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Background and aims Pollinators often drive the evolution of floral traits, but their capacity to influence the evolution of pollen colour remains unclear. Pollen colour in Campanula americana is variable and displays a longitudinal cline from prevalence of deep purple in western populations to white and light-purple pollen in eastern populations. While selection for thermal tolerance probably underlies darker pollen in the west, factors contributing to the predominance of light pollen in eastern populations and the maintenance of colour variation within populations throughout the range are unknown. Here we examine whether pollinators contribute to the maintenance of pollen colour variation in C. americana. Methods In a flight cage experiment, we assessed whether Bombus impatiens foragers can use pollen colour as a reward cue. We then established floral arrays that varied in the frequency of white- and purple-pollen plants in two naturally occurring eastern populations. We observed foraging patterns of wild bees, totalling >1100 individual visits. Key results We successfully trained B. impatiens to prefer one pollen colour morph. In natural populations, the specialist pollinator, Megachile campanulae, displayed a strong and consistent preference for purple-pollen plants regardless of morph frequency. Megachile also exhibited a bias toward pollen-bearing male-phase flowers, and this bias was more pronounced for purple pollen. The other main pollinators, Bombus spp. and small bees, did not display pollen colour preference. Conclusions Previous research found that Megachile removes twice as much pollen per visit as other bees and can deplete pollen from natural populations. Taken together, these results suggest that Megachile could reduce the reproductive success of plants with purple pollen, resulting in the prevalence of light-coloured pollen in eastern populations of C. americana. Our research demonstrates that pollinator preferences may play a role in the maintenance of pollen colour variation in natural populations.

† Background and Aims Local adaptation enables plant species to persist under different environmental conditions. Evolutionary change can occur rapidly in invasive annual species and has been shown to lead to local adaptation. However, the patterns and mechanisms of local adaptation in invasive species along colonization sequences are not yet understood. Thus, in this study the alien annual Impatiens glandulifera was used to investigate local adaptation to distinct habitats that have been consecutively invaded in central Europe. † Methods A reciprocal transplant experiment was performed using 15 populations from alluvial deciduous forests, fallow meadows and coniferous upland forests, and a greenhouse experiment was performed in which plants from these habitats were grown under treatments reflecting the main habitat differentiators (shade, soil acidity, competition). † Key Results Biomass production, specific leaf area, plant height and relative growth rate differed between habitats in the field experiment and between treatments in the greenhouse, but not between seed origins. Overall, there was no indication of local adaptation in either experiment. † Conclusions Since I. glandulifera is a successful invader in many habitats without showing local adaptation, it is suggested that the species is coping with environmental variation by means of high phenotypic plasticity. The species seems to follow a ‘jack-and-master’ strategy, i.e. it is able to maintain high fitness under a wide range of environmental conditions, but performs particularly well in favourable habitats. Therefore, the proposed colonization sequence is likely to be based primarily on changes in propagule pressure. It is concluded that invasive alien plants can become dominant in distinct habitats without local adaptation.

† Background and Aims Riparian systems are prone to invasion by alien plant species. The spread of invasive riparian plants may be facilitated by hydrochory, the transport of seeds by water, but while ecological studies have highlighted the possible role of upstream source populations in the establishment and persistence of stands of invasive riparian plant species, population genetic studies have as yet not fully addressed the potential role of hydrochoric dispersal in such systems. † Methods A population genetics approach based on a replicated bifurcate sampling design is used to test hypotheses consistent with patterns of unidirectional, linear gene flow expected under hydrochoric dispersal of the invasive riparian plant Impatiens glandulifera in two contrasting river systems. † Key results A significant increase in levels of genetic diversity downstream was observed, consistent with the accumulation of propagules from upstream source populations, and strong evidence was found for organization of this diversity between different tributaries, reflecting the dendritic organization of the river systems studied. † Conclusions These findings indicate that hydrochory, rather than anthropogenic dispersal, is primarily responsible for the spread of I. glandulifera in these river systems, and this is relevant to potential approaches to the control of invasive riparian plant species.

