The effects of bioturbation on sediment transport on an intertidal mudflat

Laboratory flume studies were conducted to determine the relative importance of various biological and physical factors controlling sediment erosion threshold and transport rate on an intertidal mudflat in the Bay of Fundy. Studies were conducted in a July period of maximum solar exposure. The upper and mid-intertidal stations of the flat were dominated by silt-clay sediments, while the lower intertidal was dominated by very fine sand. The tube-dwelling amphipod Corophium volutator was the most abundant infaunal species with densities exceeding ∼1300 ind·m⁻² based on counts of burrow openings. Sediment-penetrometry and water-content measurements indicated no change in unconsolidated shear strength and porosity, respectively, along the intertidal transect. Despite the apparently cohesive nature of the sediment, erosion occurred as small ripples. Critical shear velocities (u_crit) for erosion determined with intact cores in a laboratory flume were relatively consistent between stations and sampling dates (mean = 2.1 cm·s⁻¹ ± 0.2 SD), with no relationship to Corophium density, sediment chlorophyll a, or physical variables. Field-treatment of sediment with formalin did not cause an obvious change in u_crit as determined by flume experiments. Corophium seemed to have little effect on erosion thresholds because incipient motion could be observed between tube burrows, beyond the local influence of the amphipod. In contrast to erosion thresholds, sediment-erosion rates measured with bedload traps were negatively correlated with density of small Corophium, probably due to binding of sediment into burrows and the ambient sediment microfabric, all of which reduce the availability of sediment for transport. Adult amphipods, which occurred at low density probably due to territorial/competitive interactions, had no obvious effect on erosion rate since only a small proportion of the sediment surface was impacted by their bioturbation. Although a portion of the amphipod population exerts a stabilizing influence on sediment-erosion rates, concurrent studies at the site indicate that Corophium seasonally reduces the sediment-erosion threshold by grazing on microflora which would otherwise inhibit the initiation of grain motion. Due to this decoupling of erosion rate and threshold, it is necessary to measure both processes in assessing the effects of benthic biota on sediment transport.     

Stage Sensitivity of Mungbean (Vigna radiata L. Wilczek) to an Elevated Level of Carbon Dioxide

An experiment was undertaken using open‐top chambers to study the effect of CO₂ enrichment on crop growth and to assess the variation related to the time of CO₂ exposure. In general, enriched CO₂ conditions (600 ± 50 p.p.m.) in the chamber positively affected the growth and development of mungbean (Vigna radiata), a short‐duration (60–65 days) summer pulse crop. However, there was significant variability in the effect of the crop stage/time of CO₂ exposure. It was observed that high CO₂ exposure at an early growth stage [0–20 days after germination (DAG)] had a larger effect than that at a later growth stage (21–40 DAG). The experiment also provided a means of assessing the short‐ and long‐term effects of elevated CO₂ on the carbon exchange rate at both stages of exposure. The study revealed that the sensitivity of plants to high CO₂ was more pronounced with respect to net photosynthetic rate. The overall photosynthetic activity gave greater growth and development of plants under high CO₂.     

Impact of tail-nipping on mortality,growth and reproduction of Arenicola marina

The impact of predation by amputation of regenerating body parts (tail tips) of the lugworm Arenicola marina on species mortality, growth and reproduction has been studied under laboratory conditions by the artificial removal of tail tips at different frequencies. The loss of body weight by amputation was not compensated for by an increased growth. Within a wide range of amputation frequencies, total growth (body growth + amount of tail tip amputated) and reproduction of the lugworm were not affected. Also, both egg development and amount of energy stored in reproduction remained the same. Only at the highest frequency of amputation (once a week) did total growth decrease in the course of time, resulting even in a loss of body weight. The amount of energy stored in reproduction was also significantly less at the highest rate of amputation. Lugworms appeared to be unable to sustain this high level of amputation and the anaerobic sediment conditions in the cuvettes suggest a reduced pumping activity and food intake. Mortality in this group was also higher than in the other groups. The consequences of tail-nipping by flatfish for A. marina in the field situation are discussed.     

