Gynoecium structure and development in core Caryophyllales: a matter of proportions

This review based on a morphological and developmental perspective reveals a striking diversity in shapes and evolutionary trends in the gynoecium of core Caryophyllales that have affected the number of carpels, the formation of septa and the number of ovules. Two major developmental shifts are responsible for the diversity in gynoecial forms and are linked to the proportional development of carpellary tissue (ovary wall) and the floral apex. (1) Meristic change is caused by an expansion or reduction of the diameter of the floral apex. An expansion leads to polygyny linked with the development of more numerous small carpels; a reduction of space leads to lower carpel numbers, eventually resulting in a single carpel. (2) Different ovary shapes can be recognized at a mid-developmental stage predicting the further development of ovaries. With an equal growth of the ovary wall and floral apex, young ovaries take the shape of a salt-shaker; with more extensive development of the floral apex and delay of the ovary wall, a club-shaped ovary is formed; with faster growth of the ovary wall linked with intercalary expansion and a delayed growth of the floral apex, a (half-) inferior cup-shaped ovary develops. The different growth forms are the results of heterochronic shifts and affect the development of septa and ovule numbers. A common trend in the order implies a weakening and break-up of septa during development, leading to residual apical and basal septa and the shift to free-central and basal placentation. The club-shaped ovary is linked with an almost complete loss of septa and a reduction of the ovule number to one. The salt-shaker shape leads to ovaries with a massive placental column and several ovules. The cup-shaped ovary leads to a shift of ovules away from the floral apex. Developmental flexibility is responsible for a disconnection of carpel wall growth from ovular tissue. Subtle shifts in proportional growth lead to a high diversification of ovaries in core Caryophyllales and the establishment of predictable developmental trends. These trends clearly represent apomorphic tendencies, affecting different families of core Caryophyllales in different degrees. The ancestral gynoecium was probably pentamerous and isomerous with the other floral whorls, with ovules clearly separated from the carpellary wall and inserted on axile placentas corresponding to the central axis of the flower.

Floral ontogeny of Illicium Lanceolatum (Schisandraceae) and its implications on carpel homology

There are two competing hypotheses for the origin of carpels. The traditional hypothesis favors a phyllosporous origin and regards a conduplicate carpel as an ancestral form that is the result of longitudinal folding of a leaf bearing ovules along its margins. Alternatively, the carpel formation is interpreted as the result of a fusion between an ovule-bearing branch and its subtending leaf-like structure. Illicium is a member of the Austrobaileyales, which are one of the three ANA lines that diverged before all other extant angiosperms. This genus with apocarpous gynoecium has various ancestral morphological characteristics in terms of carpel, ovule, and floral apex. Although various aspects of Illicium morphology have been previously investigated, many evolutionary characteristics remain poorly understood. In this study, the development of carpel, ovule, and floral apex of I. lanceolatum was studied using LM and SEM. The results showed that the ovule primordium originates in the axillary position between the flower axis and carpel wall. So the carpel of Illicium is a leaf-like structure that encircles the ovule. This kind of carpel favors the carpel as a result of fusion between two parts, ovule-bearing branch and its subtending leaf-like structure.

Comparison of synandrium structure and development in three species from the Myristicaceae

Species of Myristicaceae have diverse morphology and structure of their synandria, making them an interesting group for androecium evolution research. To clarify the morphology, structure, and origin of the synandrium, scanning electron microscopy and histology were performed on staminate flowers of Horsfieldia pandurifolia H.H.Hu, H. tetratepala C.Y.Wu & W.T.Wang, and Myristica fragrans Houtt. at different developmental stages. A whorl of stamen primordia was rapidly initiated around the margin of the floral apex in groups of two in H. pandurifolia and groups of three in M. fragrans. Each stamen primordium grew longitudinally, producing a pair of anther lobes and four microsporangia, accompanying one vascular bundle in H. pandurifolia and M. fragrans. In H. tetratepala, three stamen primordia groups were formed, and each group with several anthers was supported by one vascular bundle, indicating a secondary increase of stamen. Three types of synandrium origin were observed: in M. fragrans, the central sterile column tissues originated from the elongate receptacle; in H. pandurifolia and H. tetratepala they were derived from a combination of floral meristem and fused connectives, and a combination of receptacular tissue and stamen groups. The diverse origins of the central sterile column suggest that the synandrium develops differently and independently in different genera and species of Myristicaceae.

Rozwój Narcissus poeticus L. [Development of Narcissus poeticus L.]

One generation of <i>Narcissus poeticus</i> develops during three seasons. The vegetative stage of the meristem lasts 11 months. It changes to a floral apex in the second year and initiates all the flower elements from the first days of June to mid August. The plants bloom in the third year.

