Pollination

Adaptations of flowering plants 

Flowers of wind-pollinated plants typically lack petals and sepals and produce large quantities of pollen. Pollination in these plants often occurs early in the growing season, before leaves can obstruct pollen dispersal. Examples include many trees, grasses, and sedges. In contrast, plants that rely on insects or other animals for pollination have evolved highly specialised flower parts to attract pollinators and facilitate pollen transfer. They utilise various strategies such as colour, scent, heat, nectar, edible pollen, and specific flower shapes to draw in pollinators.


Timing of flowering and the number or size of flowers also play significant roles in reproduction. Some plants have a few large, striking flowers, while others produce numerous small flowers gathered in inflorescences to maximise visibility to pollinators. The co-evolution between plants and their pollinators often leads to mutual benefits, enhancing the effectiveness of pollination.


The largest family of flowering plants, the Orchidaceae, includes up to 35,000 species with highly specialised flowers that attract insects. These orchids often produce pollen in clusters called pollinia, which adhere to insects. The flowers' shapes and scents are adapted to lure specific pollinators, with some mimicking insect pheromones or physical appearances.


Another significant group is the Asteraceae or sunflower family, comprising nearly 22,000 species. These plants have complex inflorescences, where individual flowers, or florets, are collected into heads. Depending on the arrangement and type of florets, heads can be homogamous (with all florets of one sex or bisexual) or heterogamous (with florets of different sexual forms).


In gymnosperms, pollen directly contacts the ovule, whereas in angiosperms, pollen lands on the stigma, the receptive part of the carpel. Pollination is crucial for the production of genetically diverse offspring. The study of pollination spans multiple disciplines, including botany, horticulture, entomology, and ecology. Christian Konrad Sprengel first explored the interaction between flowers and pollinators in the 18th century, highlighting its importance in horticulture and agriculture, where successful fertilisation depends on effective pollination.


Types of Pollination

Abiotic pollination

Abiotic pollination occurs without the aid of living organisms. Only about 10% of flowering plants rely on abiotic factors for pollination. The most prevalent form is anemophily, or wind pollination, which is common in grasses, many conifers, and numerous deciduous trees. Another form is hydrophily, or water pollination, seen in aquatic plants that release their pollen directly into the water. Of the plants that are pollinated abiotically, around 98% rely on wind, while only 2% are pollinated by water. In total, approximately 80% of plant pollination is biotic.


Biotic pollination

Biotic pollination involves organisms that transfer pollen from the anther to the receptive parts of the carpel or pistil. This process is facilitated by pollinators, which can be various types of animals. The traits of flowers that attract specific pollinators are referred to as pollination syndromes.


There are roughly 200,000 species of animal pollinators, with the majority being insects. Entomophily, or insect pollination, is common in plants that exhibit coloured petals and strong scents to attract a range of insects such as bees, wasps, ants, beetles, moths, butterflies, and flies. Zoophily, or vertebrate pollination, involves animals like birds and bats. Plants that rely on bats or moths often have white petals and a strong fragrance, while those that attract birds generally have red petals and minimal scent, as many birds do not have a strong sense of smell.

 

Mechanics of pollination

Pollination can occur through two primary mechanisms: cross-pollination and self-pollination.


Cross-pollination

Cross-pollination, also known as allogamy, happens when pollen from one plant is transferred to the flower of a different plant. Plants that have evolved to facilitate cross-pollination often feature longer stamens compared to carpels or employ other mechanisms to enhance the likelihood of pollen reaching flowers of other plants. For example, the European honeybee collects nectar and, in the process, pollen adheres to its body, which it then transfers to other flowers.


Self-pollination

Self-pollination occurs when pollen from one flower fertilises either the same flower or another flower on the same plant. This method is believed to have evolved in response to unreliable pollinators, and is most commonly observed in short-lived annual species and plants that spread to new locations. Self-pollination includes two main types: autogamy, where pollen moves to the female parts of the same flower, and geitonogamy, where pollen is transferred to another flower on the same plant. Plants that are adapted to self-fertilisation typically have similar lengths of stamens and carpels. Self-fertile plants can produce viable offspring through self-pollination, while self-sterile plants require cross-pollination to produce viable seeds.


Cleistogamy

Cleistogamy is a form of self-pollination that occurs before the flower opens. In these cases, the pollen is either released from the anther within the flower or grows a tube down the style to reach the ovules. Cleistogamous flowers may never open, in contrast to chasmogamous flowers, which open and are then pollinated. Cleistogamous plants are inherently self-compatible or self-fertile. Many plants, however, are self-incompatible, meaning they require cross-pollination to reproduce effectively.


Pollinisers and pollinators

It is important to distinguish between pollinators and pollinisers. Pollinators are the agents that transfer pollen, such as bees, flies, bats, moths, or birds. Pollinisers, on the other hand, are the plants that provide pollen for other plants. Self-fertile plants can pollinate themselves, while other plants have mechanisms to prevent self-pollination.


In agriculture and horticulture, effective pollination management involves selecting good pollinisers—plants that provide compatible, viable pollen and bloom simultaneously with the plants that need pollination, or whose pollen can be stored and used as needed. Hybridisation involves pollinating flowers of different species or breeding lines, and can improve crop yields and plant traits.


