Sepal Primordia Research Articles

Mechanical forces exerted on floral primordia with a novel experimental system modify floral development in Arabidopsis thaliana.

Mechanical forces play a crucial role in plant development, including floral development. We previously reported that the phyllotactic variation in the staminate flowers of Ceratophyllum demersum may be caused by mechanical forces on the adaxial side of floral primordia, which may be a common mechanism in angiosperms. On the basis of this result, we developed a novel experimental system for analysis of the effects of mechanical forces on the floral meristem of Arabidopsis thaliana, aiming to induce morphological changes in flowers. In this experimental system, a micromanipulator equipped with a micro device, which is shaped to conform with the contour of the abaxial side of the young floral primordium, is used to exert contact pressure on a floral primordium. In the present study, we conducted contact experiments using this system and successfully induced diverse morphological changes during floral primordial development. In several primordia, the tip of the abaxial sepal primordium was incised with two or three lobes. A different floral primordium developed an additional sepal on the abaxial side (i.e., two abaxial sepals). Additionally, we observed the fusion of sepals in some floral primordia. These results suggest that mechanical forces have multiple effects on floral development, and changes in the tensile stress pattern in the cells of floral primordia are induced by the mechanical forces exerted with the micro device. These effects, in turn, lead to morphological changes in the floral primordia.

Tradeoff between speed and robustness in primordium initiation mediated by auxin-CUC1 interaction

Robustness is the reproducible development of a phenotype despite stochastic noise. It often involves tradeoffs with other performance metrics, but the mechanisms underlying such tradeoffs were largely unknown. An Arabidopsis flower robustly develops four sepals from four precisely positioned auxin maxima. The development related myb-like 1 (drmy1) mutant generates noise in auxin signaling that disrupts robustness in sepal initiation. Here, we find that increased expression of CUP-SHAPED COTYLEDON1 (CUC1), a boundary specification transcription factor, in drmy1 underlies this loss of robustness. CUC1 surrounds and amplifies stochastic auxin noise in drmy1 to form variably positioned auxin maxima and sepal primordia. Removing CUC1 from drmy1 provides time for noisy auxin signaling to resolve into four precisely positioned auxin maxima, restoring robust sepal initiation. However, removing CUC1 decreases the intensity of auxin maxima and slows down sepal initiation. Thus, CUC1 increases morphogenesis speed but impairs robustness against auxin noise. Further, using a computational model, we find that the observed phenotype can be explained by the effect of CUC1 in repolarizing PIN FORMED1 (PIN1), a polar auxin transporter. Lastly, our model predicts that reducing global growth rate improves developmental robustness, which we validate experimentally. Thus, our study illustrates a tradeoff between speed and robustness during development.

Screening of flower bud differentiation conditions and changes in metabolite content of Phalaenopsis pulcherrima

Using the Phalaenopsis pulcherrima (P. pulcherrima) as the test material, the optimal conditions for flower bud differentiation were selected by setting different photoperiods and temperatures. The process of flower bud differentiation was observed, and changes in the content of carbohydrates, hormones, and other metabolites during flower bud differentiation were studied. The results showed that the optimum time of illumination was 16 h. d−1 and the temperature was 30 ℃ / 25 ℃. According to the characteristics of the flower bud development of P. pulcherrima, the differentiation process can be divided into six periods: the initial differentiation phase, inflorescence primordium differentiation phase, flower primordium differentiation phase, sepal primordium differentiation phase, petal primordium differentiation phase, and column and pollinia differentiation phase. The soluble sugar content in the leaves showed a trend of first increasing and then decreasing. The soluble sugar content was highest 30 days after the treatment, when the differentiation of the inflorescence primordium was completed. The starch content first decreased, then increased, and finally decreased. The starch content was lowest at 30 days of treatment and highest at 35 days. The soluble protein content in the leaves reached a maximum after 30 days of treatment. With the continuous differentiation of flower buds, the soluble protein content showed an overall decreasing trend. The zeatin (ZT) content reached a maximum at day 35, showing an overall decreasing trend from days 35 to 50 and reaching a minimum at day 50. The gibberellin (GA3) and auxin (IAA) content first decreased and then increased during the process of flower bud differentiation. The changes in soluble sugar and soluble protein content during flower bud differentiation are consistent, while the changes in GA3 and IAA are consistent.

Tradeoff Between Speed and Robustness in Primordium Initiation Mediated by Auxin-CUC1 Interaction.

