7.2.3 Floral biology and sexual reproduction
Angiosperm flowers are the most advanced and structurally intricate in the Plant Kingdom. Their multiple components each have one or more specialised functions, most importantly the female and male generative organs, the pistil (gynoecium) and the anthers (androecium) respectively (Sedgley and Grifﬁn 1989). Other floral organs also contribute to the success of the reproductive process. The sepals (calyx) protect the flower in bud, and in some species contribute to the floral display and even photosynthesis. The petals (corolla) are usually the main component of the floral display, which in animal-pollinated flowers provide visual and olfactory attraction. Nectaries secrete a sugar reward for many pollen vectors. In several large Australasian genera such as Eucalyptus, Acacia and Callistemon and others in the Proteaceae, the styles and stamens double up as showy visual attractants (Figure 7.19), which compensate for reduced perianth surface area. The latter may well be an adaptation to the water-limited environments in which these plants dominate.
The pistil is an integrated organ comprising stigma, style and ovary (Knox 1984). The stigma is covered with unicellular or multicellular papillae, which are modiﬁed epidermal cells (Figure 7.20a, b). The surface of the stigma may be wet or dry, meaning copious or sparse secretion respectively. These secretions contain lipids, carbohydrates, proteins and water and this is the site of pollen recognition, hydration and germination (Figure 7.20c). Pre-pollination secretion occurs in all species, and in a minority there is additional secretion in response to pollination (Sedgley and Scholeﬁeld 1980). Style structures vary but most conform to one of three patterns. Open styles with a central canal ﬁlled with mucilage are characteristic of many monocotyledons, and of some dicotyledons such as Citrus. Closed styles with solid transmitting tissue and no canal predominate in dicotyledons and some monocotyledons such as grasses. Semi-closed styles are an intermediate condition found in avocado and some eucalypts. Transmitting tissue is composed of longitudinal ﬁles of elongated cells which produce an extracellular secretion through which the pollen tubes grow.
The ovary contains one or more ovules with integuments which form the micropyle and surround the nucellus and embryo sac. After meiosis, the haploid egg cell, along with two synergid cells, a central cell containing two polar nuclei and three antipodal cells, is produced by mitosis during embryo sac formation. A normal mature embryo sac therefore contains seven cells and eight haploid nuclei (Figure 7.21). Overall, secretory cells of the stigma, style and ovary provide an extra-cellular medium for pollen tube attraction, growth and nutrition.
Double fertilisation is unique to angiosperms, and refers to the fact that both the sperm nuclei fertilise nuclei within the embryo sac. After germinating on the stigma, the pollen tube grows between the transmitting tissue cells of the style and on reaching the ovary, continues through the micropyle and nucellus, entering the embryo sac via one of the synergids.
A pore forms in the pollen tube wall, through which a small amount of cytoplasm is released along with the two sperm nuclei. One migrates to the egg cell, and fuses with the egg nucleus to form the diploid zygote which develops into the embryo. The other migrates to the central cell and there fuses with the two polar nuclei to form the triploid endosperm which acts as a food source for the developing embryo.
Pollen is produced within the anther, with pollen grains varying in size from 3.5 up to 300 µm in diameter, and even up to 3 mm long in ﬁlamentous pollen of aquatic angiosperms such as Zostera (Figure 7.22a, b). In general, small grains are wind dispersed (Figure 7.22d) and large grains are animal dispersed, and their shape may be spherical, elongated, oval, triangular or tetrahedral. At maturity, pollen grains are dehydrated propagules. Most pollen occurs as single grains, but some is aggregated into composite structures such as the polyads of Acacia with 4, 8, 16, 32 or 64 genetically identical grains (Figure 7.22c) and the pollinia of orchids consisting of hundreds or thousands of grains. Highly sculptured pollen surfaces are characteristic of animal dispersal, whereas wind-dispersed species have smoother grains. The pollen surface can be sticky which causes grains to clump, and in some species viscin threads hold the grains together. These kinds of pollen are generally animal dispersed, whereas grains with a dry surface are mostly wind dispersed.
