Plant Transport Systems

Plant Transport Systems

Introduction

Overview of the lecture, which includes plant transport and plant hormones. Some students didn't finish quiz two, and are advised to email regarding this issue.

Recap of Plant Biology

Photosynthesis
  • Photosynthesis converts carbon dioxide and sunlight into sugars within the chloroplasts of mesophyll cells or any green tissue.
  • Oxygen is produced as a waste product.
  • Light-dependent reactions: Chlorophyll captures light and turns it into chemical energy in the form of ATP and NADPH.
  • Light-independent reactions: NADPH and ATP are used in the Calvin cycle to capture carbon dioxide and turn it into sugars.
  • Rubisco is an enzyme involved, but it's not very efficient.
  • C4 photosynthesis is a modification to overcome Rubisco's inefficiency and reduce photorespiration.
Plant Organs
  • Roots: Absorb water and nutrients with a high surface area, aided by root hairs.
  • Stems: Provide support and connect roots and leaves, containing xylem and phloem.
  • Leaves: Conduct photosynthesis and transpiration with a high surface area; pores enable gas exchange (carbon dioxide in, oxygen out).
Plant Reproduction
  • Plants exhibit alternation of generations.
  • Flowers are reproductive structures; double fertilization leads to a seed with a triploid endosperm.
  • Seeds:
    • Function in distribution, protection, and nourishment.
    • Germination with dormancy.
    • Epigeal versus hypogeal germination.

Plant Transport Systems Overview

  • What gets transported around the plant.
  • The structure and function of transport elements.
  • How transport systems work.
Key Transport Processes
  • Water Transport: Water moves from roots to shoots via xylem.
  • Sugar Transport: Phloem carries sugars from leaves to the rest of the plant, moving in various directions depending on the plant's needs. Phloem moves sugars from sources to sinks.
    • Sources: Produce sugars.
    • Sinks: Accumulate or use sugars.
Additional Transport
  • Water moving up xylem carries mineral nutrients (nitrogen, potassium, phosphorus).
  • Phloem carries hormones and amino acids.

Cross-Sectional Views

Cross-sections of stems, roots, and leaves reveal the arrangement of vascular tissues.

  • Stems: Xylem and phloem are visible; distribution indicates stem or root type (monocot vs. dicot).
  • Roots: Xylem in the center, phloem in surrounding arches.
  • Leaves: Vascular bundles (veins) contain xylem and phloem.
Vascular Bundles

Vascular bundles contain both xylem and phloem.

Structure and Function of Xylem

  • Function: Transports water and mineral nutrients from roots to shoots in one direction (upwards), even against gravity.
  • Volume: Handles large volumes of water under highly negative pressures.
  • Structure:
    • Large diameter to accommodate high water volumes.
    • Highly lignified (lignin strengthens cell walls, stains pink).
    • Provides strength and support (e.g., tree trunks).

Structure and Function of Phloem

  • Function: Transports photosynthates (sugars) from sources to sinks, in multiple directions.
  • Pressure/Volume: Operates at much lower pressures and volumes.
  • Structure:
    • Smaller diameters than xylem.
    • Not lignified.
Xylem Cell Types
  • Vessel elements and tracheids.
    • Wide bore, long, with pits for water movement.
    • Highly lignified, strong cell walls.
    • Dead at maturity to allow unobstructed water flow.
  • Vessel elements:
    • Wider, with perforations at the end for continuous tubes.
  • Tracheids:
    • Tapered ends that tessellate.
    • Gymnosperms (conifers) only have tracheids; angiosperms have both.
Phloem Cell Types
  • Sieve elements and companion cells.
    • Sieve plates at the end, with sieve pores.
    • Sieve elements are not quite dead but lack organelles.
  • Companion cells:
    • Support sieve elements by handling DNA transcription, translation, and storage.
    • Carry out phloem loading (actively loading photosynthates).
  • Sieve Element Companion Cell Complex (SECC complex): Tissues work together.
Tissues vs. Cells
  • Both phloem and xylem are tissues, composed of multiple cell types.

Xylem Transport: Understanding Water

  • Water molecules exhibit cohesion (sticking to each other) and adhesion (sticking to other things) due to hydrogen bonds.
  • This allows water to be sucked up a straw, with each molecule pulling up the one behind it.
Soil Plant Atmosphere Continuum (SPAC)
  • Water pathway from soil to atmosphere through the plant.
    • Soil → root → stem → leaf → stomata → atmosphere.
Transpiration
  • Water is pulled up, not pushed, due to transpiration (evaporation of water).
    • Evaporation from mesophyll cells in the leaf reduces humidity, creating negative pressure.
    • Water is pulled out of the xylem in the leaf, drawing water up the xylem from the roots.
Water Uptake at the Roots
  • Root hairs increase root surface area for water uptake.
  • Water moves from the outside of the root through the cortex, endodermis, and into the xylem.
Pathways for Water Movement
  • Symplastic Route: Water travels through cells via plasmodesmata.
    • Also carries mineral nutrients.
  • Apoplastic Route: Water travels through cell walls (porous) until it reaches the endodermis.
  • Transmembrane Route: Water crosses in and out of cells.
Casparian Strip
  • Border control for the xylem in the endodermis.
  • A band of suberin (hydrophobic polymer) in the radial cell walls.
  • Forces water to go through cells, allowing the endodermis to control what enters.
    • Prevents entry of harmful substances (fungi, heavy metals).
    • Prevents backflow of important ions.
Essential Nutrients
  • 16 essential nutrients are taken up from the soil with water.
  • Macronutrients: Needed in larger amounts (more than 1% dry matter).
    • Include NPK (nitrogen, phosphorus, potassium).
  • Micronutrients: Needed in smaller amounts.
  • Deficiency symptoms occur when plants don't get enough of these nutrients; toxicity symptoms occur with too much.
  • Drought reduces nutrient uptake.
Root Pressure
  • Pumping of water into the xylem, creating positive pressure (mostly at night when transpiration is low and soil moisture is high).
  • Causes guttation, where sap is pushed out at the tips of leaves (hydrophodes).

Phloem Transport

Sources and Sinks
  • Sources: Tissues that make photosynthate (sugars), such as mature leaves (net photosynthesizers).
  • Sinks: Tissues that use more sugar than they make, such as seeds, fruits, and roots.
  • Immature leaves are also sinks.
  • All tissues require sugars for respiration.
  • Tissues can change from being sources to sinks and vice versa over time.
Diffusion and Osmosis
  • Diffusion: Movement of molecules from high to low concentration.
  • Osmosis: Movement of water across a semipermeable membrane to equalize concentrations.
Munch Pressure Flow Hypothesis
  • Phloem moves sugars via a pressure gradient.
  • Sugars are actively loaded into the phloem at the source, increasing sugar concentration.
  • Water from the xylem follows by osmosis, increasing pressure in the phloem.
  • At the sink, sugars are actively used, drawing sugar out of the phloem, and water moves back out of the phloem, reducing pressure.
  • This creates a pressure gradient, moving phloem sap from high-pressure (source) to low-pressure (sink).
  • Sieve plates slow down the movement of sap to maintain pressure differential.
Carbohydrate Partitioning
  • Sugars compete for available sugars.
  • Source strength: How much net sugar something is producing.
  • Sink strength: How good something is at taking up sugar.
  • Plant growth is limited by source or sink at different developmental stages.
  • A study showed that wheat plants are sensitive to source manipulation during flowering and sensitive to sink manipulation after anthesis.