Plant Nutrition

Plant Nutrition, Resource Acquisition & Transport

Learning Goals & Outcomes

  • Goal: Understand how plants absorb nutrients, water & CO2 from the environment and how soil and climate influence plant distribution and growth.

  • Outcomes:

    • Describe how plants obtain nutrients from the soil.

    • Describe nutrient transport mechanisms in plants.

    • Describe an essential nutrient.

    • Explain soil features and characteristics.

    • Explain nitrogen fixation.

    • Diagram transpiration & explain how it is regulated via stomata.

    • Diagram sugar transport from source to sink.

Resource Acquisition & Transport

  • Overview: What resources does a plant need, and where are they acquired?

Evolution of Vascular Tissue

  • Vascular tissue enables plants to carry out long-distance transport.

  • Xylem:

    • Transports water and minerals from roots to shoots.

  • Phloem:

    • Transports photosynthetic products from where they are made or stored to where they are needed.

Adaptations for Resource Acquisition

  • Shoot Architecture & Light Capture:

    • Photosynthesis occurs via leaves optimized for light capture.

    • Trade-offs exist between investing energy in height to avoid shading versus branching to maximize surface area for light capture.

    • Leaves arranged in an ascending spiral to avoid shading of lower leaves.

  • Root Architecture & Absorption:

    • More root branching occurs in soil pockets with higher nutrient availability.

    • Competition for resources is reduced within the same individual.

Plant Transport Systems

  • Plant Compartments:

    • Symplast: Cytosol of all cells & plasmodesmata (connecting pores).

    • Apoplast: Everything external to the plasma membrane, including cell walls, extracellular spaces, and the interior of vessel elements & tracheids.

  • Transport Pathways:

    • Symplastic Route: Crosses one membrane then continues cell-to-cell.

    • Apoplastic Route: Moves through cell walls and extracellular spaces.

    • Transmembrane Route: Involves transport through both cell membranes and cell walls.

Short-Distance Transport

  • Short-distance transport relies on the selective permeability of the plasma membrane.

  • Mechanisms:

    • Plant cells utilize H+ gradients for membrane potential and cotransport mechanisms.

    • Water absorption or loss by cells occurs through osmosis, defined as the diffusion of free water across a membrane.

  • Water Potential:

    • Predicts direction of water flow during osmosis, influenced by water and solute concentration and pressure.

    • Water moves from areas of high water potential to low water potential.

Long-Distance Transport

  • Bulk Flow: Movement of liquid driven by a pressure gradient.

  • It is independent of concentration.

  • Vascular Tissue Functions:

    • Xylem: Utilizes tracheids for bulk water transport.

    • Phloem: Utilizes sieve-tube elements for transporting sap.

The Chemical Composition of Plants

  • Water, air, and soil minerals significantly contribute to plant growth.

  • Composition:

    • 80-90% of a plant's fresh mass is water.

    • 96% of a plant's dry mass comprises carbohydrates.

    • 4% of the dry mass consists of inorganic substances sourced from soil.

20 Essential Elements for Plant Growth

  • Plants require light, water, and approximately 20 elements for biochemical needs, known as essential nutrients.

  • Criteria for Essential Nutrients:

    1. Plant cannot complete its lifecycle without the element.

    2. No other element can perform the function of the element.

    3. The element is directly involved in plant nutrition.

Macronutrients and Micronutrients

  • Macronutrients: Nutrients needed in large amounts.

  • Micronutrients: Trace elements required in smaller amounts.

  • Examples of Nutrients:

    • Macronutrients: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S).

    • Micronutrients: Iron (Fe), Manganese (Mn), Boron (B), Molybdenum (Mo), Copper (Cu), Zinc (Zn), Chlorine (Cl), Nickel (Ni), Sodium (Na), Silicon (Si).

Nutrient Deficiency

  • Nutritional deficiencies can adversely affect plant growth, manifesting as:

    • Stunted growth.

    • Slow growth.

    • Chlorosis (yellowing of leaves).

  • Extreme deficiencies can lead to visible signs of cell death in leaves.

Soil Formation

  • Soil forms from biological, physical, and chemical processes.

  • Microorganisms such as bacteria, algae, and fungi remain dormant in dry soil but become active with moisture.

  • Soil distribution is heterogeneous and comprises distinct layers.

Soil Management: Fertilization

  • Fertilization: The addition of mineral nutrients to soil.