Cell number varies at anther dehiscence. Two-thirds of angiosperms release bicellular pollen grains, with a vegetative cell which controls tube growth and metabolism and a generative cell which divides after pollen germination into two sperm cells. The other third release tricellular pollen grains with a vegetative cell and two sperm cells, as the generative cell divides before pollen dispersal and germination.
The pollen coat contains lipids, proteins, carbohydrates and pigments. In insect-pollinated species it may be brightly coloured. Its functions are pollinator attraction, adhesion to pollinator bodies and to other grains, and recognition of a compatible stigma. Pollen wall exine patterning is characteristic of the species. It consists of sporopollenin, a highly resistant polymerised coloured carotenoid, and has micropores which contain a material called pollenkitt. The remarkable durability of the exine has allowed semi-fossilised pollen coats to survive in many ancient sediments, and palaeobotanists can identify the source plant types from the pollen shape and sculpturing. This enables reconstruction of vegetation histories over millions of years, often at sites where few other plant remains persist. Exine is absent in some species, whereas the pollen wall intine is unpatterned and is present in all pollen types. Intine consists of polysaccharides and contains tubules ﬁlled with proteins and enzymes. Over the germination aperture(s), which vary in number with species, the exine is thin, absent or present as a cap, whereas the intine is thicker and more complex in structure.
Pollen in the anther is surrounded by the tapetum. The tapetum provides nutrition for the developing microspores, contributes to pollen wall formation and deposits proteins and pollen coat substances into the exine pores. We will see later how these molecules can influence reproductive outcomes (Section 7.2.4). The tapetum degenerates late in pollen development, prior to anther dehiscence.
Pollination is simply the transfer of pollen from the anther to the stigma of the same or another flower. It is no guarantee that a seed or fruit will result, although the term is often used loosely also to encompass subsequent pollen germination and fertilisation. Pollination generally employs an external agent or vector. The two major types of pollen vector are wind and animals, and the most common animal vectors are insects, including bees, butterflies, moths, flies and beetles. In addition, many Australian plants are pollinated by birds or mammals. Examples include honeyeaters in the case of Eucalyptus caesia, honey possums pollinating some banksias and bats pollinating Syzygium, banana and plantain. There are also plants, such as coconut and chestnut, which depend upon both wind and insects for pollination.
Floral characteristics, pollination mechanisms and vectors
Wind-pollinated flowers, as in gymnosperms and grasses, tend to have inconspicuous or absent petals, and large anthers and stigmas for maximum pollen shed and interception. In contrast, most animal-pollinated flowers have large showy petals, often scented for attraction of vectors, and a flower shape which promotes or sometimes restricts ease of access. In the Asteraceae, the inflorescence is a flat capitulum which is both showy and accessible. Large flowers or inflorescences provide a visual cue, as does flower colour. For example, bees cannot see red, whereas birds can, so bird-pollinated flowers are often red. Some insects can also see flower markings such as ultraviolet nectar guides which indicate the position of the floral rewards, the pollen and nectar. Nectar is a sugar solution produced in specialised structures called nectaries, and functions speciﬁcally as an energy source reward to pollen vectors. Most animal-pollinated plants develop floral nectaries, but some, such as acacias, have extrafloral nectaries on the petiole or at the base of the leaf lamina. Nectar consists of sugars, mainly sucrose, fructose and glucose, but also organic acids, volatile oils, polysaccharides, proteins, enzymes, alkaloids and amino acids. Nectar varies in composition: for example, Prunus avium (cherry) nectar contains 12% sugar, whereas that of Brassica rapa (oilseed rape, canola) contains 51%. Time of day is also important, with Citrus sinensis (orange) nectar containing 20% sugar in the morning and 30% in the afternoon. Water availability, photosynthetic activity, weather conditions such as wind and humidity, age of the flower and prior insect visits all may influence nectar secretion. The other floral reward is pollen, a signiﬁcant protein source for many invertebrate vectors. In addition to being protein rich, pollen also contains lipid and starch. Pollen colour, odour, ease of collection and protein compositon all feature in attraction of animal vectors.
