Biology: Science for Life, with Physiology Lecture on Plant Growth

Learning Outcomes

  • Describe how water moves up a plant stem.
  • List the modifications of plant physiology and anatomy that reduce water loss or make plants more drought tolerant.
  • Describe the pressure flow mechanism of phloem transport.
  • Define photoperiodism and explain the mechanism by which plants respond to day length.
  • List the environmental factors that cause tropism in plants and describe how differences in cell expansion lead to directional growth.
  • Define "plant hormone" and describe ways in which auxin affects plant growth.

26.1 The Right Plant for the Place: Water Relations

Factors Influencing Plant Growth

  • Soil’s Chemistry: The chemical composition of soil affects nutrient availability and plant health.
  • Light Availability: Essential for photosynthesis; influences plant growth and development.
  • Length of the Growing Season: Includes:
      - Temperature: Affects metabolic processes.
      - Precipitation: Water availability essential for survival and growth.
  • Water Availability: Crucial for various physiological processes.

Transpiration

  • Definition: Water loss from leaves through evaporation.
  • Process:
      - Water is pulled up through xylem sap (water and dissolved minerals) from roots, through tubes of dead xylem cells in the stem, to leaves.
      - An effective process in tall trees (up to 100 m or 330 ft).
  • Evolved after land colonization approximately 400 million years ago.

Mechanism of Transpiration

  • Water Column Continuity: Maintained by:
      - Cohesion: Identical molecules stick together due to hydrogen bonding.
      - Adhesion: Unlike molecules stick together; water adheres to cellulose in plant cell walls.
      - Tension: Negative water pressure created by cohesion and adhesion forces.
  • Rate Control: Determined by the size of stomata (pores on leaf surfaces) facilitating gas exchange and water loss.

Detailed Process of Transpiration

  • Evaporation from leaves creates tension due to cohesive forces attracting water molecules together.
  • Adhesion helps maintain water continuity within the xylem under exerted tension.
  • Roots uptake water from the soil, and the tension is transmitted throughout the plant.

Mycorrhizae

  • Definition: Symbiotic fungal strands that receive carbohydrates from plants, enhancing nutrient and water absorption by increasing surface area.
  • Found in approximately 90% of land plants, particularly in soils with abundant organic matter and aeration.

Stem Damage Prevention

  • Recommendation: Avoid walking in gardens to prevent disrupting water streams; prevent air gaps that lead to wilting.
  • Best Time for Cutting Flowers: Early morning when water tension is lowest.

Adaptations Affecting Transpiration

  • Evolutionary Strategies:
      - Modifications to photosynthesis for efficient CO2 acquisition.
      - Alterations in leaf shape and stomata for better water conservation.

Photosynthetic Adaptations

  • C4 Photosynthesis:
      - Additional process before light-independent reactions where CO2 is pumped from air toward chloroplasts.
      - Advantageous for plants in hot, sunny, and dry conditions (e.g., corn, sugarcane).

  • CAM Plants:
      - Accumulate CO2 at night to minimize water evaporation, converting it into carbohydrates stored in vacuoles for use during the day.
      - Found in extremely dry environments (e.g., cacti, pineapple).

Leaf Adaptations

  • Correlation of Water Loss and Leaf Surface Area:
      - Greater water loss occurs with larger leaf areas.
      - Important to consider during transplants as leaf removal may reduce water stress.
  • Adaptations:
      - Smaller leaves lead to less photosynthesis and potential competitive disadvantages in well-watered environments.
      - Some plants modify stems (e.g., cacti) to optimize water use.

Broad-leafed Plants

  • Thrive in shaded environments due to lower evaporation and better light interception.
  • Stomata Placement: Under leaf or in hair-lined pits to trap moisture.
  • Deciduous evolution reduces transpiration rates during unfavorable conditions.

Xylem Adaptations

  • Tube Diameter: Affects flow rates.
      - Small Diameter Tracheids: Reduce embolism risk but increase friction.
      - Wide Vessel Elements: Move water to leaves faster in moist conditions (10X faster).

Overwatering Issues

  • Excess water fills soil air spaces, depriving root cells of oxygen leading to wilting.
  • Common in houseplants and clay-rich soils; can be amended with organic matter.

Water Requirements of Plants

  • Water Needs: Approximately 2.5 cm (1 in) weekly for optimal health.
  • Signs of Water Stress:
      - Limp/wilting appearance, leaf discoloration, dry soil beneath the surface.

Water Inside Plant Cells

  • Vacuole Role: Central vacuole maintains turgor pressure and support, may freeze under low temperatures, leading to possible damage.

Ice Formation Damage Mechanisms

  • Ice outside cell walls leads to osmosis disruption.
  • Ice crystals within vacuoles can puncture membranes, causing cell death.
  • Xylem ice formation causes blockage preventing water uptake.

