Capitulum Production Research Articles

Edaphic ecotypic divergence in Senecio vulgaris and the evolutionary potential of predominantly self-fertilising species

ABSTRACT Background There is continued interest in ecotypic divergence because it is informative of local adaptation and can be an important step in speciation. Aim To investigate ecotypic divergence in a predominantly self-fertilising species, Senecio vulgaris, in response to variation in soil nitrogen (N) and phosphorus (P) concentrations. Methods Responses to increased N and P were compared among populations sampled from sand dune, bird sanctuary and garden habitats, containing soils of low, high and intermediate fertility, respectively. Responses were measured in terms of growth, with capitulum production and days to seed set recorded in some experiments. Results Population differences in response to increased N and P were detected and linked to differences in soil fertility among habitats. Thus, the sand dune population exhibited the lowest growth response to increased N and P concentrations, while a bird sanctuary population showed the greatest response to increased N supplied as either NaNO3, NH4NO3 or urea. Genetic variation in response to increased N was present in both of these populations. Conclusions The results indicate the presence of edaphic ecotypes in S. vulgaris, providing further evidence that questions the premise that self-fertilisation limits adaptation to environmental change and thus is an evolutionary dead-end.

Pest management in future climates: Warming reduces physical weed management effectiveness.

Climate change alters many aspects of weed performance and may also alter the effectiveness of management practices to control pests. Despite this concern, entire categories of widely used management practices, such as physical control, remain understudied in this context. We conducted a field experiment growing the invasive pest musk thistle (Carduus nutans) at ambient and experimentally elevated temperatures. We tested mowing management strategies that varied in the timing of a single mowing event relative to thistles' stem elongation phenology and compared these with an unmowed control. Results from this experiment informed demographic models to project population growth rates for different warming/mowing scenarios. Compared to plants grown under ambient conditions, warmed thistles were more likely to survive the same mowing treatment, flowered earlier in the season, grew to taller heights, and produced more flowering capitula. Proportional reductions in plant height and capitulum production caused by mowing were smaller under warming. Warming did not change the relative ranking of mowing treatments; mowing late in the growing season (2 weeks after individuals first reached a height of 40 cm) was most effective at ambient temperatures and under warming. Warming caused significant increases in projected local population growth rate for all mowing treatments. For invasive musk thistle, warmed individuals outperformed individuals grown at ambient temperatures across all the mowing treatments we considered. Our results suggest that to achieve outcomes comparable to those attainable at today's temperatures, farmers will need to apply supplemental management, possibly including additional mowing effort or alternative practices such as chemical control. We recommend that scientists test management practices under experimental warming, where possible, and that managers monitor ongoing management to identify changes in effectiveness. Information about changes in managed weeds' mortality, fecundity, and phenology can then be used to make informed decisions in future climates.

Evaluation of the Impact of Eustenopus villosus on Centaurea solstitialis Seed Production in California.

Simple SummaryThe exotic thistle, Centaurea solstitialis, has become overly abundant in the western USA. A seed weevil, Eustenopus villosus, was released in the early 1990s to reduce seed production and abundance of the thistle. A first step in evaluating success of the biological control program is to evaluate the amount of seed destroyed by the weevil in its new area of infestation. At two sites for two years, we found that adult weevils killed 60–70% of young flower buds, forcing plants to regrow new buds which delayed flowering by 9 days and extended flowering by 4 weeks at season end. Flower heads varied in size and seed production increased linearly by size of the flower head. The seed weevil attacked larger flower heads more frequently than smaller flower heads but the probability of a flower head being attacked did not vary with plant size. Weevil larvae occurred in 27% to 49% of seed heads and resulted in 34% to 47% of the annual seed crop being destroyed. We recommend that another survey be performed to see if the seed weevil has increased in abundance since its introduction.The impact of the capitulum weevil Eustenopus villosus on Centaurea solstitialis seed production was examined at two field sites in central California. The study occurred in 1993–1995 during the early phases of the biological control program on C. solstitialis and before the current guild of capitulum insects had become widespread. Results showed that adult feeding on early flower buds resulted in 60–70% of buds failing to develop. Regrowth delayed capitulum production by 9 days and extended production by 4 weeks at season end. Between 69% and 92% of capitula were punctured from feeding or oviposition but the occurrence of larvae in capitula ranged from 27% to 49%. Seed production in C. solstitialis capitula increased linearly with size. The occurrence of larvae was proportionally higher in larger capitula (>8 mm) but the probability of attack for individual capitula did not vary with plant size. Total seed loss from larval feeding ranged from 34 to 47%. It is recommended that another survey be performed to determine if the level of infestation of E. villosus has increased since its initial introduction.