In many crops the commercial advantages of cross-pollination by insects are increased yield via both larger fruit and greater fruit number. Sometimes crops are earlier and more uniform, and fruit quality can be improved. Cultivation inevitably disrupts the ecology of an area, and this has consequences for natural insect populations, often because nesting and foraging habitats have been destroyed. Likewise, agricultural chemicals, particularly insecticides, have deleterious effects on beneﬁcial insect populations, even when used sparingly. Synchronous monocultures may have more flowers than local insect popu-lations can work efﬁciently. Consequently, there is a need to introduce pollen vectors, generally honeybees, into the cultivation system for most insect-pollinated crops.
Pollinator varieties, also termed pollenisers, are often inter-planted with commercial cultivars for yield improvement. An effective polleniser needs to produce large numbers of flowers with viable pollen and be compatible with the commercial cultivar. Anthesis of both cultivars must coincide. Ideally, both polleniser and recipient should be useful commercial varieties, with crops harvested simultaneously. Orchard layouts also influence efﬁciency of cross-pollination: a 1:1 ratio of cultivars, either within rows or as alternating rows, is often recom-mended. An alternative to interplanting is to graft a branch of the polleniser into the commercial tree. Crops requiring pollinator cultivars and insects include almond, apple and kiwifruit. Macadamia does produce a crop in single-cultivar (i.e. self-pollinated) plantings but beneﬁts greatly from cross-pollination. Wind-pollinated crops requiring pollinator cultivars include walnut, hazel and pistachio.
Honeybee (Apis mellifera)
Honeybees as pollen vectors have many advantages, principally their social behaviour which allows artiﬁcial hiving and hence facilitates placement of suitable numbers of insects. Large amounts of protein (pollen) and sugar (nectar) are required to feed the young in a colony, so worker honeybees are frenetic foragers, and therefore effective pollinators, and can travel 10 km from their hive. Pollen readily adheres to the honeybee’s hairy body, and its eyes are sensitive to colours from yellow through blue to ultraviolet. Its sensory powers also include shape recognition and a good olfactory system, and it can communicate the location of a good food source to other hive members. Honeybees have some disadvantages. They often keep to one cultivar or species, and frequently forage along rows, especially if the foliage of adjacent plants touches. They also show species preferences, for example citrus over mango and in some cases weeds over the crop.
The number of bees needed for maximal pollination varies from one per thousand flowers for apple up to 250 per thousand flowers for sunflower oil crops, and from one hive per hectare for peach and grapefruit up to 10 to 12 hives per hectare for cucumber and rockmelon (Crane and Walker 1984). Hives usually need to be conditioned to a new crop to prevent visits to previous food sources, and this is often achieved by transporting the hives a long distance from the previous crop. Hives are placed in the crop at full bloom, and the colony may be fed sugar syrup containing flowers of the target crop. In addition, it is possible to increase the pro-portion of pollen gatherers by removing the pollen store from the hive or by providing extra brood. Pollination can be further enhanced by ﬁtting hives with pollen inserts of the pollinator variety.
Australian native bees
Many Australian bees are pollen vectors for native genera, but none is currently used for commercial pollen transfer. Many are solitary rather than social, and so cannot be readily hived. One exception is the tropical genus Trigona, which is social and has been hived successfully. It pollinates mango and Macadamia, which produces the macadamia nut, the only Australian native food crop traded internationally to any large extent. Trigona bees in the future may be adopted for certain crops, and they have an added advantage of being stingless.
In most species, pollen is either removed directly from anthers by foraging fauna or is dislodged by wind or water. Some plants, however, have specialised flowers which facilitate pollen removal by improving its accessibility to vectors. These pollen presentation mechanisms often involve hairs which hold the pollen grains. The hairs develop from the corolla in Astroloma (Epacridaceae), from the receptacle in quandong (Santalaceae) or from the style in Verticordia (Myrtaceae). Sometimes adhesive material is secreted from anther glands (e.g. Thryptomene and some eucalypts), which causes pollen to clump together and ensures transfer of large numbers of pollen grains onto the vector’s body.
