Plant Stress Exam 2

-where carbon is located in plants -expose to radioactive carbon dioxide in air then radioactive carbon enters plant and follow it over time - study translocation -if translocation is affected then wouldn't be moving

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Effects of Severe Stress -Translocation- the movement of metabolites —from sources to sinks— occurs in phloem tissue -carbohydrates need to be distributed throughout the plant Below -1.5MPa the translocation is disrupted —each process is affected differently photosynthesis problem could show up at a different MPa then translocation

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The effect of drought on dry weight and leaf area -soy bean plants —if suffering from drought then leads to decreased growth or completely stops growth

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Water may be lost from saturated soil by:

- as water is lost from the soil, soil pores fill with air -drainage -evaporation -absorption drainage - caused by gravity pulling water out of soil - some water retained against drainage by water adhering to charged surfaces of soil particles

The Ψ of leaf tissue is often used to indicate the degree of drought stress the plant is experiencing. At what Ψ is a plant considered to be experiencing mild drought stress? Moderate drought stress? Severe drought stress?

- degree of stress defined by leaf Ψ (which decreases as cells become dehydrated) Mild Stress: Ψleaf cell > - 0.5 MPa moderate stress: Ψleaf cell < - 0.5 MPa, > - 1.5 MPa severe stress: Ψleaf cell < - 1.5 MPa

What factor has the greatest effect on the water potential of water vapor? How does the water potential of air generally compare to the water potential of solutions?

- depends mostly on RELATIVE HUMIDITY of the air, and temperature -at 100% RH, Ψ = 0 MPa -at RH < 100%, Ψ < 0 MPa general equation: Ψ = - 1.06 T log10 (100/RH) -small changes in relative humidity can cause large changes in Ψ --water molecules are -less likely to move into a different area when there are solute molecules - associate with solute molecules so water molecules are less active

What is cuticular resistance? What factors cause variation in the extent of cuticular resistance?

- resistance to diffusion across the cuticle - varies with thickness of the cuticle -thicker the cuticle the more resistance

Water in Plants

- water makes up ~ 85 - 90 % of fresh weight of herbaceous plants (woody parts of plants weigh more, but contain less water) Functions of Water in Plants 1)as a solvent 2) as a reactant in many biochemical reactions 3) to provide turgor pressure in plant cells a) produces rigidity of cells/plant (prevents wilting) b) important in cell expansion & plant growth ** functions associated with turgor pressure are the most sensitive to drought stress

High Irradiance Stress

-- occurs if too much light is absorbed by antenna molecules, especially in photosystem II

What is hydraulic conductivity? How is the hydraulic conductivity of a soil influenced by soil texture? How is it influenced by the amount of water present in the soil?

--= ability of water to flow through soil --influences the ability of plant to absorb soil water - higher in soil with more large pores --larger pores and only the water particles close to soil particles are sticking to them the other water in the space will be free to flow - lower in drying soil-- even if you have large pores (interference by air-filled spaces)--theres more drainage because creates air pockets which act as a barrier

To what category of molecules do violaxanthin, antheraxanthin, and zeaxanthin belong? What happens to light energy absorbed by violaxanthin? What happens to light energy absorbed by antheraxanthin and zeaxanthan?

--Carotenes-- Carotenoids -at low light intensity, most xanthophyll is violaxanthin - which passes absorbed energy back to chlorophylls -at higher light intensity, violaxanthin is converted to antheraxanthin & zeaxanthin ---these do NOT pass energy to chlorophylls - they allow excess energy to be lost as harmless heat -takes too much energy to pass energy to chl b so just losses it as heat

What are guard cells? Explain, in general, the mechanism by which guard cells regulate the opening and closing of stomates. What environmental or internal factors influence guard cell behavior, and thus influence stomatal opening/closing?

--Each stomate is a pore (opening) surrounded by guard cells -guard cells are different shapes - some curved C Shaped—if curved a lot then bigger opening in stomate — some shaped like dumbbells—extent to which the ends of guard cells determines if open or closed —Stomatal aperture-opening between guard cells —size increases as turgor in guard cells increases (caused by changes in cell shapes as they swell). -changes in turgor are caused by water entry or exit -stomates open when water is available Water fills guard cells and they get stiff and keep it open --stomatal aperture = opening between guard cells - size increases as turgor in guard cells increases (caused by changes in cell shape as they swell) - changes in turgor are caused by water entry/exit --stomatal opening influenced by several factors, including -light -plant water content -all must influence diffusion of water into or out of guard cells - caused by changes in guard cell solute conc., ▪ when solute conc. of guard cells increases, stomate opens ▪ when solute conc. of guard cells decreases, stomate closes

What other group of enzymes is frequently involved in protecting plants from reactive oxygen species?

