Endogenous Rhythms in Plants

Endogenous Rhythms/Plant Movements

• Many plant behaviors exhibit periodic oscillations, controlled by an internal time-measuring system (endogenous biological clock).
• Manifestations: Diurnal rise and fall of leaves (sleep movements), diurnal photosynthesis.
• Criteria to distinguish simple periodic phenomena and rhythm driven by an endogenous clock: Persistence in the absence of external cues, resourcing by external signals (light/temperature), no lasting effect of temperature on the timing.
• Classification of Biological Rhythms: Circadian rhythm (24 hours), e.g., bean movement(hyponasty, epinasty), Fertility Tree of UPLB; Lunar rhythm (28 days), e.g., flowering of fire trees, cherry blossoms, Kapok; Ultradian rhythm (<24 hours).
• Plant Movements: Growth movements (irreversible, due to differential growth), turgor movements (reversible, but not all the time, due to changes in cell volume).
• Plant Responses to Stimuli: Tropisms (growth towards or away from a stimulus), positive tropisms (growth toward), negative tropisms (movement away), phototropism (towards light). Nastic movements (movement not oriented towards stimulus), photoasty, thigmotropism, thigmonasty, gravitropism, chemotropism, chemonasty, hydrotropism, hydronasty, thermonasty, photonasty.

The Biological Clock

• Many aspects of plant behavior exhibit periodic oscillations controlled by an internal time-measuring system (endogenous biological clock).
• Plant behaviors are controlled by an internal time measuring system.

Plant Movements - Categories

• Growth Movements: Result of differential growth within an organ or between two different organs. Irreversible.
• Turgor Movements: Result from volume changes in cells due to changes in osmotic pressure. Reversible (but not all the time).

Plant Responses to Stimuli

• Tropisms: Result from differential growth of specific plant organs in response to a stimulus.
  • Positive tropisms- Growth toward the stimulus (e.g., bending toward light).
  • Negative tropisms- Growth away from the stimulus.
• Phototropism: Movement in response to light.
• Nastic Movements: Movement not oriented toward or away from an environmental stimulus.

Gravitropism

• Shoots are negatively gravitropic (grow against gravity).
• Roots are positively gravitropic (grow towards earth's center).

Thigmotropism

• Directional response of plant organs to touch or physical contact with a solid object. (e.g., curling of vines)

Heliotropism

• Solar tracking (e.g., sunflowers).

Photoperiodism

• The response of an organism to seasonal changes in day length.
  • Long-day plants: Flower when day length is longer than a critical value.
  • Short-day plants: Flower when day length is shorter than a critical value.
  • Day-neutral plants: Flower when they become mature, regardless of day length.

Dormancy

• A slowdown in an organism's metabolic rate.
• Seed dormancy: Inability of a viable seed to germinate.
• Parthenocarpy: Fruit development without fertilization.

Senescence

• The collective term for aging processes that lead to the death of a plant or plant part.
• Leaf colors are an example, and a result of changes in light quality and photoperiod.

Hormones and Growth Regulation

• Phytohormones: Organic substances, other than nutrients, that modify plant growth and development. Effective at low concentrations.
• Auxins: Synthesized in shoot apical meristems, young leaves, seeds, and fruits.
  • Promote growth.
  • Toxic in large concentrations.
  • Apical dominance (inhibiting activation of lower buds)
  • Promote lateral and adventitious root development.
• Indoleacetic Acid (IAA): A key auxin, chemically produced or synthesized from indole or tryptophan.

Cytokinins

• Phenyl urea derivatives of adenine (a molecule in DNA)..
• Found in actively dividing tissues (seeds, fruits, leaves, root tips, wound tissue).
• Transported through xylem.
• Promote cytokinesis, cell expansion, and axillary bud growth.

Gibberellins

• Reversal of genetic dwarfism in plants.
• Bolting of biennials (production of flowers during the first growing season)
• Promote parthenocarpy (fruit development without fertilization).

Abscisic Acid (ABA)

• Inhibits growth activities during environmental stress (e.g., drought).
• An antagonist to other growth-promoting hormones.
• Promotes seed dormancy.
• Promotes stomata closure during water deficit.

Ethylene

• Synthesized from the amino acid methionine.
• Promotes flowering and leaf senescence.

Environmental Factors

• Abiotic Factors: Climate (weather), edaphic (soil) factors (e.g., drought, salinity, acidity, lahar, La Niña, El Niño), greenhouse effect, global warming.
• Biotic Factors: Beneficial organisms (pollinators, decomposers, natural enemies), pests (arthropods, pathogens, weeds, vertebrate pests), allelopathy.

Weather versus Climate

• Weather: Atmospheric conditions in a specific area at a given time (hours or days).
• Climate: Long-term weather pattern of an area (averaged over long periods). Seasonal patterns are part of climate.

Solar Radiation

• Solar Constant: 2.0 cal/cm^2/min (with 15% fluctuation annually).
• Amount of solar radiation that reaches the outer atmosphere.

PAR (Photosynthetically Active Radiation)

• Used by plants for CO2 fixation during photosynthesis.
• Classified based on light intensity requirements. (Heliophytes vs. sciophytes)

Albedo

• Ratio of reflected radiation to total incident light.
• Reflects how quickly and how much a surface is heated. (e.g., 50-60% in the polar regions).

Environmental Factors Affecting Crop Production

• Phenotype: Genotype + Environment + (G*E).
• G: Sets the limits for the phenotype, and E: Modifies the expression of the phenotype.
• Weather (sun, cloud, temperature, wind, rain), variations from patterns, is part of weather (short term events), while climate is the long-term average weather pattern.

Photoenergetic effect:

• Direct effect on photosynthesis.

Photocybernetic effect:

• Effect on plant development.

Climate Factors

• Solar Radiation: Most important in agriculture.
• Temperature: Important in every chemical, physical, and biological process in plants. Minimum T below which rxn=0, optimum reaction at maximum T, Maximum T -above which reaction =0
• Wind: Effects on lodging, grain shattering, transpiration, effects on plant form, sterility due to pollination loss, reduced CO2 levels and spread of diseases.
• Precipitation (Rainfall): Water falling from the air.
• Relative Humidity: Directly influences plant growth and disease or pest incidence.
• Mist and Fog.

Roles of Water in Plant Growth:

• Reactant in many biological processes.
• Medium of transport for nutrients and other substances.
• Temperature regulation.

Classification based on moisture need

• Xerophytes: Desert plants
• Hydrophytes: Aquatic plants
• Mesophytes: Land plants

Soil structure

• Grouping of soil particles (sand, silt, clay) into larger aggregates.
• Arrangement of particles affects movement of air, water and roots.
• Stable aggregates- allow rapid exchange of air and water, needed for plant growth.

Soil Texture & Structure

• Texture= size and relative proportions of sand, silt and clay.
• Structure: Arrangement of soil particles.

Soil Depth:

• Thickness of soil materials (topsoil and subsoil).

Topography:

• Physical configuration of the soil surface (drainage, erosion run-off etc...).

Infiltration Rate:

• Velocity of water entering the soil. Affected by soil surface permeability, soil moisture content, soil surface conditions & plant cover, time and quantity of chemical use.

Chemical Properties

Explore the fascinating world of plant movements and their internal rhythms. This quiz covers the criteria to distinguish between simple periodic phenomena and rhythms driven by an endogenous clock, along with various classifications such as circadian and lunar rhythms. Test your knowledge on how plants respond to stimuli and exhibit growth movements.