UTA Plant Science Exam 4

he is green for an amazing reason

adhesion

molecule attract to other molecules of DIFFERENT kind

cohesion

molecule attract to other molecules of the SAME kind

bulk flow (mass flow)

• all molecules move together

• based on differences in potential energy

  • lots of water = high potential

  • very little water = low potential

diffusion

random movement of solutes from high to low concentration

• active transport: move against gradient, need ATP

which way will water move in regards to solute diffusion?

high water potential => low water potential

• water will move OPPOSITE direction of solute diffusion

hypotonic

low solute concentration

hypertonic

high solute concentration

osmosis

water moves from:

1. high to low water potential

2. low to high solute concentration

3. high to low osmotic potential

these things are DISTINCT

explain turgor pressure

hydrostatic pressure in a cell

• plant has high concentration of soln. in vacuoles

• this creates hypertonic soln.

• water moves into vacuole, vacuole pushes against cell wall

wall pressure

force of cell wall opposing hydrostatic pressure

• cell walls strong enough to resist breaking by water absorption

• growing cells with malleable walls enlarge rather than bursting 

plasmolysis 

• water leaves vacuole

• protoplast pulls away from cell wall, shrinks, cell becomes flaccid

• if FULLY pulled away, cell becomes [plasmolyzed]

  • cell death!

transpiration

the loss of water vapor from plants

• occurs from the shoot of the plant

• leaves are main culprit 

• plants can lose up to 99% of their water through this

stomatal transpiration

water vapor (blue arrows) diffuses from the leaf to the atmosphere through stomata

1. [Evaporation] of water from cell wall surfaces bordering the extracellular space

2. [Diffusion] of resulting water vapor into the atmosphere via the stomata 

how does a stomata open and close

• high turgor pressure [turgid]: open

• low turgor pressure [flaccid]: closed

radial micellation

radial orientation of cellulose microfibrils in the guard cell walls is required for Pore Opening

• prevents lateral expansion of guard cells

• promotes longitudinal expansion

guard cells are attached at their ends to each other 

effect of temperature on transpiration 

rate of water evaporation doubles for every 10^o increase in temperature 

effect of humidity on transpiration 

high humidity lessens the concentration gradient of water between leaf and environment

• leaves in humid environments are big, have no fear of losing water!

effect of air currents on transpiration

lowers the local humidity (at the leaf’s surface) 

how does water travel through a plant?

water is pulled to the tops of trees

explain the cohesion-tension theory

water is “pulled” up the plant through a series of water potential changes across cells

• water is lost through transpiration

• lost water is replaced from within the cell

• conc. of solutes is now higher, creates gradient potential between this cell and neighbors 

chain reaction…water is cohesive AND adhesive!

bubbles can break the continuity of water in xylem

[surface tension] in pit pairs prevents embolism from spreading

• in conifers the [torus] prevents embolism from spreading!

the limit of tree height

• tensile strength of water has a breaking point

• water stress on leaves due to gravity and increasing path-length resistance leads to poor photosynthesis 

  • the maximum tension is close to the point of embolism!

how does water enter root hairs

enters directly through:

• epidermis => cortex => endodermis => vascular cylinder 

apoplastic

around protoplast, does NOT cross plasma membrane

• cell wall

symplastic 

via plasmodesmata from protoplast to protoplast

• connected! through plasma membrane, not through vacuole yet

transcellular

goes from cell to cell, across plasma membranes and tonoplast (vacuole)

• pass any membrane 

root pressure

in absence of transpiration , roots generate positive pressure

• driving force in water uptake in roots is difference in water potentials in soil and xylem

• can be enhanced by secretion of ions/solutes INTO the xylem

  • endodermis prevents movement of ions out of the xylem (bc of Casparian strip)

root pressure drives water into xylem!

translocation (assimilate transport)

movement through the phloem

• “source-to-sink” movement

[translocation]: source

photosynthetic/storage tissue

[translocation]: sink

tissue that cannot meet their own nutritional needs

[translocation]: developing fruit

sinks that monopolize assimilates

how is phloem transport driven?

driven by osmotically generated pressure flow

pressure-flow hypothesis 

assimilates are transported from source to sink along a gradient of turgor pressure developed osmotically 

pressure-flow hypothesis mechanism

• sugars move into sieve tubes increasing conc. gradient (Phloem Loading)

• water (from xylem) enters sieve tubes via osmosis which increases turgor pressure 

