Plant Adaptations

• Adaptations aid in the acquisition of resources, including water, minerals, carbon dioxide, and light

• Early nonvascular (no xylem & phloem) land plants lived in shallow water and had aerial shoots

  • Nothing could live on land until plants colonized it

  • Xylem: transports water and minerals from roots to shoots

  • Phloem: transports photosynthetic products from where they are made to where they are needed

Shoot Architecture and Light Capture

• Trade-off between growing tall & branching

  • More energy invested in branching→ less energy available for height growth

• Water availability signals leaf growth

• Phyllotaxy: arrangement of leaves on a stem; a species-specific trait important for light capture → more leaves

  • Angle between leaves is 137.5 degrees

    • Minimizes shading of lower leaves

  • If there are too many top leaves, will shade bottom leaves

    • Competition among plants

• Self-pruning: shedding of lower shaded leaves

  • Occurs when they respire more than they photosynthesize

    • Costs more than it benefits

  • Community: multiple different species living in the same area & interacting

• Leaf-area index: ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows

  • 7→ shading so self-pruning occurs

  • less leaves→ smaller leaf area index

Root Architecture and Acquisition of Water & Minerals

• Roots are less competitive with other roots from the same plant than with roots from different plants

  • Nitrates necessary for growth

• Mycorrhizae: mutualistic associations formed between roots and hyphae of soil fungi (roots/extensions of soil fungi)

  • ex/ increase surface area for absorbing water and minerals, especially phosphate (necessary for making DNA & phospholipids)

Transport

• 2 major transport pathways

  • Apoplast: everything external to the plasma membrane

    • Includes cell walls, extracellular spaces, and interior of dead cells like vessel elements and tracheids (water-conducting cell in xylem)

  • Symplast: consist of cytosol of all living cells & plasmodesmata

    • everything inside plasma membrane

• 

• Apoplastic route: through cell walls and extracellular spaces

• Symplastic route: where water and solutes cross a plasma membrane once and then travel through cytosol

• Transmembrane route: water and solutes repeatedly cross membranes when they pass from cell to cell

  • Can also occur in cells with the same plasmodesmata (some nutrients may not want to travel through the cytosol)

• Active transport: need ATP

• Proton pumps establish membrane potential by pumping H+ and establishing a proton gradient (plants)

• Membrane potential is established by pumping Na+ by sodium-potassium pumps (animals)

• Plant cell membranes have ion channels that only allow certain ions to pass

• Osmosis: the diffusion of water into our out of a cell that is affected by solute concentration and pressure

• Water potential: quantity that includes effects of solute concentration and physical pressure

  • Determines direction of movement of water

  • Flows from regions of higher water potential to regions of lower water potential

  • Potential→ refers to water’s capacity to perform work

  • Unit of pressure is a megapascal

• Diffusion: high to low concentration

• Solute potential: osmotic potential; directly proportional to molarity

• Pressure potential: physical pressure on a solution (can be positive or negative)

• Turgor pressure: positive pressure exerted by plasma membrane against cell wall & cell wall against protoplast

  • Protoplast: living part of cell, includes plasma membrane

  • Turgor loss results in wilting, which can be reversed by watering the plant

• Flaccid: limp/deflated cell

  • If flaccid cell placed in environment with higher solute concentration, cell will lose water and undergo plasmolysis

    • Plasmolysis: protoplast shrinks and pulls away from cell wall

  • If flaccid cell placed in solution with lower solute concentration, cell will gain water and become turgid (plump, lots of water)

• Aquaporins: transport proteins in cell membrane that facilitate passage of water

  • Opening & closing of aquaporins affect rate of osmotic water movement across membrane

  • Hydrophilic head & hydrophobic tail

• Bulk-flow: the movement of a fluid driven by a pressure gradient (long-distance transport)

  • Water and solutes move through tracheids and vessel elements (in xylem, tube-like components) of xylem & sieve-tube elements (tube-like element of phloem) of phloem

  • Enhanced by structural adaptations of xylem and phloem

    • Mature tracheids and vessel elements have no cytoplasm; inside of cells empty to move water and minerals (dead cells)

    • Sieve-tube elements have few organelles in their cytoplasm (alive)

  • Perforation plates connect vessel elements & porous sieve plates connect sieve-tube elements

Absorption of Water & Minerals by Root Cells

• Transpiration: bulk flow up, using xylem; evaporation of water from a plant’s surface

• Water and mineral absorption occurs near root tips, where there are root hairs and the epidermis is permeable to water

Transport of Water and Minerals into the Xylem

• Endodermis: innermost layer of cells in the root cortex (external part of root)

