production,nutrient dynamics and initial decomposition of floating leaves of Nymphaea alba L. and Nuphar lutea (L.) Sm. (Nymphaeaceae) in alkaline and acid waters

Production, turnover and nutrient dynamics of floating leaves of Nymphaea alba L. and Nuphar lutea (L.) Sm. were studied in four aquatic systems in The Netherlands, differing strongly in water quality. Production was 108–447 g AFDW.m⁻² for N. lutea and 319–348 g AFDW.m⁻² for N. alba. Turnover ranged from 3.6 to 4.4 without much difference between the sites and the species. During senescence 60–70% of the N and P from the leaves was resorbed by both of the plant species. The nutrient flow from the floating leaves into the detritus food chain differed considerably between the species and sites studied, mainly because of the differences in production. The data suggest that production is strongly influenced by the environment, whereas turnover and nutrient resorption during senescence seem to be plant characteristics. There was little difference in dynamics of the chlorophyll-α concentration in the leaves of the two species, irrespective of the growing site. Floating leaves of both species lost about 70% of their area due to fragmentation in the alkaline waters, whereas in the acid water very little fragmentation was observed.     

Seed dormancy and control of germination in Sisymbrella dentata (L.) O.E. Schulz (Brassicaceae)

• Seed germination responsiveness to environmental cues is crucial for plant species living in changeable habitats and can vary among populations within the same species as a result of adaptation or modulation to local climates. Here, we investigate the germination response to environmental cues of Sisymbrella dentata (L.) O.E. Schulz, an annual endemic to Sicily living in Mediterranean Temporary Ponds (MTP), a vulnerable ecosystem.
• Germination of the only two known populations, Gurrida and Pantano, was assessed over a broad range of conditions to understand the role of temperatures, nitrate, hormones (abscisic acid – ABA and gibberellins – GA) and after‐ripening in dormancy release in this species.
• Seed germination responsiveness varied between the two populations, with seeds from Gurrida germinating under a narrower range of conditions. Overall, this process in S. dentata consisted of testa and endosperm rupture as two sequential events, influenced by ABA and GA biosynthesis. Nitrate addition caused an earlier testa rupture, after‐ripening broadened the thermal conditions that allow germination, and alternating temperatures significantly promoted germination of non‐after‐ripened seeds.
• Primary dormancy in S. dentata seeds likely allows this plant to form a persistent seed bank that is responsive to specific environmental cues characteristic of MTP habitats.

     

In situ andin vivo studies on the breakdown ofNymphaea alba L. (Nymphaeaceae)

Summary The breakdown of floating leaves ofNymphaea alba was studied with litterbags in two aquatic systems,viz., an alkaline, eutrophic former riverbed (Oude Waal) and an acid, oligotrophic pool (Voorste Choorven). It was found that the floating leaves from the Oude Waal decomposed at a faster rate than those from the Voorste Choorven when they were incubated in their natural system. In order to get insight in the parameters which determine the observed differences in the field, laboratory experiments were performed. Leaf discs ofN.alba were incubated in staged concentration gradients of some important chemical parameters (bicarbonate, ammonium, nitrate, orthophosphate, sulphate, pH, combinations of nitrate and phosphate; nitrate and bicarbonate, and bicarbonate and sulphate). It appeared that the differences in decomposition rate as observed in the field could be largely explained by the pH and alkalinity of the system-water, and the chemical composition and physiological state of the plant material.Contribution nr. 36 of the nymphaeid project.     

Investigating seed dormancy in cotton (Gossypium hirsutum L.): understanding the physiological changes in embryo during after‐ripening and germination

