Lecture 3, full

When does a plant suffer from Trockenstress (drought stress)?

When water consumption is greater than Wasseraufnahme (water uptake)

How does water deficit (Wassermangel) affect secondary growth (sekundäres Dickenwachstum) in trees?

• Secondary growth (sekundäres Dickenwachstum) is greatest in early summer (Frühsommer) → growth rate (Wachstumsrate) is highest.

• In midsummer (Hochsommer), the soil (Boden) is often too dry (zu trocken).

• Because of this, trees cannot fully rehydrate at night (nachts nicht voll rehydrieren).

• As a result, cells cannot build enough turgor pressure (Turgordruck).

• Since cell expansion requires turgor, secondary growth decreases or stops under water shortage.

How is stomatal conductance (stomatäre Leitfähigkeit, gs) regulated in response to water availability?

• Stomatal conductance (stomatäre Leitfähigkeit, gs) regulates transpiration (Transpiration) by controlling stomatal opening.

• Plants adjust transpiration to water availability (Wasserverfügbarkeit) through stomatal control (stomatäre Kontrolle).

• When water availability is low, stomata close partially or fully to reduce water loss.

• Therefore, transpiration can be greatly reduced even at the same VPD (Vapor Pressure Deficit / Dampfdruckdefizit).

How does adaptation of the phenotype (Anpassung des Phänotyps) via the root system (Wurzelsystem) help plants under water stress?

• A larger root system (grösseres Wurzelsystem) allows the plant to access more soil water (Bodenwasser).

• A deeper root system (tieferes Wurzelsystem) enables the plant to reach new water sources (Wasserquellen) located deeper in the soil.

• Both adaptations improve water uptake (Wasseraufnahme) during drought (Trockenheit).

What is osmotic adjustment (osmotische Anpassung, OA) and how does it help plants during water stress?

• Osmotic adjustment (osmotische Anpassung, OA) involves the accumulation (Anreicherung) of osmotically active substances (osmotisch aktive Substanzen) in the cell.

• This delays loss of turgor (Turgorverlust) and protects biomembranes (Biomembranen) and proteins (Proteine).

• Its primary benefit (primärer Nutzen) is maintaining metabolism (Stoffwechsel) and growth (Wachstum) even at low leaf water potentials (niedrige Blattwasserpotentiale).

• The effect on improving water uptake (Wasseraufnahme) through a stronger potential gradient (Potentialgradient) is only small (gering).

What is the difference between Stress ausweichen, Stress vermeiden, and Stress ertragen in plants? Give examples.

• Stress ausweichen (escape stress)
  = The plant avoids the stressful period by changing its life cycle / timing.
  Examples:

  • Desert annuals complete their life cycle before drought starts

  • Seeds remain dormant until rain comes

  • Some plants lose leaves during dry seasons

• Stress vermeiden (avoid stress)
  = The plant prevents its cells from actually experiencing severe stress.
  Examples:

  • Closing stomata to reduce water loss

  • Deep roots to reach groundwater

  • Thick waxy cuticle to reduce evaporation

• Stress ertragen (tolerate stress)
  = The plant remains alive despite stress by tolerating damage or protecting cells.
  Examples:

  • Accumulating sugars/proline during drought or freezing

  • Salt-tolerant plants storing ions in vacuoles

  • Frost-resistant plants producing antifreeze proteins

Easy memory aid:

• ausweichen → wait or escape the bad period

• vermeiden → keep stress away from cells

• ertragen → survive even under stress

What happens to plant and soil water status during prolonged drought without precipitation?

• The water content of plant cells (Wassergehalt der Zellen) decreases.

• The water potential of the leaves (Blattwasserpotential, ψ der Blätter) decreases / becomes more negative.

• Without precipitation (ohne Niederschlag), the water content of the soil (Wassergehalt des Bodens) also decreases.

• Therefore, the soil water potential (Bodenwasserpotential, ψ des Bodens) also decreases / becomes more negative.

• The plant can no longer fully rehydrate at night (nachts nicht mehr voll aufsättigen).

• Eventually, the water potential reaches the Permanent Wilting Point (PWP, permanenter Welkepunkt).

• At PWP, the plant stays wilted (verwelkt) because it cannot absorb enough water anymore.

What determines the difference between predawn ψ and midday ψ in plants?

The difference between predawn ψ (ψ vor Sonnenaufgang) and midday ψ (ψ am Mittag) depends on:

• Transpiration rate (Transpirationsrate)

  • Higher transpiration → more water loss → midday ψ drops more (becomes more negative).

• Hydraulic conductivity (hydraulische Leitfähigkeit)

  • Describes how efficiently water moves through soil, roots, and xylem.

  • Higher hydraulic conductivity → water is replaced more easily → smaller ψ difference.

  • Lower hydraulic conductivity → harder to transport water → larger ψ difference.

Important:

• Predawn ψ reflects the plant’s water status after nighttime rehydration and is often close to soil water potential (Bodenwasserpotential).

• Midday ψ is usually lower because transpiration is highest during the day.

What is the formula for Wasserpotential (water potential)?

It is the sum of osmotisches Potential (osmotic potential) and Turgordruck (turgor pressure)

What term describes water potential equilibrium between the plant and soil before sunrise?

Predawn-Wasserpotential (predawn water potential)

Define the Permanenter Welkepunkt (Permanent Wilting Point, PWP).

The soil water potential threshold, roughly -1.5 MPa, where plants can no longer extract water

What does the Turgorverlustpunkt (Turgor Loss Point, TLP) represent?

The point where turgor pressure reaches zero, indicating critical dehydration and potential irreparable cell damage

Which physiological process is the most sensitive to water deficiency?

