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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 structure 2)   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, sodification, & 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

World food production faces a serious decline within the century due to climate change

UN FAO

IMPORTANT: 30-40% decline in India, 20-30% in Africa, with some countries experiencing some gain (mostly temperate countries). Sudan and Senegal could experience collapse: >50% decline

By the 2080’s 5-20% decline in agricultural output globally

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

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Plant Nutrition: N has the greatest impact

•  It’s in: –  proteins –  nucleic acids –  chlorophyll –  enzymes (remember

the giant rubisco) –  & more!

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

Let’s Talk About Getting Nitrogen

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-fixing bacteria

Organic material (humus)

NH3 (ammonia)

NH4+

(ammonium)

H+ (From soil)

NO3–

(nitrate) Nitrifying bacteria

Denitrifying bacteria

Root

NH4+

Soil

Atmosphere Nitrate and nitrogenous

organic compounds exported in

xylem to shoot system

Ammonifying bacteria

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!

Staghorn fern

EPIPHYTES

PARASITIC PLANTS

CARNIVOROUS PLANTS

Mistletoe - photosynthetic Dodder - nonphotosynthetic

Host’s phloem

Haustoria

Indian pipe - nonphotosynthetic

Venus’ flytrap Pitcher plants Sundews

Dodder

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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 have evolved ‘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 plants revisited

• Plant defenses & 2° compounds

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

Adaptations to water stress

•  Water is an important factor influencing plant growth and development

•  Plants exhibit structural and physiological adaptations to water supply

•  We’ll see some in lab…

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

Maple trees: genus Acer Roses

Mesophytic grasses

Hydrophytes: wet habitats, wet soil, sometimes partially submerged. Water lily, Elodea

Waterlettuce (Pistia stratiotes)

La jacinthe d' eau (Eichhornia crassipes)

Water Lily: Nymphaeaceae, basal angiosperms

Structural adaptations of hydrophyte leaves and plants

•  Air sacks in leaves (for floatation) •  Stomata on the upper side of the leaf

(often) and almost always open •  Thin cuticle (don’t need to prevent water

loss) •  Leaves often flat for surface area •  Less rigid structure (water holds them up)

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

Saguaro Cactus Carnegiea gigantea (Cereus giganteus)

BOOJUM TREE (Idria columnaris)

Structural adaptations of xerophyte 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: stomatal crypts on the underside of the leaves

Halophytes: salty soils - makes water osmotically unavailable to them - resemble xerophytes. Pickleweed, mangroves

Common Sea-lavender (Limonium serotinum)

Pickle weed: Salicornia virginica

Batis maritima Red mangrove: Rhizophora mangle

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

loss), in part through stomatal morphology and placement

•  Stomatal density is under both genetic and environmental control

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

Environmental control of stomatal density •  During development, light intensities and CO2 levels = stomatal densities

What might that mean???

Studies show that CO2 leads to of stomata which leads to an transpiration. This has implications for cooling, xylem flow etc.

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20 µm

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

•  They close when they become flaccid

Transpiration •  Plants can wilt if too much water is lost

•  Higher rates of photosynthesis can lead to increased sugar production

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

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

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

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

•  Stomata are usually open during the day and closed at night: minimizes water loss when photosynthesis 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 epidermal tissue

Trichomes (“hairs”)

Cuticle Upper epidermal tissue

Stomata 100 µm

Cues for stomatal opening and closing OPENING: •  Redlight receptors in Chlorophyll and Bluelight receptors in

Xanthophyll 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

•  CLOSING: •  Darkness •  ABA (Abscisic Acid, a hormone) •  High internal CO2 concentration •  Circadian rhythm.

Stomatal opening

•  Proton pumps activate to pump H+ out of the cell •  This triggers gated inward specific K+ channels

to open. K+ moves down its electrochemical gradient

•  Cl- diffuses in to balance the positive charge of the K+

•  It is the accumulation of the ions that lowers the water potential of the cells, causing water to move inward, swelling the guard cells and opening the stomatal pore.

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Stomatal closure

•  A build up of ABA causes Cl- anions to move towards the cell wall, and the closure of the inward specific K+ channels and opening of outward specific K+ channels.

•  K+ moves out of the cells, again down its electrochemical gradient.

•  This increases water potential in the cell, and water will follow the K+ out, collapsing the guard cells and closing the pore

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 plants revisited

• Plant defenses & 2° compounds

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

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

Figure 10.17 The thylakoid membrane.

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-carbon compounds

•  Energy from ATP and NADPH is used to re-arrange 3-carbon compound into higher energy G3P

•  G3P used to build glucose, other organic molecules

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

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Rubisco •  The key enzyme in the Calvin Cycle or

“C3 pathway”

•  World’s most abundant enzyme!

