Plant Stress Physiology & Defense

Overview

Every mechanism covered earlier in this section — stomatal control, ABA signaling, hormone balance, mineral uptake — is also a tool the plant deploys under stress. This closing page integrates them into whole-plant stress responses, first abiotic (drought, salinity, temperature) and then biotic (pathogen and herbivore defense), the two broad categories a stationary organism must survive without the option of fleeing either.

Key Concepts

Drought Stress

Water deficit triggers a coordinated response built almost entirely from mechanisms already established: root and leaf ABA synthesis rises sharply, driving the stomatal closure covered in full on Stomatal Physiology & Gas Exchange to limit further transpirational water loss, at the direct cost of restricting CO₂ entry and therefore photosynthetic rate — a whole-plant example of the same photosynthesis-transpiration trade-off introduced on that page, now pushed toward its water-conserving extreme. Sustained drought also triggers osmotic adjustment: cells accumulate compatible solutes (proline, glycine betaine, soluble sugars) that lower cellular solute potential without disrupting enzyme function at high concentration (unlike ionic solutes, which become toxic at the concentrations needed to meaningfully shift Ψₛ), helping cells retain water and maintain turgor even as external/soil water potential falls.

Salinity Stress

High soil salinity presents a double challenge: an osmotic component (a saline soil has a very negative water potential, making it physically harder for roots to extract water at all, mechanistically continuous with the water potential framework on Water Transport & Transpiration) and an ionic toxicity component (excess Na⁺ specifically disrupts K⁺-dependent enzyme function and membrane integrity once it accumulates intracellularly).

Source: not specified

Plants respond by excluding Na⁺ at the root (selective ion channels favoring K⁺ uptake over Na⁺) or, where exclusion is incomplete, compartmentalizing absorbed Na⁺ into the vacuole (away from cytoplasmic enzymes) using tonoplast Na⁺/H⁺ antiporters — a cellular-scale solution conceptually similar to CAM’s vacuolar malic acid storage (see Carbon Fixation) in that both rely on the vacuole as a large-capacity storage compartment isolated from cytoplasmic metabolism.

Source: ResearchGate / Trends in Plant Science, fig. 5, “Overview of cellular Na+ transport mechanisms and the salt stress response network”

Temperature Stress

Heat stress induces heat-shock proteins (HSPs), molecular chaperones that bind partially denatured proteins and prevent irreversible aggregation, buying time for normal folding to resume once temperature falls — the same chaperone principle covered for animal/general protein folding elsewhere, applied here as an inducible, stress-triggered response rather than a constitutive one.

Source: MDPI

Cold stress below freezing threatens cells primarily through extracellular ice formation drawing water out of the cytoplasm (worsening cellular dehydration) and, if ice forms intracellularly, direct membrane puncture by ice crystals; cold-acclimated tissue increases membrane unsaturated-fatty-acid content (maintaining membrane fluidity at low temperature, preventing the phase transition that would otherwise rupture the membrane) and accumulates the same compatible solutes used in drought response, since freeze-induced cellular dehydration and drought-induced water deficit present the plant with an overlapping physiological problem.

Biotic Defense: Constitutive and Induced Barriers

Plants maintain constitutive defenses present regardless of attack — the cuticle and cell wall as physical barriers, and preformed antimicrobial/antifeedant secondary metabolites (e.g. tannins, which bind and precipitate proteins, reducing herbivore digestive efficiency and palatability) — alongside induced defenses activated specifically upon attack, which are faster to deploy only when actually needed and less costly to maintain constitutively across the plant’s whole lifetime.

The Hypersensitive Response and Systemic Acquired Resistance

Localized pathogen recognition (via plant immune receptors detecting pathogen-associated molecules) can trigger the hypersensitive response (HR): rapid, localized programmed cell death at and immediately around the infection site, which deprives biotrophic pathogens (which require living host tissue to feed) of the living cells they depend on, physically containing the infection to a small necrotic lesion rather than allowing it to spread through continuously living tissue.