Impacts of a biocontrol agent on invasive Ageratina adenophora in Southwest China: Friend or foe?

Classical biological control of invasive plants depends on the introduction of host-specific natural enemies. Many natural enemies have provided successful control, but the impacts of herbivorous insects on their hosts are highly variable and context dependent. Under some circumstances, overcompensation can even occur, with plant reproduction temporarily increasing rather than decreasing in response to herbivory. The stem-galling fly Procecidochares utilis has been widely introduced to help control Ageratina adenophora, a globally significant weed, but its impact has been inconsistent and in places ineffective. Its galling is known to stimulate production of side branches in plants under laboratory conditions. We examined the abundance and impact of P. utilis at four A. adenophora sites of Southwest China that were invaded at different times and support different densities of the insect. Stems with galls were more likely to be branched and produced more capitula. Furthermore, seed numbers in capitula from galled and un-galled stems were similar, and galling resulted in only slightly reduced seed weights. However, the increase in above-ground vegetative biomass associated with branching resulted in less efficient capitulum production relative to vegetative growth. Clearly Procecidochares utilis is not providing effective biocontrol of Ageratina adenophora in Southwest China so far. In the long term, galled plants may display reduced competitive ability and lower lifetime reproductive success, but the short term increases in seed production it generates suggest it may be favouring its host’s range expansion. The fly has been a successful biocontrol agent in dry area of Hawii, so understanding the factors that determine its effectiveness represent a major challenge for the future.

Growth of chamomile (Matricaria chamomilla L.) and production of essential oil stimulated by arbuscular mycorrhizal symbiosis

This study aimed to evaluate the influence of arbuscular mycorrhizal fungi on the growth and production of capitula and essential oil of chamomile plants, under different growth conditions. The symbiosis between fungi of the phylum Glomeromycota and plant roots may cause changes in secondary metabolism, including the synthesis of essential oils. Increasing biomass production associated with gains in oil content promoted by mycorrhization, makes soil biota management sustainable. Two assays were carried out in a greenhouse, in plastic pots (3 L), for 150 days after emergence of the seedlings. In the first assay, plants were inoculated with soil cultivated with organic chamomile, in three levels (0, 150 and 300 g), with pH adjusted at 6.5, for evaluating the influence of the quantity of inoculum on the growth and production of plants. In the second assay (factorial design with repetition at the central point), three levels of soil pH (6.5, 7.0, 7.5) and three levels of auxin (0; 10; 20 mg L−1) were tested to identify the influence of pH and auxin on the production of chamomile capitula and essential oil. The arbuscular mycorrhizal fungi present as spores in the soil-inoculum were composed of 22 species. In the first assay, mycorrhization favored the growth and production of capitula and such benefits tended to be higher with the increase in concentration of inoculum. In the second assay, the production of capitula and essential oil was promoted by mycorrhization, with the highest content at neutral pH (0.7 mL oil in 30 g of capitula), in synergy with the highest amount of inoculum. It was concluded that the presence of arbuscular mycorrhizal fungi in the soil is the most influential variable in the production of capitula and essential oil of chamomile, but subtle differences on soil pH influenced these parameters. The levels of auxin did not influence significantly plant growth and arbuscular mycorrhizal fungi.

