Modes of Reproduction and Pollination in Crop Plants

In this article we will learn about different Modes of Reproduction and Pollination in Crop Plants.

Modes of Reproduction in Crop Plants

Crop plants exhibit two primary modes of reproduction: asexual and sexual. Each mode has distinct mechanisms and implications for plant breeding and agriculture.

A. Asexual Reproduction

Asexual reproduction does not involve the fusion of male and female gametes. Instead, new plants develop from vegetative parts or from embryos that form without fertilization. This mode can be subdivided into vegetative reproduction and apomixis.

1. Vegetative ReproductionVegetative reproduction occurs when new plants arise from vegetative parts of the parent plant. This process can involve several modifications of stems, leaves, or roots:
   • Underground Stems: These modifications act as storage organs and contain buds that can develop into new shoots. Examples include:
     • Tuber: Seen in potatoes (Solanum tuberosum).
     • Bulb: Found in onions (Allium cepa) and garlic (Allium sativum).
     • Rhizome: Present in ginger (Zingiber officinale) and turmeric (Curcuma domestica).
     • Corm: Found in crops like colocasia (Colocasia antiquorum) and arwi (Colocasia esculenta).
   • Sub-aerial Stems: These structures, such as runners, stolons, and suckers, propagate plants by extending horizontally. Examples include:
     • Runner: Used by mint (Mentha sp.).
     • Sucker: Found in date palms (Phoenix dactylifera).
   • Bulbils: These are modified flowers that develop directly into new plants without seed formation. For instance, the lower flowers in garlic naturally produce bulbils. Scientists have also succeeded in inducing bulbil formation in cardamom through tissue culture techniques.
   • Artificial Vegetative Reproduction: Techniques like stem cuttings, layering, budding, and grafting are employed to propagate many crop species. Tissue culture is another method used for vegetative multiplication, even though natural vegetative reproduction may not occur in some species. This method allows the propagation of crops such as sugarcane, grapes, and roses.
   Significance: Vegetative reproduction enables the multiplication of desirable plant varieties and ensures uniformity. It also allows the preservation of beneficial traits in crops, bypassing the genetic variability associated with sexual reproduction.
2. ApomixisApomixis involves the formation of seeds without fertilization, resulting in progeny genetically identical to the parent plant. Apomixis can be categorized into:
   • Adventive Embryony: Embryos develop directly from vegetative cells in the ovule, such as the nucellus or integument. Examples include mango (Mangifera indica) and citrus species.
   • Apospory: Vegetative cells in the ovule develop into embryos, bypassing the production of the embryo sac. This occurs in species like Hieracium and Crepis.
   • Diplospory: In this type, the megaspore remains diploid and undergoes mitotic divisions to form the embryo. Diplospory can lead to parthenogenesis or apogamy.
   • Parthenogenesis: The embryo develops from an unfertilized egg cell. Depending on whether the embryo sac is haploid or diploid, parthenogenesis can be haploid (e.g., in Solanum nigrum) or diploid (e.g., in Taraxacum).
   • Apogamy: Synergids or antipodal cells in the embryo sac develop into an embryo. Apogamy can be haploid or diploid, depending on the state of the embryo sac.
   Significance: Apomixis is advantageous for maintaining genetic uniformity in breeding programs. However, it can hinder efforts to create new hybrids or inbred lines. It is particularly useful in preserving desirable genotypes.

B. Sexual Reproduction

Sexual reproduction involves the fusion of male and female gametes to form a zygote, which then develops into an embryo. This process occurs within flowers, the specialized reproductive structures of plants.

