Floral biology

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Floral biology is an area of ecological research that studies the evolutionary factors that have moulded the structures, behaviour and physiological aspects involved in the flowering of plants. The field is broad and interdisciplinary and involves research requiring expertise from multiple disciplines that can include botany, ethology, biochemistry, entomoloniques. A slightly narrower area of research within floral biology is sometimes termed as pollination biology or anthecology. [1]

Contents

Flowers are structures that are produced by angiosperms and their evolution is intricately associated with their pollinators, particularly insects. Flowers are costly structures that target pollinators by offering them rewards so as to enhance cross-pollination. The evolution of the size of flowers, their structure and the nature of rewards and the way these signals are transmitted and perceived by potential pollinators are typically examined in terms of the costs incurred and the benefits accrued. The timing of flowering, the duration of flowering and the mode by which they cease to function once their role is fulfilled are all areas of research within the field of floral biology. [2]

Studies in floral biology can have applications since pollination and fruit set are key factors that affect yield in all crop plants.

History of the field

Cover of Sprengel's 1793 book Sprengel Geheimniss der Natur.jpg
Cover of Sprengel's 1793 book

The beginnings of the field of floral biology is generally traced to Christian Konrad Sprengel's Entdeckte Geheimniss der Natur im Bau in der Befruchtung der Blumen (The Secret of Nature in the Form and Fertilization of Flowers Discovered) (1793). [3] Sprengel may however have been influenced by the earlier work of Joseph Gottlieb Kölreuter in 1761. [4] Sprengel began his studies in 1787 starting with the wood cranesbill Geranium sylvaticum. He noted that the lower portions of the petals had soft hairs. He believed in the wisdom of the "Creator" and that not even a single hard could be without purpose. He suggested that the hairs were present to protect the nectar from rain like eyebrows and eyelashes preventing sweat to flow into the eyes. It took him six years of observation in which time he examined 461 plants. He observed that orchids lacked nectar but had nectar guides. He called these as false nectar flowers and observed that the flowers of Aristolochia trapped insects. His book included twenty five illustrations. Sprengel's work was favourably viewed by Carl Ludwig Willdenow who incorporated some of the results in his Grundriss der Kräuterkunde zu Vorlesungen (1802). [5] Sprengel noted, contrary to popular belief of his time, that flowers were aimed to prevent self-fertilization. Sprengel identified the patterns on the petals as nectar guides ("Saftmale") for pollinators. [6] At that time flowers were considered as the place for the marriage of the stamens and pistils and nectar was thought to aid the growing seeds. Bees were thought of as thieves. Sprengel's work was criticized by Johann Wolfgang Goethe. Sprengel's work however got wider coverage in the English speaking world only after Charles Darwin credited him in his Fertilisation of Orchids (1862). [7] [8]

Aspects

Flowering plants, angiosperms, are relatively recent among the plants. The oldest flower structures date to about 140 million years ago. Flowering plants underwent a major diversification after this period. Darwin saw this as an "abominable mystery" in a letter to Joseph Hooker in 1879. The earliest flowers were principally actinomorphic or having radial symmetry with multiple axes of symmetry. From these evolved flowers with bilateral symmetry or zygomorphy. It is thought that the attraction of insect pollinators led by visual cues had an influence in the evolution of zygomorphy. [9]

The earliest groups of flowering plants among the Magnoliids and the families Choranthaceae, Ceratophyllaceae, Nymphaeaceae, Annonaceae, and Aristolochiaceae are bisexual with both male and female parts present and functional within the usually large floral structure. The main pollinators of these flowers were beetles, flies and thrips. They evolved mechanisms to reduce self-pollination by changing the timing of maturity of the male and female parts. This altered timing mechanism or dichogamy was principally expressed by protogyny or the early maturation of the female parts and only rarely by protandry or the early maturation of the male parts. [10] The early flowers were principally flat and dish-like with the evolution of deeper corolla tubes being a later innovation and principally associated with long-tongued pollinators such as moths. [11] There are a large number of other mechanisms that enhance cross-pollination and prevent self-pollination. The forces that lead to the evolution of such systems as bearing male and female flowers on separate kinds of plants is still unclear. [12]

The evolution from solitary flowers to the production of inflorescences is also thought to be influenced by pollinator behaviour. Clusters of flowers may increase the visitation rates of pollinators. It may also reduce the risk of damage to individual flowers. [13]

The production of volatile chemicals by flowers is targeted towards insects. Some evidence shows that there is significant overlap between the chemicals produced by plants and those used by insects for their communications, especially for mating. [14] In the classic case of orchids in the genus Ophrys , the volatiles mimic the female sex pheromone of bees which attempt to copulate with the flower and thereby pollinate them. [15] A study of the evolution of volatile chemicals in scarab beetles and flowers that attract them in the family Araceae showed that the insects had evolved the chemicals in the Jurassic Period while the plants evolved the attractants later in the Cretaceous Period. [16]

The colours of flowers are another area of enquiry. Some distinct patterns have been noted, for instance bird-pollinated flowers are predominantly red while night-flowering plants tend to be white. The colours of flowers are produced by a variety of pigmentary mechanisms and they are meant to signal messages to pollinators. Flowers that have been pollinated often quickly wither and the nutrients resorbed by the plant. In some case parts of the flower may undergo colour changes to indicate their being unfit for visitation by pollinators. [17]

See also

Related Research Articles

Pollinator

A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.

