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PHYSIOLOGICAL ECOLOGY:Plant Adaptations To Their Needs

• Nutrition– Soil conditions– Essential nutrients– Root mutualists

• Other stresses– Sunlight– Heat– Cold– Low Oxygen

• Water– Water stress– Role of stomata– C4 & CAM plants

•Plant defenses & 2° compounds

Plant Nutrition:Soil Quality Impacts Plant Vigor

• Two soil factors:1) Texture - its general structure2) Composition - its organic &

inorganic components

Plant Nutrition:Topsoil Loss Is Critical

• Mix of rock (inorganic)& organic matter(humus breakdown)

• grasslands accumulate most• 100t/km2/yr

• Its loss is important• From 1700-5000 t/km2/yr• 50,000 km2/ yr of arable land

to wind & water erosion,salination, sodication,& desertification.

Plant Nutrition:Topsoil Loss Is Critical

• Mix of rock (inorganic)& organic matter(humus breakdown)

• grasslands accumulate most• 100t/km2/yr

• Its loss is important• From 1700-5000 t/km2/yr• 50,000 km2/ yr of arable land

to wind & water erosion,salination, sodication,& desertification.

• Precautions reduce loss• Role of grazers

Plant Nutrition:Essential Elements

• 9 Macronutrients– need large amounts

• 8 Micronutrients– need small amounts

• Deficiencies are visible– Main ones are P, K, N

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

Plant Nutrition:N has the greatest impact

• It’s in:– proteins– nucleic acids– chlorophyll– enzymes (rubisco)– & more!

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

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Plant Nutrition:Bacteria Fix Atmospheric N2

• Soils have:– Nitrogen-fixers making

nitrogenous minerals• ammonia, ammonium & nitrate

N2

Soil

N2 N2

Nitrogen-fixingbacteria

Organicmaterial (humus)

NH3 (ammonia)

NH4+

(ammonium)

H+

(From soil)

NO3–

(nitrate)Nitrifyingbacteria

Denitrifyingbacteria

Root

NH4+

Soil

AtmosphereNitrate and nitrogenous

organiccompoundsexported in

xylem toshoot system

Ammonifyingbacteria

Plant Nutrition:Bacteria Fix Atmospheric N2

• Legumes have:– root nodules w/ Rhizobium– A mutualistic relationship

• Soils have:– Nitrogen-fixers making

nitrogenous minerals• ammonia, ammonium & nitrate

N2

Soil

N2 N2

Nitrogen-fixingbacteria

Organicmaterial (humus)

NH3 (ammonia)

NH4+

(ammonium)

H+

(From soil)

NO3–

(nitrate)Nitrifyingbacteria

Denitrifyingbacteria

Root

NH4+

Soil

AtmosphereNitrate and nitrogenous

organiccompoundsexported in

xylem toshoot system

Ammonifyingbacteria

Root Mutualists:Rhizobium In Nodules

• Legumes have:– root nodules w/ Rhizobium– A mutualistic relationship

• Crop rotation– Grow various crops

• that deplete soil N– But rotate in a legume

• to refresh soil N

Root Mutualists:Mycorrhizal Root/Fungus Mutualism• Fungus gives plant:

– ⇑ water & nutrient uptake by

– ⇑ root surface area w/ hyphae

• Plant give fungus:– Sugars!

What You’ve Learned So Far:Plant Nutrition

• Soils provide nutrients– So soil loss is important– Texture

• Mix of rock & organics– Composition

• Esp. P, K, N

• Nitrogen is critical– Plentiful in air– “Fixed” by bacteria

• In soil, make– ammonia– ammonium– Nitrate

• Agricultural benefits

• Root Mutualisms– Rhizobium in legume nodules

– Crop rotation ⇑ soil nitrogen

– Mycorrhizal fungi• Ecto & endomycorrhizae• Translocate water/nutrients• Get sugars

• Some plants haveevolved ‘special’nutritional modes

PHYSIOLOGICAL ECOLOGY:WHAT PLANTS NEED

• Nutrition– Soil conditions– Essential nutrients– Root symbionts

• Water– Adaptations to

water stress– Special role of

stomata– Photosynthesis C4

and CAM plantsrevisited

•Plant defenses & 2° compounds

• Other stresses– Sunlight– Heat– Cold– Low Oxygen

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Adaptations to water stress

• Water is an important factor influencingplant growth and development

• Plants exhibit structural and physiologicaladaptations to water supply

• We’ll see some in lab…

Mesophytes:moderate water supply temperate forests and grasslands - shade and sunforms.

