Solar radiation at the Earth’s surface contains ultraviolet (UV) radiation in the UVB (~295–315 nm) and UVA (315–400 nm) wavebands. Currently, atmospheric ozone removes shorter, more damaging UV radiation and reduces levels of UVB, but before the formation of the ozone layer, UV radiation levels would have been higher, while the recent ‘ozone hole’ increased UV radiation. UV radiation is strongly attenuated in water, but aquatic organisms can be damaged to extents that depend on the species and conditions. The targets of damage include proteins in the photosystems of photosynthesis, DNA and oxidative damage caused by the production of free radicals and reactive oxygen species. Defence against damage involves the production of new proteins, repair to the DNA and the production of antioxidants. UV stress interacts, positively and negatively, with other environmental changes such as rising temperature and CO2, ocean acidification and nutrient stress. Further research is needed to forecast responses to future environmental change.

The evolution of oxygenic photosynthesis had profound effects on the biogeochemistry of the planet. The increase in atmospheric oxygen levels brought about alterations to a range of biogeochemical processes involving changes in the availability of a host of elements, including nitrogen, sulfur and many metal ions such as iron and manganese, central to biological activities. Critically for photosynthetic organisms, the increase in oxygen levels in the atmosphere following the evolution of oxygenic photosynthesis and the Great Oxidation Event had consequences for the assimilation of inorganic carbon via the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco). Although there are a number of alternative pathways leading to autotrophic CO2 assimilation, 99% of primary productivity on the planet is carried out by processes that involve Rubisco and the Benson–Calvin–Bassham cycle. The accumulation of O2 in the atmosphere also had major repercussions for increasing the energetic yield of the catabolism of photosynthate by allowing oxidative respiration, with a much greater ATP yield than from anaerobic fermentative processes. The interaction of O2 with UVC radiation led to the production of UVC- and UVB-absorbing O3. This also significantly influenced life on Earth and facilitated the colonisation of the upper ocean and terrestrial surface.

Water is essential for life on Earth, but many organisms are subject to water loss under certain environmental conditions and this can cause biological stress. However, some cyanobacteria and algae are capable of coping with periodic exposure to potentially desiccating conditions. Thus, phototrophs in biological soil crusts can survive in desert environments, even when the only source of water is dew. Other aquatic plants and algae can be exposed to emersion following seasonal changes in water level in rivers or lakes and, importantly, during the daily emersion of intertidal species. Seaweeds living in the intertidal are poikilohydric, and each time they are emersed, they risk water loss. Dehydration can lead to inhibition of photosynthesis and respiration as well as disruption to nutrient availability and assimilation. However, intertidal seaweeds have evolved a range of adaptations/acclimations that allow them to cope with exposure to air. These include morphologies that minimise surface area:volume ratio and biochemical changes that involve, for example, enhanced capacity for detoxification of reactive oxygen species. The extent to which seaweeds can recover function following re-immersion and differences in their capacity for nutrient uptake during restricted periods of immersion appear to be correlated with the zonation of species in the intertidal.

The origin of life could have involved autotrophy, but this is most probably chemolithotrophic rather than photolithotrophic. There is evidence, from the natural abundance of carbon isotopes, of autotrophy involving Rubisco and the Benson–Calvin–Bassham cycle from about 4 Ga. However, other autotrophic CO2 fixation pathways could also have occurred. Evidence on the evolution of photosynthetic reactions suggests an early origin of the photochemical reaction centre, with the possibility of the occurrence of two photosystems in series (photosystem II plus photosystem I) and the possibility of oxygenic photosynthesis, before the origin of the single photosystem (reaction centre I or reaction centre II) photosynthesis in the multiple clades of anoxygenic photosynthetic bacteria. The origin of photosystem II and photosystem I preceded the origin of cyanobacteria and the subsequent Great Oxidation Event at about 2.4–2.3 Ga. The occurrence of oxygenic photolithotrophy is a necessary, but not sufficient, condition for the occurrence of the Great Oxidation Event and the Neoproterozoic Oxidation Event. There is no consensus on what other factors are involved in initiating the Great Oxidation Event and the Neoproterozoic Oxidation Event.