CHAMOMILE PRODUCTION USING SUPPLEMENTARY IRRIGATION AND ORGANIC FERTILIZATION IN SANDY SOILS

ABSTRACT: The use of medicinal plants in the herbal medicine of Brazilian Health System has intensified production and the need for developing efficient agricultural techniques that promote greater productivity of these species. The objective of this work was to evaluate the chamomile production at field conditions as a function of irrigation depths and organic fertilizer rates. The experiment was conducted in the Universidade do Oeste Paulista (Campus II experimental area), in Presidente Prudente, São Paulo State, Brazil. A triple factorial experimental design was used, consisting of irrigation rates (150, 100, 75, 50, 25 and 0% of the reference evapotranspiration - ETo), organic manure types (poultry and cattle manure) and manure rates (0, 3 and 5 kg m-2), with four replications. The capitula production per plant, capitula dry weight and yield per water input (water use efficiency) were evaluated. The supplementary irrigation combined with organic manure fertilization provided the highest capitula yield for chamomile crop in the Presidente Prudente region. The combination of poultry manure at rate of 5 kg m-2 with water depth equal to 150% of the ETo resulted in higher average values of capitula fresh and dry weight and water use efficiency.

Reproductive Variation in Naturally Occurring Populations of the WeedParthenium hysterophorus(Asteraceae) in Australia

Parthenium weed, an annual herb native to tropical America, causes severe economic, human, and animal health and environmental impacts in Australia and in many countries in Asia, Africa, and the Pacific. There is little known about variation in reproductive output in naturally occurring populations of this weed. This information is vital to develop plant population models, devise management strategies to reduce seed output, and formulate parthenium weed pollen-induced human health (e.g., dermatitis and hay fever) risk assessment. Here, the variations in the number of capitula produced by the parthenium weed at two sites in Queensland, Australia, over a 4-yr period are reported. Under field conditions, parthenium weed produced up to 39,192 capitula per plant (> 156,768 seeds per plant), with majority of the plants (≈ 75%) producing between 11 and 1,000 capitula, and less than 0.3% of the plants producing more than 10,000 capitula (> 40,000 seeds per plant). The number of capitula per plant in the field (297 ± 22) was much lower than those reported from glasshouse and laboratory studies. Plant biomass contributed to 50 to 80% of the variation in capitulum production between plants within plots at each site, and weed density accounted for 62 to 73% of the variation in capitulum production between plots within each site. As plant size is directly correlated with reproductive output, plant size distributions in parthenium weed can be used to estimate effective population size. Information on variation in reproductive output will be used to implement management strategies to reduce parthenium weed seed output, resulting in reduced soil seed bank and weed seed spread.

Integrating multiple disturbance aspects: management of an invasive thistle, Carduus nutans

Disturbances occur in most ecological systems, and play an important role in biological invasions. We delimit five key disturbance aspects: intensity, frequency, timing, duration and extent. Few studies address more than one of these aspects, yet interactions and interdependence between aspects may lead to complex outcomes. In a two-cohort experimental study, we examined how multiple aspects (intensity, frequency and timing) of a mowing disturbance regime affect the survival, phenology, growth and reproduction of an invasive thistle Carduus nutans (musk thistle). Our results show that high intensity and late timing strongly delay flowering phenology and reduce plant survival, capitulum production and plant height. A significant interaction between intensity and timing further magnifies the main effects. Unexpectedly, high frequency alone did not effectively reduce reproduction. However, a study examining only frequency and intensity, and not timing, would have erroneously attributed the importance of timing to frequency. We used management of an invasive species as an example to demonstrate the importance of a multiple-aspect disturbance framework. Failure to consider possible interactions, and the inherent interdependence of certain aspects, could result in misinterpretation and inappropriate management efforts. This framework can be broadly applied to improve our understanding of disturbance effects on individual responses, population dynamics and community composition.

