Plant Physiology: Detailed Notes

1. Photoperiodism and Flowering

Photoperiodism is the physiological response of plants to the length of day or night. It is a crucial mechanism that regulates various developmental processes, most notably flowering, to ensure they occur at the optimal time of year for reproductive success.

Plant Classification based on Photoperiodism:

• Short-Day Plants (SDPs): Flower when the day length is shorter than a critical period (long-night plants).
  • Examples: Chrysanthemums, rice, soybeans, poinsettias.
• Long-Day Plants (LDPs): Flower when the day length is longer than a critical period (short-night plants).
  • Examples: Spinach, lettuce, potatoes, radishes.
• Day-Neutral Plants (DNPs): Flower irrespective of day length, relying on other cues like age.
  • Examples: Tomatoes, cucumbers, roses, corn.

Molecular Mechanism of Flowering Control:

The decision to flower is a major developmental transition in plants and is controlled by a complex network of interacting pathways. The photoperiod pathway is a key part of this network.

• Perception and Signaling: The perception of the photoperiod occurs in the leaves. The key components are the circadian clock and photoreceptors.
  • Circadian Clock and CONSTANS (CO): The plant's internal circadian clock measures day length by controlling the expression of the CONSTANS (CO) gene. In long-day plants, the circadian clock causes CO to be expressed in the late afternoon. The CO protein is only stable in the presence of light. On long days, CO protein accumulates and activates the expression of the FLOWERING LOCUS T (FT) gene.
  • Florigen (FT Protein): The FT (FLOWERING LOCUS T) protein is the mobile flowering signal, or florigen. It travels from the leaves to the shoot apical meristem.
• Activation of Flowering: At the meristem, the FT protein forms the Florigen Activation Complex (FAC) with the proteins FD and 14-3-3. This complex activates floral identity genes, such as APETALA1 (AP1) and LEAFY (LFY), which are the master regulators of flower development. This activation commits the meristem to producing flowers instead of leaves.
• Other Flowering Pathways: Besides the photoperiod pathway, other pathways also influence flowering time, including the autonomous pathway (which promotes flowering in the absence of specific environmental cues), the vernalisation pathway, and the gibberellin pathway.

2. Phytochrome and Light Perception

Phytochrome is a family of photoreceptor proteins that plants use to detect light, particularly red and far-red light. They also contribute to temperature sensing.

Types and Forms:

• Forms: Phytochromes exist in two interconvertible forms: the inactive Pr form (absorbs red light) and the active Pfr form (absorbs far-red light). The Pfr/Pr ratio provides information about the light environment.
• Types: In Arabidopsis, there are five phytochromes: PhyA, PhyB, PhyC, PhyD, and PhyE. PhyA is most sensitive to far-red light, while PhyB is the primary red-light receptor and plays a major role in shade avoidance.

3. Night Break and Day Break

• Night Break: A brief exposure of light during the dark period can manipulate flowering. A red-light night break inhibits flowering in SDPs and promotes it in LDPs by altering the Pfr/Pr ratio.
• Day Break (Dawn Signal): The transition from dark to light at dawn, perceived by phytochromes and cryptochromes, resets the circadian clock and initiates a cascade of gene expression for photosynthesis and other daytime activities.

4. Seed Dormancy

Seed dormancy is a state where a seed is unable to germinate, even under favorable conditions. This is a crucial survival mechanism.

Hormonal Control:

The balance between abscisic acid (ABA) and gibberellins (GA) is the primary regulator of seed dormancy.

• Abscisic Acid (ABA): Induces and maintains dormancy.
• Gibberellins (GA): Promote germination.
• ABA/GA Ratio: A high ABA/GA ratio promotes dormancy, while a low ratio promotes germination.

5. Vernalisation

Vernalisation is the induction of flowering by prolonged exposure to cold temperatures.

Epigenetic Mechanism:

Vernalisation involves the epigenetic silencing of the FLOWERING LOCUS C (FLC) gene, a potent floral repressor.

• Silencing FLC: During cold exposure, the VIN3 protein is induced and interacts with the Polycomb Repressive Complex 2 (PRC2) to deposit repressive histone marks (H3K27me3) at the FLC locus. The non-coding RNA COOLAIR also contributes to this repression.
• Maintaining the Memory of Winter: After the cold period, the silenced state of FLC is maintained by other proteins, including VERNALIZATION 1 (VRN1) and VERNALIZATION 2 (VRN2). This ensures that the plant does not flower until after winter has passed. This epigenetic memory is reset in the next generation.

6. Plant Hormones (Phytohormones)

Plant hormones are chemical messengers that regulate various aspects of plant growth and development. They often work in complex interactions with each other.

• Auxins (e.g., IAA): Primarily involved in cell elongation, apical dominance, root formation, and tropic responses (phototropism and gravitropism).
• Gibberellins (GAs): Promote stem elongation, seed germination, and flowering.
• Cytokinins: Promote cell division (cytokinesis), shoot formation, and delay leaf senescence.
• Abscisic Acid (ABA): A general inhibitor of growth. It promotes seed dormancy, stomatal closure during water stress, and senescence.
• Ethylene: A gaseous hormone that promotes fruit ripening, leaf abscission, and senescence.
• Brassinosteroids: Steroid hormones that are involved in cell elongation and division, photomorphogenesis, and stress responses.
• Strigolactones: Involved in inhibiting shoot branching and promoting symbiotic relationships with mycorrhizal fungi.

7. Allied Topics

Circadian Rhythms in Plants

Plants possess an internal biological clock that allows them to coordinate their biological processes with the 24-hour day-night cycle. This rhythm is crucial for optimizing photosynthesis, regulating flowering time, and managing energy reserves.

Shade Avoidance Syndrome (SAS)

This is a set of responses to avoid being shaded by other plants. It is triggered by a low red to far-red light ratio and involves rapid stem elongation, upward leaf movement, and accelerated flowering, all primarily driven by an increase in auxin.

Thermoperiodism

This refers to a plant's response to daily temperature fluctuations. Many plants exhibit optimal growth with a difference between day and night temperatures, which can influence flowering, growth, and development.

Plant Movements

Plants are not static; they exhibit a variety of movements in response to environmental stimuli.

• Tropisms: These are directional growth movements in response to an external stimulus.
  • Phototropism: Growth towards a light source. Mediated by the redistribution of auxin.
  • Gravitropism: Growth in response to gravity. Shoots grow upwards (negative gravitropism) and roots grow downwards (positive gravitropism), also regulated by auxin.
  • Thigmotropism: Growth in response to touch, such as a vine coiling around a support.
• Nastic Movements: These are non-directional movements in response to a stimulus.
  • Nyctinasty (Sleep Movements): The daily opening and closing of leaves or flowers, often regulated by the circadian clock and changes in turgor pressure.
  • Thigmonasty: A rapid response to touch, such as the folding of the leaves of the sensitive plant (Mimosa pudica). This is also due to rapid changes in turgor pressure.