Lecture 15

Overview of Flower Development and Life Cycle

• Discussion on what happens to flowers after pollination and fertilization.

• Focus on how embryos develop into seeds and what leads to the formation of flowers.

Changes Post-Pollination

• Pollination and Fertilization: After fertilization, significant transformations occur within the flower structure.

  • Ovules turn into seeds.

  • The ovary wall transforms into fruit.

Function of Fruits

• Purpose of Fruits: To aid in the dispersal of seeds.

  • Typically, it's disadvantageous for a plant to be eaten; however, some plants evolve to produce fruits attractive to animals.

  • Animals consume the fruit, digest it, and excrete the seeds in a new location, promoting seed dispersal.

Types of Fruits

1. Fleshy Fruits: Such as apples, which are appealing to animals.

   • Example: Eating an apple and discarding the core allows seeds to be planted elsewhere.

2. Dry Fruits: Utilize mechanisms for mechanical dispersal.

   • Example: Milkweed pods that release seeds carried by the wind.

   • Burdock fruits adhere to animal fur, inspired the design of Velcro.

Seed Development and Dormancy

• Embryo Growth: After fertilization, embryos proliferate and develop cotyledons.

• Dormancy Phase: Most plants cease development after a certain embryonic stage to conserve resources.

  • Dormancy helps protect seeds with low water content (as low as 10% compared to 80-85% in growing plants).

  • ABA (abscisic acid) is the hormone that regulates dormancy.

  • Some seeds can remain dormant for thousands of years, as demonstrated with the Arctic ground squirrel's 32,000-year-old seeds.

Germination Process

• The transition from dormancy to growth involves changes triggered by gibberellic acid, counteracting ABA's effects to initiate germination.

Flower Formation Mechanisms

1. Vegetative Growth: New seedlings grow leaves before transitioning to reproductive growth.

2. Reproductive Growth: Involves the sequential formation of various flower parts (sepals, petals, stamens, and carpels).

   • This phase completion significantly influences plant lifecycle and longevity.

   • Plants can be classified as annuals (complete lifecycle in one year), biennials (two years), or perennials (produce flowers across multiple seasons).

Factors Triggering Flowering

• Phase Change: To transition from juvenile (non-flowering) to adult (capable of flowering).

• Environmental Cues: Include photoperiodism (response to daylight duration) and vernalization (response to chilling) for certain species.

Photoperiodism

• Mechanism that helps plants respond to changing lengths of daylight to optimize flowering times.

• Plant Categories Based on Photoperiodism:

  • Short-Day Plants (SDP): Flower when days are shorter than a certain length.

  • Long-Day Plants (LDP): Require longer daylight to initiate flowering.

  • Day-Neutral Plants (DNP): Flower irrespective of day length.

Experiments with Tobacco Plants and Photoperiodism

• Research showed Maryland Mammoth tobacco could produce flowers in a controlled greenhouse environment due to modified light exposure.

• The critical day length is essential for flowering; plants respond variably based on their light exposure conditions.

Night Break Experiments

• Experimental evidence showed short-day plants require longer nights for flowering, and this process can be disrupted by brief light exposure during the night.

Florigen: The Flowering Hormone

• Florigen: A protein produced in leaves that induces flowering in the shoot apical meristem.

  • Controlled by another transcription factor (Constans), which is regulated by photoperiod.

  • Phytochrome helps set the internal clock used for measuring light.

Conclusions on Plant Flowering

• The photoperiod response largely hinges on dark-light cycles, implicating Florigen in communication for triggering the shift from vegetative to reproductive stages.

• The mechanisms governing flowering showcase the interplay of various environmental triggers and internal pathways, illustrating the complexity of plant life cycles.

Overview of Flower Development and Life Cycle

Introduction to Flower Development

The life cycle of flowering plants, or angiosperms, encompasses intricate processes that begin with pollination and culminate in seed dispersal. Each stage is crucial for reproduction, ensuring the continuation of plant species.

Post-Pollination Changes

Pollination and Fertilization

After a flower is successfully pollinated, significant transformations take place within its structure.

• Fertilization leads to the merging of male gametes (pollen) and female gametes (ovules), resulting in the formation of a zygote, which will develop into an embryo.

• Ovules undergo development to become seeds, encapsulating the genetic material necessary for the next generation.

• The ovary wall thickens and transforms into the fruit, which plays a pivotal role in seed protection and dispersal.

Function of Fruits

Purpose of Fruits

Fruits serve a dual purpose in flowering plants:

1. Protection: They safeguard developing seeds from predators and environmental factors.

2. Dispersal: They facilitate the spreading of seeds over various distances to reduce competition and promote colonization of new areas.

   • Many plants evolve fruits that are attractive to animals, enhancing the chances of seed dispersal through animal interactions.

   • When animals consume the fruit, they often excrete the seeds in new locations, allowing for successful germination away from the parent plant.

Types of Fruits

• Fleshy Fruits: These fruits are soft and often sweet, making them appealing to animals (e.g., apples, berries).

  • Example: An apple's core contains seeds that, once discarded, can germinate elsewhere.

