Plant Tissue Systems

• Plants have two systems: the shoot system (above ground) and the root system (underground).
• These systems have tissues for gas exchange, material transport, and photosynthesis.

Tissue Types

• Dermal tissue forms boundaries, protects, interacts with the environment, and facilitates absorption like gas exchange, example being the epidermis.
• Ground tissue performs life functions: metabolism, photosynthesis, support, and energy storage.
• Vascular tissue transports materials.

Mesophyll Tissue

• Photosynthesis happens in mesophyll between leaf epidermal layers, containing palisade and spongy cells.
• Palisade mesophyll cells are tightly packed, rectangular, rigid cells with many cells exposed to sunlight just below the upper epidermis of leaves, optimized for photosynthesis.
• Spongy mesophyll cells are loosely packed, irregularly shaped and spaced out above the lower epidermis, facilitating gas exchange, taking in CO2 from stomata to palisade layer and transferring O2 out.

Meristematic Tissue

• This tissue contains undifferentiated cells dividing via mitosis.
• Apical meristems are at shoot and root tips, facilitating growth in length.
• Lateral meristems are along stems and roots, responsible for growth in diameter.

Micropropagation

• Plants constantly grow and regenerate from meristematic tissue.
• Micropropagation is an in vitro technique, producing many genetically identical plants.
• Meristematic tissue from a stock plant is cut into explants and grown in media with auxin and cytokinin.
• A ratio of auxin to cytokinin being equal results in a callus (undifferentiated mass).
• A 10:1 ratio of auxin to cytokinin results in roots, known as rooting media.
• A ratio of less than 10:1 auxin to cytokinin results in shoots, known as shooting media.
• Micropropagation allows sterile production of virus-free plants, is faster, requires less space, and helps preserve endangered species.
• Micropropagated plantlets can be stored indefinitely in liquid nitrogen (cryopreservation).

Dermal Tissue

• Shoot system dermal tissue is for gas exchange; root system dermal tissue absorbs water and minerals.
• The epidermis is the outer, one-cell-thick layer of herbaceous plants and is replaced by cork and bark in woody plants.
• Root hairs on the epidermis absorb water and minerals from the soil, increasing surface area for absorption.
• Guard cells are specialized epidermal cells that control stomata openings and closings for gas exchange.
• The cuticle is a waxy substance from leaves and stems, protecting against microorganisms and water loss.

Ground Tissue

• Ground tissue, between epidermis and vascular tissues, makes up most of the plant.
• Spaces in ground tissue facilitate gas exchange.
• Functions include strength, support, water/food storage in roots, and photosynthesis in leaves.
  • Parenchyma:
    • Basis of plant functions
    • Majority of cells in a plant.
    • Thin, flexible cell walls
    • Examples: Mesophyll, cortex and pith cells
  • Collenchyma:
    • Supports growing areas of a plant
    • Elongated, irregularly thickened cell walls
    • provides structural support
  • Schlerenchyma:
    • Hard, thick cell walls made of lignin and cellulose
    • Provides structural support
    • Dead at maturity due to thick walls preventing transfer; found in non-growing regions like bark.

Vascular Tissue

• Vascular tissue transports water, minerals, sugars, and sap.
• Xylem transports water and dissolved minerals.
• Phloem transports sugars and sap.
• Xylem and phloem can be organized together in a vascular bundle.

Xylem

• Xylem transports water and minerals upwards from roots to stems and leaves for photosynthesis.
• As xylem matures, cells lose protoplasts (nucleus and cytoplasm) and develop thick, lignin-strengthened walls to withstand low pressure, creating space for water flow.
• Xylem consists of non-living cells.
  • Tracheids:
    • Thin, elongated tubes without perforations
    • Pits connect tracheids for water and solute transport
  • Vessel elements:
    • Wider tubes with perforated ends
    • Major role in water and mineral transport and are building blocks of xylem, and are also connected by pits.
• Tracheids and vessel elements contain lignin in their cell walls for strength and support.

Phloem

• Phloem transports sugars or sap in both directions throughout the plant, originating in the leaves.
• Phloem includes sieve tube elements, companion cells (control conduction), and fibers and sclereids (support).
  • Sieve tube cells:
    • Lose nucleus and organelles to conduct sap
    • Retain smooth ER near the cell membrane and cytoplasm.
    • Connected with sieve plates with holes for passage
  • Companion cells:
    • Contain organelles and a nucleus that directs the activities of the sieve tubes.
• Sieve tube elements and companion cells are connected through plasmodesmata (cytoplasmic connections between cell walls) for transport like ATP.
• These are living cells.
  • Fibers and Sclereids:
    • Unspecialized cells that are heavily lignified for support.

Water Transport in the Xylem

• Water's polarity allows attraction to other water molecules (cohesion) and substances (adhesion).
• Water transport is done via the cohesion-tension theory.
  • Cohesion-Tension Theory:
    • Water molecules move together due to cohesion with adhesive properties allowing water to move up the xylem.
  • Transpiration Pull:
    • Transpiration maintains low water concentration at the top, "pulling" water up by osmotic forces.
    • Negative pressure (tension) is created within the xylem, resulting in water potential.
    • The low pressure in the xylem compared to atmospheric pressure outside, makes the water move against gravity.
• Cohesion prevents cavitation of water (breaking of a liquid column).
• Xylem's lignified walls prevent collapse due to negative pressure, creating a transpiration space, linking water flow through vascular bundles from roots to stems and leaves.