Robustness is the reproducible development of a phenotype despite stochastic noise. It often involves tradeoffs with other performance metrics, but the mechanisms underlying such tradeoffs were largely unknown. An Arabidopsis flower robustly develops four sepals from four precisely positioned auxin maxima. The development related myb-like 1 (drmy1) mutant generates noise in auxin signaling that disrupts robustness in sepal initiation. Here, we found that increased expression of CUP-SHAPED COTYLEDON1 (CUC1), a boundary specification transcription factor, in drmy1 underlies this loss of robustness. CUC1 surrounds and amplifies stochastic auxin noise in drmy1 to form variably positioned auxin maxima and sepal primordia. Removing CUC1 from drmy1 provides time for noisy auxin signaling to resolve into four precisely positioned auxin maxima, restoring robust sepal initiation. However, removing CUC1 decreases auxin maxima intensity and slows down sepal initiation. Thus, CUC1 increases morphogenesis speed but impairs robustness against auxin noise. Further, using a computational model, we found that the observed phenotype can be explained by the effect of CUC1 in repolarizing PIN FORMED1 (PIN1), a polar auxin transporter. Lastly, our model predicts that reducing global growth rate improves developmental robustness, which we validated experimentally. Thus, our study illustrates a tradeoff between speed and robustness during development.

Floral Initiation, Organogenesis, and Flowering of Mature Paphiopedilum Clair de Lune ‘Edgard Van Belle’ Award of Merit/American Orchid Society Plants Derived from Divisions

Paphiopedilum Clair de Lune ‘Edgard Van Belle’, an excellent Maudiae-type hybrid that has been propagated by artificial division for a long time. We studied its flower bud initiation, development of floral organs, and flowering habits with a view to providing information for flowering control and efficient commercial production. According to our research, the flower bud initiation phase of this cultivar begins in February every year, and 80% of the plants completed sepal primordium differentiation in March, The flower bud differentiation lasts for 6 to 7 months, until flowering in August. Within 1 to 3 months after flower bud differentiation, all tested plants differentiated lateral buds. After 5 to 6 months, the new, aboveground vegetative shoots reached their maximum growth, with an average plant height of 20 cm, five leaves, and a shoot dry weight of more than 3 g. From February to April of the following year, a new cycle of flower development and vegetative growth began. In addition, this cultivar was notably sensitivity to high ambient temperature during the late phase of flower development, with a flower bud drop rate as high as 33.3% under average day/night temperatures of 29.0/26.5 °C.

Development of Flowers Buds and Mixed Buds in the Dichasial Inflorescence of Geranium koreanum Kom. (Geraniaceae).

Flower bud differentiation is of great significance for understanding plant evolution and ecological adaptability. The development of flower buds and mixed buds in the dichasial inflorescence of Geranium koreanum was described in this paper. The morphogenesis, surface structure, and organ morphology at different growth stages of G. koreanum buds were examined in detail using scanning electron microscope and stereo microscope. The development of mixed buds started from the flattened apical meristem. The stipule and leaf primordia arose first. Subsequently, the hemispherical meristem was divided into two hemispheres, forming a terminal bud and floral bud primordia, followed by lateral bud differentiation. The formation of the terminal and lateral buds of G. koreanum was sequential and their differentiation positions were also different. The floral bud primordia would develop into two flower units and four bracts. The primordia of a flower bud first formed the sepal primordia, then the stamen and petal primordia, and finally the pistil primordia. Compared to the stamen primordia, the growth of the petal primordia was slower. Finally, all organs, especially the petals and pistil, grew rapidly. When the pistil and petals exceeded the stamens and the petals changed color, the flower bud was ready to bloom.

Sympetaly in the mimosoid clade (Leguminosae, Caesalpinioideae): An unusual trait in the rosid group