• Dry Fruits: These fruits may have mechanisms that facilitate mechanical dispersal, such as:

  • Milkweed Pods: These pods open to release seeds that are carried by the wind.

  • Burdock Fruits: They cling to animal fur, exemplifying nature's innovation that inspired the design of Velcro.

Seed Development and Dormancy

Embryo Growth and Dormancy Phase

After fertilization:

• Embryo Development: Embryos grow and develop cotyledons, which serve as the initial leaves.

• Dormancy Phase: Many plants enter a dormant state post-fertilization to conserve energy and protect seeds from harsh conditions.

  • Seeds can have water content as low as 10%, contrasting with the 80-85% water content in actively growing plants, which facilitates longevity and resilience.

  • Abscisic Acid (ABA): This plant hormone plays a critical role in regulating dormancy and preventing premature germination.

  • Remarkably, some seeds can remain viable for thousands of years—such as seeds recovered from the Arctic ground squirrel that were 32,000 years old!

Germination Process

The transition from dormancy to active growth involves hormonal changes triggered by:

• Gibberellic Acid: This hormone counteracts the effects of ABA, signaling the seed to break dormancy and initiate growth.

• Conditions like moisture, temperature, and oxygen levels are also vital for this process, indicating the right time for germination.

Flower Formation Mechanisms

Vegetative and Reproductive Growth

• Vegetative Growth: Seedlings exhibit growth primarily focused on developing leaves and stems to maximize sunlight absorption before gearing up for reproduction.

• Reproductive Growth: Following vegetative growth, plants enter a stage of forming flowers which involves the sequential development of flower parts:

  • Sepals: Protect developing buds.

  • Petals: Attract pollinators.

  • Stamens and Carpels: Involved directly in reproduction. This phase's successful completion is vital as it determines the plant's reproductive success and overall life cycle longevity. Plants can be categorized based on life cycles:

• Annuals: Complete their life cycle within one year.

• Biennials: Require two years to complete their cycle.

• Perennials: Flower and produce seeds over multiple seasons, returning year after year.

Factors Triggering Flowering

Phase Change and Environmental Cues

Plants transition from a juvenile (non-flowering) to an adult (flowering) phase through various triggers:

• Environmental Cues: This includes factors such as photoperiodism and vernalization, which signal the season for flowering.

Photoperiodism

Photoperiodism is a crucial mechanism that allows plants to respond to varying lengths of daylight:

• Short-Day Plants (SDP): Flower when day lengths are shorter than a specific duration, often in late summer or fall.

• Long-Day Plants (LDP): Require longer day lengths to trigger flowering, typically in late spring or early summer.

• Day-Neutral Plants (DNP): Flower regardless of day length, relying more on other environmental factors.

Experiments with Tobacco Plants and Photoperiodism

Research conducted on Maryland Mammoth tobacco plants illustrated that successful flowering could occur in controlled environments with modified light exposure, showcasing plant adaptability to conditions. The findings emphasize the importance of a critical day length necessary for flowering, as plants respond variably based on their light exposure conditions.

Night Break Experiments

Experiments have demonstrated that:

• Short-day plants require prolonged darkness for flowering, and that brief light exposure during the night can disrupt this process, thereby delaying flowering.

Florigen: The Flowering Hormone

Role of Florigen in Flowering

• Florigen: This protein, produced in leaves, is key in inducing flowering at the shoot apical meristem. Its production is influenced by a transcription factor called Constans, which is regulated by photoperiod.

• Additionally, Phytochrome plays a pivotal role in helping plants set their internal clocks for measuring light, thereby influencing flowering timing.

Conclusions on Plant Flowering

The complex interplay between environmental triggers and internal pathways—especially concerning dark-light cycles—underpins the plant's ability to adapt to its surroundings effectively. Florigen is crucial for facilitating the transition from vegetative to reproductive stages, illustrating how various factors interconnect within the lifecycle of plants, showcasing the sophistication of floral development and plant reproduction.

Short-Day Plants (SDP)

• Definition: These plants require shorter day lengths, typically blooming when daylight is less than a critical length, often occurring in late summer or fall.

• Light Requirement: The specific day length varies by species, generally requiring long nights (more darkness) to induce flowering.

Long-Day Plants (LDP)

• Definition: Unlike short-day plants, long-day plants require longer day lengths to flower, usually triggered in late spring or early summer when daylight extends beyond a certain threshold.

• Light Requirement: The critical length varies among species, with these plants needing shorter nights to promote flowering.

Studies on Light Requirements

• Experiments with Tobacco Plants: Research conducted on Maryland Mammoth tobacco plants demonstrated that the flowering response could be manipulated in controlled environments by adjusting light exposure. These experiments confirmed the importance of critical day length for flowering.

• Night Break Experiments: Studies have shown that short-day plants have specific dark requirements. Brief exposure to light during the night can disrupt the flowering process by providing a 'break' to the uninterrupted darkness needed for blooming.

These findings highlight the intricate connection between light exposure and the flowering mechanisms in plants, illustrating their adaptability to environmental changes.