Stomata

• Opening and closing of stomata is important for transpiration.
• Each stoma is flanked by two guard cells with unevenly shaped cell walls and more cellulose on the side adjacent to the stoma.
• In light, potassium ions are pumped into guard cells, drawing in water by osmosis, making the guard cells turgid and causing them to take on a "sausage" shape.
• At night, potassium ions exit causing the guard cells to become flaccid and close the stomata.

External Stimuli

• These influence stomata opening and closing:
  • Light causes stomata to open.
  • Low carbon dioxide levels cause stomata to open.
  • Wind usually makes the stomata close unless humidity is increased.
  • Decreased humidity generally causes the closing of the stomata.
  • High temperatures may close stomata under water stress or open to encourage transpiration when there is low water stress.
  • ABA stops other external stimuli by allowing guard cells to leak potassium ions; this prevents photosynthesis.

Counteraction

• Transpiration results in negative tension in the xylem sap (water with nutrients and hormones) which draws this sap up the xylem. This tension is strong enough to draw water out of root cells and into the xylem (despite the solute potential of root cells).
• If transpiration stops, positive pressure is created in the sap.
• To counteract this, cells next to the xylem move material into the sap, generating root pressure.

Sugar Transport in the Phloem

• Translocation occurs when sugars from photosynthetic sources travel up and down the phloem to sinks that need (actively growing / metabolizing parts like meristematic tissues or specialized structures for storage like fruits and tubers).
• Phloem loading/unloading moves sugars via the pressure flow hypothesis. The reason sieve tube cells must be living cells because an active membrane and active transport are required to maintain the concentration gradient. Because they require energy, the cell has to be alive to produce that. The rigid cell walls allow the establishment of pressure and perforated walls of sieve plates allow the flow of sap through the phloem through the sieve tubes. Furthermore, the plasmodesmata of sieve tubes and companion cells are larger than plasmodesmata in other parts of the plant.

Pressure Flow Hypothesis

• Sugars are actively transported into sieve tube elements from photosynthetic sources
• As water moves up the xylem, some water is drawn into the phloem by the high solute concentration resulting from the active transport of sugars to the phloem.
• The water and solutes create a higher osmotic potential within the phloem at the sources, which drives movement to sinks which have lower osmotic potentials (passive transport).
• Active transport of the sugars to the sink causes a high concentration of water in the phloem at the sink. Water then moves by osmosis from the phloem back to the xylem.

Xylem Movement Vs Phloem Movement

• Xylem: Negative tension (due to transpiration); Water potential difference within xylem; Passive transport; Unidirectional (upwards); Sap is water, dissolved minerals, and ions
• Phloem: Positive pressure (hydrostatic); Water potential difference between xylem and phloem; Active and passive transport; Bidirectional; Sap consists of sugars, amino acids, and hormones

Plant Growth

• Plant growth occurs in response to stimulus like light and gravity.
  • Phototropism
    • Response to light
    • Positive phototropism- growth towards the light (eg. stem)
    • Negative phototropism- growth away from the light (eg. root)
  • Gravitropism
    • Positive gravitropism- growth towards gravity (eg. root)
    • Negative gravitropism- growth away from gravity eg. (stem)
• Growth is initiated by the chemical hormone auxin (indole -3- acetic acid or IAA)

Importance of Plant Hormones

• Phytohormones are responsible for changes in plants.
• Gibberellin promotes stem growth.
• Ethylene promotes fruit ripening.
• Jasmonic acid triggers digestive enzyme release.

Auxin

• Auxin is a plant hormone required for growth. It promotes stem cell elongation by loosening cell wall connections. It is synthesized in stem tips and transported through the stem to stimulate growth.
• Light, detected by phototropins, stimulates auxin movement to the shadier side, resulting in higher concentration of auxin at the shadier side causing the stem to curve towards the light.
• Auxin promotes cell division at apical tips and, with growth factors, causes cell differentiation, acting as a transcriptional regulator.
• Regulation of auxin is involved with auxin being produced only in certain cells in certain conditions of the plan and the translocation of auxin, resulting in an uneven distribution of auxin (usually from peak of shoots to peak of roots).
• Detection of different levels of auxin on various areas of the plant allows the plant to respond to its environment and mediate various tropisms.
• Auxin is produced in the shoot tips and is transported down the shoots into the roots through the phloem, and cytokinin is produced in the root tips and transported up the roots to the shoots in the xylem. In some instances, both can work together to stimulate a process (ie. have a synergistic effect) while in others, they have opposing effects (antagonistic effects).

Auxin Vs Cytokinin

• Cell Division in the apices (tips) of stems and roots: Auxin- Stimulated if cytokinin is present. Cytokinin- stimulated, Interaction- synergistic

• Cell enlargement in the apices of stems and roots: Auxin- stimulated, Cytokinin- Stimulated if auxin is present, Interaction- synergistic

• Development of branches and roots or new roots: Auxin- stimulated, Cytokinin- inhibited, Interaction- antagonistic

• Development of lateral buds into branches of the stem: Auxin- inhibited, Cytokinin- stimulated, Interaction- antagonistic

• Apical dominance occurs where auxin inhibits axillary bud growth. The further the node from the apical meristem, the lower the auxin, and less inhibition. *If shoot tips are removed, less root growth will occur (due to less auxin) and growth of lateral buds (which is promoted by cytokinin) will no longer be inhibited allowing the growth of new buds to replace the main shoot.