2.5: Plant Nutrition and Transport

Nutrition & Transport

• Plants exchange energy and materials within their environment.
• Water and minerals are obtained from the soil, while carbon dioxide from the air.

Macronutrients and Micronutrients

• More than 50 chemical elements have been identified in plants, although not all are essential.
• An element is deemed essential to a plant if it is needed to complete its life cycle and carry out normal metabolic functions.
• Macronutrients include: H, C, O, N, K, Ca, Mg, P, S, and Si.
• Micronutrients include: Cl, Fe, B, Mn, Na, Zn, Cu, Ni, and Mo.

Pathways for Survival

• Vascular plants evolved differentiation into roots and shoots.
• Vascular tissue enables the transport of water and nutrients.
• Xylem transports water and inorganic ions.
• Phloem transports organic substances in aqueous solutions.

Plant Transport

• Transport in vascular plants happens at three scales:
• Water and solute transport by individual cells, such as in root hairs.
• Short-distance transport from cell to cell within tissues and organs.
• Long-distance transport in xylem and phloem throughout the plant.

Three Major Compartments of Vacuolated Plant Cells

• Transport of molecules is controlled by compartmental structures.
• The plasma membrane regulates molecule traffic in and out of the protoplast.
• The plasma membrane separates to major compartments called the cell wall and the cytosol.
• A vacuole is a large organelle that occupies up to 90% or more of a mature plants cell's volume.
• The vacuolar membrane, known as the tonoplast, regulates transport between the cytosol and the vacuole.

Symplast and Apoplast Functions

• Cell walls and cytosols are continuous between cells in most plant tissues.
• The cytoplasmic continuum is known as the symplast.
• The continuum of cell walls and extracellular spaces is known as the apoplast.
• Water and minerals traveling through a plant can take three routes:
  • Transmembrane route: substance exits one cell, crosses the cell wall, and enters another cell.
  • Symplastic route: substance travels through the continuum of cytosols.
  • Apoplastic route: substance moves through cell walls and extracellular spaces.

Mass Flow in Long-Distance Transport

• Fluid movement in the xylem and phloem results from pressure differences at either end of xylem vessels and sieve tubes.

Water & Nutrient Ion Transport

• Roots absorb water and minerals from the soil.
• Water and mineral salts enter the plant via the epidermis of roots, flowing to the shoot system.
• Water and minerals are absorbed mainly near root tips, where the epidermis is water-permeable and root hairs are present.

Root Hairs and Cortical Cells

• Root hairs increase the surface area of roots.
• The surface area of cortical cell membranes enables water uptake and selection of minerals.

Root Endodermis

• Water is able to cross the cortex via symplast or apoplast pathways.
• Endodermis is the root cortex's innermost layer of cells.
• It surrounds the vascular cylinder and is the last checkpoint for mineral entry into the vascular tissue.
• The endodermal wall has a waxy Casparian strip (with suberin) that blocks apoplastic transfer into the vascular cylinder.
• Passage cells, located in the endodermis, facilitate ion transport across the plasma membrane.
• Lateral roots develop from the meristematic pericycle layer inside the endodermis, aiding in ion transport.

Water Ascent Through Xylem

• Water is conducted up from root xylem to stem xylem, and finally to leaf xylem.
• Plants lose a lot of water through transpiration.
• Transpired water needs replenishing, thus water is transported up from the roots.

Forces Driving Ascent of Xylem Sap

• Xylem sap can rise to heights of over 100 m in tall plant species.
• Xylem sap rises depending on:
  • If the sap is being pushed up from the roots
  • If the sap is being pulled up from the leaves

Root Pressure and its Effects

• At night, root cells pump mineral ions into the xylem of the vascular cylinder.
• Water flows in from the root cortex, creating root pressure.
• Root pressure is a consequence of the osmotic pressure of xylem sap (minerals and water) that enters the stele.
• Guttation, or water droplet exudation on grass blades or eudicot leaf margins (via hydathodes), can result from root pressure.

The Transpiration-Cohesion Tension Mechanism

• Water is pulled up by negative pressure present in xylem (transpiration pull).
• Water vapor in leaf tissues diffuses into airspaces and exits via stomata during transpiration.
• Transpiration results in tension in the leaf. This exerts pulling force on water inside the xylem, drawing water from the root into the leaf.

Cohesion & Adhesion

• Transpirational pull on xylem sap reaches from the leaves down to the root tips, and even into the soil solution.
• Transpirational pull depends on water properties called cohesion and adhesion.

Stomata

• Stomata regulate transpiration rates.
• Broad leaf surfaces and high surface-to-volume ratios increase photosynthesis but can also increase water loss through stomata (called transpiration).
• Evaporative cooling is a result of transpiration. It lowers leaf temperature and prevents enzyme denaturation during photosynthesis and metabolism.

Factors that Affect Transpiration

• Relative humidity
• Temperature
• Wind Velocity
• Light Intensity
• Soil Water Availability

Xerophyte Adaptations That Reduce Transpiration

• Xerophytes are plants suited for arid climates.
• The leaf modifications reduce transpiration rates.
• Stomata are often located on the lower leaf surface in depressions, which offers shelter from dry winds.

Transport of Sugars

• Organic nutrients (like sugars) travel through the phloem.

Movement from Sugar Sources to Sugar Sinks

• Phloem sap consists of an aqueous solution that contains mostly sucrose.
• It moves from a sugar source to a sugar sink.
• Sugar source is an organ producing more sugar than it needs, either through sugar production or starch breakdown.
• Sugar sink is an organ that consumes more sugar than it produces via depository or storing.
• Phloem sap is transported in both directions between a plants shoots and roots.
  • Photosynthetic leaves typically function as sources.
  • Growing roots, buds, stems, flowers, and fruits are typically sugar sinks.
    • Tubers or bulbs can be sources or sinks based on development stage.
    • Expanding leaves also function as sugar sinks.

Phloem Loading

• Sugar must be loaded into the sieve-tube members before reaching the sinks (called phloem loading).
• In plants, sugar travels using symplastic and apoplastic pathways.
• Phloem loading often needs active transport in plants for sieve tube entry.
• Proton pumping and cotransport via H+ enable cells to build up sucrose.

Pressure Flow

• The sap moves through sieve tubes through bulk flow driven by positive pressure, known as pressure flow in angiosperms..
• Sugar loading into the sieve tube, at the source, reduces the water potential within the sieve tube elements; osmosis then causes the tube to take up water.
• Water absorption causes positive pressure, which forces sap to flow.
• Pressure subsides due to sugar unloading and subsequent water loss at the sink.
• During leaf-to-root translocation, the xylem moves water from the sink back to the source.
• The pressure flow hypothesis describes why phloem sap moves from source to sink.
• Experiments have affirmed translocation in angiosperms occurs due to pressure flow.