1. Flower Structure:
   • Perfect Flowers: Contain both stamens (male organs) and pistils (female organs).
   • Staminate Flowers: Contain only stamens.
   • Pistillate Flowers: Contain only pistils.
   • Monoecious Species: Have both staminate and pistillate flowers on the same plant (e.g., maize, Colocasia).
   • Dioecious Species: Have staminate and pistillate flowers on separate plants (e.g., papaya, date palm).
2. Sporogenesis:
   • Microsporogenesis: The production of microspores (male gametes) in the anthers. A microspore mother cell undergoes meiosis to produce four haploid microspores, which mature into pollen grains.
   • Megasporogenesis: The production of megaspores (female gametes) in the ovules. A megaspore mother cell undergoes meiosis to produce four haploid megaspores, with one surviving to form the embryo sac.
3. Gametogenesis:
   • Microgametogenesis: Involves the development of male gametes within the pollen grains. Pollen grains consist of a vegetative (tube) nucleus and generative nucleus, which divides to form two sperm cells.
   • Megagametogenesis: The process within the embryo sac includes the formation of one egg cell, two synergid cells, three antipodal cells, and a diploid secondary nucleus.
4. Fertilization:
   • Syngamy: The fusion of one sperm with the egg cell to form a diploid zygote.
   • Triple Fusion: The fusion of the second sperm with the secondary nucleus to form a triploid primary endosperm nucleus, which develops into the endosperm.
   Significance: Sexual reproduction is crucial for creating genetic diversity through recombination of genes from two parents. This diversity is essential for crop improvement and adaptation. Sexual reproduction forms the basis of most plant breeding programs, enabling the development of new varieties with desirable traits.

Determination of the Mode of Reproduction in a Species

Determining the mode of reproduction in a species involves several key steps that help identify whether a plant primarily relies on self-pollination, cross-pollination, or other mechanisms. The process encompasses both direct observation of floral characteristics and experimental isolation techniques.

1. Examination of Floral Structures

The initial approach to determining a species’ mode of reproduction is to analyze its floral anatomy and mechanisms. Key floral traits to examine include:

• Dioecy: Flowers are either staminate (male) or pistillate (female), located on separate plants. This indicates obligate cross-pollination as the species requires pollen transfer between different plants.
• Monoecy: Both male and female flowers are present on the same plant but in separate inflorescences. This condition, observed in plants like maize (Zea mays) and cucurbits (Cucurbita sp.), suggests the potential for both self- and cross-pollination.
• Protandry: In hermaphroditic flowers, stamens mature before pistils. This temporal separation, found in crops like maize and sugar beets (Beta vulgaris), facilitates cross-pollination.
• Protogyny: Pistils mature before stamens, as seen in some species like bajra (Pennisetum typhoides), also favoring cross-pollination.
• Cleistogamy: Flowers remain closed, ensuring self-pollination as foreign pollen cannot reach the stigma. This trait is observed in certain grasses and cereals.

By identifying these characteristics, researchers can infer the likely mode of pollination and reproduction for a species.

2. Experimental Isolation

The next step involves experimental validation through isolation techniques to determine the species’ actual reproductive behavior. This includes:

• Space Isolation: Plants are grown at a sufficient distance from one another to prevent natural cross-pollination. This method is preferred as it closely simulates natural conditions and minimizes the risk of unintended cross-pollination.
• Isolation by Bags or Cages: This method involves enclosing plants to prevent pollinator access. However, this can sometimes create an unnatural environment that may affect pollination and seed set. Therefore, while effective in controlling cross-pollination, it may not always provide accurate results regarding the plant’s natural reproductive strategy.

Seed Set Evaluation:

• No Seed Set in Isolation: If plants fail to produce seeds when isolated, it typically indicates a reliance on cross-pollination. This is because the lack of pollen transfer between different plants hinders fertilization.
• Seed Set in Isolation: Successful seed production in isolation suggests that the plant can self-pollinate. However, this result must be interpreted cautiously, as cross-pollination can still occur if the isolation is not complete.

3. Analysis of Inbreeding Depression

Another factor to consider is the presence or absence of inbreeding depression. Inbreeding depression is a decrease in fitness and vigor that occurs when self-pollination leads to increased homozygosity. This is particularly relevant for:

• Cross-Pollinators: Typically, these plants exhibit inbreeding depression when self-pollination occurs, as they are adapted to a higher level of genetic diversity.
• Self-Pollinators: These plants generally do not show significant inbreeding depression, as they are adapted to maintaining genetic stability through self-pollination.

Modes of Pollination

Pollination is the process by which pollen grains are transferred from the male structures (anthers) to the female structures (stigmas) of flowers. This transfer is essential for fertilization and subsequent seed production. Pollination can occur in various ways, and understanding these mechanisms is crucial for both natural plant reproduction and agricultural practices. There are three primary modes of pollination: self-pollination, cross-pollination, and geitonogamy.