•  Contains lots of Nitrogen

•  Catalyzes two competing and opposite reactions

Photosynthesis and photorespiration

‘Normal’ reaction:

‘Photorespiration: non-productive and wasteful:

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. at high temperatures

Some plants solve this problem with a CO2-concentrating mechanism: The C4

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

Rubisco, essentially eliminating photorespiration

•  Downside: it takes extra energy to do this, therefore…

•  Only beneficial at high temperatures

Big Bluestem-a “C4 plant”

Figure 10.19 C4 leaf anatomy and the C4 pathway

C4 plants fix CO2 in the mesophyll using the enzyme PEP Carboxylase, which has a much higher affinity for CO2 than does Rubisco.

CO2 is then shunted into the isolated bundle-sheath cells to join the Calvin Cycle.

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 not catalyze a reaction with O2

•  4-carbon compound transported to bundle-sheath cell

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

•  Energetically costly

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Advantages of C4 pathway at higher temperatures

1. More efficient use of light energy

(from Pearcy & Ehleringer 1984)

Advantages of C4 pathway at higher temperatures

2. Higher Water Use Efficiency (WUE)

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10

20

30

40

50

0 160 320 480 640

C3 C4

Net

Pho

tosy

nthe

sis (µ

mol

m-2

s-1)

Leaf Conductance (mmol m-2 s-1)

Advantages of C4 pathway at higher temperatures

3.  Higher Nitrogen Use Efficiency (NUE)

Why? Less Rubisco is needed per gram of leaf

Question: how might litter quality differ between C4 and C3 plants?

Ecological advantages for C4 plants

•  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 grasses dominate the summer season

  In grasslands of South Africa, C4 grasses dominate, except at higher altitudes

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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 (µ

mol

m-2

s-1)

Intercellular CO2 (ppm)

700 ppm CO2

350 ppm CO2

200 ppm CO2

The advantage of C4 plants at high temps is negated at high [CO2]!

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

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

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

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

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Figure 10.20 C4 and CAM photosynthesis compared

Spatial separation of carbon fixation from the Calvin cycle

Temporal separation of carbon fixation from the Calvin cycle

Crassulacean Acid

Other adaptations to environmental stresses

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

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

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

What You’ve Learned So Far:

Water, heat and CO2 Adaptations to water

stress –  Mesophytes,

hydrophytes halophytes, and xerophytes have specific adaptations to water availability

Role of stomata –  Regulate water loss

and CO2 uptake –  Density and

placement are important

–  Stomata open and close with specific cues

C4 and CAM photosynthesis –  Photorespiration can

be a bad thing –  The C4 pathway

helps at high temperatures, but not high CO2!

–  The CAM photosynthetic pathway works in dry conditions

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 and development

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

•  Biological (biotic) factors including competition, herbivory, symbiosis

•  Competition can involve chemicals (allelopathy)

Allelopathy: chemical warfare

Chara Eucalyptus (blue) forest

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Forms of defense against herbivores:

Trichomes, spines etc.

Ant mutualists (especially African acacias)

Poisons “Secondary compounds” “Secondary metabolites”

Derived from offshoots of the biochemical pathways that produce “primary metabolites” like amino acids.

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

Bull-horn Acacia species (Americas, Africa

Pseudomyrmex ants (in central America) Obligate mutualism?

Ant acacias lack alkaloid defenses present in species lacking ant mutualists

Ants are extremely aggressive predators

What about pollination? (Willmer 1997)

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

Terpenes

Insect-deterrents Citronella Pyrethrum

Sagebrush Mint family

Peppermint (menthol) Oregano Basil Catnip

25,000 different kinds Fragrances

(Aromatherapy)

Taxol - Pacific Yew, Cancer

Plant secondary compounds

Phenolics Phenol unit

8000+ kinds, 4500 flavonoids

Flavonoids: in fruits Anthocyanin 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

Caffeine 12,000+ types Nitrogen-containing compounds

Anti-herbivore and anti-pathogen defenses Active on nervous system Most psychoactive drugs Toxic in high doses

Medicinal uses: morphine, quinine, codeine…

Heroin-- From the opium poppy

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, or you put energy into something else (e.g. competing

with your neighbor or making lots of offspring.)

Allelopathic effects

•  Most often inhibit seed germination or seedling growth

•  May act directly on competing plants, or inhibit their growth via effects on soil microbes (eg mycorrhizae) or nutrient availability

•  Proving importance of allelopathy in nature can be tricky…

Identifying allelopathy in nature

•  Step 1: isolate presumed allelochemicals, prove that they inhibit seedling germination 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

How to compete with neighbors

•  Grow faster (above ground) and monopolize light resources

•  Grow faster (below ground) and monopolize soil resources

•  Poison them - allelopathy