Source: Wikipedia

HR is typically accompanied by a burst of reactive oxygen species and local salicylic acid accumulation, which — beyond its local role — also functions as a signal (itself or via a derived mobile signal) that travels systemically to uninfected parts of the plant, inducing systemic acquired resistance (SAR): a primed, heightened defensive state (including elevated expression of pathogenesis-related (PR) proteins, many with direct antimicrobial activity such as chitinases degrading fungal cell walls) across the whole plant, providing broad, long-lasting resistance to subsequent infection by a wide range of pathogens, not just the one that triggered HR initially — an induced, whole-plant analog of an immune “memory,” though mechanistically unrelated to (and evolutionarily independent of) the adaptive immune memory covered for vertebrates on Immune Physiology.

Source: ScienceDirect (topic page, “Systemic Acquired Resistance”)

Herbivory Defense

Beyond constitutive deterrents like tannins, many species mount induced anti-herbivore responses: mechanical damage or herbivore saliva components trigger jasmonic acid signaling (a hormone pathway distinct from but interacting with the six covered on Plant Hormones), which upregulates production of defensive secondary metabolites — protease inhibitors (impairing the herbivore’s digestive enzymes), alkaloids, or, in some species, volatile organic compounds released from damaged tissue that can attract the herbivore’s own natural predators or parasitoids (an indirect defense, recruiting a third party rather than acting on the herbivore directly) or warn neighboring undamaged tissue/plants to preemptively upregulate their own defenses before being attacked themselves.

Source: ResearchGate, fig. 2, “Schematic picture of induced plant defense”

Comparative Structures

Common Exam Questions

• “Explain why ABA-driven stomatal closure under drought stress necessarily reduces photosynthetic rate, referencing the trade-off established on Stomatal Physiology.”
• “Distinguish the osmotic and ionic components of salinity stress, and describe one plant response to each.”
• “Explain why the hypersensitive response is an effective defense specifically against biotrophic pathogens, referencing what those pathogens require from living host tissue.”
• “Explain the relationship between the hypersensitive response and systemic acquired resistance, naming the mobile signal connecting them.”
• “Distinguish a constitutive plant defense from an induced defense, and explain the trade-off that favors maintaining some defenses constitutively and others only upon induction.”
• “Explain how a volatile organic compound released from herbivore-damaged tissue can function as an indirect defense.”

Visual Reference

Interactive

• Drought response cascade (click-through, extends the Stomatal Physiology diagram) — clicking through rising ABA levels shows stomatal closure, reduced CO2 uptake, and osmotic adjustment occurring in sequence, with a running photosynthesis-rate readout showing the water-conservation/carbon-gain trade-off numerically as ABA rises.

• Hypersensitive response / SAR spread simulator (SVG/JS) — a leaf diagram where clicking an infection site triggers a localized necrotic lesion (HR) and then an animated systemic signal spreading to the rest of the plant, activating a visibly “primed” state (PR protein icon) in distant, uninfected tissue.

(Static images are placed inline in Key Concepts above, next to the concept each one illustrates, rather than collected here. No image was sourced for the Drought Stress section’s compatible-solute mechanism specifically.)

Practice Problems

1. Explain why a drought-stressed plant’s photosynthetic rate falls even before any visible wilting occurs, tracing the causal chain from soil water potential to stomatal aperture to CO2 availability.
2. A plant growing in saline soil accumulates high internal Na+ concentrations but shows no toxicity symptoms. Propose a cellular mechanism explaining this tolerance.
3. Explain why the hypersensitive response would be an ineffective strategy against a necrotrophic pathogen (one that feeds on and benefits from dead tissue), even though it works well against biotrophic pathogens.
4. A plant with an experimentally infected leaf shows heightened pathogen resistance in leaves on the opposite side of the plant that were never directly infected. Explain the mechanism responsible.
5. Explain why maintaining tannins constitutively but reserving jasmonic-acid-induced defenses for actual herbivore attack represents an energetic trade-off, rather than the plant simply deploying its strongest defense at all times.