Sympetaly is a notable feature within the mimosoid clade (Leguminosae: Caesalpinioideae), specially as it is uncommon in rosids. The way the petals are organized, forming a tubular corolla by the union of the petals directly affects the perception of pollinators. This study examines whether the petal union leading to a sympetalous corolla in the mimosoid clade is widespread and whether there is variation in type and extent. For this purpose, floral buds and flowers of 16 species from 13 genera were collected, fixed, and processed for analysis by light and scanning electron microscopy. Most species studied display a pentamerous sympetalous corolla with free lobes. The petal primordia are individualized and emerge simultaneously on the floral apex, alternating with the sepal primordia. Petals remain free at the beginning of the intermediate stages of development, and subsequently, the petals curve and approach each other, and their margins touch. Epidermal papillae are found on the petals’ apical margin interconnecting the lobes, which enclose the inner organs of the floral bud. The structure of the sympetalous corolla is variable regarding the type and extent of the union. Four different types of sympetaly were found and classified as follows: 1. full connation - petals fully united along their length; 2. connation-coherence - petals genuinely united in the basal portion and coherent in the median and apical portion; 3. full coherence - petals intertwined with papillae throughout; and 4. partial coherence - free petals at the base and intertwined with papillae in the middle and apical portion. Thus, sympetaly of mimosoid legumes results from postgenital union of the petals at the apical, median, and basal regions during their development by tissue union (connation), intertwining of epidermal papillae (coherence), or both processes. The mucilage cells found along the petals guarantee the stabilization and flexibility of the tubes, in addition to contributing to the protection of the internal organs of the flower. Although more studies on floral development are needed to understand the evolution of this unusual trait, connation appears to be a derived trait within mimosoids.

Morphological Development and Nutritional Metabolism during Floral Bud Differentiation in Acca Sellowiana (Feijoa)

ABSTRACT To provide scientific and technological support for the smooth development of flowers, regulation of florescence, and cultural management, among others, this study analyzes the corresponding relationship between bioclimatology and the characteristics of growth, development, and reproductive biology of A. sellowiana. Berg (Feijoa). Therefore, through optimized paraffin sections, this study examined the floral bud differentiation in five-year-old A. sellowiana trees and analyzed the dynamic changes in carbohydrate and nitrogenous compound metabolism during floral bud differentiation and flower development under natural growth conditions. The results showed that there were six stages in the differentiation process of A. sellowiana floral buds: the initiation of differentiation, inflorescence primordium differentiation, sepal primordium differentiation, petal primordium differentiation, stamen primordium differentiation and pistil primordium differentiation. Differentiation of floral buds began in late March in the Mianyang region, China, and lasted approximately 30 days. Following bud differentiation, the growth enters a period of bud development lasting approximately five months. Throughout the entire differentiation process, the flower buds contained significantly higher levels of total sugar, starch, soluble sugar, and nucleic acid than those in the leaves, and a peak of total nitrogen, nitrate nitrogen, and sucrose during the physiological transition period. Total nucleic acid and RNA content both increased, with RNA content increasing more significantly.

Is seasonal flowering time of Paphiopedilum species caused by differences in initial time of floral bud differentiation?

Members of the genus Paphiopedilum are world-famous for their large, colourful flowers, unique floral morphology and long floral lifespan. Most Paphiopedilum species bloom in spring or autumn. The control of flowering time is of great significance to the commercial production of floral crops, because it affects the sales and prices of flowers. However, the mechanism that regulates when Paphiopedilum species bloom is unclear. In the present study, floral bud initiation and development of P. micranthum (spring-flowering species with one flower per stalk), P. dianthum (autumn-flowering species with multiple flowers per stalk) and P. henryanum (autumn-flowering species with one flower per stalk) were investigated by morphological and anatomical methods. We divided Paphiopedilum floral bud differentiation into six phases: the initiation of differentiation, inflorescence primordium differentiation, flower primordium differentiation, sepal primordium differentiation, petal primordium differentiation and column primordium differentiation. We found that the timing of floral bud differentiation for the three species was synchronized when experiencing the same environment, while the period from initiation to flowering largely differed. In addition, initiation of floral bud differentiation in P. dianthum was earlier at a warmer environment. The difference in flowering time of three species was mainly caused by the duration of floral bud development, rather than the initiation time. The findings were of great significance for the cultivation and flowering regulation of Paphiopedilum species.