Cold Hardiness of Plants

  • Definition: Ability to tolerate cold temperatures, influenced by cellular solutes that lower freezing points (e.g., sugar).
  • Example Hardiness Temperatures:
      - Tomatoes: -1°C (31°F)
      - Apples: -4°C (25°F)
      - Cabbage: -9°C (16°F)

Hardened Plant Requirements

  • To resist cold, plants need gradual cooling and uninterrupted cold periods.
  • Damage may occur from warm temperatures during cold periods causing transpiration without soil moisture.

Hardiness Zones

  • Defined based on minimum temperature tolerances of landscape plants; ranges from below -50°F to above 40°F.

Special Care for Less Cold Tolerant Plants

  • Location plays a key role; north sides of buildings offer shade.
  • Additional protection methods include mulching, plastic coverings during frost events, and ensuring adequate water reserves.

26.2 A Beautiful Garden: Translocation and Photoperiodism

Translocation Overview

  • Definition: Movement of phloem sap (sugars and nutrients) from source to sink.
  • Mechanism: Driven by pressure differences within phloem tubes; water may move passively or actively.

Pressure Flow Mechanism

  • Process Description:
      - Sugar is actively loaded into phloem at the source.
      - Water follows into phloem, increasing pressure, while sugars move into sink cells.

Managing Translocation

  • Objectives:
      - Promote flowering and increase produce quantity and quality.
      - Prepare plants for winter through proper management of flower types: annuals, biennials, perennials.

Annual Plants

  • Characteristics: Complete life cycle in one season and survive one year (e.g., petunia, pansy).
  • Strategy: Produce many seeds to maximize pollination chances.

Biennial Plants

  • Life Cycle: Live for 2 years, producing leaves the first year and flowering the second year (e.g., hollyhock).

Perennial Plants

  • Definition: Live multiple seasons and flower every year (e.g., phlox, tulips).

Woody Plants

  • Classification of perennials with stiffened stems not used as garden perennials.

Evolutionary Strategies for Perennials

  • Balanced allocation to flowering and storage due to longer life spans.

Perennial Plant Care

  • Techniques: Deadheading minimizes fruit production, ensuring nutrient storage before dormancy; reduce crowding for optimal resource access.

Increasing Produce Quality

  • Best practices for annual crops include encouraging leaf growth and managing flowering times for maximum yield and quality.

Tobacco Plant Study

  • Unique findings related to the Maryland mammoth plant, which produced larger leaves and delayed flowering due to daylight hours influences.

Photoperiodism

  • Definition: Biological response to changes in light and dark duration affecting processes such as flowering, fruit ripening, and leaf drop.
  • Key Protein: Phytochrome undergoes conformation changes in response to light exposure, affecting flowering.

Short-Day vs. Long-Day Plants

  • Short-Day Plants: Flower in shorter day lengths (e.g., tobacco, chrysanthemums).
  • Long-Day Plants: Require longer day lengths for flowering (e.g., lettuce).
  • Day-Neutral Plants: Do not exhibit photoperiodism.

Critical Night Length

  • Minimum darkness required for flowering inhibition varies among plant species.

Abscission Process

  • Triggered by increasing night length leading to chlorophyll production cessation, resulting in leaf drop.

Plant Hardy Matching

  • Geographic Adaptation: Different critical night lengths may lead to flowering irregularities for plants planted out of their native environments.

26.3 Pleasing Forms: Tropisms and Hormones

Tropism Overview

  • Definition: Directional growth in response to environmental stimuli.
  • Types of Tropism:
      - Gravitropism: Growth direction influenced by gravity; positive in roots, negative in stems.
      - Phototropism: Growth direction influenced by light.
      - Thigmotropism: Growth in response to touch (e.g., vining plants).

Indoor Gardening with Tropisms

  • Best Practices: Rotate plants to promote even growth and touch leaves to encourage sturdy stem development via thigmotropism.

Plant Hormones

  • Regulate internal environments and responses to external conditions similarly to animal hormones (produced in small amounts with significant effects).
  • Types and Applications of Plant Hormones:
      - Auxin: Promotes cell expansion and various growth processes; used for rooting and thinning produce.
      - Gibberellin: Stimulates growth and breaks seed dormancy; used in brewing for uniform germination.
      - Cytokinin: Promotes cell division; commercial applications in plant growth from genetically engineered cells.
      - Ethylene: Triggers ripening and abscission; used in harvesting to ensure fruit uniformity.
      - Abscisic Acid: Promotes dormancy; prevents seed germination.

Auxin Functionality

  • Mechanism: Auxin causes cell elongation and can have differing effects depending on concentration and location in the plant.

Apical Dominance

  • Definition: Dominance of uppermost bud over lower buds, primarily mediated by auxin.
  • Effects: Enhances height growth and suppresses lateral branch growth.

Reducing Apical Dominance

  • Can be achieved through light pruning, thus releasing lateral buds for growth.

Interactions with Other Hormones

  • Auxin reduces sensitivity to ethylene, delaying abscission and maintaining fruit on plants longer.

Conclusion

  • Sustainability Message: Emphasizes humanity’s responsibility to cultivate gardens in harmony with natural processes and the biological world around us.