ASPECTOS AGRONÔMICOS E PRODUTIVOS DOS HÍBRIDOS DE GIRASSOL (Helianthus annus L.) HELIO 251 E HELIO 360

This study had as objective to quantify achenes, leaves, stem and capitulum production and participation, as well as green and dry matter production of two sunflower hybrids for silage production. Two sunflower hybrids, Helio 251 (H251) and Helio 360 (H360), were used and implanted under oat stubble. The counting of twelve points with five linear meters/hectare of each hybrid was carried out to evaluate hybrid production and five samples were taken to evaluate plant composition. A complete randomized experimental design was used and the averages were analyzed by Tukey test. H251 hybrid presented higher plants per linear meter and per hectare (3.99 vs. 3.12 plants/m; 49,953.95 vs. 38,342.10 plants/ha, for H251 and H360, respectively). H360 capitulum were heavier than the ones from H251, when expressed in green (303.79 g/plant) or dry (56.88 g/plant) matter basis, consequently, it showed heavier achenes (151.09 and 100.12 g/plant, respectively, for green and dry matter). H360 presented higher stem weight, when expressed in green (120.95 vs. 61.27 g/plant) or dry (48.58 vs. 29.01 g/plant) matter basis. The higher weights of capitulum, achene and stem presented by H360 resulted in heavier plants and superiority on dry matter production per hectare. KEYWORDS: achenes, capitulum; dry matter; green matter; leaves; stem.

Formas de aplicação de cama-de-frango no crescimento e produção de capítulos florais de Calendula officinalis L.

Este trabalho objetivou avaliar características fisiológicas, morfológicas e a produção de inflorescências em cultura de Calendula officinalis em função do fornecimento de cama-de-frango de corte semidecomposta. O experimento conduzido em Latossolo Vermelho distroférrico contou com quatro tratamentos, a testemunha, cama incorporada, cama em cobertura e cama em cobertura (50%) mais incorporada (50%) na dose 20 t ha-1. Empregou-se o delineamento experimental de blocos casualizados em parcelas subdivididas, com seis repetições. Os valores de matéria fresca e seca dos capítulos florais decresceram com o tempo de cultivo. As formas de fornecimento da cama-de-frango não interferiram na síntese de clorofila (44,812 µg cm-2), no metabolismo fotossintético (20,938 µmol m-2 s-1) e na transpiração das plantas (6,750 Mmol m-2 s-1), com valores mais elevados aos 112 dias após o transplantio. A maior produção total de capítulos foi observada sob cultivo com cama-de-frango 50% incorporada e 50% em cobertura (1361,16 kg ha-1) e a menor no tratamento testemunha (939,28 kg ha-1).

Influência do substrato e do tamanho da célula de bandejas de poliestireno expandido no desenvolvimento de mudas e produção de calêndula (Calendula officinalis L.)

A calêndula (Calendula officinalis L.) é uma planta medicinal anual de origem Mediterrânica que apresenta propriedades antiinflamatórias, antivirais, antigenotóxicas. A propagação desta planta normalmente é feita por sementes, sendo o uso de bandejas de poliestireno uma forma econômica de produção de mudas. Foram conduzidos dois experimentos com o objetivo de avaliar a interferência de dois tamanhos de célula em bandeja de poliestireno expandido (40 cm³ e 12 cm³) e de dois substratos (solo + esterco bovino curtido e substrato comercial tipo Plantmax para hortaliças®) sobre a qualidade de mudas e a produção de calêndula. O delineamento estatístico nos dois experimentos foi em blocos casualizados, dispostos em arranjo fatorial 2 x 2 (dois substratos e dois tamanhos de células) com seis repetições. Os tratamentos, referentes aos tamanhos das células foram bandeja com 128 células (40 cm³ de volume) e, bandeja com 288 células (12 cm³ de volume). Os substratos utilizados foram solo de cerrado + esterco bovino curtido (1:1) e substrato comercial tipo "Plantmax para hortaliças®". Parte das mudas foi transplantada para avaliação da produção de capítulos. Houve diferenças significativas entre os substratos para as variáveis: matéria seca de parte aérea, matéria seca da raiz e matéria seca total. O uso do substrato formulado com solo de cerrado e esterco bovino proporcionou o melhor desenvolvimento das mudas de calêndula. Contudo, os diferentes tipos de substrato e bandejas de poliestireno não influenciaram significativamente a produção de capítulos das mudas transplantadas.