• The dormancy of seeds of upland cotton can be broken during dry after‐ripening, but the mechanism of its dormancy release remains unclear.
• Freshly harvested cotton seeds were subjected to after‐ripening for 180 days. Cotton seeds from different days of after‐ripening (DAR) were sampled for dynamic physiological determination and germination tests. The intact seeds and isolated embryos were germinated to assess effects of the seed coat on embryo germination. Content of H₂O₂ and phytohormones and activities of antioxidant enzymes and glucose‐6‐phosphate dehydrogenase were measured during after‐ripening and germination.
• Germination of intact seeds increased from 7% upon harvest to 96% at 30 DAR, while embryo germination improved from an initial rate of 82% to 100% after 14 DAR. Based on T₅₀ (time when 50% of seeds germinate) and germination index, the intact seed and isolated embryo needed 30 and 21 DAR, respectively, to acquire relatively stable germination. The content of H₂O₂ increased during after‐ripening and continued to increase within the first few hours of imbibition, along with a decrease in abscisic acid (ABA) content. A noticeable increase was observed in gibberellic acid content during germination when ABA content decreased to a lower level. Coat removal treatment accelerated embryo absorption of water, which further improved the accumulation of H₂O₂ and changed peroxidase content during germination.
• For cotton seed, the alleviation of coat‐imposed dormancy required 30 days of after‐ripening, accompanied by rapid dormancy release (within 21 DAR) in naked embryos. H₂O₂ acted as a core link between the response to environmental changes and induction of other physiological changes for breaking seed dormancy.

     

Seed bank persistence of clonal weeds in contrasting habitats: implications for control

The ability of weeds to form a seed bank is important for their population dynamics and management because it provides a refuge enabling reinvasion after established target plants have died. However, knowledge of the differential seed behaviour of individual species over multiple years and varying environmental conditions is surprisingly rare but necessary for effective control of diverse weed populations. We established a seed burial experiment in alpine habitats differing in management regime (i.e., forest, hay meadow and pasture) to determine whether seeds of the unpalatable perennial weeds, Veratrum album (white hellebore) and Gentiana lutea (yellow gentian) were able to delay germination and remain viable over 3 years. Our study shows that both species formed a short-term persistent seed bank; in the third-year, the soil seed banks of both species were nearly depleted, having declined to <5% of their original size. Both species had strikingly different germination strategies: G. lutea seeds mainly germinated in their first-year, whilst the majority of V. album seeds germinated in their second-year. The fraction of dormant G. lutea seeds increased with seed age, indicating that seeds remained viable after forgoing germination in the previous year. Habitat-specific differences in seed germination increased with seed age, with germination fractions being lowest in moist hay meadows. This suggests that the negative effects of anoxic conditions became more pronounced as seeds aged in hay meadows. Conversely, seed dormancy was equal among habitats. The absence of a long-term persistent seed bank has important implications for the management of both nuisance and endangered-plant populations. In the case of V. album and G. lutea, re-colonization of habitats from the seed bank is unlikely after established plants have been removed.     

Factors affecting the dormancy and germination of bleeding heart [Lamprocapnos spectabilis (L.) Fukuhara] seeds

• Information on the optimal conditions to promote the germination of Lamprocapnos spectabilis (L.) Fukuhara seeds is limited; consequently, this study was conducted to establish the requirements to break seed dormancy and promote germination.
• The selected seeds had morphophysiological dormancy and had not begun embryo development. To study the dormancy breaking and embryo development processes, seeds were subjected to constant or changing temperature treatments during moist stratification.
• High temperature and humidity resulted in vigorous embryo growth, with the longest embryos occurring after 1 month of incubation at 20 °C. At 4 °C, the seeds required incubation period of at least 3 months to germinate. Embryo growth and germination were higher with changing high and low temperatures than under a constant temperature, and changing temperatures also considerably changed the endogenous hormone levels, embryo development and germination. Bioactive gibberellin (GA) content was higher in seeds incubated at 20 °C for 1 month, then at 4 °C for 2 months. The content of endogenous abscisic acid in seeds subjected to the same treatment decreased by 97.6% compared with that of the untreated seeds.
• Embryo growth and seed germination require changing high and low temperatures; however, exogenous GA₃ could substitute for high temperatures, as it also causes accelerated germination. In this study, the seeds of L. spectabilis were identified as an intermediate simple type, a sub‐level of morphophysiologically dormant seeds.

     

Germinability and fungal occurrence in seeds of Abies alba Mill. populations in southern Italy

Abstract

Germination tests were carried out on seeds from three Italian populations of Abies alba. Seed germinability was up to 38%. HgCl₂, NaClO and plant preservative mixture were used as seed‐sterilising agents. Despite chemical treatments, Papulaspora rubida and Chaetomium globosum were isolated. The possible role of these fungi in the physiology of silver fir seed germination process was discussed.     

