Halophytic PlantsHalophytic Plants

Biology 561 Barrier Island EcologyBiology 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)• Most halophytes prefer saline conditions but can

survive in freshwater environments• Most halophytes are restricted to saline environments

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).

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

Hypothetical Glycophyte/Halophyte Growth

in Various Salinities

Salinity

Gro

wth

Glycophyte

Intolerant Halophyte

Facultative Halophyte

Obligate Halophyte

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

Cyanobacterium Nostoc sp.

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

Angiosperm Halophyte Types

• Marine angiosperms

• Mangroves

• Coastal strand

• Salt marshes

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

Osmotic potentials of some halophytes of the eastern coast of United States

Species Osmotic pressure (atm)

Seawater (New Jersey) 23.2

Spartina glabra 31.1

Spartina patens 31.1

Spartina michauxiana 31.1

Salicornia europaea 31.1

Distichlis spicata 28.8

Limonium carolinianum 28.8

Juncus gerardii 28.8

Baccharis halimifolia 26.1

Atriplex hastata 26.1

Hibiscus moschuetos 12.2

Contribution of NaCl to the osmotic potential (OP) of glycophytes and halophytes

Osmotic potential of plant sap (atm)

Species

OP of soil solution (atm)

OP calculated as NaCl

OP due to other substances Total OP

Halophytes

Atriplex portulacoides

27.7 36.4 4.7 41.1

Salicornia fruticosa

20.6 31.7 9.6 41.3

Inula crithmoides 17.0 17.6 7.1 24.7

Statice limonium 10.5 18.5 5.0 23.5

Juncus acutus 9.3 11.9 7.5 19.4

Plantago coronopus

4.0 7.7 4.0 11.7

Glycophytes

Pistacia lentiscus A 4.5 20.1 24.6

Phillyrea latifolia A 3.4 19.7 23.1

Pinus pinaster A 6.9 15.0 21.9

Quercus ilex A 2.2 24.6 26.8

A Osmotic potential was not measured but is presumably very low.

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)

Components of Water Potential

w total water potential

m matric potential

ss osmotic (solute) potential osmotic (solute) potential

p pressure (turgor) potential

g gravitational potential

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

Typical Glycophyte

w = m + ss + p + g

w = m + ss + p + g

w = 4.0 + (-0.2) + 0 + (-4.0)

w = -0.2

w = 0 + (-0.2) + 0.5 + 0

w = -0.3

Plant

Soil

Water

Typical Halophyte

w = m + ss + p + g

w = m + ss + p + g

w = 4.0 + (-3.0) + 0 + (-4.0)

w = -3.0

w = 0 + (-4.5) + 1.0 + 0

w = -3.5

Plant

Soil

Water

Regulation of Salt Content in Shoots

• Secretion of salts– Salt exported via active

transport mechanism– Excretion includes Na+ and Cl-

as well as inorganic ions

Leaf surface containing salt gland of Saltcedar (Tamarix ramiosissima)

Two celled salt gland of Spartina

Photograph and schematic diagram of salt gland of Aeluropus litoralis

Salt Glands in Black Mangrove (Avicennia marina)

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

a

b

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

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

Responses to Increased Salts

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

• Increased growth Reduces cellular solute concentrations

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

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

Physiological Response in Halophytes

• Switch from Carbon-3 photosynthesis to CAM (crassulacean acid metabolism)– Stomates closed duringthe day– CO2 fixation during the night– Sugars accumulate in cells

• Decrease osmotic pressure with organic ions (proteins)

Summary