Floral ontogeny ofDelphinium anthriscifolium(Ranunculaceae) and development of intrafloral and symmetrical resupinated organs

AbstractDelphinieae (Ranunculaceae) are characterized by zygomorphic spiral flowers and show a remarkable diversity of perianth organization. Floral structure and development have been investigated in most lineages of the tribe, mainly focusing on the establishment of symmetry and on perianth development. In this study, floral organogenesis and morphogenesis in Delphinium anthriscifolium, a member of the recently erected Delphinium subgenus Anthriscifolium, were investigated and compared with those of other species of Delphinieae in a phylogenetic context. In D. anthriscifolium, zygomorphy of the spiral flower is established soon after the initiation of sepal primordia. Among the four fully developed petals of the dorsoventralized corolla, the two dorsalmost ones become spurred, and the lateral petals resupinate before anthesis and are mirror images of each other. Resupination of petals, long overlooked in morphological studies of Delphinieae, is here reported in Ranunculaceae for the first time. The role of resupinated petals, possibly acting as a landing platform for nectar foragers, may be of high adaptive significance. Based on our new findings regarding floral ontogeny and morphology in D. anthriscifolium, we provide an updated picture of floral evolution in Delphinieae.

AINTEGUMENTA and AINTEGUMENTA-LIKE6 directly regulate floral homeotic, growth, and vascular development genes in young Arabidopsis flowers.

Arabidopsis flower primordia give rise to organ primordia in stereotypical positions within four concentric whorls. Floral organ primordia in each whorl undergo distinct developmental programs to become one of four organ types (sepals, petals, stamens, and carpels). The Arabidopsis transcription factors AINTEGUMENTA (ANT) and AINTEGUMENTA-LIKE6 (AIL6) are required for correct positioning of floral organ initiation, contribute to the specification of floral organ identity, and regulate the growth and morphogenesis of developing floral organs. To gain insight into the molecular mechanisms by which ANT and AIL6 contribute to floral organogenesis, we identified the genome-wide binding sites of both ANT and AIL6 in stage 3 flower primordia, the developmental stage at which sepal primordia become visible and class B and C floral homeotic genes are first expressed. AIL6 binds to a subset of ANT sites, suggesting that AIL6 regulates some but not all of the same target genes as ANT. ANT- and AIL6-binding sites are associated with genes involved in many biological processes related to meristem and flower organ development. Comparison of genes associated with both ANT and AIL6 ChIP-Seq peaks and those differentially expressed after perturbation of ANT and/or AIL6 activity identified likely direct targets of ANT and AIL6 regulation. These include class B and C floral homeotic genes, growth regulatory genes, and genes involved in vascular development.

Arabidopsis sepals: A model system for the emergent process of morphogenesis.

During development, Arabidopsis thaliana sepal primordium cells grow, divide and interact with their neighbours, giving rise to a sepal with the correct size, shape and form. Arabidopsis sepals have proven to be a good system for elucidating the emergent processes driving morphogenesis due to their simplicity, their accessibility for imaging and manipulation, and their reproducible development. Sepals undergo a basipetal gradient of growth, with cessation of cell division, slow growth and maturation starting at the tip of the sepal and progressing to the base. In this review, I discuss five recent examples of processes during sepal morphogenesis that yield emergent properties: robust size, tapered tip shape, laminar shape, scattered giant cells and complex gene expression patterns. In each case, experiments examining the dynamics of sepal development led to the hypotheses of local rules. In each example, a computational model was used to demonstrate that these local rules are sufficient to give rise to the emergent properties of morphogenesis.

Variation of Endogenous Hormones during Flower and Leaf Buds Development in ‘Tianhong 2’ Apple

Hormones have an important role in apple flower bud differentiation; therefore, it is necessary to systematically explore the dynamic changes of endogenous hormones during flower and leaf bud development to elucidate the potential hormone regulation mechanism. In this study, we first observed the buds of ‘Tianhong 2’ apple during their differentiation stage using an anatomical method and divided them into physiologically differentiated stages of spur terminal buds, flower buds, and leaf buds. Then, we determined the contents of zeatin riboside (ZR), abscisic acid (ABA), auxin (IAA), and gibberellin (GA3) in these various types of buds using an enzyme-linked immunosorbent assay. The results showed that the content of ZR and the ratio of ZR to IAA in spur terminal buds decreased significantly during physiological differentiation. The contents of ZR, IAA, and GA3 in leaf buds culminated at the initial differentiation stage. The content of ZR in flower buds was significantly higher than that in leaf buds after formation of the inflorescence primordium and sepal primordium. Before the appearance of stamen primordium, the content of GA3 in flower buds was remarkably lower than that in leaf buds. The ratios of ABA/IAA and ZR/IAA in flower buds were significantly higher than those in leaf buds before the appearance of flower organ primordium. Moreover, ABA content, ABA/ZR, and ABA/GA3 in flower buds were higher than those in leaf buds throughout the whole flower bud morphological differentiation process. Therefore, the reduced ZR content was beneficial to floral induction. The low content of GA3, and high ratios of ABA/IAA and ZR/IAA were conducive to early morphological differentiation. In addition, high ratios of ABA/GA3 and ABA/ZR were beneficial to the morphological differentiation of flower buds. Moreover, the high ABA content was beneficial to floral induction and morphological differentiation of flower buds. Our results shed light on the mechanisms of hormonal regulation of apple flower bud differentiation and could potentially strengthen the theoretical basis for artificial regulation of apple flower bud development using exogenous plant hormones.