--Peroxidases

How do carotenes help to prevent damage in plants exposed to high irradiance light?

--Tolerance Mechanisms -protective carotenoids and the xanthophyll cycle -every photosystem includes chlorophyll molecules AND protective carotenoids -at high light intensity, a) some carotenoids (carotenes) will directly absorb some of the excess light energy (instead of antenna chlorophylls) b) some carotenoids (particular xanthophylls) absorb excess excitation energy from antenna chlorophylls --Protect Chlorophylls -they are there in the way so absorb some light and it doesn't make it to chlorophyll -nonphotochemical - pass excitation energy on (not electrons)

What is a saturated soil? What is the approximate Ψ of the soil water in a saturated soil? Explain why this is true in terms of the solute and pressure conditions present in a saturated soil.

--a soil with all of its pore space filled with water Ψ=0 MPa Pressure is 0MPa because equal to atmospheric pressure Solute concentrations are relatively zero because there

What is the function of superoxide dismutase? What is the function of the Asada-Halliwell Pathway (ascorbate-glutatione-redox chain), and how is this function accomplished?

--converts superoxide radical to H2O2 --Asada-Halliwell Pathway, or (ascorbate-glutathione-redox chain)--reduces H2O2 to water by a series of rxns --Electrons from photosystem 1 go into ascorbate-glutathione-redox chain

What effects (responses) are seen in plants experiencing mild drought stress?

--decreased cell expansion caused by decreased turgor in growing regions - can cause permanent decrease in size if stress continues * has major impact on productivity of crop plants,especially if effect is on fruits or grains - can decrease leaf size, size of plant's canopy - has secondary effect on photosynthesis

What is a reactive oxygen species (ROS)? What conditions lead to their formation? What is their effect on photosynthesis?

--highly reactive molecules which tend to oxidize other biological molecules usually damaging them --damages chlorophyll -they target the D1 cells --the absence of functional photosynthetic electron transport may cause excited electrons to interact with oxygen and form (ROS) -at high irradiance, light energy is often absorbed faster than it can be used in photosynthetic transport so it is likely that an excited electron could get passed to a molecule of O2 creating ROS -Superoxide O-2- participates in the production of these other ROS -Singlet Oxygen -Hydrogen Peroxide -Hydroxyl Radical --Decreases the rate of photosynthesis-- light intensity leads to ROS production which leads to D1 damage which limits the light reactions-- leads to photoinhibition

What mechanisms do plants use to avoid high irradiance damage?

--increased light reflection - common in plants that are often exposed to high irradiance (e.g., alpine plants) - can be accomplished by increased leaf pubescence; or changes in cuticle waxes-- (made up of cutin and some other waxes)- layers of wax can be laid out in different ways that will be more reflective --heliotropism--Elastic Response - change in leaf position to alter light absorption --chloroplast movements- Elastic Response --most chloroplasts are around the perimeter -low irradiance chloroplasts would be mostly parallel to surface -high irradiance chloroplasts would be mostly along walls that are perpendicular to the surface -changes constantly- over hours- different positions throughout the day

How does soil texture influence water availability to plants?

--soil if not filled with water can have more air --coarse--lots of air-bad at water retention -clay-not a lot of air because spaces filled with water

What types of things create pores within a soil? What is soil porosity, and why is it important to a plant growing in that soil?

--spaces between particles/aggregates - filled with water and/or air -earthworm & termite channels, cracks in drying soil -diameter ~ a few cm --spaces between aggregates -dia. ~ a few mm --spaces within aggregates dia. ~ a few mm (or less) pore space = total space taken up by all soil pores --soil porosity - % of soil volume occupied by pore space (usually ~ 40 - 60 %) --important because thats where they do gas exchange — space filled with water

What is a D1 protein? Describe, in detail, the movement of electrons from the oxygen-evolving complex through photosystem II, to the cytochrome complex. What is the role of D1 in this process, and how is it related to photoinhibition?

