Plant Experimental Ecology
H1: Functioning of plants
Plants perform many functions to grow, survive, reproduce, & interact with their environment. Main plant functions:
• Intercept light for photosynthesis.
• Protect themselves from excess radiation.
• Communicate with neighboring plants => example: By releasing volatile gases (such as smell released when
grass is mowed).
• Discourage herbivores & pathogens using chemical defenses.
• Assimilate C & synthesize biochemical compounds.
• Form new leaves & adjust leaf anatomy.
• Transport substances & info throughout plant.
• Occupy space & explore resource-rich patches.
• Store & recycle carbohydrates & mineral nutrients.
• Take up water & nutrients from soil.
• Disperse seeds & attract pollinators for pollen transfer.
• Remobilize nutrients during leaf senescence.
• Influence soil processes through litter production & root exudates (= rhizodeposition).
• Communicate with roots of neighboring plants & recognize their own roots.
Plants can adjust their enzyme activity depending on number of neighboring plants. This suggest that they can sense
or “count” number of neighbors around them.
• Example: Plants grown under low P produce few seeds, which can strongly affect next generation.
1.1: Nutrients
Plants do not need all resources in equal quantities, even relative to their demand
Law of Minimum = Plant growth controlled by most limiting resource (= resource that is scarcest relative to
plant demand) => Concept illustrated with Liebig’s barrel, where shortest plank determines how much water
it can hold (shortest barrel = limiting nutrient, which determines plant growth or biomass production).
Plant growth & biomass can be limited by environmental factors (such as temperature, water, light, CO₂) & availability
of essential nutrients. There is competition for space between plants, but space itself is not consumed so it’s not
considered limiting resource in same way as nutrients or water.
Different barrel “planks” (aka resources) are not independent, changes in 1 resource affect availability / use of others.
1.1.1: Macronutrients
Plants require 6 major mineral macronutrients = N, P, K, Ca, Mg, S => Functions:
• Nitrogen (N) = component of proteins, nucleic acids, hormones, chlorophyll => absorbed as nitrate (NO₃⁻) &
ammonium (NH₄⁺)
• Potassium (K) = enzyme cofactor, important for protein synthesis & water balance (regulates stomatal opening
& closing).
• Calcium (Ca²⁺) = important for cell wall formation & stability (maintains membrane structure & permeability),
activates enzymes, regulates cell signaling & responses to stimuli.
• Magnesium (Mg²⁺) = central component of chlorophyll, enzyme cofactor, activates metabolic enzymes.
• Phosphorus (P) = component of ATP, nucleic acids, phospholipids & coenzymes => absorbed as
dihydrogenphosphate (H₂PO₄⁻) & hydrogenphosphate (HPO₄²⁻)
• Sulfur (S) = component of proteins & coenzymes => absorbed as sulfate (SO₄²⁻)
Plants also require large amounts of C, O & H => Functions:
, • Carbon (C) = major component of organic compounds => absorbed mainly as CO₂
• Oxygen (O) = essential for organic molecules & respiration => obtained from CO₂
• Hydrogen (H) = component of organic molecules => obtained from water (H₂O)
1.1.2: Micronutrients
Plants require 8 micronutrients in smaller amounts = Cl, Fe, Mn, B, Zn, Cu, Ni & Molybdenum (Mo)
Functions of Micronutrients
• Chlorine (Cl⁻) = involved in photosynthetic water splitting & contributes to water balance.
• Iron (Fe²⁺ / Fe³⁺) = component of cytochromes, enzyme cofactor & required for photosynthesis.
• Manganese (Mn²⁺) = activates enzymes, involved in amino acid synthesis & in photosynthetic water splitting.
• Boron (B) = involved in chlorophyll synthesis, carbohydrate transport & cell wall function => absorbed as H₂BO₃⁻
• Zinc (Zn²⁺) = involved in chlorophyll formation, enzyme cofactor & required for DNA transcription.
• Copper (Cu⁺ / Cu²⁺) = component of redox enzymes & involved in lignin synthesis.
• Nickel (Ni²⁺) = enzyme cofactor in nitrogen metabolism.
• Molybdenum (MoO₄²⁻) = essential for N-fixation by symbiotic bacteria & involved in nitrate reduction.
Some plant species also require Sodium (Na), Cobalt (Co) & Silicon (Si)
1.2: Photosynthesis & C-assimilation
Photosynthesis = process where plants use sunlight energy to convert CO₂ into organic compounds.