  • surrounds vascular cylinder & is last checkpoint for selective passage of minerals from cortex to vascular tissue

• Casparian strip: barrier at checkpoint of the endodermal wall, blocking apoplastic transfer of minerals from the cortex to the vascular cylinder

• Endodermal cells discharge water and minerals from their protoplasts into their own cell walls

• Xylem Sap: fluid in xylem containing water and dissolved minerals

  • transported from roots to leaves by bulk flow

Pushing Xylem Sap: Root Pressure

• Water flows in from root cortex, generating root pressure (push of xylem sap)

  • Sometimes results in guttation

    • Guttation: exudation of water droplets on tips or edges of leaves

• Positive root pressure is weak and is a minor mechanism of xylem bulk flow

Pulling Xylem Sap: The Cohesion-Tension Hypothesis

• Cohesion-tension hypothesis: transpiration and water cohesion pull water from shoots to roots

  • Cohesion: water sticks to water

  • Adhesion: water sticked to other surfaces

    • prevents water from falling back down

  • Surface tension: water won’t spill over & bugs don’t sink in water

• Xylem sap is normally under negative pressure/tension

• Transpirational pull: positive + negative pressure

  • Water vapor in the air spaces of leaf diffuse down water potential gradient and exits via stomata

    • Air-water interface retreats into mesophyll cell walls

    • Creates negative pressure potential

  • Negative pressure potential lowers water potential

    • Water molecules pulled from more hydrated areas of lead by negative pressure potential created by air-water interface

  • Positive pressure → pushes up

  • Negative pressure → pulls up

• Drought stress or freezing can cause a break in chain of water molecules through cavitation

  • Cavitation: formation of water vapor pocket; break in hydrogen bond

Xylem Sap Ascent by Bulk Flow

• How does bulk flow differ from diffusion?

  • Driven by differences in pressure potential rather than solute potential

  • Occurs in hollow dead cells rather than membranes of living ells

  • Moves the entire solution instead of just water or solutes

  • Much faster

Mechanisms of Stomatal Opening and Closing

• Changes in turgor pressure open & close stomata

  • Turgid→ guard cells bow outward and pore opens (expand & leave gap)

  • Flaccid→ guard cells become less bowed and pore closes (deflate & cover surface)

• Generally open during day and close at night to minimize water loss

  • Opening triggered by light, CO2 depletion, and internal clock in guard cells

    • Circadian rhythms: 24 hr cycles; genes that control hormone release are exact

      • ex/ most heart attacks if at risk occur at 10am

  • Hormone Abscisic Acid (ABA) produced in response to water deficiency; causes closure of stomata

• Sunny, warm, dry, and windy conditions cause evaporation & increase transpiration

  • More water molecules lost to atmosphere

    • Water sometimes leaks through cuticle

  • If uptake & transport can’t replace lost water, plant wilts

• Transpiration→ evaporative cooling (lower temp of leaf)

  • Trade-off with losing water

  • Water has a high specific heat

Adaptations that Reduce Evaporative Water Loss

• Xerophytes: plants adapted to dry conditions and climates

  • Crassulacean acid metabolism (CAM): stomatal gas exchange occurs at night

    • Reverse reaction

    • Change CO2 into a 4-carbon molecule and store it

Movement from Sugar Sources to Sugar Sinks

• Translocation: opposite of transpiration; goes down using phloem

• Phloem sap: aqueous solution high in sucrose

  • Travels from sugar source to sugar sink

  • Phloem made of sieve-tube elements

  • Moves through bulk flow driven by positive pressure called pressure flow

• Sugar source: organ that is a net producer of sugar (ex/ mature leaves)

  • Pressure is high

• Sugar sink: organ that is a net consumer or depository of sugar (ex/ roots, buds, fruits)

  • Sugar must be loaded into sieve-tube elements before being exported to sinks

  • Pressure is low

• Companion cells: enhance solute movement between apoplast and symplast

• Self-thinning: dropping of sugar sinks (ex/ flowers, seeds, fruits)

  • Occurs when there are more sugar sinks than sources can support

  • Usually don’t want to get rid of flowers because they attract pollinators

Misc Info

• Plasmodesmata (plant can have multiple) open and close in response to turgor pressure, cytosolic Ca2+ levels or cytosilic pH

  • plant viruses can cause it to dilate, allowing viral RNA to pass between cells

    • plants try to stop spread of virus by closing plasmodesmata

• Phloem is key for transport of macromolecules and viruses

  • Systemic communication through phloem allows the integration of plant functions

  • Electricity: movement of any charge (ion)

    • Triggers cells to act/behave differently