--they are in the photosystem II Reaction Centers --oxygen-evolving complex (the OEC) accepts four electrons from the oxidation of water --passes electrons one at a time to P680, as P680 is oxidized -- P680 passes electrons one at a time to D2 cells which passes one at a time electrons to D1 cells ----D1 protein passes two electrons to plastoquinone - reduces plastoquinone and allows it to carry electrons to the cytochrome complex --damage to (D1 proteins) in PSII reaction centers is a source of photoinhibition --individual D1 molecules can only undergo oxidation-reduction a limited # of times before becoming dysfunctional --damaged D1 interferes with photosynthetic electron transport --the absence of functional photosynthetic electron transport may cause excited electrons to interact with oxygen and form reactive oxygen species (ROS) -Superoxide -Singlet Oxygen -Hydrogen Peroxide -Hydroxyl Radical

Water diffuses from areas of higher Ψ(water potential ) to areas of lower Ψ. What factors influence the Ψ liquid water in a particular area, and thus influence the direction in which water diffuses?

--water content of plant determined by the balance between - absorption of water by roots - loss of water by transpiration from leaves -when rate of absorption ≥ rate of transpiration, - plant can maintain sufficient water content -when rate of absorption < rate of transpiration, -plant will suffer net water loss, eventual water stress water potential of liquid water is influenced by: a) solute concentration b) pressure

Explain why conditions of high light intensity along with slow Calvin cycle reactions can lead to photoinhibition.

--when the rate of electron transport through the photosystems is higher than the rate at which NADPH is used in carbon fixation --if NADPH molecules build up, no NADP+ is available to "accept" electrons - electrons are instead passed to O2 - may again create ROS

Initial photoinhibition vs Chronic Photoinhibition

-Initial photoinhibition may be rapidly reversed -Continued high irradiance may lead to chronic photoinhibition --results in decreased number of functional photosynthesis -no xanthophylls present or not enough of them - long term chronic photoinhibition can result in reduced growth, even death

What is photobleaching of plants?

-ROS oxidizes and permanently damages chlorophyll molecules -if have a plant inside during winter and then move it outside in summer too much light - need to be gradual move to shade and let it adjust - light intensity inside is a lot less than outside

What types of damage are caused by UV radiation? by X ray radiation? by gamma radiation? What types of resistance do plants have against these types of damage?

-UV-A causes photooxidation of photosynthetic pigments; -also prevents operation of the xanthophyll cycle -UV-B also damages DNA, -results in protein damage and membrane damage -x-rays and gamma radiation can cause problems, but less common -can come from space, or from radioactivity in earth's crust - additional exposure from radioactive waste, nuclear power plants, etc. -produces free radical formation-can damage biological molecules =resistance to UV radiation stress: provided by -waxes in the cuticle - reflect UV radiation -flavonoids - especially anthocyanins--(water soluble- dissolved in water) - strongly absorb UV-B radiation -synthesis of these is induced by exposure to UV radiation (again, common in alpine plants - likely to be exposed to high UV levels) -more hairs on surface of leaves-pubescence

What is the pressure potential of water at atmospheric pressure? Under what circumstances might you expect the pressure of potential of water to be greater than 0 MPa? Under what circumstances might you expect it to be less than 0 MPa?

-at atmospheric pressure, Ψp = 0 MPa (typical of water in the soil) -when actual pressure > atm. press., Ψp > 0 MPa (common in plant cells under turgor pressure) -when actual pressure < atm. press., Ψp < 0 MPa (can occur in plant's water transport cells - xylem) Ψp = actual pressure - atmospheric pressure -water tends to diffuse to areas of lower pressure

What is boundary layer resistance? What factors can cause a leaf's boundary layer resistance to increase?

-boundary layer resistance— resistance to diffusion through the boundary layer Boundary Layer-unstirred air outside of stomates -has higher RH (and higher Ψ) than surrounding air -slows diffusion through stomate -can allow the water molecules to accumulate right outside of the stomate so increasing relative humidity outside of -resistance varies with thickness of unstirred layer— wind can stir this air and eliminate portions of the boundary layer stomate— making sort of buffer area -water loss is unfortunate side effect

D1 Protein Turnover When does chronic photoinhibition occur?

-damaged D1 is recognized, degraded and replaced --there is generally a high D1 protein turnover rate -chronic photoinhibition occurs when rate of damage is greater than rate of repair - results in decreased # of functional photosystems

What is meant by gravitational water? What is meant by capillary water?

-gravitational water = water lost from soil by drainage -capillary water = water retained in pores --most water available to plants is capillary water

What is photoinhibition?