• carried passively to a “sink”

• sugar is removed from sieve tube

• water moves back to xylem

transpiration stream

water and inorganic ions taken up by root travels through this 

• move laterally into tissues

• carried into growing plant parts

• transferred to phloem in leaves, and along with sucrose, carried in [assimilate stream]

assimilate stream

movement of substances

do growing plants get more water and ions from assimilate streams or from transpiration streams?

growing plant parts get more water and ions from assimilate stream than from transpiration stream

who discovered auxins?

charles and francis darwin in 1881

explain auxin collar experiment

• placed collar on plant tip

• plant with collar on tip did not show phototropism

• plant with collar placed somewhere that WASN’T the tip bent towards light 

concluded that an “influence” is transmitted from tip to region of bending

where are auxins produced?

• root and shoot meristems (initially)

• leaf primordia

• young leaves

• developing fruits and seeds

• anywhere tissue develops

what type of transport do auxins do?

both non-polar and polar transport

how does non-polar transport work in auxins?

• goes in both directions

• is carried in phloem

• based on sink/source movement

how does polar transport work in auxins?

• goes in ONE direction only

• carried in vascular parenchyma

• movement is independent of phloem transport based on changing sinks/sources

which plant hormone can move polarly? (only one!)

auxins are the only plant hormones that can move polarly 

basipetal transport

away from the apex

acropetal transport

towards the apex

what does auxin developed in young leaf primordia do?

• induces formation of the midvein

• induces basipetal (tip to base) development of smaller minor veins

auxins in role in wound repair 

• development delayed if buds and leaves above damage are removed 

auxin and apical dominance 

• basipetal flow of auxin from apical merstem inhibits lateral bud development

• doesn’t act directly on bud, uses a secondary messenger 

auxins role in development of lateral and adventitious roots 

• auxin from shoot travels nonpolarly through phloem

• auxin from root travels polarly through parenchyma

• polar auxin primes the lateral root founder cells in the pericycle

• nonpolar auxin transportation triggers these cells to turn into lateral root

auxins role in fruit development

developing embryo and seed are a source of auxin

• remove seed => prevent fruit from forming

parthenocarpic fruit 

fruit made without fertilization, can be formed when carpal is treated with auxin

• how we get seedless fruits 

synthetic auxins

• most mechanisms not known

• some block photosynthetic electron flow

• some not broken down naturally, last longer and contribute to lethality 

cytokinins chemical nature:

• breakdown product of DNA, resembles Adenine!

• [Zeatin]: most active naturally

• [Kinetin]: first discovered synthetic form

where are cytokinins synthesized?

in the root tips 

describe cytokinin transport

through the xylem to:

• actively dividing tissues

• seeds, fruit, leaves, root tips

what are cytokinins’ main role?

regulates root and shoot production

cytokinins role in leaf senescence

delays leaf senescence 

• if leaves are treated with kinetin => remain green

• lots of cytokinin in leaves, but NOT synthesized in leaf organs

cytokinin:auxin ratio

• high auxin: root growth

• high kinetin: bud growth

• both equal amounts of auxin + kinetin = undifferentiated cells (meristem maintenance)

callus tissue

growth of undifferentiated plant cells

where was cytokinin discovered?

discovered in coconut milk 

• endosperm, contained growth stimulator

in what crop was the cytokinin zeatin isolated?

Zea mays (corn)

chemical nature of ethylene

simple hydrocarbon

• C₂H₄

where is ethylene synthesized?

• in all organs of higher plants

• more in young developing leaves than fully expanded leaves 

• high concentration in ripe fruits

ethylene’s main role is?

inhibitory effect on cell expansion

Triple Response

1. decrease in longitudinal growth

2. increase in radial expansion of epicotyl and roots

3. horizontal orientation of epicotyls

this allows for seedlings to overcome obstacles!!!!

ethylene promotional effect on stem growth

• allows aquatic plants to keep pace with flood waters (think rice crop)

• increase in aerenchyma in submerged tissues

ethylene role in ripening fruit

• [climacteric phase] of fruit development: increase in cellular respiration during ripening

• triggered by increase in ethylene synthesis

• agricultural importance for timing the ripening of fruit

  • cut fruit when its green, so that its nice and ripe in the store

ethylene role in promoting abscission

triggers enzymes that cause cell wall dissolution

used to loosen cherries, blackberries, and grapes from their branches

ethylene role in sex expression

• in Cucurbitaceae, stimulates female flowers in monoecious plants

• [gibberellins] stimulate male flowers

how was ethylene discovered?

