Lecture 14 - Transpiration, Transport, and Nutrition in Plants

Transpiration, Transport, and Nutrition in Plants

Overview of Plant Nutrient Acquisition

• Sources of Nutrients:

  • Plants acquire essential nutrients from three main sources: air, water, and soil.

  • Water and Mineral Sources:

    • Water and minerals are taken up from the soil.

    • Oxygen is also absorbed through the soil.

  • Photosynthesis:

    • Plants synthesize sugars from carbon dioxide (CO2), water (H2O), and light energy during photosynthesis.

  • Cellular Respiration:

    • This process requires sugars for energy release which supports various plant functions.

Mechanisms of Nutrient Transport in Plants

• Transport Pathway:

  • Water and minerals are transported from the roots to the shoots and leaves.

  • Sugars can be moved in both directions between shoots and roots.

Root Structure and Function

• Plasma Membrane Functionality:

  • The plasma membrane of root cells regulates the uptake of solutes.

  • Root Hairs:

    • These structures significantly enhance the absorptive surface area of roots.

    • Root hairs absorb water, dissolved ions, metals, and inorganic nutrients from the soil.

  • Microbial Interaction:

    • Roots engage with various microbes and fungi present in the soil.

Water and Nutrient Transport Mechanisms

• Movement of Water:

  • Water ascends from the roots through specialized tissue structures known as xylem vessels.

  • Transpiration Process:

    • Defined as the loss of water vapor from the leaves through evaporation, transpiration creates a suction force that pulls water upward.

  • Cohesion and Adhesion:

    • Cohesion:

      • The attraction between water molecules (H₂O), which maintains a continuous column of water.

    • Adhesion:

      • The attraction of water molecules to the sides of xylem vessels enhances upward movement.

• Xylem Structure:

  • Contains two primary types of water-conducting cells: tracheids and vessel elements.

  • Cell characteristics:

    • Both types possess rigid, lignin-rich secondary cell walls and are dead upon maturity, forming continuous tube chains for effective water transport.

Transpiration and Water Flow Dynamics

• Regulation of Transpiration:

  • Stomata, or leaf pores, control the rate of transpiration and can open or close based on environmental conditions, influenced by:

    1. Guard cells surrounding the stomata regulate opening.

    2. Environmental factors including sunlight and atmospheric CO₂ concentrations affect guard cell activity.

    3. Guard cells possess a biological clock maintaining their rhythmic opening and closing.

• Transpiration and Tension Harmony:

  • The tension generated from transpiration indirectly pulls water and minerals from the soil via xylem through a continuous water chain from roots to leaves.

Leaf Structure Adaptations for Photosynthesis

• Photosynthesis Structures:

  • Leaves contain specialized structures to maximize photosynthesis efficiency:

    1. Palisade Mesophyll:

    • Primary site for photosynthesis with a high concentration of chloroplasts.

    1. Stomates:

    • Located on the lower leaf surface for optimal gas exchange.

    1. Spongy Mesophyll:

    • Contains intercellular air spaces that facilitate gas exchange and assist in transporting photosynthates to vascular bundles.

    1. Vascular Bundles:

    • Transport water/minerals to mesophyll and distribute photosynthates throughout the plant.

Guard Cell Functionality and Water Regulation

• Guard Cell Functions:

  • Stomates generally open during the day and close at night to minimize water loss.

  • Factors influencing guard cells:

    1. Daylight stimulates the accumulation of potassium ions (K+), resulting in stomate opening.

    2. Low CO₂ concentrations signal stomate opening.

    3. An internal biological clock regulates the daily opening and closing cycles.

  • Water loss dynamics:

    • Approximately 400 molecules of water (H₂O) are lost for every molecule of carbon dioxide (CO₂) that is gained during photosynthesis.

Phloem Function and Sugar Transport

• Phloem Structure:

  • Phloem consists of food-conducting cells called sieve-tube elements responsible for the transport of sugars, amino acids, and hormones.

  • Each sieve-tube element is associated with a companion cell that assists in nutrient transport and maintenance.

• Sugar Transport Mechanism:

  • Phloem sap is conducted in various directions.

  • The flow occurs from sugar sources (areas of sugar production) to sugar sinks (areas of sugar utilization or storage) based on concentration gradients.

Pressure Flow Mechanism in Phloem

• Mechanics of Sugar Flow:

  • At the sugar source, sugars are loaded into the phloem, increasing solute concentration, which draws water into the phloem tube by osmosis, enhancing pressure within the tube.

  • As sugars are utilized or stored at the sugar sink, decreased solute concentration causes water to exit the phloem and re-enter the xylem, lowering water pressure in the sieve tubes.

Plant Nutrients and Soil Quality

• Essential Nutrients:

  • Plant health relies on obtaining essential inorganic nutrients:

    • Carbon dioxide, inorganic substances, and others are crucial for successful growth and reproduction.

  • Macronutrients:

    • Include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and phosphorous (P), collectively comprising approximately 98% of dry plant weight.

    • About 1.7% comes from potassium (K+), calcium (Ca²+), and magnesium (Mg²+).

  • Micronutrients:

    • These function mainly as cofactors and are present in less than 0.3% of dry weight: chlorine (Cl-), iron (Fe²+), manganese (Mn²+), boron (B), zinc (Zn²+), copper (Cu²+), nickel (Ni²+), molybdenum (Mo).

Fertilization and Nutrient Deficiencies

• Fertilizer Impact:

  • Fertilizers can alleviate nutrient deficiencies in soils, impacting plant health and growth.

  • Observations of plants indicate varying deficiencies, such as nitrogen (N)-deficient, phosphorus (P)-deficient, and potassium (K)-deficient plants.

Nutrient Acquisition and Symbiotic Relationships

• Plant-Nutrient Bacteria Connection:

  • Due to high atmospheric nitrogen levels (about 80%), plants struggle with nitrogen deficiency mainly because they cannot absorb N₂ directly.

  • Nitrogen Fixation Process:

    • Soil bacteria convert atmospheric nitrogen (N₂) into forms usable by plants, primarily through:

      • Nitrogen-fixing bacteria that convert N₂ to ammonia (NH₃).

      • Ammonifying bacteria break down organic matter to produce ammonium (NH₄+).

      • Nitrifying bacteria convert ammonium to nitrates (NO₃-), which are preferred by plants.

• Importance of Symbiosis:

  • Many plants engage in mutualistic relationships with nitrogen-fixing bacteria, particularly legumes (peas, beans, alfalfa).

    • These bacteria live in root nodules and convert atmospheric nitrogen into ammonium ions (NH₄+).

  • Mycorrhizae:

    • Approximately 80% of plants form symbiotic associations with fungi, enhancing water and mineral absorption capabilities.

    • Mycorrhizal fungi expand the root area and were critical in helping plants colonize terrestrial environments.