BACKGROUND AND AIMS Invasiveness of some alien plants is associated with their traits, plastic responses to environmental conditions and interpopulation differentiation. To obtain insights into the role of these processes in contributing to variation in performance, we compared congeneric species of Impatiens (Balsaminaceae) with different origin and invasion status that occur in central Europe. METHODS Native I. noli-tangere and three alien species (highly invasive I. glandulifera, less invasive I. parviflora and potentially invasive I. capensis) were studied and their responses to simulated canopy shading and different nutrient and moisture levels were determined in terms of survival and seedling traits. KEY RESULTS AND CONCLUSIONS Impatiens glandulifera produced high biomass in all the treatments and the control, exhibiting the 'Jack-and-master' strategy that makes it a strong competitor from germination onwards. The results suggest that plasticity and differentiation occurred in all the species tested and that along the continuum from plasticity to differentiation, the species at the plasticity end is the better invader. The most invasive species I. glandulifera appears to be highly plastic, whereas the other two less invasive species, I. parviflora and I. capensis, exhibited lower plasticity but rather strong population differentiation. The invasive Impatiens species were taller and exhibited higher plasticity and differentiation than native I. noli-tangere. This suggests that even within one genus, the relative importance of the phenomena contributing to invasiveness appears to be species' specific.

• Background and Aims: The family Balsaminaceae is essentially herbaceous, except for some woodier species that can be described as ‘woody’ herbs or small shrubs. The family is nested within the so-called balsaminoid clade of Ericales, including the exclusively woody families Tetrameristaceae and Marcgraviaceae, which is sister to the remaining families of the predominantly woody order. A molecular phylogeny of Balsaminaceaeis compared with wood anatomical observations to find out whether the woodier species are derived from herbaceous taxa (i.e. secondary woodiness), or whether woodiness in the family represents the ancestral state for the order (i.e. primary woodiness). • Methods: Wood anatomical observations of 68 Impatiens species and Hydrocera triflora, of which 47 are included in a multigene phylogeny, are carried out using light and scanning electron microscopy and compared with the molecular phylogenetic insights. • Key Results: There is much continuous variation in wood development between the Impatiens species studied, making the distinction between herbaceousness and woodiness difficult. However, the most woody species, unambiguously considered as truly woody shrubs, all display paedomorphic wood features pointing to secondary woodiness. This hypothesis is further supported by the molecular phylogeny, demonstrating that these most woody species are derived from herbaceous (or less woody) species in at least five independent clades. Wood formation in H. triflora is mostly confined to the ribs of the stems and shows paedomorphic wood features as well, suggesting that the common ancestor of Balsaminaceae was probably herbaceous. • Conclusions: The terms ‘herbaceousness’ and ‘woodiness’ are notoriously difficult to use in Balsaminaceae. However, anatomical observations and molecular sequence data show that the woodier species are derived from less woody or clearly herbaceous species, demonstrating that secondary woodiness has evolved in parallel.

d Background and Aims China is one of the centres of geographical distribution of Impatiens L. Studies of the pollination ecology of this genus in China have, until now, been unreported. Impatiens reptans, a species endemic to China, was studied. The aims were to examine the pollination ecology and pollination system of this species, to compare its pollination ecology with other Impatiens species growing in Sumatra and Japan, and to discuss possible reasons for its limited distribution. d Methods The pollination ecology of I. reptans was studied by carrying out continuous observations within three naturally growing populations. Its pollination system was studied using different pollination methods, marking and counting pollen grains, assessing pollen viability and observing pollinator behaviour. d Key Results The flowering phase of the protandrous I. reptans lasted for 89 d. The life span of an individual flower was 3·6 d. Primary pollinators were honey-bees and bumble-bees. Secondary pollinators were diurnal hawk moths and butterflies. Bombus briviceps and Bombus sp. were nectar gatherers. The mean nectar sugar concentration was 29·5 %, and the mean value of sucrose/glucose + fructose was 0·82. The proportion of seed set ranged from 0·857 to 0·873. Distances that seeds were ejected ranged from 0·58 to 1·17 m. Percentage seed germination under controlled conditions was 23·1. Pollen viability was highest on the day of anthesis and thereafter decreased. Ratios of pollen : ovules ranged from 958·8 to 970·6. d Conclusions Impatiens reptans reproduces by means of cross-pollination. Its dependence on a specialized habitat, a narrow environmental niche, a low percentage of seed germination, and habitat loss could be reasons for its limited distribution and endemism.