1800’s street lamps defoliated trees, caused by gas leaks with ethylene

1901 shown to influence most/all aspects of growth and development in pea seedlings including:

• growth of most tissue

• fruit maturation

• fruit and leaf abscission

• senescence

abscisic acid chemical nature

isoprene unit

where is abscisic acid synthesized?

• from carotenoid intermediate

• in almost all cells containing chloroplasts or amyloplasts

• found in every tissue and organ including seeds

abscisic acid transport 

• through phloem from leaves

• through xylem from roots

what is abscisic acid’s main role?

primary function is to limit growth/reproduction in response to stress (water)

abscisic acid role in preventing seed germination

• high levels = production of storage proteins in seeds

• prevents premature germination

• breaking seed dormancy corresponds to a decline in ABA concentrations 

vivaparious mutants 

mutant embryos that cannot become dormant 

• corn on the cob

• either reduced sensitivity to ABA or inability to make hormone

abscisic acid role in root-to-shoot signaling

• prevents water loss in water stress conditions 

• roots respond by increasing biosynthesis of abscisic acid by closing to reduced transpiration 

• can also prevent pathogen entry via stomata 

how was abscisic acid discovered?

1949 discovered as dormin, a growth inhibitor in ash buds and potatoes

• later in 1960 discovered as abscisin, as it appeared to accelerate abscission in leaves and fruits

however!

1969 Cracker and Abeles, 2013 Evert and Eichorn 2013 found that it had NO DIRECT ROLE IN ABSCISSION, but instead increases ethylene production

chemical nature of gibberellins

• gibberellic acid

• four carbon rings + COOH groups

synthesis of gibberellins occurs where?

synthesized in roots (debated)

transport of gibberellins

transported in xylem and phloem

gibberellins role in stimulating cell division/elongation

• seen when applied to dwarf plants

• suggests gibberellin closely associated with Growth

• [gibberellin-deficient]: genes that regulate biosynthesis of GA is affected

• [response affected]: GA is present, but plants don’t response to it

GA = Gibberellin

gibberellins role in breaking seed dormancy

• promotes growth of seed dormancy in absence of special environmental conditions 

• enhances cell elongation, allowing roots to penetrate seed coat

• used for uniform germination in barley malt production

• also stimulate seeds to synthesize hydrolytic enzymes that can break down stored food into absorbable molecules 

gibberellins role in bolting (fast growth of plant)

• stem elongation followed by flowering 

• triggered by exposure to long days, cold, or both

• cell division and elongation

• can be used for early seed production

gibberellins role in fruit development

• can cause parthenocarpic fruits

• commercial application causes larger grapes in looser clusters in a cultivar of seedless grapes

foolish seedling disease (rice plants)

• causes plant to grow rapidly, fall over, and look spindly, pale-colored, and sickly

how were gibberellins discovered?

1934 Yabuta and Sumiki

• studied diseased rice plants and identified gibberellin (GA) produced from a fungus Gibberella

1956 MacMilan isolated it from a bean plants

• since then, believed to be in nearly all plants

• more than 136 naturally occurring

• most plants contain >10

tropism

the bending or curvature of a plant part towards or away from a stimulus

positive tropism

towards a stimulus

negative tropism

away from a stimulus

phototropism

growth in response to a directional light source

phototropism experiment 1

• removed coleoptile tips from oat seedlings, and placed them on agar

• place that agar on one side of decapitated shoot

• observed cell elongation on side with agar

concluded:

1. “influence” caused by auxin

2. chemical accumulated on the side OPPOSITE to the light source 

H₁

light destroys auxin

H₂

auxin migrates

phototropism experiment 2

coleoptile tips on agar 

conditions:

• dark/light

• divided coleoptile tip Dark/Light

• divided/partially-divided coleoptile tip AND divided agar

what was concluded from phototropism experiment 2?

• auxin moves unilaterally at the tip

• then moves basipetally to the elongation zone

• mediated by photoreceptors

  • a pigment-containing protein that absorbslight and converts it into a biochemical response 

gravitropism

like phototropism, is also caused by redistribution of auxin

• auxin affects roots and shoots differently

roots have what kind of gravitropism?

positive gravitropism

shoots have what kind of gravitropism?

negative gravitropism