Petal Part of most types of flower

Petals are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called corolla. Petals are usually accompanied by another set of modified leaves called sepals, that collectively form the calyx and lie just beneath the corolla. The calyx and the corolla together make up the perianth. When the petals and sepals of a flower are difficult to distinguish, they are collectively called tepals. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. Conversely, genera such as Rosa and Phaseolus have well-distinguished sepals and petals. When the undifferentiated tepals resemble petals, they are referred to as "petaloid", as in petaloid monocots, orders of monocots with brightly colored tepals. Since they include Liliales, an alternative name is lilioid monocots.

Coevolution Two or more species influencing each others evolution

In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well as gene-culture coevolution.

Pollination Biological processes occurring in plants

Pollination is the transfer of pollen from a male part of a plant to a female part of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents are animals such as insects, birds, and bats; water; wind; and even plants themselves, when self-pollination occurs within a closed flower. Pollination often occurs within a species. When pollination occurs between species it can produce hybrid offspring in nature and in plant breeding work.

Entomophily Form of pollination by insects

Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, sometimes with conspicuous patterns leading to rewards of pollen and nectar; they may also have an attractive scent which in some cases mimics insect pheromones. Insect pollinators such as bees have adaptations for their role, such as lapping or sucking mouthparts to take in nectar, and in some species also pollen baskets on their hind legs. This required the coevolution of insects and flowering plants in the development of pollination behaviour by the insects and pollination mechanisms by the flowers, benefiting both groups.

Zoophily

Zoophily is a form of pollination whereby pollen is transferred by animals, usually by invertebrates but in some cases vertebrates, particularly birds and bats, but also by other animals. Zoophilous species frequently have evolved mechanisms to make themselves more appealing to the particular type of pollinator, e.g. brightly colored or scented flowers, nectar, and appealing shapes and patterns. These plant-animal relationships are often mutually beneficial because of the food source provided in exchange for pollination.

Nectar Sugar-rich liquid produced by many flowering plants, that attracts pollinators and insects

Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists, which in turn provide herbivore protection. Common nectar-consuming pollinators include mosquitoes, hoverflies, wasps, bees, butterflies and moths, hummingbirds, honeyeaters and bats. Nectar plays a crucial role in the foraging economics and evolution of nectar-eating species; for example, nectar foraging behavior is largely responsible for the divergent evolution of the African honey bee, A. m. scutellata and the western honey bee.

Flower Structure found in some plants; aka: blossom

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants. The biological function of a flower is to facilitate reproduction, usually by providing a mechanism for the union of sperm with eggs. Flowers may facilitate outcrossing resulting from cross pollination or allow selfing when self pollination occurs.

Ornithophily Pollination by birds

Ornithophily or bird pollination is the pollination of flowering plants by birds. This sometimes coevolutionary association is derived from insect pollination (entomophily) and is particularly well developed in some parts of the world, especially in the tropics, Southern Africa, and on some island chains. The association involves several distinctive plant adaptations forming a "pollination syndrome". The plants typically have colourful, often red, flowers with long tubular structures holding ample nectar and orientations of the stamen and stigma that ensure contact with the pollinator. Birds involved in ornithophily tend to be specialist nectarivores with brushy tongues and long bills, that are either capable of hovering flight or light enough to perch on the flower structures.

Palynivore Group of herbivorous animals

In zoology, a palynivore /pəˈlɪnəvɔːɹ/, meaning "pollen eater" is an herbivorous animal which selectively eats the nutrient-rich pollen produced by angiosperms and gymnosperms. Most true palynivores are insects or mites. The category in its strictest application includes most bees, and a few kinds of wasps, as pollen is often the only solid food consumed by all life stages in these insects. However, the category can be extended to include more diverse species. For example, palynivorous mites and thrips typically feed on the liquid content of the pollen grains without actually consuming the exine, or the solid portion of the grain. Additionally, the list is expanded greatly if one takes into consideration species where either the larval or adult stage feeds on pollen, but not both. There are other wasps which are in this category, as well as many beetles, flies, butterflies, and moths. One such example of a bee species that only consumes pollen in its larval stage is the Apis mellifera carnica. There is a vast array of insects that will feed opportunistically on pollen, as will various birds, orb-weaving spiders and other nectarivores.

Pollination syndrome

Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic or biotic, such as birds, bees, flies, and so forth through a process called pollinator-mediated selection. These trait includes flower shape, size, colour, odour, reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc.

Christian Konrad Sprengel German botanist (1750-1816)

Christian Konrad Sprengel was a German naturalist, theologist, and teacher. He is most famous for his research on plant sexuality. Sprengel was the first to recognize that the function of flowers was to attract insects, and that nature favoured cross-pollination. Along with the work of Joseph Gottlieb Kölreuter he set the foundations for the modern study of floral biology and anthecology although his work was not widely recognized until Charles Darwin examined and reconfirmed several of his observations.

