Plant Ecology [EXAM 1]

Based off of the Study Guide and what I believe may be added.

What is ecology?

The scientific study of how organisms interact with each other and with their environment

Why is ecology a successful way of knowing the world?

It uses observation, hypotheses and experiments, and its results are shared, tested by others and revised

How does ecology approximate truth?

Through repeated testing and revising of models; findings describe patterns across many observations rather than exact laws

What are the weaknesses of ecology?

Systems are complex and variable, hard to control and replicate, results depend on context, and models and averages simplify reality

How did plant ecology begin as a subdiscipline?

It came later than animal ecology; many of its theories (such as r and K selection) were borrowed from animal ecology and adapted to plants

Other ways of knowing that are not science?

Ethics, art, and religion

True or false: Fields that cannot conduct manipulative experiments are not science.

False - natural and observational experiments can still test hypotheses

Rank experiment types by weight in testing a hypothesis (1 = strongest)

1) Manipulative experiments 2) Natural experiments 3) Observational experiments

Who coined the term "oecology"?

Ernst Haeckel

Ecology is a ______ subject.

Synthetic

Nature of light

Light behaves as both particles (photons) and waves; wavelength relates to the amount of energy

PAR

Photosynthetically active radiation: the visible part of the spectrum that plants use for photosynthesis

Light quality

The wavelength composition of light; plants absorb some wavelengths (red and blue) much more than others, and reflect green

Why are plants green?

Chlorophyll reflects green light rather than absorbing it

First Law of Thermodynamics and photosynthesis

Energy cannot be created or destroyed, so light energy is transferred into stored chemical energy (sugar)

Inputs of photosynthesis

Light, carbon dioxide and water (plus soil nutrients)

Outputs of photosynthesis

Glucose and oxygen

Where do the light (light-dependent) reactions happen, and what do they do?

On the thylakoid membrane (phospholipid bilayer) inside the chloroplast, where light is used to pump hydrogen ions and build the gradient that makes ATP

Where does the Calvin cycle (dark / light-independent reactions) happen, and what controls its rate? [IMAGE SLIDE 40]

In the stroma, the fluid outside the thylakoid (the cellular soup); it runs at the rate the plant supplies the ingredients (CO2, ATP, NADPH)

Chlorophyll a and b

Main photosynthetic pigments; b mostly absorbs blue light, a absorbs red and other wavelengths

Carotenoids

Orange accessory pigments that pass light energy to chlorophyll and act as antioxidants by trapping free radicals

Photosystem II [IMAGE SLIDE 33]

Where light excites electrons and water is split, releasing oxygen, H+ ions and electrons

Where does the oxygen released by photosynthesis come from? [IMAGE SLIDE 33]

From the splitting of water at photosystem II, not from CO2

Photosystem I [IMAGE SLIDE 33]

Second photosystem; with more light energy it produces NADPH

Products of the light reactions

ATP, NADPH and oxygen

ATP and NADPH

Cell energy currency made in the light reactions and used by the Calvin cycle

How is ATP made on the thylakoid membrane? [IMAGE SLIDE 34]

A hydrogen ion gradient builds up inside the thylakoid and flows through ATP synthase, which makes ATP

How does shade limit a plant?

Less light means less hydrogen ion pumping, so less ATP and NADPH, which limits the Calvin cycle and growth

Calvin cycle [IMAGE SLIDE 39]

Light-independent (dark) reactions that use ATP and NADPH to fix CO2 into sugar

Three phases of the Calvin cycle [IMAGE SLIDE 39]

Carbon fixation, reduction, regeneration

Rubisco

The enzyme that fixes atmospheric CO2 into sugar; called the most important enzyme in the world because it makes the dark reactions and nearly all life possible

Photorespiration (why Rubisco is an inefficient enzyme)

Rubisco fixes oxygen instead of CO2 because O2 competes with CO2 for the active site; it produces nothing usable and is more likely at high O2 to CO2 ratios and high temperatures

Why did Rubisco evolve to fix oxygen sometimes?

It evolved when atmospheric CO2 was very high; with today's CO2 to oxygen ratio it occasionally fixes oxygen, which gives toxic or useless products

Stomata

Openings in the leaf surface controlled by guard cells that let CO2 in but also let water vapor out

Stomata trade-off

CO2 diffuses in while water vapor diffuses out, so open stomata risk water loss and wilting

Do stomata have to be in the same place on every leaf?