Fire responses of three co-occurring Asteraceae shrubs in a temperate savanna in South America

South American temperate savannas have undergone significant woody plant encroachment through changes in their disturbance regimes. We studied fire strategies and the spatial variation in individual fire responses of Eupatorium buniifolium, Baccharis medullosa and B. dracunculifolia, the dominant shrub species at El Palmar National Park, in areas with different fire history. In recently burnt sites, all Baccharis dracunculifolia individuals died, whereas all E. buniifolium and B. medullosa individuals sprouted. The relative growth rate was higher in E. buniifolium, which invested more biomass in shoots, and in B. medullosa, which invested more biomass in leaves. At the unburnt site, B. dracunculifolia showed the highest growth rate and leaf biomass. B. medullosa produced capitula immediately after fire, whereas E. buniifolium reproduction was delayed for 1 year. In both species, production of capitula and reproductive effort were higher in recently burnt sites. For B. dracunculifolia, production of capitula was lower and the reproductive effort was higher in burnt than in unburnt sites. Seedling establishment was extremely low for E. buniifolium, low for B. medullosa and high for B. dracunculifolia in both recently burnt sites. E. buniifolium behaved as a sprouter, B. medullosa as facultative sprouter and B. dracunculifolia as obligate seeder species. In order to control the increase and/or attain desirable population levels of these species, fire management practices in these savannas should consider individual species’ responses, particularly to the time since the last fire.

Nutrient addition extends flowering display, which gets tracked by seed predators, but not by their parasitoids

Although phenological matching between two and three trophic interactions has received some attention, it has largely been disregarded in explaining the lack of strong cascade dynamics in terrestrial systems. We studied the response of the specialist seed predator, Paroxyna plantaginis (Tephritidae) and associated generalist parasitoids (Chalcidoidea) to controlled fertilisation of individuals of naturally growing Tripolium vulgare (Asteraceae) on four island populations (Skeppsvik Archipelago, Sweden). We consistently found evidence of nutrient limitation: fertilised plants increased their biomass, produced more capitula (the oviposition units for tephritid flies), were more at risk of attack by the tephritids, and puparia were heavier in fertilised plants. During some parts of the season tephritids became more heavily parasitized, supporting the presence of cascade dynamics, however net parasitism over season decreased in response to nutrient addition. We found no evidence that capitulum size complicated parasitoid access to the tephritids, however the extended bud production prolonged the flowering season. Thus, tephritids utilized the surplus production of capitula throughout the entire season, while parasitoids did not expand their oviposition time window accordingly. Implications for top down regulation and cascade dynamics in the system are discussed.

Branching responses in Silphium integrifolium (Asteraceae) following mechanical or gall damage to apical meristems and neighbor removal

Branching in plants increases plant access to light and provides pathways for regrowth following damage or loss of the apical meristem. We conducted two experiments in an eastern Kansas tallgrass prairie to determine how apical meristem loss (by clipping), apical meristem damage (by insect galling), and increased light availability affected growth, reproduction, and branching in Silphium integrifolium (Asteraceae). The first experiment compared clipping with galling. Clipping increased axillary shoot numbers, while galling increased axillary shoot lengths, reflecting different allocation responses among damage types and inhibition of branching by galls. However, total capitulum production was less in all gall/clip treatments than in intact shoots. The second experiment compared clipping with mowing the surrounding vegetation to increase light availability. Mowing increased total leaf, total capitulum, and axillary shoot length and axillary capitulum production in clipped and unclipped plants and in large vs. small shoots. The presence of the neighboring canopy, not of an intact apical meristem, was therefore the stronger limitation on leaf and capitulum production. These experiments suggest that damage and light competition affected both branching frequency and the partitioning of resources among shoots, branches, and leaves. Because Silphium's growth form is widespread, similar responses may occur in other grassland forbs.