Reproduction is a very important stage in the life-history of a species, being essential for its survival and sustenance. Different organisms adopt different strategies as they attempt to maximize their reproductive success and produce a favourable number of new individuals. Reproduction in plants can be achieved by either vegetative or sexual means or a combination of both. The seeds and propagules produced by asexual and sexual modes of reproduction have differing implications on the perpetuation of the species. Asexual means (such as vegetative reproduction) in plants is a quicker reproductive strategy that leads to production of new individuals genetically identical to parents. However, there is a limitation of genetic variability in vegetative reproduction and this may affect the long-term survival of a species. On the other hand, reproduction by sexual means brings genetic heterogeneity in progeny resulting in their wider adaptability and better survival. Sexual reproduction in angiosperms is a complex process involving several sequential events which take place in different organs of a flower. Thus, flower is a unit of sexual reproduction in angiosperms.

Plant reproductive biology is the study of the mechanisms of both sexual and asexual reproduction in plants. It involves the study of interactions of plants with biotic factors (such as pollinators, seed dispersal agents) and abiotic components (such as soil, space, climate) in the environment. With the integration of the many aspects of ecology, reproductive biology of flowering plants is now also known as Reproductive Ecology of Flowering Plants.

Different aspects of Reproductive Biology of Flowering Plants

Study of reproductive biology of plants broadly includes observations on phenology, structural and functional floral biology, sexual system, pollination biology, mating system, pollen–pistil interactions, fertilization, embryo-endosperm development, seed formation, seed dispersal and seed recruitment. These events may also be considered as the series of steps neccessary for the formation of a perfect new sporophyte. These aspects being interconnected, each of these is discussed sequentially in the subsequent sections.

Introduction

Once a flower is fully developed, most of the plants display their sex organs to carry out reproduction. This is accomplished by opening of the flower bud which is known as floral anthesis. Opening of flower is followed by pollination, a very important step in plant reproduction. Pollination is the transfer of pollen from an anther of a flower to a stigma of the same or a different flower. The transfer of pollen grains to the conspecific (belonging to the same species) stigma is the primary step in reproduction or seed formation. Pollination also forms the basis for genetic heterogeneity in plants. Study of methods of pollination and subsequent fertilization in plants is referred to as pollination biology.

Considering the fact that plants are immobile, most angiosperm species have to rely on external agents for the transfer of pollen from anther to stigma. Pollination services are rendered by various biotic and abiotic agencies and their presence is essential for optimal reproductive performance of a flowering plant. In case of biotic pollination, the plant attracts a particular type of pollinator and when the same pollinator visits the next flower there are chances that its pollen is carried to another flower of the same species. In exchange for this service, the pollinator gets access to the food (pollen and nectar) offered by the flowers. Thus, by visiting a particular type of flower, the pollinator gets important food resources and the plant gets pollinated in return. Several groups of insects, birds, bats and other animals are dependent on plants for their nutritional needs, especially during their breeding seasons. Such mutually beneficial relationship between the angiosperms and the pollinators has led to co-evolution and adaptations among these groups over millions of years. Plants and pollinators have co-evolved and undergone changes in their physical forms to increase the chances of successful interaction.

An array of floral features like color, form, nectar and scent exhibited by angiosperms is associated with different modes of pollination. Such correlation between floral features and pollinating agency is referred to as pollination syndrome. The pollination services provided by biotic and abiotic agents are not only essential for fertilization but are also required to maximize dispersal distances and increase genetic variability in plants.