Pollen presentation is most sophisticated in a southern hemisphere family, the Proteaceae. The terminal portion of the style is adapted to cause pollen to be deposited onto a specialised area which is often swollen or ridged (Figure 7.23) to aid adhesion. Even within a single genus such as Banksia, pollen presenter structure varies enormously in length, shape and surface, and can be used as a taxonomic character. More importantly perhaps, pollen presentation in the Proteaceae is part of an outbreeding mechanism (see below). Flowers are protandrous: anthers dehisce before the stigma is receptive, and indeed before the flower opens. The pollen presenter sits adjacent to the anthers inside the bud so pollen is deposited directly onto the presenter, from where it can be collected by insect, bird or mammal foragers. A further specialisation is the reduction of the stigma’s receptive surface to a small area located inside a groove well away from the presenter structure. This distance greatly reduces the probability of self-pollination within a single flower, even when not all the pollen is removed by foragers before the stigma matures. Pollen transferred from other flowers will germinate provided it is placed inside or near this groove.
Figs (Ficus spp.) have an intriguing pollination mechanism involving a symbiotic relationship between the plant and its pollinator, a wasp called Blastophaga. The ﬁg relies upon the wasp for seed production, and the wasp undergoes most or all of its life cycle within the ﬁg inflorescence or syconium. The reproductive cycles of ﬁg and wasp are synchronised. Fig syconia consist of numerous individual flowers borne on the inner surface of a curved receptacle with a single opening, the ostiole (Figure 7.24). Female and male unisexual flowers are produced, and the female flowers mature before the males. The cycle starts when a female wasp carrying her fertilised eggs and ﬁg pollen enters a female stage syconium. Within the syconium, the female wasp lays her eggs and pollinates the female flowers. There are two types of female flower within the syconium, short styled and long styled. The wasp penetrates the short-styled flowers with its ovipositor and lays an egg in the ovary. These short-styled flowers become galls as the developing wasp larvae feed on the ovary tissue. The style of the long-styled flower is longer than the ovipositor, and these flowers are pollinated by the female wasp with pollen collected from a male stage syconium. The syconia and its seed then develop slowly as the wasp larvae grow. When the wasps, both female and male, have emerged from their galls within the syconium, the male flowers of the syconium are mature. The wasps mate within the syconium, and the males then die, having spent their whole life in this enclosure. Fertilised females collect pollen from the male flowers, leave the male stage syconium and carry the pollen to a female stage syconium, entering via the ostiole. The life cycle is thus completed.
This sequence is characteristic of most ﬁg species, although fresh ﬁg cultivars of Ficus carica grown in Australia, the USA and Europe are parthenocarpic and do not need wasp pollination (Ferguson et al. 1990). The major drying ﬁg cultivar, Smyrna, on the other hand is dependent on Blastophaga psenes, and must be carefully managed for optimum yield. Smyrna trees are female and have no male or short-styled flowers, and so cannot sustain the life cycle of the wasp pollinator. They need a pollinator cultivar, the capriﬁg, which has non-commercial syconia with male flowers and both long- and short-styled female flowers. It supports the life cycle of the wasp, and growers pollinate the Smyrna by hanging male-stage syconia of capriﬁg in the canopy of the Smyrna trees. This method is termed capriﬁcation.
Other ways of enhancing pollination
A more elusive example of matching of correct pollinator is provided by the oil palm (Elaeis guinensis) and Elaeidobus weevils. Oil palm is native to West Africa, but is now cultivated throughout the tropics, particularly in Southeast Asia. For many years, yields in Asia were much lower than in Africa, and not until 1979 was the dependence on weevil pollination discovered. Introduction of Elaeidobus has now solved the low-yield problem.
For some commercial crops it is cost effective to assist pollination manually to improve yields. Hand pollination is the most direct intervention, and is employed for Annona spp. (cherimoya and types known in Australia as custard apple) and vanilla orchids (Vanilla planifotus). Vanilla orchids were cultivated by the Aztecs, and may have been hand pollinated for centuries. Vanilla is now produced in many tropical countries, such as Madagascar, Java and Polynesia, and in all cases hand pollination is essential for set of the vanilla pod.
In crops such as tomato, mechanical vibrators are sometimes used to assist pollination, and pollen sprays are increasingly used on kiwifruit in New Zealand. Placing male flower bouquets of the pollinator variety in female date trees is an ancient method depicted in Egyptian tombs and is still used today.