On the population dynamics of dab,sole and flounder within bridgwater bay in the lower severn estuary,England

A ten year study of the population dynamics of the flatfish dab Limanda limanda, sole Solea solea and flounder Platichthys flesus was undertaken within Bridgwater bay in the Bristol Channel, England. Fish abundance was estimated from the rate of capture on power station intake filter screens at Hinkley Point Nuclear Power Station. The Bay was used as an 0-group nursery by all three species while flounder were alone in regularly using the area as adults. Using the coefficient of variation (CV) in annual catch as a measure of population stability these three flatfish were found to have some of the most stable populations within a fish community of 21 common species. When these species were classified according to their use of space as benthic, proximo-benthic or pelagic it was found that the benthic fish had the lowest CV values. It is argued that this stability is linked to their use of space. A notable feature was the differing seasonal occupancy of the Bay by the benthic fish which would act to reduce potential inter-specific competition. This suggests that spatial competition could only be intra-specific or against invertebrates. A search was made for biotic or physical factors correlated with year class abundance in the flatfish. The only significant correlation found was for sole, the abundance of which increased with increasing water temperature.     

Exposure,vegetation and sediment as primary factors for mobile epibenthic faunal community structure and production in shallow marine soft bottom areas

Mobile epibenthic fauna was quantitatively assessed in 22 shallow-water (0–1.5 m), soft-bottom areas on the Swedish west coast. Relationships between faunal structure, density, biomass and annual production on the one hand, and physical factors such as exposure, vegetation cover and sediment structure on the other, were investigated. The areas were grouped according to their physical characteristics into 4 categories: exposed, semi-exposed and sheltered with little or no vegetation and vegetated.Species number, density, biomass and annual production were shown to be significantly higher in vegetated areas (Zostera marina L.) than in mainly unvegetated ones. The epibenthic production in Z. marina meadows was 6 g (AFDW)-m⁻²·y⁻¹. Species composition was also different and Carcinus maenas L. was the only dominant species occurring in both vegetated and unvegetated areas. In all unvegetated areas the number and composition of species were about the same, regardless of exposure, whereas biomass and production varied with the degree of exposure. Highest production (3 to 5.5 g (AFDW)-m⁻²·y⁻¹) in these areas was found in semi-exposed areas. In exposed and sheltered areas production was 1.1 to 3.3 g and 0.6 to 1.7 g (AFDW)-m⁻²·y⁻¹, respectively.Degree of exposure, vegetation cover and sediment structure are suggested as the primary factors affecting the composition and quantity of mobile epibenthic fauna in the investigated areas. Biotic factors e.g. predation and competition are generally considered to be subordinate to these physical factors.     

Environmental characteristics and distribution of macrobenthos in a mudflat of the west coast of Korea (Yellow Sea)

The spatio-temporal changes of environmental factors and the distribution patterns of macrobenthos were investigated on an intertidal mudflat near Panweol on the west coast of Korea. Temperatures of the surface sediments and salinities of the intertitial water were highest in summer and lowest in winter, while water content of the sediment was lowest in summer and highest in winter. Hourly variations of sediment temperature, water content and interstitial salinity during the daytime depended largely upon both the timing of ebb tide and daylight intensity.Temperature, water content and interstitial salinity showed a marked variation with sediment depth. Temperatures decreased sharply from the surface to 10 or 15 cm depth, and below this depth they were nearly constant, except in summer. Water content and interstitial salinity underwent a rapid decline in this upper layer and began to rise from this depth. From the high tide mark toward the main tidal channel, water content increased but salinity decreased. The sand fraction increased with the decrease of clay content toward the main tidal channel. This tidal flat could be divided into two zones: the “upper intertidal zone” and the “lower intertidal zone”, based on the spatial differences of the water content, interstitial salinity and grain-size composition in the surface sediment.There were three distinct zones of macrobenthos. The upper intertidal zone was dominated by burrowing deposit feeders, such as Ilyoplax dentimerosa, Helice tridens sheni, Cleistostoma dilatatum and Perinereis vancaurica tetradentata. The middle intertidal zone was characterized by other burrowing deposit feeders, such as Ilyoplax pingi, Macrophthalmus japonicus and Periserrula leucophryna. And the lower intertidal zone was dominated by suspension feeders represented by Laonome tridentata and Potamocorbula amurensis. Each zone was named after these characteristic species.Although the species composition of the upper intertidal community was different from that of the middle intertidal, the way of living and the feeding behaviour of the species occurring in these two zones were very similar. But the lower intertidal zone showed a characteristic community structure differing not only in species composition but also in ways of living and feeding of the species. Some sets of environmental factors are considered to explain the zonation. Water content of the sediment and interstitial salinity will be the main factors causing the observed zonation.     