Hoary cress (Cardaria draba (L.) Desv.) seed germination ecology,longevity and seedling emergence

The effects of gibberellic acid (GA₃), potassium nitrate (KNO₃), prechilling, temperature, salt stress and osmotic potential on seed germination and sowing depth on seedling emergence and burial depth on seed viability of hoary cress (Cardaria draba (L.) Desv.), were studied in a series of laboratory, glasshouse and outdoor experiments. The optimal temperature for hoary cress seed germination was 20°C, both in light/dark and darkness regimes. Seed germination of hoary cress at 400 ppm concentration of GA₃ in a light/dark regime was maximal. Potassium nitrate concentrations increased the percentage of germination in comparison with the control treatment. Increasing the duration of dry prechilling to 30 and 45 days promoted the seed germination of hoary cress. Germination of hoary cress markedly decreased as salt and drought stress increased. Seed germination of hoary cress occurred at a range of pH from 3 to 11. Seedling emergence significantly decreased as planting depth increased. Total seed viability decreased with increasing burial depth. The maximum increase in mortality occurred in seeds that were buried at 5‐cm depth.     

Gas transfer in floating-leaved plants

Pressurized gas transport with flow rates of 1.1 to 1.81 gas h^-1 plant^-1 have been detected in the floating-leaved aquatic macrophyte Euryale ferox on sunny days. The younger leaves gave the highest pressurization, but the gas flow was initiated mainly by the middle-aged leaves of the plants. The gas through-flow was shown to be highly beneficial for floating-leaved plants. It improved the oxygen level in the aerenchyma of the submerged organs of Nymphaea alba and Nymphoides peltata, and doubled the ATP level in the root tissues. On the other hand, the sulphur dioxide fumigation experiments on Nymphaea odorata indicated that the pressurized gas transport may adversely affect the floating-leaved plants, due to an increased contamination of the leaves leading to increased photo bleaching and impaired photosynthesis.Abbreviations ATP adenosine triphosphate - FW fresh weight - SO₂ sulphur dioxide     

Germination ecology and seed population dynamics of Digitalis purpurea

Summary The germination ecology and the dynamics of the generative reproduction in populations of Digitalis purpurea L. were investigated in the field as well as in experiments. Germination of fresh seeds in the dark on moist filter paper appeared to differ between populations. These differences were eliminated when a moist natural soil functioned as germination substrate. An interaction between the spectral composition of light and the germination substrate was present. Germination in gradients of light, temperature and soil moisture revealed some clear-cut results. Germination proved to be strongly dependent on the percentage of vegetation cover. During two years of burial in litter bags, the number of buried viable seeds did not decrease. From one generation of seeds produced in a natural population, 18% was introduced into the buried seed bank, 10% germinated in autumn and 24% was present as a enforced dormant surface seed bank in late autumn.The results are discussed in relation to secondary succession. can be derived from Milton (1936), Salisbury (1942) and Thompson and Grime (1979). Soil disturbance and germination seem to be correlated in D. purpurea (Grime 1979). The purpose of this study is to analyse the dormancy and germination behaviour of D. purpurea in relation to the relevant environmental factors in order to explain the mechanisms of entry into, and the escape of D. purpurea seeds from a seed bank. Furthermore, an attempt will be made to quantify seed rain as well as the fate of different germinating and non-germinating seed rain fractions in space and time per unit area, in different stages of succession.     

Dormancy and viability of Ferocactus peninsulae (Cactaceae) seeds

Knowledge on seed dormancy is crucial for the understanding of plant population dynamics, as it controls seed germination and seed bank formation. Dormant seeds have high potential to establish in soil seed banks, but such information within Cactaceae is scarce, although it is essential for conservation programs. The aim of this study was to determine if seeds of Ferocactus peninsulae showed any kind of dormancy and to test their germination capacity after storage. This was assessed with 15 seed sowing experiments done over 4 years with seeds stored under room conditions (20 ± 2°C). We demonstrated the existence of physiological dormancy in F. peninsulae seeds that is broken with an after-ripening period. Germination was low during the first 3 months of storage (d = 0.206) but increased after 10 months of storage (d = 0.654), and seeds maintained their viability at 48 months (d = 0.707). Also, their speed of germination increased with storage time. Ferocactus peninsulae seeds are positively photoblastic, and the requirement for light for germination persisted over all experiments. The results provide crucial information for propagation and conservation research and may allow us to infer that F. peninsulae seeds are able to form a persistent soil seed bank, as they maintained their viability after dormancy is released.     