-increases in light intensity cause decreased rate of photosynthesis

UV-A vs UV-B

-wavelengths lower then visible light range below 400 -higher energy- shorter wavelength -UV radiation includes - UV-A (315 - 400 nm), and -UV-B (280 - 315 nm) -UV-B is potentially more damaging-absorbed by atmosphere; more UV-A reaches plants - more of both types reaching earth as ozone layer thins

Evaporation

-when liquid water moves into air down a Ψ gradient

What materials make up the "soil particles" in a soil? What characteristic is used to define a soil as being "loamy" or "sandy" or some other type?

-mineral particles --clay (diameter < 0.002 mm) --silt (dia. = 0.002 - 0.02 mm) --sand (dia. = 0.02 - 2 mm) --organic matter (humus) decaying material from plants, animals, etc --aggregates: mineral particles bound together by organic matter or clay particles Loamy 40% sand, 40% silt, 20% clay sandy soil less than 15% silt and clay (more than 85% sand) clay soil more than 40% clay particles

Describe the xanthophyll cycle and how it helps to protect plants from high irradiance. What is the role of xanthophylls in the acclimation of plants to high irradiance?

-response: conversion between violaxanthin and zeaxanthin occurs on daily basis, as light intensity varies acclimation: overall amount of xanthophylls, and extent of conversion, varies with type of plant and growing conditions - allows short-term response (daily conversions among xanthophyll types) and longer term acclimation (changes in overall xanthophyll conc.) --—plant growing in the sun — produces lots of xanthophylls because its going to need the protection in the middle of the day —plant growing in the shade— produces less xanthophylls because won't need them so don't want to inhibit photosynthesis —plant growing in the winter (Douglas Fir)- low temperatures slow down the rate at which proteins can be replaced so plant produces more xanthophylls -plant growing in the summer- doesn't need as many xanthophylls uses the first form of protection from sun by replacing d proteins — plants grown in low nitrogen — less chlorophyll available to it so same amount of light hitting the plant but less chlorophyll so more likely to damage the chlorophyll so produce more xanthophylls -high nitrogen- has more chlorophyll so doesn't need as much xanthophyll protection

What is meant by the permanent wilting point of a soil?

-soil water content at the point when Ψsoil = Ψroot, there is no possibility of recovery from wilting by reduction of transpiration --at this point, plant recovery requires increase in soil water content

What is transpiration? Describe how this process occurs?

-the process by which moisture is carried through plants from roots to small pores on the underside of the leaves where it changes to vapor and is released into the atmosphere --factor determining plant water content --depends on: 1) change in water potential between plant interior and surrounding air 2) resistance of plant surface to water loss --- usually occurs at leaf surfaces —lots of surface area -Palisade mesophyll- long cells lined up right next to each other -spongy mesophyll- less orderly...randomly arranged cells fill up lower layers of the leaf— lots of spaces between these so things can move around —most water exits through stomates- there are stomates on the upper surface just a lot more in lower levels -vein include xylem cells which is where the water enters - most water exits through stomates 1) liquid water in leaf spaces evaporates into substomatal space (along Y gradient, until R.H. ~ 100%) -liquid water in apoplastic portions of leaf are actually what's evaporating 2) water vapor diffuses from substomatal space to surrounding air (also along Ψ gradient) Ψ of water vapor in substomatal space ~ 0 MPa (RH ~ 100%) Ψ in surrounding air varies with RH of the air 0 MPa, if RH = 100% - 3 MPa, if RH = 99% - 100 MPa, if RH = 50% --in winter here indoors RH is usually 20-30% --therefore, Ψ of surrounding air will usually be lower than that in the leaf --the lower the outside RH (and Ψ), the greater the change in Ψ, therefore the faster the rate of transpiration --plant must limit transpiration to survive

How does the presence of root hairs improve the water absorbing ability of a root?

1) ability to reach areas between soil particles 2) increased volume of soil reached

Water can diffuse into roots if:

1) there is an active root surface 2) there is a water potential gradient between the soil solution and the root (the Ψ in the root is lower than the Ψ in the soil solution)

What is meant by the relative water content of a plant tissue?

= (Wact/Wsat) · 100 Actual water content compared to the water content of the plant tissue at saturation

What is the solute potential of pure water (i.e., water with no solutes present)? How does the solute potential change as solutes are added to water?

Pure Water: (solute concentration = 0 M), Ψs = 0 MPa --Ψs decreases (becomes more negative) as solute concentration increases water diffuses to areas of high solute conc.