Flowering in snow tussock (Chionochloa spp.) is influenced by temperature and hormonal cues

Snow tussocks (Chionochloa spp.) in New Zealand exhibit extreme mast (episodic) seeding which has important implications for plant ecology and plant–insect interactions. Heavy flowering appears to be triggered by very warm/dry summers in the preceding year. In order to investigate the physiological basis for mast flowering, mature snow tussock plants in the field and younger plants in a glasshouse and shadehouse were subjected to a range of manipulative treatments. Field treatments included combinations of warming, root pruning and applications of two native gibberellins (GAs) GA3, which is known to be highly floral inductive and GA4, which is associated with continued floral apex development in another long-day grass. Warming, GA3 alone and especially warming + GA3, significantly promoted flowering, as did applications of GA4 alone and GA4 + CCC (2-chloroethyltrimethylammonium chloride, which is a known synergist of GA3-induced flowering in the annual grass, Lolium temulentum L.). Our results provide support for the concept that mast flowering events in tussock species are causally related to high temperature-induced increases in endogenous gibberellin levels. It is likely that GAs (endogenous or applied) promote the continued development of a previously long-day induced floral apex. In addition to the promotion of flowering, applied GA3 also disturbed the plant’s innate resource threshold requirements, as shown by the death, over winter, of many non-flowering tillers. Applied GA4 did not show this effect, likely due to its rapid catabolic metabolism to an inactive form. High temperature-induced flowering mediated by elevated levels of endogenous floral-promotive GAs could have important implications for regulating the evolutionary interaction between these masting plants and their seed predators.

Inflorescence and floral ontogeny in Osteospermum ecklonis (Asteraceae)

Development of the capitulum inflorescence with different types of florets in Asteraceae is an interesting issue in the field of plant evolution and development. In this study, ontogeny of the inflorescence and florets of Osteospermum ecklonis (DC.) Norl., an ornamental and evergreen subshrub, was investigated using epi-illumination light microscopy. The initiation and subsequent development of florets on the highly convex inflorescence apex occurred acropetally, except for the ray florets, which showed a lag in initiation. Organogenesis in disc florets started with unidirectional initiation of corolla lobes from the adaxial side and then proceeded by simultaneous appearance of five stamen and finally two median carpel primordia. Significant developmental features included the lack of pappus differentiation, formation of nonsyngenesious stamens, and formation of the ovule-less ovary. Ray florets showed significant differences from disc florets as reflected by the zygomorphic shape of floral apex and shift of floral merosithy from pentamery to tetramery. Also, expansion of corolla lobes to form the ligule and the formation of staminodia were observed. It is hypothesized that the actinomorphic pentamerous disc florets are most primitive among the family from which the tetramerous ray florets are derived. Accordingly, ray florets evolved from disc florets under long-term selective pressure and play a crucial role in enhancing reproductive success.

Aspects of inflorescence and floral development in the putative basal angiosperm Amborella trichopoda (Amborellaceae)

Amborella has woody axes whose development is intrinsically plagiotropic and determinate. The tree habit is achieved through overtopping of older axes by basally produced younger axes, as in Mangenot's model. Inflorescence units, which are produced in the axils of distal leaves, may be described as extended cymes, each branch ending in a flower. Basal bracteoles have a decussate arrangement, which is modified to an alternate phyllotaxis distally. Flowers produce one or more additional bracteoles with a rapid transition to the spiral phyllotaxis of the broad overlapping tepals. In this transition the initially conical floral apex becomes invaginated to form a floral cup, with subsequent appendages appearing on its inner margin. The floral apex then forms the flat base of the cup but retains a discrete single tunica layer. The receptacular bowl is deepened and narrowed by the basal meristem of each appendage, the last formed floral organs usually consuming the floral meristem. Sexual parts are more numerous in male than female flowers, accounting for their size difference, but primordia of stamens and carpels are initially very similar. Floral symmetry is largely a consequence of close packing of appendages within the floral cup. In its initial stages of development the flower does not conform to any conventional floral model in angiosperms and is better regarded as highly specialized rather than ancestral in its synorganization. This is not unexpected in a lineage of such long independent evolution.Key words: Amborella, basal angiosperm, development, inflorescence, primitive flower.

Ontogeny of floral organs and morphology of floral apex in Phellodendron amurense (Rutaceae)

The ontogeny of floral organs and the morphology of floral apex in the dioecious Phellodendron amurense Rupr. were investigated by light microscopy (LM), scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). Investigations indicated that P. amurense is hermaphroditic in its organisation and a common set of floral organs (sepals, petals, stamens and carpels) arise in all flowers during the early stages of development. Later, selective abortion of gynoecium and androecium occurs resulting in dimorphic unisexual flowers. The carpels in male flower buds become different from those in female flower buds soon after their initiation. The stamens of female flowers are not differentiated into anthers and filaments before abortion. The poorly differentiated carpel of male flowers never develops normal structures. Floral morphological evidence supports that Zanthoxylum, Tetradium and Phellodendron are related to one another in a linear sequence. LSCM revealed some interesting features on the apical meristem surface such as zonal differentiation, a triangular or sectorial cell, radiating cell files and linear rows of anticlinal cell walls fluorescing relatively brightly. The concept of carpel-enhancing meristem in the floral apex is tentatively proposed to account for the different fates of carpel development in male and female flowers in P. amurense.