BOT Exam 2: Abiotic stress and water

Photosynthetic Organisms

Organisms that can produce their own food by performing photosynthesis. These organisms originated in water

Plant Adaptation to Land Methods

Reproducing in dry environment - alternation of generations, heterospory, seeds. Structural support. Acquiring water and transporting it throughout the organism. Avoiding desiccation

Desiccation

The act of drying out

Nonvascular Evolution

The cuticle evolved to prevent desiccation. Lignin and vascular system evolved as well

Vascular Evolution

Evolution of heterospory and seeds

Diffusion

The process of spread of a feature or trend from one place to another over time. Example: Water in bryophytes moved from cell to cell

Vascular Tissue

Provides a conduit for water to move from the soil to the leaves.

Water Potential (Ψ)

Difference in potential energy between pure water and water in a system (helps move the water against gravity up a stem) Water moves from areas with higher to lower Ψ

Gradient in Ψ

Provides the driving force for water to move from soil, through the plant, into the atmosphere.

Osmotic Potential (-Ψπ)

From dissolved solutes - major component in cells!

Pressure Potential (ΨP)

From hydrostatic and pneumatic pressures - turgor

Matric Potential (Ψm)

Cohesive forces of water to other objects

Gravitational Potential (Ψg)

Gravity

Mesophytes

Grow in regions with moderate soil moisture. Affected by short, mild water shortages. Respond by closing stomata

Xerophytes

Grow in regions with frequent, extended droughts. Possess adaptations to balance photosynthesis with water conservation

Hygrophytes

Grow in areas almost permanently saturated with water. Must deal with hypoxic conditions

Drought Adaptation - Shading

Plants use some parts to shade other parts from sunlight (e.g. ribs in cacti) to reduce photosynthesis and evaporation.

Drought Adaptation - Avoidance Strategy

Long-term seed dormancy can help plants avoid extensive droughts (e.g. desert superbloom)

Drought Avoidance - Leaf Abscission

During dry season: Helps plants minimize photosynthesis (e.g. kapok tree)

Drought Avoidance - Vegetative Dormancy

Some herbaceous plants die back during dry season, and only retain belowground structures (e.g. rhizomes, corm)

Bulb

Short, flattened stem bearing fleshy, food-storage leaves (e.g. onions)

Corm

Short, swollen underground stem with thin, scaly leaves protecting it (cormlets - asexual reproduction)

Fleshy Rhizome

Thick underground horizontal stem, serves as water and food storage (e.g. ginger)

Tuber

Enlarged tip of a rhizome containing stored foods (e.g. potato)

Morphological Adaptations

Adaptation of roots structure, modified leaves, etc

Anatomical Adaptations

Adaptation of photosynthetic stems, epidermal hairs, etc.

Physiological Adaptations

Adaptation of osmoregulation, CAM photosynthesis, etc

Adaptations - Roots

Xerophytes have a greater root-to-shoot ratio than mesophytes (i.e. they invest more in roots). Shallow and extensive to take advantage of brief rains (e.g. cacti). Deep roots reaching water table (phreatophytes)

Adaptations - Succulent Tissue in Leaves or Stems

Reduced surface-to-volume ratio: reducing water loss while increasing water storage capacity.

Adaptations - Photosynthetic Stems

Can be only photosynthetic organ (e.g. cacti) or secondary for drought (e.g. palo verde)

Physical Adaptations - Leaf Abscission

In some species, water deficit triggers hormonal changes that lead to abscission of older leaves, preserving water for younger ones.

Physical Adaptations - Osmoregulation

Involves the synthesis of solutes (e.g. proline) to lower the water potential of cells; preventing loss of turgor

Halophytes

"Salt plants"

Physical Adaptations - Resurrection Plants

Plants survive cellular desiccation; via the coordinated modification of the cytoplasm and cell membrane

Crassulacean Acid Metabolism (CAM)

A specialized form of C4 photosynthesis. Temporarily separates the absorption of CO2 (nighttime) from the light-dependent reactions and Calvin cycle (daytime)

Malic Acid

Organic acid that is used to store carbon dioxide during the night and is released during the day to allow photosynthesis when the stomata are closed

Hypoxia

Coping with wet conditions. This is dangerous because as soil becomes hypoxic, bacterial anaerobic metabolism produces toxic waste products, adding another stress

Waterlogging

Only the root system is under anaerobic conditions

Partial Submergence

All roots are immersed in water while just a portion of the shoot is covered by water

Complete Submergence

All plant is under the water level. Water depth and turbidity are important factors defining this scenario

Morphological Adaptations - Aerial Root Formation

Several species form adventitious roots that remain above water and can transfer oxygen to submerged roots

Morphological Adaptations - Cypress Knees

Vertical protuberances of roots that serve the same role as aerial roots

Morphological Adaptations - Aerenchyma

Modified spongy parenchyma that allows for gas exchange between shoots and roots. This forms in the root cortex to allow diffusion of oxygen from the shoot.

Aerenchyma Mechanisms

Lysigeny, Schizogeny, and Expansigeny

Lysigeny

Collapse and death of cells in the cortex

Schizogeny

The expansion of intercellular spaces

Expansigeny

Cell division and expansion

Radial O2 Loss (ROL) Barrier

Prevents diffusion of oxygen to anoxic soil, retaining O2 in the root.

Fermentative Bypasses

Ameliorate poisoning via accumulation of toxic byproducts

Activation of Antioxidant Enzymes

Helps buffer accumulation of reactive oxygen species