PHYSIOLOGICAL ECOLOGY: Plant Adaptations To Their Needs ...

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1. PHYSIOLOGICAL ECOLOGY: Plant Adaptations To Their Needs. • Nutrition. – Soil conditions. – Essential nutrients. – Root mutualists. • Other stresses.

PHYSIOLOGICAL ECOLOGY: Plant Adaptations To Their Needs

Plant Nutrition: Soil Quality Impacts Plant Vigor • 

•  Nutrition

•  Other stresses

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

–  Soil conditions –  Essential nutrients –  Root mutualists

–  –  –  – 

Sunlight Heat Cold Low Oxygen

Two soil factors: 1)  Texture - its general structure 2)  Composition - its organic & inorganic components

• Plant defenses & 2° compounds

Plant Nutrition: Topsoil Loss Is Critical •  Mix of rock (inorganic) & organic matter (humus breakdown)

Plant Nutrition: Topsoil Loss Is Critical •  Mix of rock (inorganic) & organic matter (humus breakdown)

•  grasslands accumulate most

•  grasslands accumulate most

•  100t/km2/yr

•  100t/km2/yr

•  Its loss is important

•  Its loss is important

•  From 1700-5000 t/km2/yr •  50,000 km2/ yr of arable land to wind & water erosion, salination, sodification, & desertification.

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

Plant Nutrition: Essential Elements •  9 Macronutrients –  need large amounts

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

•  8 Micronutrients

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

–  need small amounts Phosphate-deficient




Plant Nutrition: N has the greatest impact •  It’s in: –  proteins –  nucleic acids –  chlorophyll –  enzymes (remember the giant rubisco) –  & more!


Let’s Talk About Getting Nitrogen




Plant Nutrition: Bacteria Fix Atmospheric N2 •  Soils have:

Root Mutualists: Rhizobium In Nodules

•  Legumes have:

–  Nitrogen-fixers making nitrogenous minerals

•  Legumes have:

–  root nodules w/ Rhizobium –  A mutualistic relationship

–  root nodules w/ Rhizobium –  A mutualistic relationship

•  ammonia, ammonium & nitrate N2


Atmosphere Soil



Nitrogen-fixing bacteria NH3 (ammonia)

H+ (From soil) NH4+ (ammonium)

Denitrifying bacteria

•  Crop rotation

Nitrate and nitrogenous organic compounds exported in xylem to shoot system

–  Grow various crops •  that deplete soil N

–  But rotate in a legume


Nitrifying bacteria

NO3– (nitrate)

•  to refresh soil N

Ammonifying bacteria

Organic material (humus)



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

•  Plant give fungus:

–  ⇑ water & nutrient uptake by –  ⇑ root surface area w/ hyphae

Staghorn fern


–  Sugars!

Host’s phloem Dodder Haustoria

Mistletoe - photosynthetic

Dodder - nonphotosynthetic

Indian pipe - nonphotosynthetic


Venus’ flytrap

Pitcher plants



What You’ve Learned So Far: Plant Nutrition •  Soils provide nutrients –  So soil loss is important –  Texture •  Mix of rock & organics

•  Agricultural benefits •  Nutrition •  Root Mutualisms –  Rhizobium in legume nodules

–  Composition

–  Crop rotation ⇑ soil nitrogen

•  Esp. P, K, N

–  Mycorrhizal fungi

•  Nitrogen is critical

–  ammonia –  ammonium

–  Nitrate

–  Soil conditions –  Essential nutrients –  Root symbionts

•  Ecto & endomycorrhizae •  Translocate water/nutrients •  Get sugars

–  Plentiful in air –  “Fixed” by bacteria •  In soil, make


•  Some plants have evolved ‘special’ nutritional modes

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…

•  Water –  Adaptations to water stress –  Special role of stomata –  Photosynthesis C4 and CAM plants revisited

•  Other stresses –  –  –  – 

Sunlight Heat Cold Low Oxygen

• Plant defenses & 2° compounds

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

Maple trees: genus Acer


Mesophytic grasses

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

La jacinthe d' eau (Eichhornia crassipes) Water Lily: Nymphaeaceae, basal angiosperms Waterlettuce (Pistia stratiotes)

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)



Structural adaptations of xerophyte leaves

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

Saguaro Cactus Carnegiea gigantea (Cereus giganteus)

•  •  •  • 

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

BOOJUM TREE (Idria columnaris)

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

Pickle weed: Salicornia virginica

Oleander: stomatal crypts on the underside of the leaves

The stomata •  Stomata help regulate the rate of transpiration (water loss), in part through stomatal morphology and placement

Common Sea-lavender (Limonium serotinum)

Red mangrove: Rhizophora mangle

Batis maritima

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

•  Stomatal density is under both genetic and environmental control

What might that mean???

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

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

CO2 levels


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

Transpiration •  Plants can wilt if too much water is lost

•  They close when they become flaccid

•  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 20 µm

The role of potassium in stomatal opening •  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

•  The stomata of xerophytes –  Are concentrated on the lower leaf surface –  Are often located in depressions that shelter the pores from the dry wind Cuticle

Upper epidermal tissue

•  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

Lower epidermal tissue

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.