Maple trees: genus AcerRoses

Mesophytic grasses

Hydrophytes:wet habitats, wet soil, sometimes partiallysubmerged. Water lily, Elodea

Waterlettuce (Pistia stratiotes)

La jacinthe d' eau (Eichhornia crassipes)

Xerophytes:seasonal or persistent drought - arid andsemiarid. Cactus, succulents

Saguaro CactusCarnegiea gigantea(Cereus giganteus)

BOOJUM TREE (Idria columnaris)

Structural adaptations ofxerophyte leaves

• Small leaves (reduced surface area)• Thick cuticle and epidermis• Stomata on underside of leaves• Stomata in depressions (protected from

wind) or buried in hairs• Reflective leaves• Hairs

Oleander

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Halophytes: salty soils - makes waterosmotically unavailable to them - resemblexerophytes. Pickleweed, mangroves

Common Sea-lavender(Limonium serotinum)

Pickle weed:Salicornia virginica

Batis maritimaRed mangrove: Rhizophora mangle

The stomata• Stomata help regulate the rate of transpiration (water

loss), in part through stomatal morphology andplacement

• Stomatal density is under both genetic andenvironmental control

• Desert plants (xerophytes) have lower stomatal densitiesthan water lilies (hydrophytes)

Environmental control of stomatal density

• During development, light intensities and CO2 levels = stomatal densities

What might thatmean???

20 µm

• Guard cells take in water and buckleoutward due to cellulose microfibrils,opening the stoma

• They close when they become flaccid

Cells flaccid/Stoma closedCells turgid/Stoma open

Radially oriented cellulose microfibrils

Cellwall

Vacuole

Guard cell

The radial orientation of the Cellulose microfibrils causes the cells to increase in length more than width when turgor increases. The two guard cells are attached at their tips, so the increase in length causes buckling.

Transpiration• Plants can wilt if too much water is lost

• Higher rates of photosynthesis can lead toincreased sugar production

• Transpiration also results in evaporative cooling:prevent the denaturation of enzymes involved inphotosynthesis and other metabolic processes

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• Changes in turgor pressure that open and close stomata resultprimarily from the reversible uptake and loss of potassium ions bythe guard cells

• These are driven by active transport of H+ = membrane potential

• Accumulation of K+ (lowers water potential) results in water gainthrough osmosis - opens stoma

• Stomata are usually open during the day and closed at night:minimizes water loss when photosunthesis is not possible

The role of potassium in stomatal opening • The stomata of xerophytes– Are concentrated on the lower leaf surface– Are often located in depressions that shelter

the pores from the dry wind

Lower epidermaltissue

Trichomes(“hairs”)

Cuticle Upper epidermal tissue

Stomata 100 µm

Cues for stomatal opening and closing

• Bluelight receptors stimulate the proton pumps=uptake of potassium

• Depletion of CO2 in leaf as photosynthesis begins

• ‘internal clock’: circadian rhythm (approximately 24 hours)

• Environmental stresses can cause stomata to close during the day

PHYSIOLOGICAL ECOLOGY:WHAT PLANTS NEED

• Nutrition– Soil conditions– Essential nutrients– Root symbionts

• Water– Adaptations to

water stress– Special role of

stomata– Photosynthesis C4

and CAM plantsrevisited

•Plant defenses & 2° compounds

• Other stresses– Sunlight– Heat– Cold– Low Oxygen

Figure 10.5 An overview of photosynthesis: Cooperation of the light reactionsand the Calvin cycle (or C3 Cycle) (Layer 3)

Figure 10.17 The thylakoid membrane.

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Figure 10.18 The Calvin cycle (Layer 3)

Calvin Cycle• Begins with Rubisco catalyzing reaction of

3 CO2 and 3 RuBP to form 6 3-carboncompounds

• Energy from ATP and NADPH is used tore-arrange 3-carbon compound into higherenergy G3P

• G3P used to build glucose, other organicmolecules

• Cyclic process: one G3P (of 6) releasedeach pass through cycle, rest (5)regenerate (3) RuBP

Rubisco• The key enzyme in the Calvin Cycle or

“C3 pathway”

• World’s most abundant enzyme!