In plants experiencing moderate drought stress, the rate of photosynthesis is significantly decreased, resulting in slow growth and decreased agricultural yield. What causes the decreases in the rate of photosynthesis?

a) decreased chlorophyll synthesis b) decreased leaf size c) decreased CO2 uptake --again, causes decreased plant growth; negative effects on agricultural yield

What is meant by active root surface? Where is the active root surface in a typical root? Why are other parts of the root surface not "active"?

active root surface = -root surface capable of absorbing water ▪ is present in younger portions of root ▪ is absent in older portions of a root -(present in root hairs, not in older root tips) -continual new root growth is needed to maintain active root surface - branching is especially important (produces more growing root tips) -for active root surface to contact soil water, the direction of root growth is also important - different patterns of growth reach soil water in different regions of soil

What is meant by dynamic photoinhibition?

decreased the rate that electrons are being passed to the ETC- a lot of the energy is being given off as heat energy

What is meant by a soil at field capacity? What is the approximate Ψ of the water in a soil at field capacity? Again, explain.

field capacity = maximum amount of water a given soil can retain field capacity : ▪ is high in soil with small pores (lots of clay, humus)-- closely packed together so less pores-less space for drainage ▪ is low in soil with large pores (sand)-more spaces so more drainage Ψ=-0.033 Water content for different soils is different because its at which point the soil particles are holding water against the gravitational pull but field capacity is always the same

What are hydrophytes, mesophytes, and xerophytes?

hydrophytes = adapted to moist soils -root Ψ not usually lower than - 1 MPa mesophytes = adapted to soil with moderate water content ▪ root Ψ can be between - 0.2 MPa and - 4 MPa ▪ typically approx. - 1.5 MPa xerophytes = adapted to very dry soil conditions can adjust root Ψ down to - 6 MPa

Overall Value of Water Potential

Ψ = Ψs + Ψp -the overall value of water potential is the sum of the two effects- solute potential and pressure potential -most often the Ψ of liquid water is between - 3 MPa and +1 MPa

Is it likely that a plant root will absorb water from a saturated soil? From a soil at field capacity? Explain.

Ψ soil at saturation = ~ 0 MPa P = ~ 0 MPa, S = ~ 0 MPa, M = 0 MPa ▪ in a saturated soil Ψsoil = ~ 0 MPa Ψroot = ~ - 1.5 MPa Ψroot < Ψsoil ▪ root will absorb water Ψ soil at field capacity = ~ 0.033 MPa (exactly what is needed to counteract the force of gravity) in a soil at field capacity (after drainage) Ψsoil = ~ - 0.033 MPa Ψroot = ~ -1.5 MPa change in Ψ= ~ - 1.467 MPa ▪ root will still absorb water

What is stomatal resistance? What factors cause stomatal resistance to vary?

—Stomatal Resistance -resistance to diffusion through stomates -varies with: -density of stomates— how many stomates are there in a given space-packed close together or spaced out— some circumstances where stomates being packed closely together will increase resistance -opening and closing of stomates— controls how big the opening is —closing more resistance to transpiration Stomates-where gases enter and exit the plants —no cuticle covering just opening

What kind of damage to proteins is caused by severe drought stress in plants? What kind of damage to membranes is caused by severe drought stress in plants?

—protein damage- Protein metabolism and amino acid synthesis disrupted —results from inhibition of various enzymes -nitrate reductase activity strongly inhibited Nitrate reductase is the enzyme that converses nitrate absorbed from soil to ammonium needed in amino acids synthesis —very important enzyme for maintaining plant growth -inhibition can produce symptoms of nitrogen deficiency --membrane damage— having enact membranes is REQUIRED! occurs in final stages of plant damage before plant death -water content in cells may no longer be sufficient to maintain stable bilayers— need water on both sides to maintain membrane—stable because the water is in contact with hydrophilic layer— membranes made of phospholipid bilayer - phospholipids rearrange into hexagonal structures —pop out of phospholipid bilayer and clump together with hydrophobic tails together in hexagonal structure - recovery only possible if small # of phospholipids have been affected— if water is added and the plant can rehydrate- only if slight damage visible effects - large central vacuole fragments into many small vesicles— breaks up into smaller membrane bound sacs - chloroplast thylakoids, mitochondrial cristae swell & decompose— disitintegrate - nuclear membrane is disrupted disintegration of membranes results in cell death

What is meant by the term nonphotochemical quenching?

—when use nonphotochemical reactions —stopping fluorescence because some energy is being passed to xanthophylls instead -results in short term photoinhibition aka "dynamic photoinhibition"- decreased the rate that electrons are being passed to the ETC- a lot of the energy is being given off as heat energy