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)
• 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
Facultative
Halophyte
Intolerant
Halophyte
Obligate Halophyte
Glycophyte
Salinity 
Halophytism in Higher Plants
• Early plants developed in oceanic (i.e., high
salinity) environments
– Marine algae
– Phytoplankton
– Cyanobacteria
Cyanobacterium
Nostoc sp.
Marine algae
(Codium sp.) grow
and reproduce in
waters with
elevated salt
content
• Land plants seem to have lost the
ability to thrive under high salt
conditions; most land plants are glycophytes
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
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
Glycophytes
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
s osmotic (solute) potential
p pressure (turgor) potential
g gravitational potential
Total water potential (w ) = m+s+p+ g
Typical Glycophyte
w = m + s + p + g
Plant
w = 0 + (-0.2) + 0.5 + 0
w = -0.3
Water
Soil
w = m + s + p + g
w = 4.0 + (-0.2) + 0 + (-4.0)
w = -0.2
Typical Halophyte
w = m + s + p + g
Plant
w = 0 + (-4.5) + 1.0 + 0
w = -3.5
Water
Soil
w = m + s + p + g
w = 4.0 + (-3.0) + 0 + (-4.0)
w = -3.0
Regulation of Salt Content in Shoots
• Secretion of salts
– Salt exported via active
transport mechanism
– Excretion includes Na+ and Clas well as inorganic ions
Photograph and schematic diagram of salt gland of
Aeluropus litoralis
Leaf surface
containing salt
gland of Saltcedar
(Tamarix
ramiosissima)
Two celled salt gland of Spartina
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
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 during
the day
– CO2 fixation during
the night
– Sugars accumulate in cells
• Decrease osmotic pressure with organic ions
(proteins)
Summary