The light and temperature dependence of growth rate and elemental composition of Phaeocystis globosa scherffel and P. Pouchetii (HAR.) Lagerh. in batch cultures

The light and temperature dependence of growth, cellular elemental composition (C, N, P) and chlorophyll content during exponential growth and phosphate starvation of batch cultures of the temperate algal species Phaeocystis globosa were investigated. Temperature-dependent minimum generation time and cell phosphorus content were compared with those of the cold water species P. pouchetii. The variation in growth and cell carbon content was due to the different origin of seawater used for the culture media. Coastal, estuarine seawater enabled shorter doubling times than “offshore” and oceanic-influenced seawater. At nutrient-saturated growth, P. globosa was able to compete with diatoms only if high temperature (14°C) and high irradiances (180 μE·m⁻²·s⁻¹) predominated. P. pouchetti showed markedly shorter generation times than P. globosa did at temperatures below 10°C. At 6°C, the cell carbon content exceeded the value at 18°C by a factor of 1.7. High C/N atomic ratios of 6.9±0.7 and high C/Chl a ratios of ≥ 54.0 during exponential growth could not entirely be explained by the carbon content of the colony mucus. There was no significant light and temperature dependence of the : N : P ratio during exponential growth. At phosphate starvation (subsistence P quota of cells) this was so only at temperatures higher than 10°C. With their stored phosphorus quota P. globosa and P. pouchetii were capable of only two cell divisions. Hence, because of their limited ability to store phosphorus, the two Phaeocystis species cannot compete with planktonic diatoms for growth. The frequent change in nutrient concentrations (particularly of silicate and phosphate) in coastal and estuarine waters at summer temperatures and light conditions causes a rapid alternation of smaller blooms of P. globosa and diatoms.     

Breeding of polyploid heather (Calluna vulgaris)

Calluna vulgaris is an important landscaping plant in Northern Europe. As C. vulgaris is the only species within the genus Calluna, the available gene-pool for breeding is rather narrow and phenotypic variation is limited. Hence, a breeding program for polyploids was set up in order to broaden phenotypic variation in this important ornamental crop. Therefor basic genetic characteristics of polyploid C. vulgaris were analyzed using the progeny of a spontaneously occurring tetraploid genotype. With the help of morphological and molecular marker analysis, it was determined that the autotetraploid plant probably arose from unreduced gametes resulting from first division restitution. Tetraploids displayed tetrasomic inheritance with free combination of homologous and homeologous chromosomes. Triploids were semi-fertile with a high rate of aneuploids in their progeny. As the so-called bud-blooming flower type is a trait of outstanding interest in breeding of C. vulgaris, a strategy for efficient breeding of triploid bud-bloomers was deduced.     

Settlement,abundance, growth and mortality of juvenile flatfish in a subtropical tidal estuary (Georgia,U.S.A.)

The population dynamics of juveniles of some flatfish species were studied in the Duplin River, a tidal creek in a subtropical salt-marsh area in Georgia, U.S.A. from April until September 1990. Seven species were found. Paralichthys dentatus, Paralichthys lethostigma, Paralichthys oblongus and Trinectus maculatus were relatively rare. Etropus crossotus, Citharichthys spilopterus and Symphurus plagiusa were abundant and settled during the period studied. E. crossotus was the most abundant species with a mean abundance of 18 ind·10⁻² m⁻² (max. 287). Demersal settlement of E. crossotus took place in shallow areas and over sandy bottoms from mid-May to August. Prolonged settlement hampered the calculation of growth rate and instantaneous mortality rate. However, laboratory growth experiments indicated a mean growth of about 0.50 mm·d⁻¹ at 24–28°C. Juveniles of C. spilopterus were already present in the Duplin River in March. Settling continued until the end of April with a mean abundance of 3.5 ind·10⁻² m⁻² (max. 183). With increasing size the juveniles of this species tended to migrate to deeper waters and to the mouth of the river, possibly as a reaction to increasing water temperatures. Maximum growth rate was 1.4 mm·d⁻¹ at about 26°C. The mean instantaneous mortality rate (z) was estimated at 0.03·d⁻¹. Settling of S. plagiusa occurred from mid-May onwards. The mean abundance was 10.3 ind·10⁻² m⁻² (max. 98.3). Newly settled juveniles were most abundant on muddy sediments in the shallow river areas. The maximum growth rate was 1.3 mm·d⁻¹ at about 28°C. The mean instantaneous mortality rate (Z) decreased from 0.04·d⁻¹ in April to 0.01·d⁻¹ in August. At all sites the abundance of juveniles of this species decreased with increasing water depth. Predation experiments indicated that blue crabs (Callinectes similis and C. sapidus) and sea robins (Prionotus sp.) are potential predators on juvenile flatfish. The high abundances of juvenile flatfish indicate that the tidal creeks are an important nursery area. The correspondence between growth rates estimated from field data and those observed in the laboratory suggests that growth in the nursery is mainly related to water temperature and not food limited.     