Trichomes (“hairs”)


100 µm

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.


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

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

Figure 10.18 The Calvin cycle (Layer 3)

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

•  Other stresses –  –  –  – 

Sunlight Heat Cold Low Oxygen

• Plant defenses & 2° compounds

Figure 10.17 The thylakoid membrane.

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


Rubisco •  The key enzyme in the Calvin Cycle or “C3 pathway”

Photosynthesis and photorespiration ‘Normal’


•  World’s most abundant enzyme! •  Contains lots of Nitrogen •  Catalyzes two competing and opposite reactions

Photosynthesis and photorespiration •  O2 has an inhibitory effect on photosynthesis •  Competition between O2 and CO2 on the Rubisco enzyme



productive and wasteful:

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…

•  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

Figure 10.19 C4 leaf anatomy and the C4 pathway Light reactions (and O2 production) only in mesophyll

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

•  Only beneficial at high temperatures

Big Bluestem-a “C4 plant”

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


Advantages of C4 pathway at higher temperatures

Advantages of C4 pathway at higher temperatures 2. Higher Water Use Efficiency (WUE) Net Photosynthesis (µmol m-2 s-1)

1. More efficient use of light energy

50 40

C4 C3

30 20 10 0 0





Leaf Conductance (mmol m-2 s-1)

(from Pearcy & Ehleringer 1984)

Advantages of C4 pathway at higher temperatures

Ecological advantages for C4 plants •  At higher temperatures, C4 plants:

3.  Higher Nitrogen Use Efficiency (NUE)

–  Use light more efficiently –  Use water more efficiently –  Use nitrogen more efficiently

Why? Less Rubisco is needed per gram of leaf

•  Examples:   In North American tallgrass prairie, C3 grasses dominate during cool seasons, while C4 grasses dominate the summer season


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

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

Another ecological challenge for plants: dry air. Solution: CAM photosynthesis C3

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

Net Photosynthesis (µmol m-2 s-1)


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



40 30

700 ppm CO2


200 ppm CO2

350 ppm 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

0 -10 0







Intercellular CO2 (ppm)


Figure 10.20 C4 and CAM photosynthesis compared

Crassulacean Acid

•  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

Temporal separation of carbon fixation from the Calvin cycle

Spatial separation of carbon fixation from the Calvin cycle

What You’ve Learned So Far: Water, heat and CO2

Other adaptations to environmental stresses

Plant physiological ecology

C4 and CAM photosynthesis

–  Photorespiration can be a bad thing –  The C4 pathway helps at high Adaptations to water temperatures, but not stress high CO2! –  Mesophytes, Role of stomata –  The CAM hydrophytes –  Regulate water loss photosynthetic halophytes, and and CO2 uptake pathway works in dry xerophytes have conditions specific adaptations –  Density and placement are to water availability important –  Stomata open and close with specific cues

Plant defenses and secondary compounds •  •  •  • 

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

Ecological factors influencing plant growth and development

Allelopathy: chemical warfare

•  Fall into two broad categories: physical and chemical (abiotic factors), including … •  Biological (biotic) factors including competition, herbivory, symbiosis •  Competition can involve chemicals (allelopathy)

Eucalyptus (blue) forest



Bull-horn Acacia species (Americas, Africa

Forms of defense against herbivores: Trichomes, spines etc.

Pseudomyrmex ants (in central America) Obligate mutualism? Ant mutualists (especially African acacias)

Ant acacias lack alkaloid defenses present in species lacking ant mutualists

First defense = Physical structures. Second defense = Chemical poisons. Poisons “Secondary compounds” “Secondary metabolites”

Ants are extremely aggressive predators

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

Plant secondary compounds

--> In 1999, $400million for St. Johns wort in the U.S.

(an antidepressant).

What about pollination? (Willmer 1997)

Plant secondary compounds

Phenolics Phenol unit


8000+ kinds, 4500 flavonoids

Taxol - Pacific Yew, Cancer

25,000 different kinds Fragrances

(Aromatherapy) Insect-deterrents


Pyrethrum Sagebrush Mint family

Flavonoids: in fruits

Anthocyanin pigments Herbivore deterrents: Lignans: in grains and veggies (prevent cancer) Tannins: in leaves and unripe fruits

[oak family]

Peppermint (menthol) Oregano Basil Catnip

Capsaicin: in chili peppers.

Function--to deter mammals from eating seeds. Have receptors in mucous membranes --> PAIN. vs.

Do NOT have receptors.

But does act as a laxative -->Improves dispersal.

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… Nicotine, caffeine

What is different about this cactus? Heroin-- From the opium poppy

Peyote cactus (Lophophora)

No spines!

Chemical defense instead of mechanical defense

(25 different alkaloids)


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.)

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

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…

How to compete with neighbors •  Grow faster (above ground) and monopolize light resources •  Grow faster (below ground) and monopolize soil resources •  Poison them - allelopathy


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