Differences in flowering time maintain species boundaries in a continental radiation of Viburnum.

PREMISE: We take an integrative approach in assessing how introgression and Pleistocene climate fluctuations have shaped the diversification of the core Lentago clade of Viburnum, a group of five interfertile species with broad areas of sympatry. We specifically tested whether flowering time plays a role in maintaining species isolation. METHODS: RAD-seq data for 103 individuals were used to infer the species relationships and the genetic structure within each species. Flowering times were compared among species on the basis of historical flowering dates documented by herbarium specimens. RESULTS: Within each species, we found a strong relationship between flowering date and latitude, such that southern populations flower earlier than northern ones. In areas of sympatry, the species flower in sequence rather than simultaneously, with flowering dates offset by ≥9 d for all species pairs. In two cases it appears that the offset in flowering times is an incidental consequence of adaptation to differing climates, but in the recently diverged sister species V. prunifolium and V. rufidulum, we find evidence that reinforcement led to reproductive character displacement. Long-term trends suggest that the two northern-most species are flowering earlier in response to recent climate change. CONCLUSIONS: We argue that speciation in the Lentago clade has primarily occurred through ecological divergence of allopatric populations, but differences in flowering time were essential to maintain separation of incipient species when they came into secondary contact. This combination of factors may underlie diversification in many other plant clades.

Sterile marginal flowers increase visitation and fruit set in the hobblebush (Viburnum lantanoides, Adoxaceae) at multiple spatial scales.

Background and Aims: Enlarged sterile flowers on the periphery of inflorescences increase the attractiveness of floral displays, and previous studies have generally demonstrated that these have positive effects on insect visitation and/or reproductive success. However, experiments have not specifically been designed to examine the benefits of sterile flowers under conditions that reflect the early stages in their evolution, i.e. when plants that produce sterile flowers are at low frequency. Methods: Over three years, three experiments were performed in natural populations of Viburnum lantanoides, which produces sterile marginal flowers (SMFs). The first experiment established that fruit production in V. lantanoides increases with the receipt of outcross pollen. The second tested the role of SMFs under extant conditions, comparing fruit production in two populations composed entirely of intact plants or entirely of plants with the SMFs removed. The third was designed to mimic the presumed context in which SMFs first evolved; here, SMFs were removed from all but a few plants in a population, and rates of insect visitation and fruit set were compared between plants with intact and denuded SMFs. Key Results: In comparing whole populations, the presence of SMFs nearly doubled fruit set. Under simulated 'ancestral' conditions within a population, plants with intact SMFs received double the insect visits and produced significantly more fruits than denuded plants. There was no significant effect of the number of inflorescences or fertile flowers on insect visitation or fruit set, indicating that the presence of SMFs accounted for these differences. Conclusions: The presence of SMFs significantly increased pollinator attraction and female reproductive success both in contemporary and simulated ancestral contexts, indicating that stabilizing selection is responsible for their maintenance, and directional selection likely drove their evolution when they first appeared. This study demonstrates a novel approach to incorporating historically relevant scenarios into experimental studies of floral evolution.

Use of phytoremediated sediments dredged in maritime port as plant nursery growing media.

We evaluated the potential of a phytoremediated sediment (TR) dredged from maritime port as peat-free growth substrate for seven ornamental plants, in comparison with an untreated sediment (NT), in a greenhouse experiment. The studied plants were Quercus ilex, Photinia x fraseri, Viburnum tinus, Cistus albidus, Raphiolepis indica, Westringia fruticosa and Teucrium fruticans. Plant growth was monitored for ten months, and the changes in the physico-chemical properties, toxicity, microbial biomass and enzyme activities involved in the C, P and N cycles were also monitored during the plant growth period. The results showed that the studied ornamental plants could grow on both NT and TR sediments, but that the growth was higher on TR sediment. The plant growth induced changes in the sediment chemical functional groups, with clear separation between NT and TR sediments for each of the studied plant. Microbial biomass and enzyme activities significantly increased during the plant growth, more in TR than in NT sediment. Toxicity was detected in NT sediments during the plant growth whereas it was not observed in NT sediments during the whole growth period. We concluded that phytoremediation converted the dredged maritime sediments into suitable substrates for growing ornamental plants, and that the re-use by plant nursery industry can be a sustainable management and valorization for remediated sediments.

Whole-plant allocation to storage and defense in juveniles of related evergreen and deciduous shrub species.