Developmental basis for flower sex determination and effects of cytokinin on sex determination in Plukenetia volubilis (Euphorbiaceae)

Key messageCytokinin might be an important factor to regulate floral sex at the very early stage of flower development in sacha inchi.Sacha inchi (Plukenetia volubilis, Euphorbiaceae) is characterized by having female and male flowers in a thyrse with particular differences. The mechanisms involved in the development of unisexual flowers are very poorly understood. In this study, the inflorescence and flower development of P. volubilis were investigated using light microscopy and scanning electron microscopy. We also investigated the effects of cytokinin on flower sex determination by exogenous application of 6-benzyladenine (BA) in P. volubilis. The floral development of P. volubilis was divided into eight stages, and the first morphological divergence between the male and female flowers was found to occur at stage 3. Both female and male flowers can be structurally distinguished by differences in the shape and size of the flower apex after sepal primordia initiation. There are no traces of gynoecia in male flowers or of androecia in female flowers. Exogenous application of BA effectively induced gynoecium primordia initiation and female flower development, especially at the early flower developmental stages. We propose that flower sex is determined earlier and probably occurs before flower initiation, either prior to or at inflorescence development due to the difference in the position of the female and male primordia in the inflorescence and in the time of the female and male primordia being initiated. The influence of cytokinin on female primordia during flower development in P. volubilis strongly suggests a feminization role for cytokinin in sex determination. These results indicate that cytokinin could modify the fate of the apical meristem of male flower and promote the formation of carpel primordia in P. volubilis.

On organ development in larvae of Diplosoma migrans (Menker & Ax, 1976) (Tunicata, Ascidiacea, Didemnidae)

Development in the colonial ascidian genus Diplosoma is unusual in producing two zooids already during the free-swimming larval period. The development of D. listerianum had been investigated in two studies. The present study reports embryonic development of a second species D. migrans. In D. migrans the pharynx of the oozooid develops from two separate primordia. The intestinal tract develops as well from two distinct primordia that fuse and subsequently connect to the prospective pharynx. The pharynx of the blastozooid develops from the paired epicardia of the oozooid. In the oozooid the primordia of the peribranchial sacs often derive from the endodermal wall of the prospective pharynx, but occasionally develop from thickenings of the ectodermal epidermis. No atrial siphon is formed; instead, the body wall retracts and the gill slits open directly to the exterior. In the oozooid the rectum develops from the initial prospective intestine, while the remaining intestine develops from a diverticulum of the stomach. In the blastozooid the main part of the intestinal tract develops from the initial prospective intestine, whereas the rectum develops as a diverticulum of the initial intestine. Due to this developmental mode, the intestinal tracts of oozooid and blastozooid are transitorily connected at the respective border of rectum and proximal part of posterior intestinal tract. The paired epicardia of the two zooids derive from the epicardial outgrowths of the pharyngeal primordia of the oozooid that are also the primordia of the heart and pericardia. Budding in the larva and the adult is highly similar, in particular concerning the development of a new thorax and the heart from paired epicardial Anlagen and the origin of the distal part of the posterior intestine. Discrepancies in the descriptions of the embryonic development of D. listerianum and differences to the development in D. migrans are discussed. The development of the pericardium-heart-complex from epithelially organized tissue constitutes the plesiomorphic condition within Didemnidae and Aplousobranchiata. The development from mesenchymatic cell masses is a secondarily derived simplification.