Introduction

The pistil of a flower is exposed to all types of pollen grains in the atmosphere irrespective of whether they belong to the same species or not. However, mere landing of pollen on the stigma is not enough to effect fertilization. As we learnt in the last chapter, there are cellular interactions or cross talk that take place between the pollen and the pistil before successful fertilization. These specific interactions between pollen and pistil facilitate selection of the right type of pollen grains by the pistil and limit fertilization between incompatible gametes.

The inability of a functional male gamete and female gamete to fuse with each other and achieve fertilization is termed as sexual incompatibility. Sexual incompatibility may be interspecific or intraspecific. Following pollination, the ability of a pistil (or stigma) to reject pollen grains from other species is termed as inter-specific incompatibility. This type of incompatibility prevents the formation of inter-specific hybrids and maintains the identity of a biological species. The inter-specific incompatibility is controlled by several genes and is also referred to as heterogenic incompatibility. Interestingly, in nature there are several incidences where pistil carrying functional female gametes are unable to set fruits even when pollinated by viable and fertile self-pollen grains. Scientific investigation have established that the failure of fruit set in these plants is due to genetic factors which impose a physiological barrier to self-fertilization. This phenomenon of failure of a male gamete and a female gamete to achieve self-fertilization is termed as intra-specific incompatibility or more specifically, self-incompatibility (SI). In other words, self-incompatibility is the inability of a fertile hermaphrodite plant to set seeds when self-pollinated. The term self-incompatibility was first coined by Stout (1917); it allows flowering plants to avoid inbreeding and involves genetic mechanisms which prevent self-fertilization and promote out-crossing.

In a self-incompatible plant, whenever its own pollen grains reach stigma either pollen germination or pollen tube growth is terminated which results in failure of seed-set. Yet, there are incidences where self-pollen are able to germinate, and self-pollen tubes are even able to penetrate the ovules. In these cases either fertilization fails to occur, or if at all occurs, the zygote gets aborted after syngamy. This type of SI is called Late Acting Self-incompatibility (LSI).

Introduction

The protective seed habit is a significant feature in the evolutionary success of angiosperms. The seed, encloses an undeveloped miniature plant ‘the embryo’ and acts as a functional unit which links the successive generations. Developmentally, seed is a fertilized ovule and, a typical angiospermous seed consists of an embryo, some storage tissue (mostly endosperm) and a seed-coat. While the embryo and the endosperm are the products of double fertilization, the seed-coat develops from the integument/s of the ovule. Embryos accomplish their early development before seed germination, protected by the surrounding seed coats and sustaining on the stored food in the endosperm. Protection provided to embryo by seed coat increases its chances of survival, and establishment of subsequent generations. Generally, seeds develop as discrete units attached to the inside of the fruit wall through a stalk called the funiculus. However, in many plants, seeds are associated with some other structures that help in their dispersal. In such cases, a single entity of the seed and the structure assisting in dispersal are together described as dispersal units or diaspores. For example, in the members of Asteraceae, the outer integument of the ovule is completely fused with the ovary wall and the diaspore is called a cypsella. Some other examples of diaspores are the seeds with the elaiosomes, achene (dry indehiscent fruits), and caryopsis (fruit type seen in grasses).

Here, one must acknowledge that all structures and processes associated with reproduction in angiosperms are directly responsible for the formation of seed. Seeds perform a wide variety of functions including dispersal, perennation (surviving seasons of stress such as winter), dormancy (a state of arrested development), and most importantly, perpetuation of a plant species.

A huge variation in the size, shape, color, seed coat, weight and dispersal mechanism can be observed among angiospermous seeds. The smallest known seeds are those of orchids which are about 85 micrometers in size and weigh about 0.8 micrograms, thus appearing similar to dust particles. Double coconut or Lodoicea maldivica has the largest (nearly 0.5 meter) and the heaviest (weighing up to 25 kg) known seeds in the world (Fig. 12.1). The size of the seed in a plant depends on the size of the embryo and also on whether the seed at maturity is endospermous or non-endospermous. Seeds in Orchids are small as the endosperm formation is completely suppressed, and also, the embryo is highly reduced.