In evergreen plants, old leaves may contribute photosynthate to initiation of shoot growth in the spring. They might also function as storage sites for carbohydrates and nitrogen (N). We hence hypothesized that whole-plant allocation of carbohydrates and N to storage in stems and roots may be lower in evergreen than in deciduous species. We selected three species pairs consisting of an evergreen and a related deciduous species: Mahonia aquifolium (Pursh) Nutt. and Berberis vulgaris L. (Berberidaceae), Prunus laurocerasus L. and Prunus serotina Ehrh. (Rosaceae), and Viburnum rhytidophyllum Hemsl. and Viburnum lantana L. (Adoxaceae). Seedlings were grown outdoors in pots and harvested on two dates during the growing season for the determination of biomass, carbohydrate and N allocation ratios. Plant size-adjusted pools of nonstructural carbohydrates in stems and roots were lower in the evergreen species of Berberidaceae and Adoxaceae, and the slope of the carbohydrate pool vs plant biomass relationship was lower in the evergreen species of Rosaceae compared with the respective deciduous species, consistent with the leading hypothesis. Pools of N in stems and roots, however, did not vary with leaf habit. In all species, foliage contained more than half of the plant's nonstructural carbohydrate pool and, in late summer, also more than half of the plant's N pool, suggesting that in juvenile individuals of evergreen species, leaves may be a major storage site. Additionally, we hypothesized that concentration of defensive phenolic compounds in leaves should be higher in evergreen than in deciduous species, because the lower carbohydrate pool in stems and roots of the former restricts their capacity for regrowth following herbivory and also because of the need to protect their longer-living foliage. Our results did not support this hypothesis, suggesting that evergreen plants may rely predominantly on structural defenses. In summary, our study indicates that leaf habit has consequences for storage economics at the whole-plant level, with evergreen shrub species storing less carbohydrates (but not N) per unit plant biomass than deciduous species.

Deep simple epicotyl morphophysiological dormancy in seeds of two Viburnum species, with special reference to shoot growth and development inside the seed.

BACKGROUND AND AIMS: In seeds with deep simple epicotyl morphophysiological dormancy, warm and cold stratification are required to break dormancy of the radicle and shoot, respectively. Although the shoot remains inside the seed all winter, little is known about its growth and morphological development prior to emergence in spring. The aims of the present study were to determine the temperature requirements for radicle and shoot emergence in seeds of Viburnum betulifolium and V. parvifolium and to monitor growth of the epicotyl, plumule and cotyledons in root-emerged seeds. METHODS: Fresh and pre-treated seeds of V. betulifolium and V. parvifolium were incubated under various temperature regimes and monitored for radicle and shoot emergence. Growth of the epicotyl and cotyledons at different stages was observed with dissecting and scanning electron microscopes. KEY RESULTS: The optimum temperature for radicle emergence of seeds of both species, either kept continuously at a single regime or exposed to a sequence of regimes, was 20/10 °C. GA(3) had no effect on radicle emergence. Cold stratification (5 °C) was required for shoot emergence. The shoot apical meristem in fresh seeds did not form a bulge until the embryo had grown to the critical length for radicle emergence. After radicle emergence, the epicotyl--plumule and cotyledons grew slowly at 5 and 20/10 °C, and the first pair of true leaves was initiated. However, the shoot emerged only from seeds that received cold stratification. CONCLUSIONS: Seeds of V. betulifolium and V. parvifolium have deep simple epicotyl morphophysiological dormancy, C(1b)B (root)-C(3) (epicotyl). Warm stratification was required to break the first part of physiological dormancy (PD), thereby allowing embryo growth and subsequently radicle emergence. Although cold stratification was not required for differentiation of the epicotyl--plumule, it was required to break the second part of PD, thereby allowing the shoot to emerge in spring.

[Characteristics of foliar delta13C values of common shrub species in various microhabitats with different karst rocky desertification degrees].

By measuring the foliar delta13C values of 5 common shrub species (Rhamnus davurica, Pyracantha fortuneana, Rubus biflorus, Zanthoxylum planispinum, and Viburnum utile) growing in various microhabitats in Wangjiazhai catchment, a typical karst desertification area in Guizhou Province, this paper studied the spatial heterogeneity of plant water use at niche scale and the response of the heterogeneity to different karst rocky desertification degrees. The foliar delta13C values of the shrub species in the microhabitats followed the order of stony surface > stony gully > stony crevice > soil surface, and those of the majority of the species were more negative in the microhabitat soil surface than in the others. The foliar delta13C values decreased in the sequence of V. utile > R. biflorus > Z. planispinum > P. fortuneana > R. davurica, and the mean foliar delta13C value of the shrubs and that of typical species in various microhabitats all increased with increasing karst rocky desertification degree, differed significantly among different microhabitats. It was suggested that with the increasing degree of karst rocky desertification, the structure and functions of karst habitats were impaired, microhabitats differentiated gradually, and drought degree increased.