Overall reaction = 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
This process occurs in 2 different parts of chloroplasts:
• Light reactions occur in thylakoids
o Inputs = light & H₂O
o Outputs = ATP, NADPH & O₂ (from water splitting)
• Carbon assimilation (Calvin cycle) occurs in stroma => uses ATP &
NADPH to convert CO₂ into sugar.
In most plants (C3 plants), C assimilation occurs through Calvin cycle.
1.2.1: C3 mechanism
Calvin Cycle = standard mechanism for C assimilation in plants.
Calvin cycle has 3 phases:
1. Carbon fixation = CO₂ combines with RuBP (ribulose-1,5-bisphosphate) &
forms 3-phosphoglycerate (3-PGA) => this is done by enzyme Rubisco
2. Reduction = 3-PGA converted into G3P (glyceraldehyde-3-phosphate)
using ATP & NADPH (some G3P leaves cycle to form sugars) => aka CO₂ is
converted into sugar molecules.
3. Regeneration = Remaining G3P regenerates RuBP, so that cycle can
continue.
CO₂ enters cycle & 1st compound formed has 6 carbon atoms, which quickly splits into 2x 3-carbon molecules. => To
produce 1 G3P molecule, cycle must run 3 times (aka must fixing 3 CO₂ molecules).
,C assimilation linked to light availability:
C assimilation depends on light intensity, at
• low light intensity => C assimilation increases linearly with light & photosynthesis is light-limited.
o Slope of this curve part is called Quantum yield = C assimilation/unit of absorbed light.
• high light intensity => Photosynthesis becomes limited by CO₂ diffusion into leaf & carboxylation capacity
(Rubisco activity). Here curve levels off.
Parameters on graph:
• Light Compensation Point (LCP) = Light level where photosynthesis = respiration (AKA
net C gain = 0)
• Dark respiration (Rd) = C loss due to respiration in dark.
• Amax = Max photosynthetic rate (aka max C assimilation rate)
Exact curve differs depending on plant species & environmental conditions. => Example:
Atriplex triangularis shows plants grown under different light intensities.
• Sun-grown plants => have high Amax & require high light to reach max photosynthesis
• Shade-grown plants => have lower Amax & are efficient at low light levels
This is called light acclimation = leaves adapt to light conditions in which they grow.
• Shade Leaves characteristics =
o thinner leaves, larger leaf area
o More N in light-harvesting complexes.
=> Adaptations for max light capture & absorption, but low carboxylation capacity (because
shade plants need better light capture efficiency)
• Sun Leaves characteristics =
o thicker leaves, extra mesophyll cell layers, more chloroplasts & higher
concentrations of enzymes involved in photosynthesis.
o More N in Calvin cycle enzymes (especially rubisco) & electron transport
=> Adaptations for max photosynthetic capacity, as in high light conditions
photosynthesis becomes CO₂-limited & so more space is needed for Calvin cycle
enzymes to increase photosynthetic capacity.
C assimilation linked to water availability:
Stomata regulate water loss, CO₂ uptake & O₂ release.
Figure: Photosynthesis rate vs leaf water potential (ΨL) shows that as water potential increases
(wetter conditions), photosynthesis increases => this is because
• During drought water potential decreases & stomata close to prevent water loss → less CO₂
enters leaf & O₂ accumulates (aka CO₂ concentration decreases & O₂ concentration
increases) → carbon assimilation decreases → stimulates photorespiration.
Photorespiration = loss of carbon & energy, due to oxygenase activity of Rubisco.
Rubisco has 2 functions:
• During ‘normal’ photosynthesis => Carboxylase Activity = Rubisco fixes CO₂ &
produces 2 molecules of PGA (Both continue in Calvin cycle).
• During photorespiration => Oxygenase Activity = Rubisco reacts with O₂ instead
of CO₂, this produces 1 PGA & 1 glycolate (= toxic C₂ compound, which is recycled
through photorespiration & this leads to loss of ATP, fixed C & reduced
photosynthetic efficiency).
, 1.2.2: C4 Plants
C4 plants (such as corn) evolved to reduce this photorespiration, via spatial
separation of CO₂ fixation & Calvin cycle. => C4 Mechanism
1. Mesophyll cells => CO₂ is fixed by PEP carboxylase into malate (= C4
compounds) & is transported to bundle sheath cells. PEP carboxylase
has very high affinity for CO₂ & thus does not bind O₂.