1. Self-Pollination

Self-pollination occurs when pollen from the anther of a flower fertilizes the stigma of the same flower or another flower on the same plant. This mode of pollination is advantageous in stable environments where cross-pollination might be less reliable. Several mechanisms facilitate self-pollination:

• Cleistogamy: Flowers do not open, ensuring that pollen from the same flower fertilizes the stigma. This mechanism is observed in crops such as wheat (Triticum sp.), oats (Avena sp.), and barley (Hordeum vulgare).
• Chasmogamy: Flowers open only after pollination has occurred, though some cross-pollination may still happen. This is common in cereals like wheat, barley, and rice.
• Position of Anthers and Stigmas: In some plants like tomatoes (Solanum lycopersicum) and brinjal (Solanum melongena), the anthers are positioned close to the stigma, promoting self-pollination even after the flower opens.
• Hidden Floral Organs: In certain legumes, such as peas (Pisum sativum), the stigma is enclosed by floral organs, which favors self-pollination.
• Receptive Stigmas: Some species have stigmas that elongate and become receptive through staminal columns, enhancing self-pollination.

Genetic Consequences:

• Homozygosity: Self-pollination often leads to increased homozygosity in plant populations, making them highly uniform genetically.
• Heterosis: Self-pollinated species may exhibit limited heterosis (hybrid vigor). Therefore, breeding methods often aim to develop homozygous varieties.

2. Cross-Pollination

Cross-pollination occurs when pollen from one plant is transferred to the stigmas of flowers on a different plant. This process increases genetic diversity and is essential for the evolution of many species. Cross-pollination can be facilitated by various mechanisms:

• Anemophily: Pollination by wind. Plants such as corn (Zea mays) and certain grasses rely on wind to transfer pollen.
• Hydrophily: Pollination by water. Aquatic plants use water currents to move pollen from one flower to another.
• Entomophily: Pollination by insects. Many plants, including many crops, depend on insects such as bees for pollen transfer.

Mechanisms Facilitating Cross-Pollination:

• Dicliny: The presence of either staminate (male) or pistillate (female) flowers on the same plant or different plants.
  • Monoecy: Both types of flowers are present on the same plant but in separate inflorescences, as seen in maize (Zea mays) and cucurbits (Cucurbita sp.).
  • Dioecy: Male and female flowers are present on separate plants, as seen in papaya (Carica papaya) and date palms (Phoenix dactylifera).
• Dichogamy: Temporal separation of stamen and pistil maturity, which reduces self-pollination.
  • Protandry: Stamens mature before pistils, observed in crops like maize and sugar beets (Beta vulgaris).
  • Protogyny: Pistils mature before stamens, seen in species like bajra (Pennisetum typhoides).
• Self-Incompatibility: The inability of pollen to fertilize the same or other flowers on the same plant. This can be:
  • Sporophytic: Determined by the plant’s genetic makeup.
  • Gametophytic: Determined by the pollen grain itself.
  • Examples: Brassica species (mustards), certain species of Nicotiana, and radish.
• Male Sterility: Absence of functional pollen in hermaphrodite flowers, which can be genetic or cytoplasmic. This is often used in hybrid seed production.

Genetic Consequences:

• Heterozygosity: Cross-pollinated species generally exhibit high genetic diversity and can display significant heterosis.
• Breeding Methods: Breeding efforts often focus on maintaining or enhancing genetic diversity while optimizing traits for agricultural purposes.

3. Geitonogamy

Geitonogamy is a form of pollination where pollen from one flower on a plant is transferred to the stigma of another flower on the same plant. This situation is common in plants with multiple flowers, such as maize. The genetic outcomes of geitonogamy are similar to those of self-pollination, although it can sometimes increase genetic diversity within the same plant.

Determination of the Amount of Cross-Pollination in a Species

To accurately assess the level of cross-pollination in a species, researchers use specific experimental techniques. These methods involve planting different strains of the same species and analyzing the genetic makeup of the resulting progeny. This approach allows for the quantification of cross-pollination rates.

1. Experimental Setup

To determine the amount of cross-pollination, the following steps are typically undertaken:

• Planting Different Strains: Two distinct strains of the species are cultivated together in a mixed stand. One strain is homozygous for a dominant allele, which is preferably associated with a visible trait (such as seed or seedling characteristics). The other strain is homozygous for the recessive allele of the same trait.
• Pollination Arrangement: Each plant of the recessive strain is surrounded by plants of the dominant strain. This setup ensures that the recessive plants receive ample pollen from the dominant strain.