The structure and roles of sterile flowers in Viburnum macrocephalum f. keteleeri (Adoxaceae).

The formation and ecological roles of sterile flowers in flowering plants are interesting issues in floral biology and evolution. Here, we investigated the morphological and anatomical characteristics of both fertile and sterile flowers of Viburnum macrocephalum f. keteleeri, a self-incompatible and insect-pollinated shrub, during different developmental stages of flowers. In addition, pollinator visitation rates and fruit set were determined in intact inflorescences and those with sterile flowers removed. The results indicate that sterile and fertile flowers were developmentally similar during early developmental stages, and that development of the flower types diverged about 15 days before flowering. In addition, pollinator visitation rates, number of pollen grains on stigmas and fruit set were significantly higher in inflorescences with sterile flowers than those without sterile flowers. The results suggest that sterile flowers of this species evolved from fertile flowers under long-term selective pressure, and play a crucial role in enhancing reproductive success through effectively attracting pollinators to the plant and thus enhancing fruit set.

Variation in xylem formation of Viburnum odoratissimum var. awabuki: growth strain and related anatomical features of branches exhibiting unusual eccentric growth.

Growth strains (GSs) and growth eccentricity in the branches of Viburnum odoratissimum var. awabuki (K. Koch) Zabel were measured. A pronounced growth promotion occurred on the lower side of some branches. Although the GS of the branches was similar to that of normal wood, a larger GS was observed on the upper side of the branches. Thus, eccentric growth occurred on the side opposite to the larger GS. In addition, there was a strong negative relationship between f-back bending and eccentric growth, indicating that eccentric growth largely precluded correction to the vertical position. To understand the function of eccentric growth on the lower side of the branches, we examined several anatomical features of the branches and found that (1) the cell walls of both sides lacked the gelatinous layer, (2) the microfibril angle measured by X-ray diffraction and polarizing light was small on both the upper and the lower sides and (3) the vessel number and the cell wall area did not change to a large extent. The anatomical features of the xylem did not differ obviously between the upper and the lower sides of the branches; however, the fibers were longer on the lower side than on the upper side. These results suggest that the growth stress pattern and formation of branch architecture in V. odoratissimum differ from those observed in other woody angiosperms.

Complex combination of seed dormancy and seedling development determine emergence of Viburnum tinus (Caprifoliaceae).

BACKGROUND AND AIMS: The shrub Viburnum tinus is widely distributed in mattoral vegetation of the Mediterranean basin. The purpose of the present study was to classify the seed dormancy type and examine the requirements for embryo growth, root protrusion and shoot emergence. METHODS: Overwintered fruits were collected in western Spain in April 2001 and prepared in three ways: entire pericarp was removed, exocarp and mesocarp were removed or fruits were left intact. Fruits treated in these three ways were subjected to artificial annual temperature cycles or to constant temperature regimes for 1.5 years. KEY RESULTS: Removal of exocarp and mesocarp was necessary for embryo growth and germination. High temperature favoured dormancy alleviation and embryo growth, intermediate to low temperatures favoured root protrusion, and intermediate temperature shoot emergence. There was substantial germination at constant temperature regimes, indicating an overlap between temperature intervals suitable for the different stages of embryo and seedling development. Functionally, V. tinus has the same root and shoot emergence pattern that is described for other Viburnum species considered to have epicotyl dormancy. However, the requirement for high and low temperatures for radicle protrusion and epicotyl emergence, respectively, was missing in V. tinus; these characters are the foundation for the epicotyl dormancy classification. CONCLUSIONS: It is concluded that V. tinus does not have epicotyl dormancy. Instead, there is a combination of a weak morphophysiological dormancy and a slow germination process, where different temperatures during an annual cycle favour different development stages. The present study suggests that the first complete seedlings would emerge in the field 1.5 years after fruit maturation in October, i.e. seed dispersal during winter, embryo growth during the first summer, root protrusion and establishment during the second autumn and winter, and cotyledon emergence during the second spring.