Given a critical day length and the number of hours of light exposure, you can determine whether a Long-Day Plant (LDP) or a Short-Day Plant (SDP) would initiate flowering:

• Long-Day Plants (LDP): These plants require a longer duration of daylight to trigger flowering. If the number of hours of light exposure exceeds the critical day length for a specific plant species, it will likely initiate flowering. For example, if the critical day length is 14 hours and the given light exposure is 16 hours, the LDP would flower.

• Short-Day Plants (SDP): In contrast, SDP require shorter daylight periods to flower. If the light exposure is shorter than their critical day length, flowering is triggered. For instance, if their critical day length is 12 hours and the light exposure is 10 hours, the SDP would initiate flowering.

Hence, by comparing the hours of light exposure to the critical day length for each plant type, one can accurately predict whether an LDP or SDP will flower under specific conditions.

Phytochrome in Seedling Growth and Flowering

Phytochrome is a photoreceptor in plants that plays a critical role in regulating various aspects of growth and development in response to light. It exists in two interconvertible forms:

1. Phytochrome Red (Pr):

   • Role: This form absorbs red light (around 660 nm) and is predominant in the dark. When Pr absorbs red light, it is converted into the active form, Pfr.

   • Function in Growth: Promotes seed germination and stem elongation in response to increased red light. This conversion helps seedlings grow taller to reach more light.

   • Impact on Flowering: This form is critical in initiating the flowering process in long-day plants by allowing them to sense the lengthening days of spring.

2. Phytochrome Far-Red (Pfr):

   • Role: This active form absorbs far-red light (around 730 nm) and rapidly converts back to Pr in dark conditions.

   • Function in Growth: Inhibits stem elongation and promotes leaf expansion and development, helping the plant balance its growth and conserve resources when light is not as plentiful.

   • Impact on Flowering: This form is crucial for short-day plants, as it helps them detect the appropriate light conditions needed to initiate flowering when day lengths shorten.

Conclusion

Both forms of phytochrome are essential for regulating seedling growth and flowering, with Pr primarily promoting growth in light and initializing flowering for LDPs, while Pfr helps inhibit excessive elongation and facilitates flowering for SDPs based on dark conditions.

Advantages of Having a Juvenile Phase in Plants

1. Resource Allocation: During the juvenile phase, plants can allocate resources toward developing a robust root system and foliage, which are vital for supporting future growth and reproduction.

2. Growth Optimization: The juvenile phase provides time for plants to optimize their growth before reproducing, ensuring they are strong enough to survive environmental stresses.

3. Increased Lifespan: By delaying reproduction until maturity, plants may enhance their overall lifespan and reproductive success, allowing for more generations of seeds.

4. Environmental Adaptation: Juvenile plants can respond to environmental conditions without the pressure to reproduce, enabling them to adapt to changing conditions more effectively.

5. Protection from Herbivory: In the juvenile phase, plants may be less attractive to herbivores, which tend to target mature plants with flowers and fruits, allowing for more energy to be invested in growth rather than reproduction.

Yes, exactly! The use of night break techniques allows growers to manipulate the timing of flowering to coincide with increased market demand. For instance, by delaying the flowering of certain short-day plants until December, producers can ensure that flowers (or fruits) are available during the winter months when demand may be higher, such as for holiday season sales. This strategic use of light manipulation helps growers maximize their profits by aligning production schedules with consumer needs.

could you rephrase this simply as well- And it might be reasonable to think, well, it's probably the meristem because that's where our flowers are produced. But it actually turns out that leaves are the site where light or its absence is perceived, and this was demonstrated by experiments where you can just, like, take a piece of of, construction paper and make a little sleeve, slide it over a leaflet on a plant that's photoperiodic, and give that individual leaf the photoperiod that you have previously determined is inductive for flowering.

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And what will happen is the entire plant will flower. So that's what's shown in this, pair of figures where a kafflebur, which is a short day plant, can bloom in response to the shortening days at the end of the growing season normally, but you can also induce it to start to bloom by just covering a leaf for, the correct amount of time. So leaves are where photoperiod is perceived, but, again, the mirror stem is where flowers are made. So that tells us it's pretty likely that there's some kind of signal that that is the these the that is made in the leaf and moved to the meristem to trigger flowering. So what is that signal?

Distinction Between Phytochrome Forms

1. PR

   • Role: Absorbs red light (around 660 nm) and predominates in dark conditions. When it absorbs red light, it converts into the active form, Phytochrome Far-Red (Pfr).

   • Function in Seedling Growth: Promotes seed germination and stem elongation in response to increased red light, facilitating taller growth towards light sources.

   • Impact on Flowering: Critical for initiating flowering in long-day plants by detecting lengthening days in spring.

2. Phytochrome Far-Red (Pfr)

   • Role: Absorbs far-red light (around 730 nm) and rapidly converts back to Pr in the dark.

   • Function in Seedling Growth: Inhibits stem elongation and promotes leaf expansion, allowing the plant to balance growth and conserve resources when light is scarce.

   • Impact on Flowering: Essential for short-day plants, helping them detect the right light conditions to flower when daylight hours shorten.