Evidence for physiological seed dormancy cycling in the woody shrub Asterolasia buxifolia and its ecological significance in fire‐prone systems

• Dormancy cycling is a key mechanism that contributes to the maintenance of long‐term persistent soil seed banks, but has not been recorded in long‐lived woody shrub species from fire‐prone environments. Such species rely on seed banks and dormancy break as important processes for post‐fire recruitment and recovery.
• We used germination experiments with smoke treatments on fresh seeds and those buried for 1 year (retrieved in spring) and 1.5 years (retrieved the following late autumn) to investigate whether Asterolasia buxifolia, a shrub from fire‐prone south‐eastern Australia with physiologically dormant seeds, exhibited dormancy cycling.
• All seeds had an obligation for winter seasonal temperatures and smoke to promote germination, even after ageing in the soil. A high proportion of germination was recorded from fresh seeds. but germination after the first retrieval was significantly lower, despite high seed viability. After the second retrieval, germination returned to the initial level. This indicates a pattern of annual dormancy cycling; one of the few observations, to our knowledge, for a perennial species. Additionally, A. buxifolia’s winter temperature and smoke requirements did not change over time, highlighting the potential for seeds to remain conditionally dormant (i.e. restricted to a narrow range of germination conditions) for long periods.
• For physiologically dormant species, such as A. buxifolia, we conclude that dormancy cycling is an important driver of successful regeneration, allowing seed bank persistence, sometimes for decades, during fire‐free periods unsuitable for successful recruitment, while ensuring that a large proportion of seeds are available for recruitment when a fire occurs.

     

Impacts of fire cues on germination of Brassica napus L. seeds with high and low secondary dormancy

• Agricultural burning is used in farm management operations; however, information about the impact of fire cues on the release and/or induction of secondary dormancy in crop seeds is scarce.
• Seeds from two oilseed rape cultivars were induced for high (HD) or low (LD) secondary dormancy using polyethyleneglycol (PEG) pre‐treatment, and their germination after exposure to various fire cues was compared to control PEG pre‐treated and non‐dormant seeds.
• Non‐dormant seed germination was unaffected by various fire cues. Low doses of aerosol smoke released secondary dormancy in HD seeds, while higher doses increased dormancy of LD seeds. Dilute smoke water also released HD seed secondary dormancy, but concentrated smke water enhanced dormancy in both LD and HD seeds. The concentrated aqueous extracts from charred oilseed rape straw only promoted germination of HD seeds, while dilution inhibited LD seed germination. Heat shock (80 °C, 5 min) released secondary dormancy in HD seeds; however, higher temperatures and/or increased exposure time was associated with seed death. GC‐MS analyses of smoke water revealed two butenolides and an array of monoaromatic hydroxybenzene compounds with potential germination inhibitor or promoter activity.
• The extent of secondary dormancy induction in seeds affects their subsequent responses to fire cues. Both aerosol smoke and smoke water have both germination promoter and inhibitor activity. Lacking any butenolides, aqueous extracts of charred straw contain a potential germination stimulating steroid, i.e. ergosterol. The significance of fire‐derived cues on behaviour of oilseed rape seeds in the soil seed bank is discussed.

     

Annual dormancy cycles in buried seeds of shrub species: germination ecology of Sideritis serrata (Labiatae)

The germination ecology of Sideritis serrata was investigated in order to improve ex‐situ propagation techniques and management of their habitat. Specifically, we analysed: (i) influence of temperature, light conditions and seed age on germination patterns; (ii) phenology of germination; (iii) germinative response of buried seeds to seasonal temperature changes; (iv) temperature requirements for induction and breaking of secondary dormancy; (v) ability to form persistent soil seed banks; and (vi) seed bank dynamics. Freshly matured seeds showed conditional physiological dormancy, germinating at low and cool temperatures but not at high ones (28/14 and 32/18 °C). Germination ability increased with time of dry storage, suggesting the existence of non‐deep physiological dormancy. Under unheated shade‐house conditions, germination was concentrated in the first autumn. S. serrata seeds buried and exposed to natural seasonal temperature variations in the shade‐house, exhibited an annual conditional dormancy/non‐dormancy cycle, coming out of conditional dormancy in summer and re‐entering it in winter. Non‐dormant seeds were clearly induced into dormancy when stratified at 5 or 15/4 °C for 8 weeks. Dormant seeds, stratified at 28/14 or 32/18 °C for 16 weeks, became non‐dormant if they were subsequently incubated over a temperature range from 15/4 to 32/18 °C. S. serrata is able to form small persistent soil seed banks. The maximum seed life span in the soil was 4 years, decreasing with burial depth. This is the second report of an annual conditional dormancy/non‐dormancy cycle in seeds of shrub species.     