Plants in general and flowering plants (angiosperms) in particular are the essential components for sustenance of life of all non-photosynthetic organisms on our planet. Plants reproduce by asexual as well as sexual means. Asexual reproduction is not congenial for long-term sustenance and evolutionary processes of the species because of genetic uniformity of the progeny. Sexual reproduction which permits genetic recombination is the dominant mode. Although Angiosperms were the last to evolve as land plants, they soon became the most successful and dominant group amongst land plants. Their success is largely due to the mode of their reproduction through the evolution of the flower and the consequent advantages it brought in. For human beings, flowering plants provide most of their essential needs – food, fibres, shelter, medicines, clean air and water. Reproduction is the basis for sustenance of any species. Thus, understanding reproductive biology of flowering plants is important not only from the fundamental point of view but also for their manipulation for human welfare. Reproductive biology of angiosperms is more complex when compared to other groups of plants because of the involvement of the flower. The progress in understanding the structural and functional aspects of reproduction has been very slow.

Initial studies on reproductive biology of angiosperms were largely confined to examining embryological details using fixed and sectioned materials. Enormous data accumulated over the years on the developmental details of the pollen grains, ovules and female gametophyte, double fertilization, embryo and endosperm, seed and fruit development. These advances were taught to the undergraduate and postgraduate students under the title embryology of angiosperms as a part of their curriculum. Following the development of electron microscopy and histochemistry, embryological details were further elaborated by using these techniques. Development of aseptic culture techniques broadened scope for experimental studies on embryological processes leading to a slow but steady understanding of the functional details of embryological structures. These developments were incorporated in some of the books of embryology under a chapter on experimental embryology. However, there was hardly any integrated account of embryological processes in relation to the structure with their function. Pre-fertilization aspects of reproductive biology covering the details of pollen, pistil, and pollen–pistil interactions, which are unique to angiosperms and play a critical role in their successful evolution, were the last to enter the field of embryology of angiosperm.

The inception of interest in understanding mechanisms of plant reproduction is as old as inception of interest in biology. The seminal work and critical observations by Charles Darwin can be regarded as a foundation for establishing a wide interest in pollinators and reproductive biology of angiosperms as a formal subject. In the last few decades, systematic field investigations, advancement of microscopy tools, and molecular techniques have taken the reproductive biology of angiosperms to a new zenith. The scope of the subject is no longer limited to just studying embryo-endosperm development and taxonomic studies but is extended to study the effect of climate change, evolution, conservation of threatened taxa, raising commercial plantations and orchards, pollinator management, seed development, population biology, phyto-geography, and much more. The reproductive biological studies are also closely linked with the understanding of, physiology, genetics and epigenetics of plants.

For a thorough understanding of the subject, a textbook summarizing the basic concepts of plant reproduction integrated with current research, is the need of the hour for both students and instructors. The aim of the present book is to provide a comprehensive account of basic concepts and recent developments in the field of reproductive biology of flowering plants with essential practical exercises. The book extensively covers all the topics from structure of a flower to seed dispersal and presents the concepts with accompanying color photographs and illustrations wherever necessary, to enhance the level of a student's perception. The new, advanced and interesting information is also provided in a box format in each chapter to reinforce learning. An elaborate glossary and questions are provided with each chapter for quick revision and concept enhancement. Boxes summarizing differences between two terms/concepts which students otherwise usually find difficult to comprehend have also been furnished in the book. This book is a blend of theoretical concepts and details of hands-on exercises in the field and laboratory. Methods for field observations, sample observation tables, and suggestions for plant materials to be used for classroom studies/demonstrations pertaining to each concept have also been provided. In addition, the observation sections under practicals are supplemented with the photographs.