Laboratory Trials of the Efficacy of the Crown Weevil Mortadelo horridus (Coleoptera: Curculionoidae) for the Biological Control of Slender Thistles Carduus pycnocephalus and C. tenuiflorus in Southwestern Australia

The hypothesis that larval crown weevil (Mortadelo horridus Alonso-Zarazaga and Sanchez-Ruiz) can suppress vegetative growth, seed set and viability in the winter annual slender thistles (Carduus pycnocephalus L. and C. tenuiflorus Curt.) was tested in laboratory gardens. Different densities (eight and 16) of M. horridus larvae and eggs were inoculated onto slender thistle rosettes between one and four times. Controls received no eggs or larvae. Larval feeding destroyed the apical meristems of treated plants, producing multi-crowned rosettes and significant declines in leaf area in both species, irrespective of the number of larvae. Treated thistles bolted at the same time and had a 4-fold increase in stem number compared to controls. The stems of control plants were up to one-third taller and thicker than attacked plants. Relative to controls, the leaves of treated plants had significantly fewer spines, decreased leaf area and decreased leaf perimeter, with less deeply lobed leaf margins. The total number of capitula produced by control and experimental groups was not significantly different, but consideration of fertile capitula only revealed a reduction in fertile capitula production following weevil attack. This was most marked in C. pycnocephalus. Control plants also produced significantly wider capitula. No control plants died, compared to 17 and 26% of the attacked plants of C. pycnocephalus and C. tenuiflorus, respectively. Implications for biocontrol of slender thistles are discussed.

Effect of phosphorus treatment on growth and yield in the medicinal herb Calendula officinalis L. (Standard Pacific) under hydroponic cultivation

The North American market for herbal medicine has been rapidly growing over the past 20 yr. This has effectively created a stro ng public and regulatory demand for high quality, uniformity and safety of medicinal plant products. Greenhouse hydroponic cultivation under controlled environmental conditions allows the grower to provide pesticide-free conditions, lacking environmental contaminants, resulting in superior product quality and consistency. Calendula officinalis is evaluated here as an alternative greenhouse crop. Growing conditions (P concentrations) were manipulated in order to maximize flower-head (capitulum) yield, as the se are the most valuable marketable structures. It was found that high P concentrations did not increase flower production, but instead produced significantly more leaf biomass.Plants receiving the lowest P concentration, with an intermittent watering reg ime, had the greatest absolute reproductive output, and greatest mass of all tissues assessed, except for leaf tissue. They also had the greatest proportion of biomass allocated to reproductive structures. Calendula officinalis shows promise as a medicinal greenhouse crop that requires low P levels for optimal capitulum (inflorescence) production (target tissue). Its xerophytic characteristics are best supported through modification of standard hydroponic systems; these modifications may be widely applicab le to medicinal plants since many share Calendula’s xerophytic characteristics. We conclude that discontinuous nutrient supply and relatively low ratios of P relative to N and K may enhance production of target medicinal tissues of Calendula officinalis. Key words:

Geigeria alata in the Namib desert: seed heteromorphism in an extremely arid environment

Abstract. Seed heteromorphism in annual Geigeria alata in the Namib Desert was investigated in relation to environmental conditions. Geigeria alata shows heteromorphism in seed dispersal distance depending on the position of the seeds on the parent plant. Seeds produced in capitula on branches (aerial) travel long distances, while seeds produced at the base of the plant remain in the vicinity of the parent plant. The ratio of aerial to basal seed production was investigated under different environmental conditions following an aridity gradient. Although there were significant differences in seed and capitula production between seasons and between different positions on the plant, no overall trend was found with increasing aridity.

Soil phosphate status modifies response of mycorrhizal and non-mycorrhizal Senecio vulgaris L. to infection by the rust, Puccinia lagenophorae Cooke.

The impact of rust infection (Puccinia lagenophorae Cooke) on growth of mycorrhizal and non-mycorrhizal Senecio vulgaris L. was examined under three levels of phosphorus fertilization. Seeds were then collected from infected and control plants and germination, seed chemistry and offspring vigour were examined. Rust infection did not become established on plants which were phosphorus limited, but on hosts fed with medium or high levels of P, the proportion of total leaf number exhibiting infection was a function of leaf production. Capitula production was decreased by rust infection and increased by mycorrhizal colonization in plants fed with a medium concentration of P. In plants fed with a high P concentration, leaf number was reduced by rust infection of mycorrhizal plants, whilst rust was deleterious to both mycorrhizal and non-mycorrhizal plants grown with a medium P supply. However, the effects of the rust were less marked in the mycorrhizal plants. Seed production and weight were unaffected by fungal treatments, although percentage germination of the resultant seeds was reduced by maternal mycorrhizal colonization and enhanced in seed from rusted, mycorrhizal plants grown at high soil P. The amount of total C, N and P was also reduced by maternal mycorrhizal infection at high soil P and increased in seeds from rusted, mycorrhizal mothers. Vigour of offspring produced from treated mother plants was influenced by mycorrhizal × phosphorus and rust × phosphorus interactions with respect to bud and leaf numbers. Mycorrhizal infection of mother plants grown with a medium P supply benefited the offspring as increased leaf and bud production was observed, but rusted mothers at the same P level produced offspring with reduced leaf number and potential fecundity.