In situ fluctuations of oxygen and sulphide in marine microbial sediment ecosystems

Laminated microbial ecosystems (microbial mats) on the island of Schiermonnikoog (The Netherlands) were studied with respect to variation in oxygen and sulphide profiles, depth distributions of photopigments and viable number and cell volume of purple sulphur bacteria. Cyanobacteria occurred in the top 2 mm, the dominant species being Microcoleus chthonoplastes. The blooming of purple sulphur bacteria below the cyanobacterial layer was observed in autumn, the dominant species being the immotile Thiocapsa roseopersicina. Cell volume of this species is indicative of its growth rate.In situ measurements showed strong diel fluctuations in oxygen and sulphide profiles. Frequently, cyanobacteria and purple sulphur bacteria were exposed to oxygen during the day, and to anoxic conditions at night. Sulphide sometimes reached the layer of the cyanobacteria. The cyanobacteria and the purple sulphur bacteria both are very well adapted to these diel fluctuations. In addition, strong seasonal variations were observed, whereas short-term fluctuations of oxygen occurred due to changing light-climate and rainfall.Attention was paid to the unusual occurrence of microbial mats on the North Sea beach during the autumn of 1987.     

Greenhouse climate control affects postharvest tomato quality

In this study, important quality properties such as firmness, sugar and acid levels were measured and analysed in tomatoes harvested from three greenhouses during a five month period and stored at 16 °C for over 20 days. Tomatoes were harvested from three identical, neighbouring, greenhouses which were either conventionally ventilated (open greenhouse) or mechanically cooled (semi-closed greenhouses). Sugar and acids levels were hardly affected by greenhouse type. Compared to the open greenhouse, semi-closed greenhouses produced heavier and less mature (firmer) fruit at the commercial harvesting stage based on colour. Fruit maturity differences could be linked to the vertical temperature gradient and to CO₂ levels in the different greenhouses. This indicates that CO₂ levels and temperature affect the synchronisation between colour and firmness maturity at harvest. The acceptance period, i.e., the time period both tomato colour and firmness are considered acceptable by consumers, will likely be positively affected when growers switch from conventionally ventilated to semi-closed production systems. Additional to greenhouse effects also effects of the harvest month were observed. The sugar to acid ratio was highest and glucose to fructose ratio was lowest in July, the month with the highest irradiance, irrespective of greenhouse type. The estimated value for the maximum firmness (F_max) varied from 17.9 N in August to 31.2 N in June. This monthly variation in F_max explains an important part of the variation found in the postharvest behaviour of tomatoes. Interestingly, the monthly variation in F_max showed the same trend as found for the monthly initial sugar levels. It might be hypothesised that the monthly variation in glucose and fructose levels causes variation in that part of firmness that is generated by cell turgor. The monthly variation in F_max, sugar and acid levels could not be linked to climate conditions and remains to be elucidated.     

Does the influence of the epibenthic predator crangon crangon L. (brown shrimp) extend to sediment microalgae and bacteria?

The effect of juvenile Crangon crangon L. (brown shrimp) on the microbial part of the food web of marine shallow-water sandy sediment was investigated in two experiments using an outdoor flow-through system. Biomass, composition and productivity of microalgae and bacteria, POC and PON content in the sediment, as well as nutrient and oxygen fluxes, were measured in the absence and presence of two natural densities (50 and 100 ind·m⁻²) of juvenile Crangon. Stimulating effects of the presence of juvenile Crangon on both microalgae and bacteria were observed after three weeks. However, the number of statistically significant effects was low, despite the conspicuous change in sediment characteristics caused by Crangon activity. To explain the possible trophic effect of Crangon via meiofaunal grazing, the results were compared with meiofaunal grazing rates (dual labelling by ¹⁴C-bicarbonate and ³H-thymidine) and changes in the meiofaunal and macrofaunal community. The reasons for the low number of significant trophic effects of Crangon on algae and bacteria were that the overall meiofaunal grazing pressure on microbiota was low, and that only part of the meiofauna (mainly harpacticoid copepods) was significantly affected by the predator. Meiofauna thus appeared to be a weak mediating link in the sandy sediment system investigated. No effect of Crangon on oxygen fluxes, or POC and PON content in the sediment was observed. Neither were the nutrient concentrations in the overlying water affected, but increased nutrient levels in the pore water were observed. According to theoretical calculations, excretion from the shrimp may imply a considerable nutrient addition which can be rapidly re-utilized by the microbes directly at the sediment/water interface. Bioturbation by Crangon appeared to counteract flaking of the microalgal mat.     