2. Bundle sheath cells => once C4 compounds arrive here, CO₂ is released
& enters Calvin cycle.
This separation leads to high CO₂ concentration & low O₂ concentration, thus
reduced photorespiration (which is efficient in high temperature & light).
C4 & Temperature => C4 plants are sensitive to low temperatures, because
enzyme pyruvate-Pi-dikinase is temperature sensitive. Therefore:
• C3 plants perform better at low temperatures
• C4 plants perform better in hot & dry environments.
=> Explains their global distribution.
C4 Plants & Atmospheric CO₂ => Experiments manipulated CO₂ concentration using cylinders placed over vegetation
showed that
• at low CO₂ concentrations → C4 plants dominate
• at high CO₂ concentrations → C3 plants become dominant.
=> C3 plants increase strongly with rising CO₂, while C4 plants have smaller response (because they
already concentrate CO₂ internally). Thus increasing atmospheric CO₂ tends to favor C3 plants.
1.2.3: CAM Plants
CAM (= Crassulacean Acid Metabolism) plants reduce water loss via temporal separation of CO₂ fixation & Calvin
cycle. These plants are typically desert succulents.
CAM Mechanism
1. Night = stomata open → CO₂ enters plant & is CO₂ fixed by PEP
carboxylase → CO₂ is stored as malate in vacuoles
2. Day = stomata close to prevent water loss → stored CO₂ is released & CO₂
enters Calvin cycle for photosynthesis.
=> This helps with water conservation in dry environments
1.3: Plant – water relationships
Plants need water for photosynthesis, nutrient transport, cell turgor & growth, cooling via transpiration, …
2 strategies of water-use in plants
• Poikilohydric “plants” = desiccation tolerant => These plants can’t regulate their internal water content &
follows environmental humidity, but they can survive complete drying. Common in bryophytes (mosses), where
water absorption occurs through epidermis & while rhizoids serve for anchorage & not water uptake.
o Example: Tortula obtusissima => Moss that absorbs water from dew or rain & can recover
physiologically within ~5 min after drying.
H1: Functioning of plants
Plants perform many functions to grow, survive, reproduce, & interact with their environment. Main plant functions:
• Intercept light for photosynthesis.
• Protect themselves from excess radiation.
• Communicate with neighboring plants => example: By releasing volatile gases (such as smell released when
grass is mowed).
• Discourage herbivores & pathogens using chemical defenses.
• Assimilate C & synthesize biochemical compounds.
• Form new leaves & adjust leaf anatomy.
• Transport substances & info throughout plant.
• Occupy space & explore resource-rich patches.
• Store & recycle carbohydrates & mineral nutrients.
• Take up water & nutrients from soil.
• Disperse seeds & attract pollinators for pollen transfer.
• Remobilize nutrients during leaf senescence.
• Influence soil processes through litter production & root exudates (= rhizodeposition).
• Communicate with roots of neighboring plants & recognize their own roots.
Plants can adjust their enzyme activity depending on number of neighboring plants. This suggest that they can sense
or “count” number of neighbors around them.
• Example: Plants grown under low P produce few seeds, which can strongly affect next generation.
1.1: Nutrients
Plants do not need all resources in equal quantities, even relative to their demand
Law of Minimum = Plant growth controlled by most limiting resource (= resource that is scarcest relative to
plant demand) => Concept illustrated with Liebig’s barrel, where shortest plank determines how much water
it can hold (shortest barrel = limiting nutrient, which determines plant growth or biomass production).
Plant growth & biomass can be limited by environmental factors (such as temperature, water, light, CO₂) & availability
of essential nutrients. There is competition for space between plants, but space itself is not consumed so it’s not
considered limiting resource in same way as nutrients or water.
Different barrel “planks” (aka resources) are not independent, changes in 1 resource affect availability / use of others.
1.1.1: Macronutrients
Plants require 6 major mineral macronutrients = N, P, K, Ca, Mg, S => Functions:
• Nitrogen (N) = component of proteins, nucleic acids, hormones, chlorophyll => absorbed as nitrate (NO₃⁻) &
ammonium (NH₄⁺)
• Potassium (K) = enzyme cofactor, important for protein synthesis & water balance (regulates stomatal opening
& closing).