Secondary seed dispersal in Montrichardia arborescens (L.) schott dominated wetlands in Laguna Grande, Venezuela

Laguna Grande, Monagas State, Venezuela, is a shallow, V-shaped lake created by the confluence of two rivers. Montrichardia arborescens (L.) Schott. dominated wetlands cover most of the north and south arms and the littoral zone of the main body of the lake. The vegetation and seed banks of Montrichardia wetland sites were sampled in the north arm, south arm and main body five times from the end of the dry season in 1991 to beginning of the rainy season in 1992. The composition of the vegetation was similar and changed very little at all three sites during the course of the study. These wetlands had 53 species. Besides M. arborescens, other common species were Hamelia patens Jacq., Mikania cordifolia (L.) Wild., Sarcostemma clausum (Jacq.) Roem. & Schult., and Vitis caribaea L. In both the vegetation and seed banks, species richness was highest during the dry season. Altogether, the seed banks contained the seeds of 61 species of which 35 were also found in the vegetation. Seeds of three tree species were found in the seed banks that did not grow anywhere in the lake. In the seed bank, seeds of Cyperus odoratus L., Eleocharis interstincta (Vahl.) R&S, Ludwigia hyssopifolia (G. Don.) Excell, L. lithospermifolia (Mich.) Hara, Polygonum acuminatum H.B.K., and Sacciolepis striata (L.) Nash were the most abundant. Mean total seed density over the entire study was 6,500, 3,800, and 6,000 seeds/m² in the north arm, south arm, and main basin, respectively. Seed production and dispersal occur primarily during the dry season, and the highest seed densities at all sites were found in the dry season when there was no or little standing water. The lowest seed densities at all sites were found during the rainy season during which seed densities declined over 80% at the north and south arm sites. In the main body of the lake, however, seed densities during the rainy season, although lower than during the dry season, actually increased significantly from 3,600 seeds/m² in August 1991 to 6,000 seeds/m² in October 1991. A significant decrease in seed density in either the north or south arms or both and a significant increase in the main body site during the rainy season occurred for 5 of the 8 species whose seeds were the most abundant, for all life-form guilds, except hydrophytes and for the entire seed bank. Secondary dispersal by water currents during the rainy season appears to be transporting seeds from the north and south arms into the seed bank of the main body of the lake.     

Autecology of Acacia Senegal (L.) Willd.

Acacia senegal (L) Willd. is a small sized tree species of Tropical arid region of Southern West Pakistan. It is well-known for producing gum arabic. Fruits from dry streams with loamy sand soil type were of bigger size whereas seed output per pod and seed weight were higher in hilly regions. The seeds possess external dormancy (seed coat), which can be broken by mechanical and chemical scarification, water (cold and boiling) and dark treatments. Seeds gave high percentage of germination in fine sand in pot culture. Germination percentage was high in May–July. The growth rate of seedlings both in pot culture and field conditions was high in April, May, June and July. Seeds collected from different localities showed considerable variation in germination capacity. Percentage and rate of germination increased with the lapse of time. Correlation of growth and abundance ofAcacia senegal was found with topography, soil texture, soil structure, soil consistence, maximum water holding capacity, wilting coefficient (sub-surface soil), soil carbonates (surface soil) soil pH (surface soil) and community associates. The plant can be propagated by shoot cuttings. Smaller and thinner cuttings are easily propagated. The plant is infested by Buffaloe tree hopper and is also infected byCladosporium herbarum. Occasionally seedlings are attacked byFusarium sp. The relative turgidity of branches was found to be high throughout the study period, but the leaves showed considerable variation.