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Halophytic Plants

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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  1. Halophytic Plants Biology 561 Barrier Island Ecology

  2. 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) • Most halophytes prefer saline conditions but can survive in freshwater environments • Most halophytes are restricted to saline environments

  3. 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).

  4. 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

  5. Hypothetical Glycophyte/Halophyte Growth in Various Salinities Facultative Halophyte IntolerantHalophyte Growth  Obligate Halophyte Glycophyte Salinity 

  6. 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 Cyanobacterium Nostoc sp.

  7. Angiosperm Halophyte Types • Marine angiosperms • Mangroves • Coastal strand • Salt marshes

  8. 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

  9. 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)

  10. 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

  11. Typical Glycophyte w = m + s + p + g Plant w = 0 + (-0.2) + 0.5 + 0 w = -0.3 Water w = m + s + p + g Soil w = 4.0 + (-0.2) + 0 + (-4.0) w = -0.2

  12. Typical Halophyte w = m + s + p + g Plant w = 0 + (-4.5) + 1.0 + 0 w = -3.5 Water w = m + s + p + g Soil w = 4.0 + (-3.0) + 0 + (-4.0) w = -3.0

  13. 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

  14. Salt Glands in Black Mangrove (Avicennia marina) a (a) sunken gland on upper epidermis; (b) elevated gland on lower epipermis b Concentrations of secreted salts is typically so high that under dry atmospheric conditions, the salts crystallize

  15. 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

  16. Responses to Increased Salts • Succulence Plant organs are thickened due to increased cellular water content • Increased growth Reduces cellular solute concentrations

  17. 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

  18. 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

  19. 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)

  20. Summary

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