    • In natural ecosystems, this process occurs through animal waste excretion and the decomposition of organic matter (humus).

    • Humus: Remains of dead organisms and organic matter.

    • Fertilizers often provide Nitrogen (N), Phosphorus (P), and Potassium (K).

Topsoil Composition

  • Inorganic Components: Cations and anions.

  • Organic Components:

    • Decomposed leaves, feces, and dead organisms.

    • Living components, including bacteria, fungi, algae, protists, insects, earthworms, nematodes, and plant roots that decompose organic materials and mix soil.

Plant – Bacteria Relationship

  • Soil bacteria benefit plants by:

    • Exchanging beneficial chemicals with plant roots.

    • Decomposing organic materials to release nutrients.

    • Converting atmospheric nitrogen into usable forms.

  • The rhizosphere is the soil layer around a plant’s roots, inhabited by rhizobacteria that depend on plant secretions.

  • Rhizobacteria contribute to:

    • Plant growth promotion.

    • Production of antibiotics.

    • Absorption of toxic metals and enhancing nutrient availability.

Nitrogen Fixation: Root and Bacteria Interactions

  • Nitrogen is a crucial macronutrient needed for nucleic acids and proteins.

  • Plants typically require nitrogen in the forms of nitrate (NO<em>3−NO<em>3^-) or ammonia (NH</em>4+NH</em>4^+).

  • Rhizobacteria can convert nitrogen to these usable forms through nitrogen fixation.

  • Example: N-fixing bacteria have a symbiotic relationship with legume roots, serving as a natural method to fertilize plants.

Nutrients from Other Sources

  • Certain plant types include:

    • Epiphytes: Plants growing on other plants.

    • Parasites: Absorb water, sugars, and minerals from host plants.

    • Carnivorous Plants: Photosynthetic yet supplement nutrients by trapping insects.

Pulling Xylem Sap (Bulk Flow)

  • Water moves to/from xylem via:

    • Symplastic or apoplastic pathways.

  • Cohesion-Tension Hypothesis:

    • Transpiration creates a pull for xylem sap ascent due to cohesion between water molecules and tension due to negative pressure in the xylem.

  • Transpirational Pull: Generated by the diffusion of water vapor from leaves, leading to negative pressure.

Transpiration Mechanism

  • Regulation by Stomata:

    • Guard cells control the diameter of stomata, influenced by water pressure within cells.

    • Turgid Condition: Guard cells are swollen and open stomata; Flaccid Condition: Guard cells are relaxed and close stomata.

  • Mechanism of Guard Cell Turgidity:

    • K+ Uptake: Leads to turgidity.

    • Release of K+: Causes guard cells to become flaccid.

Transpiration Rate Regulation

  • Factors stimulating stomata opening/closing include:

    • Light: Stimulates K+ uptake via light receptors.

    • CO2 Depletion: Signals indicate the need for more CO2 for photosynthesis.

    • Internal Clock: Circadian rhythms governing a 24-hour cycle sensed by eukaryotes.

    • Drought Stress: Triggers stomata to close due to low water, which reduces wilting and photosynthesis.

Translocation of Sugars through Phloem

  • Translocation: The transport of photosynthesis products through the phloem under positive pressure.

  • Phloem Sap Transport:

    • Travels through sieve tubes from sugar sources to sinks.

    • Sugar Source: Organ producing the sugar.

    • Sugar Sink: Organ consuming/storing the sugar.

  • Pathways utilized include symplastic or apoplastic routes, with active transport mechanisms employed for entry into sieve-tube elements.

Review Questions

  1. Which of the following is NOT a way that resources would travel before entering the vascular tissue?

    • Options: Bulk flow, Apoplastic, Transmembrane, Symplastic.

  2. Broad surfaces of leaves enhance with the consequence of increasing .

    • Options: Cellular respiration; growth / Photosynthesis; water loss / Evaporation; growth / Transpiration; cellular respiration / Diffusion; water loss.

  3. Phloem sap in the plant vascular system flows:

    • Options: One way from leaves to sites of sugar use / One way from leaves to sites of sugar storage / Two ways from leaves and sugar storage units to sites of use / Two ways from the sites of use and sugar storage units back to leaves.

  4. Which of the following phenomena occurs naturally in plant cells?

    • Options: Water moves from higher water potential to lower water potential / Water moves from lower water potential to higher solvent potential / Solute moves from higher water potential to lower water potential / Solute moves from lower water potential to higher water potential.