plants responses to environmental stresses

Stress

Any environmental condition that prevents the plant from achieving its full genetic potential

Trade offs

Physiological adjustment to abiotic stress involves trade-offs between vegetative and reproductive development

Acclimation

Short-term, reversible physiological and morphological adjustments an individual plant makes to cope with environmental changes within its lifetime

Adaptation

Longer-term, evolutionary process involving genetics changes in a population over multiple generations, leading to traits that enhance survival and reproduction in a superficial environment

Adaptation - individual or population level

Population

Acclimation - individual or population level

Individual

Adaptation caused by

Natural selection acting on allelic variation within population

Acclimation caused by

Local environmental conditions acting on genetically-determined physiological responsiveness

Adaptation heritability

Genotypic

Acclimation heritability

Generally non-heritable only; some instances of epigenetic transmission

Adaptation reversibility

Irreversible (except by further genotypic change and selection)

Acclimation reversibility

Reversible

Adaption response of homeostasis to perturbation

Mostly plastic

Acclimation response of homeostasis to perturbation

Mostly elastic

Adaptation time scale

From generation time of the organism up to evolutionary

Acclimation timescale

Short term (mins/hours): metabolic and physiological adjustments of existing components without significant change in gene expression

Long-term (up to weeks or months): altered patterns of gene expression, reallocation of resources, morphological change

Adaptation deployment in the life cycle

Strategic

Acclimation deployment in the life cycle

Tactical

3 step process of sensing and responding to abiotic stresses

1. Reception of stress signal - receptors sense

2. Signal transduction - by secondary messengers (Ca2+, ROS)

3. Change in function - changes in gene expression = changes in plant’s physiology and growth to acclimate to stress

Effects of water deficit

Water potential in soil declines

Water column in xylem → cavitation

Cell plasmolysis

Extreme water deficit → reduce membrane integrity

In seeds, LEA proteins (Late Embryogenesis abundant proteins) bind to proteins and cellular proteins to stabilise them

Environmental stress in hot desert/arid environments

Extreme lack of water (long periods of drought, unpredictable precipitation)

Wide temperature fluctuation, high soil and leaf temperature

What should plants do in hot desert/arid environments?

Acquisition of maximum amount of water

Conservation and efficient use of acquired water

Protection of cells and tissues from injury or death during severe dessication

3 strategies for plants in hot desert/arid environments

Drought avoiding plants

Drought endurers/tolerators plants

Drought resisting plants

Drought avoiding plants

Have mechanisms to minimise and avoid water loss (during drought) and maintain water status during dry periods - like thick, waxy leaves or have deep roots to access moisture from deeper soil layers

Drought avoiding plants growth and reproduction

Fast growth and reproduction cycle - ephemerals (last short time)

Grow only when water is available

Life span of weeks to months

Cooled via transpiration (can’t tolerate drought)

May not posses xeromorphic features

Xeromorphic

The structural adaptations that help plants revive in dry environments by reducing water loss

Drought avoiding plants dormancy

Dormancy as seed or spore (annuals, short lived life-strategy)

Drought avoiding plants storage

Storage organs, underground - geophyte

Drought avoiding plants survival

Survival above-ground, complete dehydration (lichens), resurrection fern (76%), deciduous leaves in hot periods

Drought endurers/tolerators

Withstand periods of dry conditions and limited water availability - continue functioning even during drought

Drought endurers/tolerators leaf traits

Xerophytic and sclerophyllous leaves - small leaves, waxy cuticle

Sunken stomata (stomata crypt)

Small, thick, waxy; sunken stomata

Drought endurers/tolerators orientation

Vertical leaves to minimise exposure, or parallel to reduce heat load

Drought tolerator example

Jojoba

Jojoba strategies for tolerating drought

Can alter leaf size and colour (pubescence) depending on season of growth

Leaf angle can respond to diurnal changes in sun angle

Phreatophytes

Plants that have adapted by growing extremely long roots (tap root) to acquire moisture or reach the water table

Drought-resistant plants

Can thrive with minimal to no water, typically for extended periods - higher level of resilience compared to tolerant plants

Drought resistant plant examples

Succulents - cacti, agave

Drought resistant plants water strategies

Store water during wet periods and use later in photosynthesis

Shall root system, fine temporary root during wet periods

Low surface-to-volume ratio

Grow slowly, but can become quite large

Photosynthetic stems

Drought resistant plants - hair and spines

Reduce water loss

Shield from sunlight

Trap moisture

Drought resistant plants - physiological adaptations

CAM physiology - high water use efficiency

Xerophytes

Plants adapted to extremely dry environments

Xerophytes phenological Adaptations

Drought evaders - grow and reproduce after rain, otherwise remain quiescent

Xerophytes - anatomical adaptations

Thick cuticle, rolled leaves, stomata crypts

Xerophytes - morphological adaptations

Deep roots, high extreme transport and/or tissue storage capacity, and tiny or absent leaves

Xerophytes - biochemical adaptations

Crassulacean acid metabolism (CAM): stomata open at night to avoid water loss

Temperature stress general effects

Disruption of plant metabolism → protein instability and enzymatic reaction

Accumulation of ROS

Effect on membrane fluidity

Destabilisation of RNA/DNA structure

Block protein degradation

Freezing temp → ice crystals, movement of water in apoplast

High temp → accumulation of heat shock proteins

Phytochrome temperature sensing

Reversion of Pfr → Pr can be accelerated by temp

Cold adaptations

Affect membrane fluidity

Formation of ice crystals → freezing injury

Cell dehydration

Increased of unsaturated fatty acid in cell membrane

Needle-like leaves, waxy cuticle, evergreen nature

Example of cold adaptation

Conifers with sunken stomata and snow-shedding leaf structure

Poikilothermic ectotherms

Organisms whose body temperature fluctuates with surrounding environment, relying on external heat sources for thermoregulation - most plants are like this

Thermogenesis plant example adaptation

Skunk cabbage - store large quantities of starch in large root and then use metabolic energy to heat flowers (generate heat) via starch metabolism

Water logging: hydrophytes structural adaptations

aerenchyma: gas chambers for buoyancy and O2 diffusion

To allow O2 diffusion in the anoxic alone, roots

Stem elongation

Adventitious roots - roots that develop from non-root tissues

stem hypertrophy - thickened, spongy stems with more aerenchyma (for gas transport)

pneumatophores - help access oxygen in water

Hydrophytes - physiological adaptation

Pressurises gas flow

Rhizospheric oxygenation - transport O2 from their aboveground parts to their roots and surrounding soil,creating an oxygenated zone

Decreased water uptake

Alterned nutrient absorption

Anaerobic respiration

Hydrophytes - whole plant adaptations

Timing of seed production

Buoyant seed and seedlings

Persistent seed bank

Resistant roots, tubers and seeds (storage organs)

Effects of high levels of sodium in plants

Dehydration

Ionic stress, destruction of biomembranes

Inhibits enzymes

Inhibit protein synthesis

Salt tolerance strategies

Die

Accumulation of amino acid (proline) as solute to lower water potential and create a gradient

Halotropism - mediated by auxin where plant grows away from regions of high salt concentration

Salt accumulators

Salt excluders

Salt accumulation

Salt accumulation: store in vacuole, specialised glands (keep cytosine concentration low)

Active transport with protein production to help remove NaCl from cytoplasm

Salt glands accumulate salt then leached through pores

Salt bladders accumulate salt then release

Salt excluders

Defoliation - leaves shed (fall off) after accumulation of salts in petiole

Abscission - leaves naturally detach

Root-level exclusion: active transport of Na+/Cl- out of root cells