No; some plants have stomata all over the leaf, others only on the underside, which affects water loss

Boundary layer

Layer of slow-moving air next to a leaf surface that slows CO2 and water exchange

Resistances to CO2 uptake

Boundary layer, still intercellular airspace, and dissolving into the liquid phase inside cells

Where and how are light, water and CO2 used in photosynthesis?

Light is absorbed by pigments on the thylakoid membrane; water is split at photosystem II to supply electrons and release oxygen; CO2 enters through stomata and is fixed by Rubisco in the Calvin cycle

C3 photosynthesis (ancestral pathway) [IMAGE SLIDE 46]

Ancestral state from which the other pathways evolved; simplest pathway, used by most plants; works best at moderate leaf temperatures

C4 photosynthesis [IMAGE SLIDE 49]

Evolved from C3; PEP carboxylase (efficient enzyme in mesophyll cells) fixes CO2 first and concentrates it near Rubisco in bundle sheath cells, separating steps in space

CAM photosynthesis [IMAGE SLIDE 68]

Stomata open only at night to take in CO2, which is stored and used by day, separating steps in time

Why do C4 and CAM pathways exist? [IMAGE SLIDE 69]

They reduce photorespiration and water loss in hot, dry conditions

What makes C4 and CAM different from C3? [IMAGE SLIDE 66]

They add an extra step with PEP carboxylase that concentrates CO2 near Rubisco, separating fixation from the Calvin cycle in space (C4) or time (CAM)

How does C4 avoid fixing oxygen? [IMAGE SLIDE 47]

PEP carboxylase makes a concentrated CO2 precursor in mesophyll cells and passes it to Rubisco in bundle sheath cells, so Rubisco rarely meets oxygen

Why do CAM plants open stomata only at night? [IMAGE SLIDE 67]

It is cooler and more humid at night, so less water diffuses out; CAM plants live in hot, dry places

Cost of C4 and CAM

Extra enzymes and steps are expensive, so C3 plants do better in cooler conditions

Typical C4 plants

Mostly grasses from hot, dry or tropical regions, such as corn

Typical CAM plants

Succulents in extreme desert conditions

True statements about C4 vs CAM pathways

C4 plants carry out the Calvin cycle in bundle-sheath cells, separate from initial carbon fixation; C4 = spatial separation; CAM = temporal separation. (False: C3 plants do NOT close stomata during the day to reduce photorespiration.)

Why did plants need adaptations for life on land?

Terrestrial plants descended from aquatic organisms and had to cope with dry environments (waxy outer layer, vascular tissue, stomata)

Why did early evolutionary plants need to be close to or in saturated environments?

Vascular systems had not evolved

Water potential

A measure of the energy state of water that predicts the direction water will move

Components of water potential

Osmotic (solute), pressure, gravitational and matric potential. (Turgor is NOT listed as a separate component.)

Direction of water movement / water potential from soil to atmosphere [IMAGE SLIDE 77]

From less negative to more negative: about zero (slightly negative) in soil, more negative in the root, stem and leaf, most negative in the atmosphere. The direction matters more than the exact values.

Osmotic potential

Dissolved solutes lower water potential, so water moves toward the more concentrated side

Matric potential

Water binding to soil particles and surfaces, so soil type affects how available water is

Transpiration

Loss of water vapor from the plant through stomata by diffusion. Equation: E = g_leaf(wv) x [Ci(wv) - Ca(wv)]

Plants lose much more water than they use

Most absorbed water is lost through transpiration; only a small fraction is used metabolically

Transpiration pull (why can water move up against gravity?) [IMAGE SLIDE 78]

Evaporation from leaves pulls a continuous water column from soil through the xylem to the stomata, like a straw, aided by cohesion and adhesion

Cohesion

Water molecules sticking to each other

Adhesion

Water molecules sticking to other surfaces

Why is water sticky?