Species strategy and N fluxes in grassland soil: A question of root litter quality or rhizosphere activity?

A clear understanding of the feedback between plant communities and soil nutrient cycles is necessary to predict the dynamics of grassland ecosystems. Nitrogen fluxes may depend on specific functional plant traits in relation to dead and living roots, which determine a litter and a rhizosphere effect of the plant community. Two species of nutrient-rich grasslands that demonstrate either a more conservative (Dactylis) or more competitive strategy (Lolium) for nitrogen were grown at a non-optimal level of N availability and at optimal water, P and K supply. In a factorial experiment, monocultures of both species were sown on soil monoliths containing the intact dead root litter system of either species. The N fluxes were measured for two years in these microcosms.The presence of a growing stand increased the N mineralisation rate of the root litter. Both the type of root litter and that of the rhizosphere significantly affected the N fluxes. A greater N harvest was recorded in the Dactylis stand and on the Lolium litter; while greater N loss by leaching occurred under the Lolium stand and from the Lolium litter. Lolium induced a greater N turnover in SOM, while Dactylis caused a faster decay rate of litter N. Dactylis favoured the “short” N cycle between stand and root litter, while Lolium favoured the “complete” N cycle between stand and SOM. The chemical composition of the root litter was similar, but the species effects on the N cycles could be accounted for by differences in morphological root traits. A set of functional root processes is proposed that differentiates species strategies towards the nitrogen cycle. Dynamic consequences for a mixture of the two species are discussed.     

Harvest index increase during seed growth of field pea

Harvest index during seed growth has been reported to increase linearly with time for many crop species. Although the rate of harvest index increase was generally stable across experiments within a species, there are indications that it is sensitive to variations in temperature. The objective of the present study was to compare the harvest index increase for field pea (Pisum sativum L.) across 31 experiments that included a wide range of temperature environments. The change in harvest index within each experiment was well described by a linear increase with time but there was considerable variability among experiments. The increase in harvest index was also calculated as a function of thermal time after flowering. The variability was only marginally reduced. Use of thermal time was, however, superior in identifying a common point for the initiation of seed growth among the experiments.     

How the food supply harvestable by waders in the Wadden Sea depends on the variation in energy density,body weight,biomass, burying depth and behaviour of tidal-flat invertebrates

For several reasons, waders in the Wadden Sea face a large seasonal and annual variation in their food supply. Observations on a tidal flat in the Dutch Wadden Sea have shown that:

• 1.- (1) The average energy density of ten invertebrate prey species varies between 21 and 23 kJ·g⁻¹ AFDW. In Scrobicularia plana and Mya arenaria, but not in Macoma balthica, the energy density is 10% lower in winter than in summer.
• 2.- (2) Depending on the species, body weights of prey of similar size are 30 to 60% lower in winter than in summer.
• 3.- (3) The year-to-year fluctuation in standing-crop biomass is larger in some species than in others, the difference depending mainly on the frequency of successful recruitment. The overall biomass of the macrobenthos in winter is half of that in summer, but the timing of the peak biomass differs per species.
• 4.- (4) The burying depth varies per species: Cerastoderma edule live just beneath the surface, while M. balthica, S. plana, M. arenaria, Arenicola marina and Nereis diversicolor bury more deeply and the majority of these prey live out of reach of the bird's bill. In all six species, burying depth increases with size. There is no seasonal variation in depth of C. edule and M. arenaria, but the four other species live at most shallow depth in early summer and most deeply in midwinter. Burying depths in winter vary from year to year, but are unrelated to temperature. Neither has temperature any effect on depth within months. For knot Calidris canutus feeding on M. balthica, the fluctuation in the accessible fraction was the main source of variation in the biomass of prey that is actually harvestable, i.e. the biomass of prey of suitable size that is accessible.Accordingly, the paper reviews the available data on the temporal variations in accessibility, detectability, ingestibility, digestibility and profitability of prey for waders. Only a small part of the prey is harvestable since many accessible prey are ignored because of their low profitability, while many profitable prey are inaccessible. The profitability of prey depends on their size and weight but also on their depth in the mud, since handling time increases with burying depth. A simple biomechanical rule explains why the handling time of small prey increases with bill length and why large, long-billed waders ignore a disproportionately larger part of the small prey. The fraction detectable for visually feeding waders is usually very low, especially when the temperature of the substrate is below 3–6°C. Waders vary their prey choice over the year in response to the changes in the availability and profitability of their different prey species. The food supply harvestable by waders is much lower in winter than in summer. For waders wintering in the Wadden Sea, the food supply may be characterized as unpredictable and usually meagre. Waders wintering in NW Europe are concentrated in coastal sites where the average surface temperature is above 3°C. This probably cannot be explained by a greater burying depth, and only partly by a lower body condition, of prey in colder areas. Yet the harvestable fraction is lower in colder sites, especially for sight-feeding waders, as invertebrates are less active at low temperatures. However, the lower energetic cost of living and reduced chances of the prey being covered by ice may also contribute to the waders' preference for warmer sites.

     

Diversity of crop development traits and nitrogen use efficiency among potato cultivars grown under contrasting nitrogen regimes

Potato (Solanum tuberosum L.) requires abundant nitrogen (N) to perform well and has low nitrogen use efficiency (NUE). We assessed phenotypic variation among 189 potato cultivars for NUE and the association between NUE and ecophysiological variables describing canopy development (CDv), under high and low N input. In 2009 and 2010, 189 cultivars were grown with N supply (soil N + fertiliser N) of 75 or 180 kg N/ha at Bant, the Netherlands. CDv was assessed weekly as the percentage of soil covered by green potato leaves (%SC). Data were analysed using a model that described CDv as a function of thermal time, based on the Beta function and estimates of cardinal temperatures. Nitrogen significantly affected model-derived, biologically relevant, curve-fit parameters for each cultivar. The t 1 (i.e., thermal time required to reach maximum soil cover (Vx)) was higher at low than at high N. Other parameters were higher at high than at low N, especially Vx and the period over which it was maintained. Nitrogen also affected tuber dry matter yield, tuber size and weight distributions, N content and N uptake but not tuber dry matter percentage. The total area under the %SC curve was highly correlated with yield in both years. Cultivars performing well under high N also performed well under low N. There was large variation in NUE component traits among cultivars; maturity type partially explained this variation. Variables of the CDv model captured this variation, N effects on light interception and its correlation with yield.     

Effects of Drought and Competition by a Ryegrass Sward on the Seedling Growth of a Range of Grassland Species

In de‐intensified grassland where an increased seedling recruitment is seen as a prerequisite to enhance plant species diversity, establishment success of introduced seeds of wildflower species is often poor. Outshading by the existing sward and dry weather spells when seedlings are at an early stage of development are major reasons for a low seedling establishment. The effects of drought and competition by an existing Lolium perenne sward on the growth of seedlings of seven wildflower species were investigated under standardized environmental conditions. Without competition (control), the relative growth rate of the wildflower seedlings varied between 103 (Tragopogon pratensis) and 184 mg g^?1 day^?1 (Leontodon autumnalis). Species differed in the extent of growth reduction by competition. Compared with the control, reduction of seedling dry weight ranged from 45 % (Trifolium pratense) up to 88 % (Daucus carota). The time range between onset of desiccation and first effects on growth varied between species, with T. pratense showing early and strong growth reduction in contrast to D. carota being least affected. It can be concluded, that those species which maintain a comparatively high growth rate supported by adapted shoot‐root allocation are most promising for establishment in grassland as they can exploit profiles of higher resource availability.     