• Contains lots of Nitrogen

• Catalyzes two competing and oppositereactions

Photosynthesis and photorespiration

‘Normal’ reaction:

‘Photorespiration: non-productive andwasteful:

Photosynthesis and photorespiration• O2 has an inhibitory effect on

photosynthesis• Competition between O2 and

CO2 on the Rubisco enzyme

• A higher ratio of O2 to CO2 favors photorespiration (which,unlike normal respiration, produces no chemical energy)

• Result: Decreased efficiency of photosynthesis, esp. athigh temperatures

Some plants solve this problem with aCO2-concentrating mechanism: The C4

photosynthetic pathway• Increases [CO2]:[O2] around

Rubisco, essentiallyeliminating photorespiration

• Downside: it takes extraenergy to do this,therefore…

• Only beneficial at hightemperatures

Big Bluestem-a “C4 plant”

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Figure 10.19 C4 leaf anatomy and the C4 pathway

C4 plants fix CO2 in the mesophyll usingthe enzyme PEP Carboxylase, which has amuch higher affinity for CO2 than doesRubisco.

CO2 is then shunted into the isolatedbundle-sheath cells to join the CalvinCycle.

Light reactions (and O2 production) only in mesophyll

Calvin cycle (and Rubisco) only in bundle-sheath cells.

C4 pathway• Physically separates light reactions (O2

production) and Calvin cycle• CO2 first fixed into a 4-carbon compound

in mesophyll by an enzyme that does notcatalyze a reaction with O2

• 4-carbon compound transported tobundle-sheath cell

• CO2 enters Calvin cycle in bundle-sheathcell, where oxygen concentration is low

• Energetically costly

Advantages of C4 pathway athigher temperatures

1. More efficient use of light energy

(from Pearcy & Ehleringer 1984)

Advantages of C4 pathway athigher temperatures

2. Higher Water Use Efficiency (WUE)

0

10

20

30

40

50

0 160 320 480 640

C3

C4N

et P

hoto

synt

hesis

(µm

ol m

-2 s-1

)

Leaf Conductance (mmol m-2 s-1)

Advantages of C4 pathway athigher temperatures

3. Higher Nitrogen UseEfficiency (NUE)

Why?Less Rubisco is needed per gramof leaf

Question: how might litter qualitydiffer between C4 and C3 plants?

Ecological advantages for C4plants

• At higher temperatures, C4 plants:– Use light more efficiently– Use water more efficiently– Use nitrogen more efficiently

• Examples: In North American tallgrass prairie, C3 grasses

dominate during cool seasons, while C4 grassesdominate the summer season

In grasslands of South Africa, C4 grassesdominate, except at higher altitudes

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Which plants, C3 or C4, should berelatively more successful if global

warming continues?

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0

10

20

30

40

50

60

0 100 200 300 400 500 600

C3

C4

Net

Pho

tosy

nthe

sis (µm

ol m

-2 s-

1 )

Intercellular CO2 (ppm)

700 ppm CO2

350 ppm CO2

200 ppm CO2

The advantageof C4 plants athigh temps isnegated at high[CO2]!

Another ecological challenge for plants:dry air. Solution: CAM photosynthesis

• In dry climates, water is lost from the stomatawhen they are open to obtain CO2

• One solution to this problem: Open stomata onlyat night, when it’s cooler & moister, and store thecaptured CO2 until daytime: CAM photosynthesis

• Found in many succulent plants (e.g. ice plant),many cacti, pineapples, and many other speciesin hot dry climates

Figure 10.20 C4 and CAM photosynthesis compared

Spatialseparationof carbonfixation fromthe Calvincycle

Temporalseparationof carbonfixation fromthe Calvincycle

Crassulacean Acid

Figure 10.20 C4 and CAM photosynthesis compared

Spatialseparationof carbonfixation fromthe Calvincycle

Temporalseparationof carbonfixation fromthe Calvincycle

Other adaptations toenvironmental stresses

• Dry conditions lead to supression of shallow roots,promotion of deep roots

• Aerial roots (pneumatophores)• Apoptosis (ethylene) leading to air pockets acting as

‘snorkels’• Salt secrection (halophytes)• Heat shock proteins - preventing denaturation• Antifreeze -high solute (eg. sugars) concentrations

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PHYSIOLOGICAL ECOLOGY:WHAT PLANTS NEED