Hydrogen bonding between water molecules (cohesion) and to other surfaces (adhesion); this controls water movement in soil and plants

Capillary action

Water moving through narrow spaces when adhesion is stronger than cohesion; limited by gravity and surface tension

Hydraulic lift [IMAGE SLIDE 79]

Deep roots pull water from deep, wet soil and release it into drier, shallower soil

Nurse plant

A plant whose shade and humidity help other plants establish (for example palo verde in the desert)

Mesophyte

Plant adapted to moderate water availability

Aquatic plant

Plant that lives in or on water

Hygrophyte

Plant adapted to wet or humid habitats

Xerophyte

Plant adapted to dry habitats with traits that reduce water loss

Halophyte

Plant adapted to salty soils or water (the type expected in a salty environment)

Desert ephemeral

Plant that completes its life cycle quickly during brief wet periods and survives drought as seeds

Components of leaf energy balance

Radiant energy input and loss, sensible heat exchange (conduction and convection), and latent heat loss (evaporation)

Sensible heat

Heat exchanged by conduction and convection

Latent heat loss

Energy lost when water evaporates, as in transpiration

What determines leaf temperature?

The balance between energy absorbed and energy lost

Long-term vs short-term responses to environment

Adaptations of species evolve over generations; physiological processes of individuals respond within their lifetime; both modify temperature, light and moisture experienced

What controls water potential in plants?

Soils are one of the key controls, through water held in soil

Wilting and permanent wilting (per the review session) [IMAGE SLIDE 77]

Plants start to wilt around negative 4 to 5 and reach permanent wilting near negative 8, when they cannot take water back in; check your slide for the units

What is soil?

A complex product of interaction between living organisms and their terrestrial substrate

Soil texture [IMAGE SLIDE 115]

Relative proportions of clay, silt and sand particles

Soil texture triangle [IMAGE SLIDE 115]

A chart that places a soil by its percentages of sand, silt and clay; you may need to find a soil on it from its composition

Sand, silt, clay [IMAGE SLIDE 116]

Mineral particles that differ in size, shape, charge and composition, and give soil different properties

Relative size of soil particles [IMAGE SLIDE 116]

Sand is the largest, silt is intermediate, clay is the smallest

Why does soil texture matter?

It controls water holding, cation exchange and organic chemistry, and so what plants can grow there

Soil organic matter

A critical component of soil structure

How do soils hold water?

Water is held in the larger and smaller pores of soil structure, and different soils hold different amounts

Why do soil differences matter?

They help determine what vegetation grows at a site and how individual plants grow

NPK / most limiting nutrients for plants

Nitrogen, phosphorus and potassium are the big three plant nutrients; nitrogen and phosphorus are the most limiting

Why is phosphorus so important to plants?

It is part of phospholipids, which make up every cell membrane; without it plants cannot build membranes

Where does nitrogen come from?

The atmosphere; nitrogen fixers and their microbes fix it into a usable form

Where does phosphorus come from?

Minerals in soil parent material, broken down over time

Nitrogen fixers / symbiotic nitrogen fixation

Microbes living with plants (such as in legume root nodules) convert atmospheric nitrogen into a usable form; these plants fix nitrogen at high rates, making it available to other plants

Nitrogen across soil age [IMAGE SLIDE 138]

Low in young soils, builds as fixers add it, peaks, then declines in very old soils

Phosphorus across soil age [IMAGE SLIDE 138]

High in young soils (plenty of parent material), declines as it is used up, eroded or lost in old soils

Why does nutrient limitation shift as soils age? [IMAGE SLIDE 138]

Young soils are N-limited (little fixed N yet; P plentiful from parent material). Old, weathered soils are P-limited: P is rock-derived, leached/used up, and has no atmospheric source, while N builds up through N fixers.

Scavenging vs. mining of nutrients (Lambers et al.) [IMAGE SLIDE 138]

Scavenging: taking up nutrients already soluble and accessible (limited by root surface area and nutrient concentration). Mining: actively releasing bound nutrients, usually via root exudates; needed when a nutrient is abundant but unavailable, like P in old soils.

Why are mycorrhizae favored in some soils and cluster roots in others? [IMAGE SLIDE 138]

Mycorrhizae = scavenging, where nutrients are fairly available. Cluster roots = mining, in old, heavily weathered soils where P is strongly bound.

How do differences in nutrient-acquisition strategies affect community composition and diversity?

Different strategies suit different soils and reduce competition for the same resources, so plants can coexist; communities differ among regions with different nutrient limitations.

Why might ecological literature overemphasize nitrogen as the limiting nutrient?

Classic research (largely Western Europe and North America) focused on young, N-limited soils; older weathered soils elsewhere are P-limited and understudied.