Pith autolysis, in the past studied only in herbaceous cultivated dicotyledonous plants, commonly occurs in wild plants. A survey of pith autolysis in wild plants was conducted in several different biomes in both North Carolina, USA and in Belgium. An objective method and a subjective method were developed and used to estimate the amount of pith autolysis in the internodes of bean (Phaseolus vulgaris L.) and the wild plant jewelweed (Impatiens capensis Meerb.) under experimental conditions. A ranking system also was used to quantify pith autolysis in other wild plants. In the field, a positive correlation was found between the rate of internode elongation and pith autolysis. Injections of sucrose solution were found to reduce the amount of pith autolysis developed in the upper internodes of jewelweed when compared to injections of control solution. Extracts from the nodal tissues of jewelweed increased the amount of pith autolysis developed by both jewelweed in the field and bean in the greenhouse. This increase in pith autolysis may be due to a principle present in the jewelweed nodes that signals or causes the supertending internodes to become hollow.

Intemodes from Impatiens sultani shoots, explanted into sterile culture, often developed a transverse separation layer after one to two weeks and the top then abscised from the bottom of the explant. Such abscission occurred more rapidly and in a greater proportion of explants when 00001 per cent auxin (IAA) was provided basally and when younger intemodes and shorter explants were used. The distance of the separation layer from the base of the explant varied little with explant length, but increased with the concentration of auxin applied basally. It seems that in this adventitious abscission the processes of positional definition and differentiation proceed without pause, whereas in normal abscission the position is defined early in development but the final stage of differentiation of the separation layer is delayed until much later when the organ senesces. To account for the results from the internodal explants and from surgical operations on shoots as well as for the characteristic position of abscission sites of leaves and fruits, we suggest that the position of abscission is controlled primarily by auxin acting as a morphogen: abscission sites occur at Y-junctions just above the base of the arm with the lower activity and auxin status, or in single axes above a region of higher auxin status. In both sites, the auxin concentration decreases in the apical direction. This hypothesis is supported by a mathematical model (see Appendix) of the interaction of diffusive and polar transport in controlling the concentration gradient along intemodes with specified auxin concentrations maintained basally. The model allows predictions concerning the site and timing of abscission which accord with observations on intemodal explants.

Impatiens balsamina L. was induced to flower by exposure to 5 short days and then made to revert to vegetative growth by return to long days. After 9 long days reverted plants were induced to re-flower by returning them to short days. Petal initiation began immediately and seven primordia already present developed into petals instead of into predominantly leaf-like organs. However, the arrangement of primordia at the shoot apex, their rate of initiation and size at initiation remained unchanged from the reverted apex, as did apical growth rate and the length of stem frusta at initiation. The more rapid flowering of the reverted plants than of plants when first induced, and the lack of change in apical growth pattern, imply that the reverted apices remain partially evoked, and that the apical growth pattern and phyllotaxis typical of the flower, and already present in the reverted plants, facilitate the transition to flower formation.

The activity of IAA-oxidase increased in the leaves of Impatiens balsamina plants receiving inductive photoperiodic cycles and in plants receiving treatments with gibberellic acid (GA3) and/or tannic acid (TA), even under non-inductive photoperiods; the activity also increased in the stem receiving inductive photoperiodic cycles (8 h). Treatment with GA3 and TA mimics the effect of SD cycles in the development of some isoenzymes of IAA-oxidase. Thus a new isoenzyme at Rf 0.48 developed in the leaves and one at Rf 0.82 developed in both the stem and the leaves of all plants receiving inductive treatments – photoperiodic or chemical – but not in water-treated controls under non-inductive photoperiods. Another isoenzyme at Rf 0.68 developed only in the stems.

GA3 increased the extension growth of Impatiens balsamina L. till 56 days under 8- and 24-h photoperiods. Cycloheximide which decreased height slightly under inductive conditions at a later stage did not affect the GA3-promoted extension growth. Both GA3 and cycloheximide caused enhancement of the rate of differentiation, although this effect was temporary in the case of GA2. Cycloheximide does not affect photoperiodic induction, whereas it hastens and increases the magnitude of GA2-induced flowering.