• Nutrition– Soil conditions– Essential nutrients– Root symbionts

• Water– Adaptations to

water stress– Special role of

stomata– Photosynthesis C4

and CAM plantsrevisited

•Plant defenses & 2° compounds

• Other stresses– Sunlight– Heat– Cold– Low Oxygen

What You’ve LearnedSo Far:

Water, heat and CO2Adaptations to water

stress– Mesophytes,

hydrophyteshalophytes, andxerophytes havespecific adaptationsto water availability

Role of stomata– Regulate water loss

and CO2 uptake– Density and

placement areimportant

– Stomata open andclose with specificcues

C4 and CAMphotosynthesis– Photorespiration can

be a bad thing– The C4 pathway

helps at hightemperatures, but nothigh CO2!

– The CAMphotosyntheticpathway works in dryconditions

Plant physiological ecology

Plant defenses and secondary compounds

• Allelopathy• Defenses against herbivory• Plant secondary compounds• Competing with neighbors: revisiting allelopathy

Ecological factors influencing plant growth anddevelopment

• Fall into two broad categories: physical andchemical (abiotic factors), including …

• Biological (biotic) factors includingcompetition, herbivory, symbiosis

• Competition can involve chemicals(allelopathy)

Allelopathy:chemical warfare

CharaEucalyptus (blue) forest

Forms of defense against herbivores:

Trichomes, spines etc.

Ant mutualists (especially African acacias)

Poisons“Secondary compounds”“Secondary metabolites”

Derived from offshoots of thebiochemical pathways thatproduce “primarymetabolites” like amino acids.

First defense = Physical structures.Second defense = Chemical poisons.

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Bull-horn Acacia species (Americas, Africa

Pseudomyrmex ants(in central America)Obligate mutualism?

Ant acacias lack alkaloiddefenses present in specieslacking ant mutualists

Ants are extremely aggressivepredators

What about pollination?(Willmer 1997)

Forms of defense against herbivores:

Trichomes, spines etc.

Ant mutualists (especially African acacias)

Poisons“Secondary compounds”“Secondary metabolites”

Derived from offshoots of thebiochemical pathways thatproduce “primarymetabolites” like amino acids.

First defense = Physical structures.Second defense = Chemical poisons.

Plant secondary compounds --> In 1999, $400million for St. Johns wort in the U.S.(an antidepressant).

Terpenes

Insect-deterrentsCitronellaPyrethrum

SagebrushMint family

Peppermint (menthol)OreganoBasilCatnip

25,000 different kindsFragrances

(Aromatherapy)

Taxol - Pacific Yew, Cancer

Plant secondary compounds

PhenolicsPhenol unit

8000+ kinds, 4500 flavonoids

Flavonoids: in fruitsAnthocyanin pigments

Herbivore deterrents:Lignans: in grains and veggies (prevent cancer)Tannins: in leaves and unripe fruits [oak family]

Capsaicin: in chili peppers.Function--to deter mammals from eating seeds.

Have receptors in mucous membranes --> PAIN.

Do NOT have receptors.But does act as a laxative -->Improves dispersal.

vs.

Plant secondary compounds

Alkaloids

Caffeine12,000+ typesNitrogen-containing compounds

Anti-herbivore and anti-pathogen defensesActive on nervous systemMost psychoactive drugsToxic in high doses

Medicinal uses: morphine, quinine, codeine…

Heroin--From the opiumpoppy

Nicotine, caffeine

What is different about this cactus?

Peyote cactus (Lophophora)No spines!Chemical defense instead of mechanical defense(25 different alkaloids)

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How come all plants don’t make all possible poisons?

Cost of defense --

TRADEOFFS: “No free lunch”Either you put energy into producing poison, oryou put energy into something else (e.g. competing

with your neighbor or making lots of offspring.)

How to compete with neighbors

• Grow faster (above ground) andmonopolize light resources

• Grow faster (below ground) andmonopolize soil resources

• Poison them - allelopathy

Allelopathic effects

• Most often inhibit seed germination orseedling growth

• May act directly on competing plants, orinhibit their growth via effects on soilmicrobes (eg mycorrhizae) or nutrientavailability

• Proving importance of allelopathy innature can be tricky…

Identifying allelopathy in nature

• Step 1: isolate presumed allelochemicals,prove that they inhibit seedlinggermination in the lab (relatively easy)

• But:– what are concentrations of these chemicals in

nature?– How do you distinguish allelopathy from

simple competition in the field?– Indirect effects