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Halophytic Plants. Biology 561 Barrier Island Ecology. Niceties. 80% of the earth is covered by saline water Very few plants are able to tolerate saline conditions without serious damage Plants that survive in saline environments are termed halophytes (c.f., glycophytes)

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Halophytic Plants

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Halophytic plants l.jpg

Halophytic Plants

Biology 561 Barrier Island Ecology

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  • 80% of the earth is covered by saline water

  • Very few plants are able to tolerate saline conditions without serious damage

  • Plants that survive in saline environments are termed halophytes (c.f., glycophytes)

  • Most halophytes prefer saline conditions but can survive in freshwater environments

  • Most halophytes are restricted to

    saline environments

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What is a halophyte?

  • The term “halophyte” has not been precisely defined in the literature:

    • Plants capable of normal growth in saline habitats and also able to thrive on “ordinary” soil (Schimper, 1903).

    • Plant which can tolerate salt concentrations over 0.5% at any stage of life (Stocker, 1928).

    • Plants which grow exclusively on salt soil (Dansereau, 1957).

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What is a halophyte?

  • Categories of halophilism:

    • Intolerant Plants grow best at low salinity and exhibit decrease in growth with increase in salinity

    • Facultative Optimal growth at moderate salinity and diminished growth at both low and high salinities

    • Obligate Optimal growth at high or moderate salinity and no growth at low salinity

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Hypothetical Glycophyte/Halophyte Growth in Various Salinities

Facultative Halophyte


Growth 

Obligate Halophyte


Salinity 

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Halophytism in Higher Plants

  • Early plants developed in oceanic (i.e., high salinity) environments

    • Marine algae

    • Phytoplankton

    • Cyanobacteria

  • Land plants seem to have lost the

    ability to thrive under high salt

    conditions; most land plants are glycophytes

Marine algae (Codium sp.) grow and reproduce in waters with elevated salt content


Nostoc sp.

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Angiosperm Halophyte Types

  • Marine angiosperms

  • Mangroves

  • Coastal strand

  • Salt marshes

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Saline Soils

  • Possess large quantities of Na+

  • Na+ adsorption on clay particles reduces Ca++ and Mg++ content of soils

  • Marsh soils are typically:

    • Low in oxygen

    • High in carbon dioxide

    • High in methane

  • Marsh soils are constantly changing due to the ebb and flow of the tides

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Water Potential

  • Water potential is a measure of the free energy (or potential energy) of water in a system relative to the free energy of pure water

  • The water potential symbol is psi, 

  • Unit of measure (pressure) = megapascals (Mpa) (10 Mpa = 1 bar [approx. 1 atmosphere])

  • Pure, free water w= 0 (the highest water potential value)

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Components of Water Potential

  • w total water potential

  • m matric potential

  • s osmotic (solute) potential

  • p pressure (turgor) potential

  • g gravitational potential

  • Total water potential (w) = m+s+p+ g

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Typical Glycophyte

w = m + s + p + g


w = 0 + (-0.2) + 0.5 + 0

w = -0.3


w = m + s + p + g


w = 4.0 + (-0.2) + 0 + (-4.0)

w = -0.2

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Typical Halophyte

w = m + s + p + g


w = 0 + (-4.5) + 1.0 + 0

w = -3.5


w = m + s + p + g


w = 4.0 + (-3.0) + 0 + (-4.0)

w = -3.0

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Regulation of Salt Content in Shoots

Leaf surface containing salt gland of Saltcedar (Tamarix ramiosissima)

  • Secretion of salts

    • Salt exported via active

      transport mechanism

    • Excretion includes Na+ and Cl- as well as inorganic ions

Two celled salt gland of Spartina

Photograph and schematic diagram of salt gland of Aeluropus litoralis

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Salt Glands in Black Mangrove (Avicennia marina)


(a) sunken gland on upper epidermis; (b) elevated gland on lower epipermis


Concentrations of secreted salts is typically so high that under dry atmospheric conditions, the salts crystallize

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Regulation of Salt Content in Shoots

  • Salt leaching

    • Not well understood, but results from transport of salts to the near epidermis of leaves; precipitation leaches salts

  • Salt-saturated leaf fall

    • Leaves shed after accumulation of salts

    • Occurs in Hydrocotyle bonariensis and others

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Responses to Increased Salts

  • Succulence Plant organs are thickened due to increased cellular water content

  • Increased growth Reduces cellular solute concentrations

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Seed Dispersal in Halophytes

  • Most seeds of halophytes are buoyant

    • Examples are glasswort (Salicornia sp.), coconut (Cocos nucifera), sea rocket (Cakile sp.), and suaeda (Suaeda maritima)

  • Marine angiosperm seeds are not buoyant

    • Examples are Thalassia and Halophila

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Germination in Halophytes

  • Germination inhibited by high salt concentrations

  • Chlorides are very toxic to germinating plants

  • Optimum germination is in freshwater

  • Germination response in salt water not necessarily correlated to later growth of a plant species under saline conditions

  • Higher temperatures slow germination in salt water

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Physiological Response in Halophytes

  • Switch from Carbon-3 photosynthesis to CAM (crassulacean acid metabolism)

    • Stomates closed during

      the day

    • CO2 fixation during

      the night

    • Sugars accumulate in cells

  • Decrease osmotic pressure with organic ions (proteins)

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