• Calcium (Ca²⁺) = important for cell wall formation & stability (maintains membrane structure & permeability),
activates enzymes, regulates cell signaling & responses to stimuli.
• Magnesium (Mg²⁺) = central component of chlorophyll, enzyme cofactor, activates metabolic enzymes.
• Phosphorus (P) = component of ATP, nucleic acids, phospholipids & coenzymes => absorbed as
dihydrogenphosphate (H₂PO₄⁻) & hydrogenphosphate (HPO₄²⁻)
• Sulfur (S) = component of proteins & coenzymes => absorbed as sulfate (SO₄²⁻)
Plants also require large amounts of C, O & H => Functions:
, • Carbon (C) = major component of organic compounds => absorbed mainly as CO₂
• Oxygen (O) = essential for organic molecules & respiration => obtained from CO₂
• Hydrogen (H) = component of organic molecules => obtained from water (H₂O)
1.1.2: Micronutrients
Plants require 8 micronutrients in smaller amounts = Cl, Fe, Mn, B, Zn, Cu, Ni & Molybdenum (Mo)
Functions of Micronutrients
• Chlorine (Cl⁻) = involved in photosynthetic water splitting & contributes to water balance.
• Iron (Fe²⁺ / Fe³⁺) = component of cytochromes, enzyme cofactor & required for photosynthesis.
• Manganese (Mn²⁺) = activates enzymes, involved in amino acid synthesis & in photosynthetic water splitting.
• Boron (B) = involved in chlorophyll synthesis, carbohydrate transport & cell wall function => absorbed as H₂BO₃⁻
• Zinc (Zn²⁺) = involved in chlorophyll formation, enzyme cofactor & required for DNA transcription.
• Copper (Cu⁺ / Cu²⁺) = component of redox enzymes & involved in lignin synthesis.
• Nickel (Ni²⁺) = enzyme cofactor in nitrogen metabolism.
• Molybdenum (MoO₄²⁻) = essential for N-fixation by symbiotic bacteria & involved in nitrate reduction.
Some plant species also require Sodium (Na), Cobalt (Co) & Silicon (Si)
1.2: Photosynthesis & C-assimilation
Photosynthesis = process where plants use sunlight energy to convert CO₂ into organic compounds.
Overall reaction = 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
This process occurs in 2 different parts of chloroplasts:
• Light reactions occur in thylakoids
o Inputs = light & H₂O
o Outputs = ATP, NADPH & O₂ (from water splitting)
• Carbon assimilation (Calvin cycle) occurs in stroma => uses ATP &
NADPH to convert CO₂ into sugar.
In most plants (C3 plants), C assimilation occurs through Calvin cycle.
1.2.1: C3 mechanism
Calvin Cycle = standard mechanism for C assimilation in plants.
Calvin cycle has 3 phases:
1. Carbon fixation = CO₂ combines with RuBP (ribulose-1,5-bisphosphate) &
forms 3-phosphoglycerate (3-PGA) => this is done by enzyme Rubisco
2. Reduction = 3-PGA converted into G3P (glyceraldehyde-3-phosphate)
using ATP & NADPH (some G3P leaves cycle to form sugars) => aka CO₂ is
converted into sugar molecules.
3. Regeneration = Remaining G3P regenerates RuBP, so that cycle can
continue.
CO₂ enters cycle & 1st compound formed has 6 carbon atoms, which quickly splits into 2x 3-carbon molecules. => To
produce 1 G3P molecule, cycle must run 3 times (aka must fixing 3 CO₂ molecules).
,C assimilation linked to light availability:
C assimilation depends on light intensity, at
• low light intensity => C assimilation increases linearly with light & photosynthesis is light-limited.
o Slope of this curve part is called Quantum yield = C assimilation/unit of absorbed light.
• high light intensity => Photosynthesis becomes limited by CO₂ diffusion into leaf & carboxylation capacity
(Rubisco activity). Here curve levels off.
Parameters on graph:
• Light Compensation Point (LCP) = Light level where photosynthesis = respiration (AKA
net C gain = 0)
• Dark respiration (Rd) = C loss due to respiration in dark.
• Amax = Max photosynthetic rate (aka max C assimilation rate)
Exact curve differs depending on plant species & environmental conditions. => Example:
Atriplex triangularis shows plants grown under different light intensities.