For instance, peaches are generally self-fertile and can be commercially produced without cross-pollination, although cross-pollination often results in better yields. Apples, however, are self-incompatible and require cross-pollination to produce a commercial crop. Many commercial fruit trees are grafted clones, making them genetically identical. This practice can limit genetic diversity, so some growers now graft a limb of a compatible polliniser onto every few trees to enhance pollination.

Controlled pollination methods

Controlled pollination involves crossing two genetically distinct plants to produce hybrid seeds. To achieve consistent F1 hybrids, the same cross must be repeated each season, typically through controlled hand-pollination. This meticulous process often results in higher costs for F1 seeds. Naturally occurring F1 hybrids, like peppermint, demonstrate how hybridization can occur without human intervention. Peppermint, a sterile hybrid of water mint and spearmint, reproduces through vegetative spreading rather than seed production.


In agriculture, the term "F1 hybrid" refers to cultivars derived from crossing two inbred parent lines, each bred for multiple generations to achieve near homozygosity. This process enhances growth and yield through heterosis, or "hybrid vigor," while ensuring uniformity in the F1 generation. For example, tomato hybrids are created annually by crossing two specific heirloom cultivars.


To produce these hybrids, two breeding populations with desired traits undergo inbreeding until homozygosity exceeds 90%, often over ten or more generations. The populations are then crossed while preventing self-fertilization, usually by removing male flowers, timing male and female flowering, or hand-pollinating.


By 1960, hybrid crops dominated in the U.S.: 99% of corn, 95% of sugar beets, 80% of spinach and sunflowers, 62% of broccoli, and 60% of onions were hybrids. Today, these figures are likely even higher. Beans and peas are exceptions, as they are automatic pollinators, making hand-pollination economically impractical.


Hybrid Generations (F1, F2, F3, F4, F5)

F1 hybrids, created through controlled pollination, are valued for their consistency and uniformity. However, F2 hybrids, resulting from the self-pollination or cross-pollination of F1 hybrids, exhibit more genetic variation and are cheaper to produce, as no intervention is required. F2 seeds are available from some companies, particularly for bedding plants where uniformity is less critical. This process can continue through subsequent generations, including F3, F4, and F5 hybrids.


Advantages

Homogeneity and Predictability: When starting with homozygous parent lines, F1 hybrids exhibit minimal genetic variation, resulting in uniform phenotypes that simplify management and mechanical processing. Once a cross is established, it can be consistently replicated.

Higher Performance: Different alleles from each parent can lead to a combination of optimal enzyme versions, enhancing performance and reducing the risk of genetic defects. This phenomenon, known as genetic heterosis, often results in higher yields compared to traditional varieties.


Disadvantages

Genetic Variation in F2: The primary drawback of F1 hybrids is that their offspring (F2 generation) will vary widely. Some F2 plants may revert to traits of the less vigorous parent, resulting in reduced yields and loss of hybrid vigor. This genetic diversity can be beneficial for seed producers, as it prevents customers from easily producing their own seeds.

Cost and Labor: Both inbreeding and crossing require significant effort, leading to higher seed costs. However, the increased yields typically justify the expense.

Uniform Maturity: F1 hybrids often mature simultaneously under consistent environmental conditions, facilitating machine harvesting. While this is advantageous for large-scale farmers, traditional varieties that crop over extended periods are often preferred by gardeners to avoid surpluses and shortages.

Types of seed germination

Seed germination is the process through which a seed embryo develops into a seedling, involving the reactivation of metabolic pathways that drive growth and the emergence of the radicle (root) and plumule (shoot). The subsequent phase, where the seedling establishes itself above the soil, is known as seedling establishment.

For germination to occur, three essential conditions must be met:



Seed Vigor encompasses seed quality, including viability, germination percentage, germination rate, and seedling strength. The germination percentage is the proportion of seeds that successfully germinate under optimal conditions. The germination rate measures the time it takes for seeds to germinate. Both percentage and rate are influenced by seed viability, dormancy, and environmental conditions. In agriculture and horticulture, high-quality seeds are characterized by high viability, which is reflected in both germination percentage and rate. For example, a seed lot with 90% germination in 20 days is considered high quality.


Dormancy refers to the state where seeds are viable but do not germinate immediately. Some seeds have no dormancy if they are dispersed and kept moist. Seeds can exhibit varying degrees of dormancy, and different seeds from the same fruit may have different dormancy levels. Dormant seeds are still viable but may have low germination rates. Environmental factors such as water, oxygen, temperature, and light play crucial roles in seed germination. The germination process includes three phases:



F1 Hybrids refer to the first filial generation of offspring resulting from a cross between genetically distinct parental types. These hybrids are often created from two inbred lines in plants and animals to produce uniform and specific characteristics. In fish breeding, F1 hybrids may arise from crossing closely related species, while in genetics, they typically result from crossing two inbred lines. Gregor Mendel's pioneering work in the 19th century demonstrated that F1 generations from true-breeding (homozygous) parents were consistently heterozygous, showing dominant traits from both parents. Mendel’s research on F1 and F2 generations established the foundation for modern genetics.