Morphological development of petals in Ranunculaceae

The petals, or the honey-leaves, are of great divergence in morphology in Ranunculaceae, i. e., tubular, bilabial, cup-shaped, flat, concaved or scaled at the base, with or without spur or succate. The previous observations showed that although the petals differ in mature morphology, they showed great similarity in the early development stage. The petal primordia are all hemispherical, rounded and much smaller than the sepal primordia, a relatively long plastochron exists between the last sepal and the first petal and differentiate into a blade and a short stalk. Thus, we assumed that the different morphology of the mature petals might be due to the morphological repatterning of petals in the development. To prove the hypothesis, the morphological development of the petals from 22 species from 20 genera, recovering all ten petalous clades and the major morphological types, in Ranunculaceae was observed by scanning electron microscope (SEM).The young petal undergoes the following developmental stages to the mature petal after it differentiates into blade and stalk. In the first stage, a depression appears at the base of the blade and the nectary tissue will appear in the depression in the later development. In the second stage, two bulges appear at the base of the depression that makes the petal bilabial and the bulges will be the upper lip of the petal and thus the blade will be the lower lip. In the third stage, two bulges become larger and fuse with one another at first and then fuse with the margins of the blade in each side, or each of the bulges fuses with the margin of the blade at first and then fuses with one another, or the bulges stop further growth and the depression deepened to form the succate or the spur. In the fourth stage, the lips, the two fused sides and the stalk growth in different speed.The divergence of development of different petals happens mainly in the third and the fourth stages and less divergence in the second and then the first stages. For example, a tubular petal of Helleborus thibetanus undergoes the following developmental stages: a depression appear at the base of the blade, then two bulges appear at the base of the depression, and then the bulges fuse with one another to form the upper lip, the upper lip fuses with two margins of the blade that makes the petal oblique cup-shaped, the growth speed of the upper lip is faster than the two fused sides and that of the later is faster than the lower lip that makes the petal to be tubular; a spurred petal of Aquilegia yabeana undergoes the following developmental stages: a depression appears at the base of the blade, then two bulges appears at the base of the depression, the depression deepens to form the spur and the bulges stop further growth.According to the molecular systematic results, the genera in two basal most clades, Glaudidium and Hydrastis, are apetalous. Coptis and Xanthorhiza are in the next basal most clade, the petal in this clade only has, or sometimes not clear, the depression at the base of the blade and the nectary tissue appears in the depression and the petals in these genera are not bilabial. The rest of the genera form the core Ranunculaceae in which two bulges appear at the base of the depression. The appearance of the bulges can be considered as a key innovation and let the petals get the diverged potential in morphology and finally resulted in the divergence of the genera.The development processes of the petals in different petalous clades, or even in different petalous genera in the same clade, are different. The bulges might be lost in Consolida clade, in this case, the spur is well developed, or only one bulge appears in Ranunculus and Nigella clades, or both the bulges and the depression might be lost in Adonis which has the flat petals without nectary tissue.The growth of the bulges which becomes the upper lip of the petal or the depression which becomes the succate or the spur can result in the hidden of the honey from the nectary tissue and thus can increase the difficulty for the nectar foraging pollinators and avoiding the non nectar foraging insects and then increase the pollination efficiency and increase the adaptability to different nectar foraging insects.

Floral Development of Parkia multijuga and Stryphnodendron adstringens, Two Andromonoecious Mimosoid Trees (Leguminosae)

Premise of research. This study compares the development of staminate and hermaphrodite flowers of Parkia multijuga and Stryphnodendron adstringens, two andromonoecious mimosoid trees, in order to determine whether diclinous flowers result from the same developmental processes. The issues involving the interesting structure of the perianth were also addressed.Methodology. Flowers and flower buds of various sizes were processed for surface and histological observations.Pivotal results. Five sepal primordia emerge in the floral meristem from the abaxial side in a sequential order in P. multijuga and from the adaxial side of the flower in a modified helical order in S. adstringens. Five petal primordia initiated simultaneously in P. multijuga and S. adstringens. The carpel primordium initiates concomitantly with five antesepalous stamen primordia in P. multijuga and after petal elongation in S. adstringens. All connections among floral organs in P. multijuga (gamosepaly, gamopetaly, epipetaly, and staminal tube) and S. adstringens (gamosepaly and gamopetaly) are postgenital. A sixth sepal primordium found in some S. adstringens flowers is formed late in the development. Early ontogenetic stages are similar between hermaphrodite and staminate flowers of both species, and only in middevelopmental stages does the carpel primordium stop elongating; no ovule, style, or stigma is formed, leaving only a small carpellodium in the center of the staminate flowers.Conclusions. Our ontogenetic studies show that carpel abortion, not carpel absence from inception, results in staminate flowers in two andromonoecious legumes.