Zellstreckungswachstum (cell elongation growth), which stops long before photosynthesis is affected

When is the Wachstumsrate (growth rate) of a leaf or stem highest?

Just before sunrise, when the plant is fully turgeszent (turgid) and turgor pressure is at its maximum

What happens to Photosynthese (photosynthesis) as water potential continues to drop?

It eventually stagnates and decreases, but it is more robust than growth

What are the two primary physical consequences of Gewebe-Dehydrierung (tissue dehydration)?

Zellschrumpfung (cell shrinkage) and Membranzerstörung (membrane destruction)

What occurs to Phospholipide (phospholipids) in membranes during severe water loss?

They undergo "packing," causing the membrane to lose its Fluidität (fluidity) and functionality

Define Xylem-Kavitation (xylem cavitation).

The formation of air bubbles, or Embolien (embolisms), in the xylem when water tension is too high, breaking the water column

Can a plant repair luftgefüllte Gefässe (air-filled vessels) after cavitation?

No, the hydraulic conductivity is permanently interrupted in those specific conduits, and the plant must grow new ones

List the three types of Stressresistenz (stress resistance) according to Levitt (1956).

Stressausweichung (stress escape), Stressvermeidung (stress avoidance), and Stresstoleranz (stress tolerance)

How do annuelle Lebensformen (annual life forms) practice stress escape?

They survive dry periods as seeds in a Samenbank (seed bank) and only germinate when moisture is sufficient

What is Trockenruhe (drought dormancy)?

A strategy of avoiding drought by dropping leaves, known as Laubwurf, to minimize transpiration

Where are Spaltöffnungen (stomata) typically located?

In the lower Epidermis (outer skin layer of the leaf)

What ion is primarily responsible for the opening of Schliesszellen (guard cells)?

Kalium-Ionen (potassium ions), which enter the vacuole to increase turgor

Which hormone signals Stomataschluss (stomatal closure) during drought?

Abscisinsäure (abscisic acid, ABA), which travels from the roots to the leaves

What is the role of Phänotypische Anpassung (phenotypic plasticity) regarding leaf area?

Reducing total Blattfläche (leaf area) to decrease total water loss through transpiration

Define Phreatophyten (phreatophytes).

Deep-rooted plants, like Populus euphratica, that reach the Grundwasserspiegel (groundwater table) to stay independent of rain

How does a larger Wurzelsystem (root system) help avoid drought?

It allows the plant to access a larger volume of soil and deeper Wasserquellen (water sources)

What is the goal of Sukkulenz (succulence)?

Building internal Wasserspeicher (water storage tissue) to prevent tissues from reaching critical dehydration levels

Define osmotische Anpassung (osmotic adjustment, OA).

The active accumulation of osmotisch aktive Substanzen (osmotic solutes) to maintain turgor and metabolism at low water potentials

What are Compatible Solutes (compatible solutes / kompatible Stoffe)?

Organic compounds like Aminosäuren (amino acids, e.g., Proline) or Zuckeralkohole (sugar alcohols) that protect cells without damaging proteins

How do compatible solutes protect Proteine (proteins) during desiccation?

They replace the Hydrathülle (hydration shell) via hydrogen bonds to prevent denaturation

What are Resurrection Plants (Auferstehungspflanzen)?

Plants like Selaginella that can survive almost complete desiccation and revive when rehydrated

Which specific sugar is often used for Austrocknungstoleranz (desiccation tolerance) in membranes?

Saccharose (sucrose), which prevents the packing of phospholipids

What happens to sekundäres Dickenwachstum (secondary thickness growth) in Swiss forests during mid-summer?

It often stops because the soil dries out, preventing trees from building the necessary Turgordruck (turgor pressure) at night

Why is the Turgorverlustpunkt (TLP) so critical for biochemistry?

Because beyond this point, high concentrations of ions and salts can damage the dreidimensionale Struktur (three-dimensional structure) of proteins

In the Jura mountains, what major drought event caused high tree Mortalität (mortality)?

The extreme Hitzesommer (heat summer) of 2018 and the dry autumn of 2023

What is the primary function of the Kran (crane) in the Hölstein forest research?

To reach the Kronenraum (canopy) of tall trees to measure growth and water potential

Why can Samen (seeds) survive such extreme dryness?

Because they contain high concentrations of compatible solutes that protect the Embryo (embryo) and cellular structures

What is the relationship between VPD (vapor pressure deficit) and stomatal regulation?

High VPD increases the "suction" from the atmosphere, prompting the plant to close stomata to prevent Welken (wilting)

Describe the packing of phospholipids.

When water is lost, phospholipids in the Zellmembran (cell membrane) pack together too tightly, causing it to become rigid and non-functional

What is Stamm-Sukkulenz (stem succulence)?

Storing water in the main stem or trunk, as seen in Kakteen (cacti)

How do Lithops (living stones) minimize water loss?

They are mostly buried in the sand, exposing only a small, light-transmitting surface while the rest is Wasserspeichergewebe (water storage tissue)

Does osmotische Anpassung (OA) significantly improve water uptake?

No, its primary benefit is maintaining Stoffwechsel (metabolism) and growth under stress, rather than increasing soil water extraction

What is the term for a plant that enters a state of dormancy during dry periods?

Trockenruhe (drought dormancy), often involving leaf shedding

Why are Tracheen (vessels) and Tracheiden (tracheids) vulnerable during drought?

Because the negative pressure needed for transport can cause them to undergo Kavitation (cavitation) and fill with gas

What is a Schleimzelle (mucilage cell)?

A specialized cell in succulents that helps in water retention.