• Sun-grown plants => have high Amax & require high light to reach max photosynthesis
• Shade-grown plants => have lower Amax & are efficient at low light levels
This is called light acclimation = leaves adapt to light conditions in which they grow.
• Shade Leaves characteristics =
o thinner leaves, larger leaf area
o More N in light-harvesting complexes.
=> Adaptations for max light capture & absorption, but low carboxylation capacity (because
shade plants need better light capture efficiency)
• Sun Leaves characteristics =
o thicker leaves, extra mesophyll cell layers, more chloroplasts & higher
concentrations of enzymes involved in photosynthesis.
o More N in Calvin cycle enzymes (especially rubisco) & electron transport
=> Adaptations for max photosynthetic capacity, as in high light conditions
photosynthesis becomes CO₂-limited & so more space is needed for Calvin cycle
enzymes to increase photosynthetic capacity.
C assimilation linked to water availability:
Stomata regulate water loss, CO₂ uptake & O₂ release.
Figure: Photosynthesis rate vs leaf water potential (ΨL) shows that as water potential increases
(wetter conditions), photosynthesis increases => this is because
• During drought water potential decreases & stomata close to prevent water loss → less CO₂
enters leaf & O₂ accumulates (aka CO₂ concentration decreases & O₂ concentration
increases) → carbon assimilation decreases → stimulates photorespiration.
Photorespiration = loss of carbon & energy, due to oxygenase activity of Rubisco.
Rubisco has 2 functions:
• During ‘normal’ photosynthesis => Carboxylase Activity = Rubisco fixes CO₂ &
produces 2 molecules of PGA (Both continue in Calvin cycle).
• During photorespiration => Oxygenase Activity = Rubisco reacts with O₂ instead
of CO₂, this produces 1 PGA & 1 glycolate (= toxic C₂ compound, which is recycled
through photorespiration & this leads to loss of ATP, fixed C & reduced
photosynthetic efficiency).
, 1.2.2: C4 Plants
C4 plants (such as corn) evolved to reduce this photorespiration, via spatial
separation of CO₂ fixation & Calvin cycle. => C4 Mechanism
1. Mesophyll cells => CO₂ is fixed by PEP carboxylase into malate (= C4
compounds) & is transported to bundle sheath cells. PEP carboxylase
has very high affinity for CO₂ & thus does not bind O₂.
2. Bundle sheath cells => once C4 compounds arrive here, CO₂ is released
& enters Calvin cycle.
This separation leads to high CO₂ concentration & low O₂ concentration, thus
reduced photorespiration (which is efficient in high temperature & light).
C4 & Temperature => C4 plants are sensitive to low temperatures, because
enzyme pyruvate-Pi-dikinase is temperature sensitive. Therefore:
• C3 plants perform better at low temperatures
• C4 plants perform better in hot & dry environments.
=> Explains their global distribution.
C4 Plants & Atmospheric CO₂ => Experiments manipulated CO₂ concentration using cylinders placed over vegetation
showed that
• at low CO₂ concentrations → C4 plants dominate
• at high CO₂ concentrations → C3 plants become dominant.
=> C3 plants increase strongly with rising CO₂, while C4 plants have smaller response (because they
already concentrate CO₂ internally). Thus increasing atmospheric CO₂ tends to favor C3 plants.
1.2.3: CAM Plants
CAM (= Crassulacean Acid Metabolism) plants reduce water loss via temporal separation of CO₂ fixation & Calvin
cycle. These plants are typically desert succulents.
CAM Mechanism
1. Night = stomata open → CO₂ enters plant & is CO₂ fixed by PEP
carboxylase → CO₂ is stored as malate in vacuoles
2. Day = stomata close to prevent water loss → stored CO₂ is released & CO₂
enters Calvin cycle for photosynthesis.
=> This helps with water conservation in dry environments
1.3: Plant – water relationships
Plants need water for photosynthesis, nutrient transport, cell turgor & growth, cooling via transpiration, …
2 strategies of water-use in plants
• Poikilohydric “plants” = desiccation tolerant => These plants can’t regulate their internal water content &
follows environmental humidity, but they can survive complete drying. Common in bryophytes (mosses), where
water absorption occurs through epidermis & while rhizoids serve for anchorage & not water uptake.
o Example: Tortula obtusissima => Moss that absorbs water from dew or rain & can recover
physiologically within ~5 min after drying.