<i>Das entdeckte Geheimnis der Natur im Bau und in der Befruchtung der Blumen</i>

Das entdeckte Geheimnis der Natur im Bau und in der Befruchtung der Blumen by Christian Konrad Sprengel was published in 1793, but received little acclaim during the author's lifetime. Sprengel's ideas were rejected by other naturalists when it was published, but the importance of this work was duly appreciated by Charles Darwin some sixty years later. Darwin's use of Sprengel's ideas and reference to this book in the seminal work on the Fertilisation of Orchids established Sprengel's book as one of the most important works in the fields of floral biology and pollination ecology and its author as a founding father of these fields.

Mimicry in plants

In evolutionary biology, mimicry in plants is where a plant organism evolves to resemble another organism physically or chemically, increasing the mimic's Darwinian fitness. Mimicry in plants has been studied far less than mimicry in animals, with fewer documented cases and peer-reviewed studies. However, it may provide protection against herbivory, or may deceptively encourage mutualists, like pollinators, to provide a service without offering a reward in return.

Anthecology The study of pollination biology

Anthecology, or pollination biology, is the study of pollination as well as the relationships between flowers and their pollinators. Floral biology is a bigger field that includes these studies. Most flowering plants, or angiosperms, are pollinated by animals, and especially by insects. The major flower-frequenting insect taxa include beetles, flies, wasps, bees, ants, thrips, butterflies, and moths. Insects carry out pollination when visiting flowers to obtain nectar or pollen, to prey on other species, or when pseudo-copulating with insect-mimicking flowers such as orchids. Pollination-related interactions between plants and insects are considered mutualistic, and the relationships between plants and their pollinators have likely led to increased diversity of both angiosperms and the animals that pollinate them.

Pollination trap

Pollination traps or trap-flowers are plant flower structures that aid the trapping of insects, mainly flies, so as to enhance their effectiveness in pollination. The structures of pollination traps can include deep tubular corollas with downward pointing hairs, slippery surfaces, adhesive liquid, attractants, flower closing and other mechanisms.

Floral scent

Floral scent or flower scent is composed of all the volatile organic compounds (VOCs), or aroma compounds, emitted by floral tissue. Floral scent is also referred to as aroma, fragrance, floral odour or perfume. Flower scent of most flowering plant species encompass a diversity of VOCs, sometimes up to several hundred different compounds. The primary functions of floral scent are to deter herbivorous and especially florivorous insects, and to attract pollinators. Floral scent is one of the most important communication channels mediating plant-pollinator interactions, along with visual cues.

Monocotyledon reproduction

The monocots are one of the two major groups of flowering plants, the other being the dicots. In order to reproduce they utilize various strategies such as employing forms of asexual reproduction, restricting which individuals they are sexually compatible with, or influencing how they are pollinated. Nearly all reproductive strategies that evolved in the dicots have independently evolved in monocots as well. Despite these similarities and their close relatedness, monocots and dicots have distinct traits in their reproductive biologies.

Robert Raguso

Robert A. Raguso is an American biologist and professor at Cornell University in the Department of Neurobiology and Behavior. He has expanded the field of chemical ecology by introducing and pioneering floral scent as a key component of plant-pollinator communication, with special focus on hawkmoths and Clarkia plants.

Floral isolation

Floral Isolation is a form of reproductive isolation found in angiosperms. Reproductive isolation is the process of species evolving mechanisms to prevent reproduction with other species. In plants, this is accomplished through the manipulation of the pollinator’s behavior or through morphological characteristics of flowers that favor intraspecific pollen transfer. Preventing interbreeding prevents hybridization and gene flow between the species (introgression), and consequently protects genetic integrity of the species. Reproductive isolation occurs in many organisms, and floral isolation is one form present in plants. Floral isolation occurs prior to pollination, and is divided into two types of isolation: morphological isolation and ethological isolation. Floral isolation was championed by Verne Grant in the 1900s as an important mechanism of reproductive isolation in plants.

References

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  3. Willis, John C. (1895). "The present position of floral biology". Science Progress (1894-1898). 4 (21): 204–215. JSTOR   43768737.
  4. Kölreuter, Joseph Gottlieb (1761). Vorläufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden Versuchen. Leipzig: In der Gleditschischen Handlung.
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  7. Bernhardt, Peter (1999). The Rose's Kiss: A Natural History Of Flowers. Island Press. pp. 122–123.
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  9. Busch, Andrea; Zachgo, Sabine (2009). "Flower symmetry evolution: Towards understanding the abominable mystery of angiosperm radiation". BioEssays. 31 (11): 1181. doi:10.1002/bies.200900081. PMID   19847818.
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  15. Schiestl, Florian P.; Ayasse, Manfred; Paulus, Hannes F.; Löfstedt, Christer; Hansson, Bill S.; Ibarra, Fernando; Francke, Wittko (1999). "Orchid pollination by sexual swindle". Nature. 399 (6735): 421. doi:10.1038/20829.
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