Introduction

Reproduction is the ultimate goal of every life-form on earth. Accordingly, flowering plants have evolved diverse and versatile strategies to ensure their reproductive success. Broadly, reproduction in higher plants can be divided into two types: sexual and asexual reproduction. Sexual reproduction in vascular plants is complex wherein, multicellular haploid and diploid generations alternate. The diploid sporophyte undergoes meiosis to produce haploid gametes which undergo fusion or syngamy to give rise to seed, the next sporophytic generation. On the other hand, asexual reproduction in plants occurs when a plant produces offspring without meiosis and syngamy. New individuals produced through asexual reproduction are genetically identical to the mother plant. Both sexual and asexual reproduction, have distinct advantages for natural plant populations. Sexual reproduction introduces genetic variability in a population and thus increases the adaptability of species to changing environments. By contrast, asexual reproduction eliminates the cost and the complexity associated with biparental sexual reproduction, and also fixes the genotype of mother plant as offsprings produced are clonal.

When vascular plants reproduce asexually, new individuals may be produced from somatic cells or somatic structures (vegetative reproduction) or through seeds that are produced without fertilization (apomixis or agamospermy). Vegetative reproduction occurs through propagules like bulbils, suckers, and tubers, which are generated from vegetative parts of a plant. Apomixis (away from mixing), is the formation of an embryo and seed from an unreduced gametophyte or sporophyte. Thus, apomixis leads to the formation of a seed without the processes of meiosis (apomeiosis), and fertilization (nuclear fusion). The discovery of apomixis in higher plants is attributed to the observation of a solitary female plant of an Australian species Alchornea ilicifolia (syn. Caelebogyne ilicifolia) by Smith (1841). This female tree would constantly form seeds at the Royal Botanic Gardens in England without any pollen donor around. The term apomixis was introduced by Winkler (1908) to denote “substitution of sexual reproduction by an asexual reproduction process without nuclear and cell fusion”. This led to the use of term apomixis to describe all forms of asexual reproduction in plants (including vegetative reproduction), but this generalization is no longer accepted.

Introduction

Plant breeding is an age-old practice of genetic improvement of plants so that they become more useful to humans. It involves combining selected parental plants to obtain the next generation with an improved genetic potential of disease-resistance, stress-tolerance or better yields. The process includes manual crosses or controlled pollination followed by an artificial selection of progeny. Plant breeding, for its enormous benefits to mankind is widely practiced. However, the process is labor intensive and it takes many years for the integration of the required gene and development of a desired progeny. Another major limitation of conventional breeding is that the gene transfer can be achieved only in genetically related species/genera. Even in conspecific plants, the incompatibility of crosses becomes a major barrier. In the last few decades, these limitations of plant breeding have largely been overcome by modern plant genetic engineering/transformation techniques which allow insertion of foreign genes from one organism into another organism. Introduction of foreign genes is relatively less time consuming and does not require recipient and donor organisms to be genetically related to each other. For example, Bt-cotton which was created by genetically altering the cotton genome using genes from the soil bacterium Bacillus thuringiensis (Pursell & Perlak 2004).

The technique of developing transgenic plants has become an integral part of crop improvement programs across the world (Eapen 2011). Although, transgenic approach for crop improvement has several advantages over conventional breeding, it suffers from some drawbacks as well. The transformation techniques used are expensive, genotype-dependent and involve time taking procedures such as in vitro culture and regeneration of explants. Therefore, alternate methods of quick and easy transformation are being developed. One such approach which offers several advantages is transformation of germ cells of plants instead of somatic cells/tissues. Germ cells of plants include sperm cells in the pollen grain (male germ cell) and egg cell in the female gametophyte (female germ cell). The process of introduction of desired genes into germ cells is known as Germline Transformation. This method of transformation has the potential to produce genetically modified plants within less time as it bypasses some tedious steps of in vitro regeneration. The genetically transformed germ cells can also be used directly for fertilization to recover transgenics without tissue culture and the procedure is known as in planta transformation.