The delay of flowering time in almond: a review of the combined effect of adaptation,mutation and breeding

Regulation of flowering time in almond, as in other Prunus species, is a complex process involving both chill and heat requirements. Following exposure to appropriate consecutive periods of cold and warm temperatures, the buds break dormancy and sprout or flower depending on bud type. To maximize flowering and subsequent vegetative growth and fruit set, chilling and ensuing warm temperature requirements have to be fully satisfied. Because of its potential for very early flowering, flowering time in almond is a major determinant of its adaptation to new environments. In colder regions, Late-flowering is often necessary to avoid frost damage during and just after flowering. Consequently, the selection of delayed flowering times remains an important objective in almond improvement programs. Flowering time is considered a quantitative though highly heritable trait. In addition, a dominant gene (Late flowering, Lb), originally identified in a spontaneous mutation of the Californian almond cultivar ‘Nonpareil’, was also described. The objective of this review is a comparative analysis of the effects of regional adaptation, breeding and mutation on the delay of flowering time in new almond cultivars. Findings indicate that the adaptation of almonds from the Mediterranean basin to colder regions in Northern Europe and America has been mainly achieved through delayed flowering. These adapted late-flowering cultivars have usually been developed by selecting desired quantitative genes within each regional germplasm. Additional progress thus appears achievable with a more comprehensive understanding of the quantitative and qualitative genetics controlling this trait. The use of molecular markers for the early selection of genes conferring late flowering, including both spontaneous mutations as well as unique regional germplasm, should allow development of even later cultivars including ultra-late cultivars flowering as into April.     

Abundance and salt tolerance of obligately aerobic,phototrophic bacteria in a marine microbial mat

Data have been collected on the abundance of obligately aerobic, bacteriochlorophyll-a-containing bacteria in a marine microbial mat on the West Frisian Island of Texel, The Netherlands. Plate counts on media rich in organic matter revealed average numbers of 3¹10⁵·cm⁻³ sediment in the top 10 mm of the mat; the number of purple non-sulphur bacteria was of the same magnitude. Due to the relatively small dimensions of obligately aerobic anoxygenic phototrophic bacteria and purple non-sulphur bacteria, compared to those of purple sulphur bacteria, the contributions of either of the two former groups to the biomass of Bchl-a-containing organisms was approximately 3%. The specific Bchl-a-content of the isolated obligately aerobic phototrophs was very low (0.8 to 1.0 μg·mg⁻¹ protein) compared to that of purple non-sulphur bacteria (16 to 20 μg·mg⁻¹ protein), and purple sulphur bacteria (27 to 30 μg·mg⁻¹). As a consequence, the relative contribution to the total Bchl a concentration of the two former groups (0.1% and 2.1%, respectively) was negligible, compared to that of the purple sulphur bacteria (97.8%).Salinities <50 had little effect on growth rate and yield of isolates; at salinities between 50 and 100 the doubling time increased progressively with a concomitant decrease in yield; no growth occurred at salinities > 140.     

Clover as a cover crop for weed suppression in an intercropping design: I. Characteristics of several clover species

Weeds often form a major problem in weakly competitive vegetable crops, particularly in low input systems. Undersown cover crops can be used to suppress weeds, but often put too high a competitive pressure on the main crop. Cover crop selection is one of the potential means that can be used to design or optimize these intercropping systems. The objective of the current research was to investigate the variability among a range of clover species in morphological and physiological traits that are considered relevant for interplant competition. To this purpose, field experiments with pure stands of eight clover species (2001) and a selection of three clover species (2002) were conducted, in which regular observations and periodic harvests were taken. Clear differences in the time in which full soil cover was obtained, total accumulated biomass, growth duration, height development and N-accumulation were observed. Persian clover (Trifolium resupinatum L.) and subterranean clover (T. subterraneum L.) were the two most contrasting species in this study, particularly differing in the period in which full soil cover was obtained. Persian clover's faster soil cover could not be attributed to a single trait, but resulted from a number of intrinsic characteristics, like light extinction coefficient, light use efficiency and specific leaf area that together determine the relative growth rate. The study also demonstrated the importance of differences in relative starting position, caused by, for instance, seed size, seeding rate and fraction establishment, for the analysis of early growth characteristics. Alsike clover (T. hybridum L.), berseem clover (T. alexandrinum L) and crimson clover (T. incarnatum L.) developed slower than Persian clover, but all produced a higher amount of accumulated dry matter, due to a longer growing period. Clear differences in height and height development between species were observed. These differences were not associated with dry matter accumulation, as the tallest (red clover; 80 cm) and the shortest species (subterranean clover; 12 cm) produced similar amounts of dry matter. A strong positive correlation between early soil cover development and N-accumulation was observed. The large variability among clover species indicates that species selection is a very important aspect of the development of cropping systems that include clover as a cover crop.