Study on Petal-Sepal Mutant of Sunflower (<i>Helianthus annuus</i>) after Space Mutation

In this paper, we studied the morphological, histological and photosynthetic characteristics of the stably inherited sunflower petal-sepal mutant, and it was obtained by the space radiation-induced mutagenesis. Afterwards, we got following results: 1) The morphological characteristics represented that the inflorescence of petal-sepal mutant maintained the appearance and structure of capitulum, whereas no explicit tubular flower or ligulate flower was differentiated. 2) The histological characteristics revealed that the petal-sepal mutant only completed the inflorescence development and the differentiation of sepal primordia and inflorescence primordia, without entering the differentiation stage of tubular flower primordia, ligulate flower primordia, stamen primordia or pistil primordia. 3) The photosynthetic characteristics showed that the photosynthetic rate, transpiration and stomatal conductance of petal-sepal mutant were relatively weaker than the control plants. In the end, we concluded that the petal-sepal mutant of sunflower had only inflorescence differentiation, while several mutant genes were caused by radiation-induced mutation, which entered an infinitely recurrent development process rather than the floral differentiation stage. We also observed a few chloroplast structures in the paraffin section, combined with the results of photosynthetic characteristics of petal-sepal mutant, and then we believed that the inflorescence of petal-sepal mutant was involved in photosynthesis to accumulate energy for plant growth.

A dynamical phyllotaxis model to determine floral organ number.

How organisms determine particular organ numbers is a fundamental key to the development of precise body structures; however, the developmental mechanisms underlying organ-number determination are unclear. In many eudicot plants, the primordia of sepals and petals (the floral organs) first arise sequentially at the edge of a circular, undifferentiated region called the floral meristem, and later transition into a concentric arrangement called a whorl, which includes four or five organs. The properties controlling the transition to whorls comprising particular numbers of organs is little explored. We propose a development-based model of floral organ-number determination, improving upon earlier models of plant phyllotaxis that assumed two developmental processes: the sequential initiation of primordia in the least crowded space around the meristem and the constant growth of the tip of the stem. By introducing mutual repulsion among primordia into the growth process, we numerically and analytically show that the whorled arrangement emerges spontaneously from the sequential initiation of primordia. Moreover, by allowing the strength of the inhibition exerted by each primordium to decrease as the primordium ages, we show that pentamerous whorls, in which the angular and radial positions of the primordia are consistent with those observed in sepal and petal primordia in Silene coeli-rosa, Caryophyllaceae, become the dominant arrangement. The organ number within the outmost whorl, corresponding to the sepals, takes a value of four or five in a much wider parameter space than that in which it takes a value of six or seven. These results suggest that mutual repulsion among primordia during growth and a temporal decrease in the strength of the inhibition during initiation are required for the development of the tetramerous and pentamerous whorls common in eudicots.

STUDY ON FLOWER BUD MORPHOLOGICAL DIFFERENTIATION OF 'TIANXIANG' STRAWBERRY

Flower bud differentiation is very important for the yield and quality of strawberry fruit. In order to identify flower bud development, the paraffin section method was adopted to observe the morphological differentiation of apical meristems of ‘Tianxiang’ strawberry (Fragaria A— ananassa Duch.) runner plants in Beijing, 2009. The results showed that top flower bud differentiation lasted for about 44 days, might be divided into seven stages. On 15 September, the buds came into the primary differentiation stage, the apical points became semi-circular, broader, and nearly 70% of the buds started differentiation on 20 September. Then the inflorescence primordia were visible on the apical points, and about 60% of the buds went into the inflorescence primordium differentiation stage on 25 September. Thereafter, the sepal primordia appeared at the base of inflorescence primordia, 60% of the buds were in the sepal primordium differentiation stage by 1 October. On 6 October, in about 90% of the buds the petal primordia could be seen inside of sepal primordial. About 10 d later, the stamen primordia developed in around 80% of the buds. On 22 October, the pistil primordia appeared on the surface of the receptacles. The main pistil primordium differentiation stage was from 22 to 29 October, and about 70% of the buds had finished their pistil primordium differentiation on 29 October. At the end of sampling on 29 October, the secondary buds were mainly in the stamen primordium differentiation stage, and the tertiary buds were mainly in the petal primordium differentiation stage. Our results provided the details of flower bud development in ‘Tianxiang’ strawberry in Beijing, which will contribute to its production and management.