The Impact of a Pathogen (Puccinia Lagenophorae) on Populations of Groundsel (Senecio Vulgaris) Overwintering in the Field: II. Reproduction

SUMMARY (1) Reproduction during the spring was studied in monocultures of groundsel (Senecio vulgaris L.) overwintered in the field. In the autumn, populations had been either inoculated with a fungal pathogen (Puccinia lagenophorae), which caused rust disease, or left as controls. The effects of rust infection were examined in relation to status within a hierarchical population (assessed by sub-classes based on shoot dry weights). (2) In control populations, the first capitulum buds were produced in late February. The onset of flowering became progressively delayed as status within the hierarchy declined. In all sub-classes of the population, rust infection during the previous autumn delayed flowering in the spring by 1-2 weeks. (3) The percentage of plants within a sub-class that were reproductive (bore buds or capitula) in March and April declined as status in the hierarchy decreased. In classes of comparable status, fewer plants flowered in inoculated than in control populations. (4) In April, sub-classes with low mean dry weights in both populations produced fewer buds and mature capitula. The number of florets per capitulum were also decreased in lower status classes. Rust inhibited all measured components of reproduction in all classes. (5) During May, many large control plants senesced and their reproductive output declined. Thus, most sub-classes of control populations produced a similar number of florets. In contrast, vegetative growth in inoculated populations continued and all measures of reproduction were far greater in the class of highest status than in any other. There was a seven-fold increase in the contribution of this class to floret production (70%) relative to its frequency in the original population. In control populations, there was no more than a two-fold difference between floret production and frequency in the parent population. (6) Infection resulted in no significant change in the percentage of florets that developed into seed or in the viability of seed. (7) Over the duration of this experiment, the potential seed production of control populations (calculated from capitulum production, florets per capitulum and percentage seed set) was >460 000 seeds m-2. Populations that had been rust-infected in the autumn potentially produced 183 000 seeds m-2, a reduction of >60% compared

Effects of density and soil nitrogen levels on growth of Galinsoga quadriradiata and G. parviflora in pure and mixed stands

The responses of two sympatric annual weeds, Galinsoga quadriradiata Ruiz & Pavon (G. ciliata (Raf.) Blake) and G. parviflora Cav., which occur abundantly in croplands and early secondary successional communities of hill regions in northeast India, were studied in relation to population density, soil nitrogen, and nature of the stand (pure versus mixed stands). Both weeds exhibited density-dependent mortality which increased at a higher nitrogen level. In the case of G. quadriradiata more individuals survived in a mixture than in monoculture, whereas G. parviflora showed the reverse trend. Growth of both weeds was better at a higher soil nitrogen level. Increased mortality and suppression of growth caused by an increase in density was also minimized in G. quadriradiata at the higher level of soil nitrogen. With an increase in population density, per-plant biomass and capitula production in both weeds showed an increase in positive skewness which, however, decreased at an increased nitrogen level. Galinsoga quadriradiata was less sensitive to density than G. parviflora regardless of soil nitrogen. At lower densities, both weeds grew better in a mixture than in pure stands, but at higher densities G. quadriradiata suppressed the growth of G. parviflora. Although the crude reproductive effort of the two weeds was independent of soil nitrogen, G. quadriradiata, in contrast to G. parviflora, exhibited a greater crude reproductive effort when grown either at higher densities or in a mixture.