2. Harvesting and Analyzing Seeds

After the plants have matured and produced seeds:

• Seed Collection: Seeds are harvested from the plants of the recessive strain.
• Genetic Analysis: The percentage of seeds carrying the dominant allele is measured. This percentage indicates the proportion of cross-pollination that has occurred. The presence of dominant alleles in the progeny suggests successful pollen transfer from the dominant strain to the recessive strain.

3. Factors Affecting Cross-Pollination

The extent of cross-pollination can be influenced by various factors, including:

• Variety: Different varieties of the same species may exhibit varying levels of cross-pollination.
• Weather Conditions: Environmental factors such as temperature, humidity, and wind can impact the effectiveness of cross-pollination.
• Location: Geographic location and local conditions can also affect the cross-pollination rates.

For example, research on safflower (Carthamus tinctorius) revealed that cross-pollination estimates ranged from 0% to 8.7% across different varieties grown in the same year and location. Similarly, the amount of cross-pollination for a single variety varied from 1.3% to 9.8% depending on the location. Therefore, to obtain reliable and comprehensive data, studies should encompass multiple varieties and locations and span over several years.

4. Determining Apomixis

To identify if a species exhibits apomixis (asexual reproduction through seeds), a different approach is employed:

• Crossing Test: A recessive strain is used as the female parent and a dominant strain as the male parent. If a high frequency of recessive offspring is produced, it suggests that the species may be apomictic.
• Consideration of Self-Pollination: During these experiments, care must be taken to avoid self-pollination, which could confound results. Ensuring that the crosses are made under conditions that prevent self-pollination is crucial for accurate assessments.

Anthesis: The Process of Flower Opening and Pollination

Anthesis refers to the critical stage in flowering when a flower first opens and becomes functionally capable of reproduction. This process is integral to plant reproduction, as it marks the transition from bud development to the flower’s readiness for pollination. The timing and characteristics of anthesis are influenced by various environmental factors and have significant implications for successful pollination and crop breeding.

Key Aspects of Anthesis

1. Definition and Significance
   • Anthesis: The period during which a flower opens and becomes capable of receiving pollen. This phase is crucial for the successful reproduction of flowering plants.
   • Significance: Understanding anthesis is important for plant breeding, as it affects the timing of crosses and the mode of pollination. Knowledge of anthesis can help optimize breeding strategies and improve crop yields.
2. Environmental Factors Influencing Anthesis
   • Humidity: High or low humidity can affect the timing and success of anthesis.
   • Temperature: Temperature fluctuations can influence the development and opening of flowers, thereby impacting the overall reproductive process.
3. Process of Anthesis
   • Opening Mechanism: The transition to anthesis involves several key changes in the flower structure:
     • Swelling of Lodicules: In certain species, such as rice (Oryza sativa), the petals and sepals are represented by two small, sac-like structures called lodicules. During anthesis, these lodicules swell, which pushes apart the surrounding bracts, known as the lemma and palea.
     • Stamen Elongation: Concurrently, the filaments of the stamens elongate, causing the anthers to emerge from the protective bracts. This exposes the anthers to the external environment.
     • Dehiscence of Anthers: As the anthers mature, they dehisce, or split open, releasing pollen grains into the flower. This process ensures that pollen is available for fertilization.
   • Pollination Timing: In rice, the pollen is typically liberated inside the flower during anthesis, and it remains viable for a short period, usually from a few minutes up to two hours. Effective pollination occurs when pollen germinates and reaches the embryo sac within this time frame.
4. Pollination Mechanisms
   • Self-Pollination: In some plants, the flower is designed to promote self-pollination, where pollen from the same flower fertilizes its own ovules.
   • Cross-Pollination: Other plants, such as rice, may experience a small percentage (2-3%) of cross-pollination, where pollen is transferred from one plant to another. This can occur due to various factors such as wind or insect activity, which facilitate the transfer of pollen between flowers.
5. Reproductive Success
   • Functional Reproduction: For successful reproduction, the timing of anthesis must coincide with the receptivity of the stigmas and the viability of the pollen. If the stigmas are not receptive or if the pollen does not germinate effectively, successful fertilization may not occur.
   • Pollination Efficiency: The efficiency of pollination during anthesis can influence the overall reproductive success of the plant. Understanding and managing these factors can improve crop yields and breeding outcomes.