Halophytes: Salt Tolerant Plants

Halophytes: Salt Tolerant Plants

Halophytes are plants that tolerate or thrive in salty conditions.  I recently finished the illustrations for a chart of seaside flowers, and got to wondering how these plants can survive in these hostile habitats?  Another job, illustrating stamps for a Seaside flowers issue, added to my interest.

This blog had me scouring the internet, and getting more and more fascinated by what I found.  However, I am no expert, and would refer interested readers to the bibliography at the end of the blog for references and further reading.

Sea Bindweed Calystegia soldanella

Salt damage

Salt damages most plants as it messes up the way cells absorb water.  A plant which isn’t adapted for salty (or haline) conditions wouldn’t last long in a salt marsh or coastal area.  Salt water can reduce plant growth and photosynthesis.  It leads to an imbalance of nutrients and ions.  It alters plant hormone production and action.  Most obviously, it makes it hard for plants to regulate their water balance.

Thrift Armeria maritima

Halophytes

Some plants have evolved to survive these harsh conditions.  These are the Halophytes.  They can tolerate a range of salty environments, from salt-marshes to dry and salty deserts.  Their adaptations help them shrug off the effects of salt spray, and allow them to live in soils saturated with salty water.  It’s not every plant that can do this.  Only 1 – 2 % of the world’s flora are halophytes.  Of these, “only 0.25% are reportedly able to complete their life cycles in Saline soils” (Flowers et al 1990, New Phytologist 1990)

(Plants which can’t tolerate salt are called Glycophytes.  This literally translates from the Latin as “Sweet loving plants”.)

Types of Halophyte

There are various classifications of Halophytes, mostly depending on what concentrations of salt they can survive.  There are Obligate halophytes, plants which need salt to grow.  An example of this is the Glasswort, Salicornia.

Common Glasswort Salicornia europaea

There are many more Faculative halophytes.  These plants can tolerate salt, but will also thrive in non-salty conditions.

Some halophytes need wet soil or salt-marshes to survive.  These are termed Hydro-halophytes.  A mangrove tree is the most obvious example.

Mangrove swamp – a haline habitat

Xero-halophytes thrive in dry and salty soils, such as deserts.  They can handle unpredictable rains as well as salty soils.  The Frankincense tree is an example.

Adaptations to Saline environments

Although there aren’t an enormous number of halophytes, they’re distributed across lots of plant families.  It’s believed that the adaptations needed to survive these inhospitable habitats have evolved independently on many occasions.  The fact that so many species have ended up with similar coping mechanisms is yet another example of convergent evolution.

Adaptations: Being a Succulent

Lots of halophytes are succulents.  This means that their stems and leaves are fleshy and watery.  Succulent plants have fewer cells, and these cells are longer than those in other plants.

Hottentot fig Carpobrotus edulis

Salt absorbs water, so it’s vital to counteract this.  In succulents, moisture is preserved using lots of these water bearing cells.  These watery cells manage to dilute the concentration of salt in the sap of the cell.  Thin cell walls allow each cell to swell and accommodate its watery burden.

English stonecrop Sedum anglicum

Adaptations: Small leaves

Halophyte leaves are excellent at counter-acting the desiccating effects of salt.  Many halophyte plants have tiny leaves.  These have a small surface area, so less water is lost through transpiration.  Lots of species have few and small stomata.  Again, this helps the plant cling onto water.

Lesser Sea spurrey Spergularia marina

Though small, leaves may be thick, and succulent.  The ratio of water-storing space to surface area is high.

Biting Stonecrop Sedum acre

Thicker epidermal layers are seen in some halophytes, and many have a thick, waxy cuticle which helps to waterproof the leaves.  However, just because plants (like sedums) have a thick waxy cuticle, this does not necessarily mean they can tolerate salty conditions.  Some can, others can not.

Remember, leaves need to keep the water inside, but they also need to protect the plant from the external damage salt spray can inflict.  The thicker epidermis and cuticle do both.

Some halophytic plants sport leaves with low levels of chlorophyll.  Perhaps this contributes to the blue-ish hue of many of their leaves?

Sea Kale Crambe maritima

Adaptations: Secreting salt & Salt glands

Salt levels can be regulated using salt glands.  These excrete salt, either direct onto the leaf surface, or into a discreet gland.  These can be vacuoles of bladder cells, and are often hidden just below the surface of the epidermis.  In some species, these glands burst; in others they break off and fall from the plant, carrying their toxic salt burden with them.

These salt bladders accommodate the build-up of salt or other ions, and allow a plant to exclude certain elements from its tissues.

Sea lavender species have salt glands just below the level of the epidermal cells.

Sea Lavender Limonium vulgare

Salt glands may be specialised Trichomes (outgrowths from the epidermis of a plant).  Lots of coastal plants have greyish blue stems and leaves.  In many cases, they are covered with a wide variety of trichomes.  Some are simple, some are un-branched.  These not only affect leaf temperature and aid water economy, but contribute to that distinctive hue.

Yellow-horned Poppy Glaucium flavum

Adaptations: Tough seeds

Seeds of halophytes have been widely researched, and their viability and ability to germinate in salty conditions is amazing.

Many have thick and waxy seed coats.  Seeds may be large.  However, it is the hormonal regulation and patterns of germination which are most interesting.

Germination times are often very fast, and times of reproduction and germination can be tightly controlled by plant hormones.  Recovery of germination after salt-stress or drought (in xerohylophtes) is rapid.  Flowers and Colmer have done extensive research on this topic.

Greater Sea spurrey Spergularia media

Greater Sea spurrey, whose seed dormancy patterns have been examined extensively by Ungar.

Adaptations: Amazing roots

Roots have an important role to play in salt regulation.  Some halophytes produce pneumatophores, structures which protrude from salty water into the air (see my blog on Root variety for more on this).

Other plants have extensive networks of roots which grow into less salty substrates.  Adventitious roots allow for horizontal growth, which could allow a plant to grow directly above saltier soils.

Marram grass Ammophila arenaria

Adaptations: Accumulate salt then die

A more extreme solution is just to accumulate salt…then die.  Some rush (Juncus) species do this.  They have no means of regulatiing their salt balance.  However, this doesn’t seem to stop them from colonising salty environments and reproducing successfully.

Why choose a salty environment?

Having looked at adaptations to this hostile environment, one has to ask, “why grow there?”  Clearly, the salt is problematic and has required an armory of evolutionary coping mechanisms.  So why spend that energy to exploit such an environment?

Firstly, there’s not a lot of competition.  As stated earlier, 95% of plants can’t survive saline habitats.  That’s 95% less potential competitors for your niche.

Haline habitats may also be lower in predators, and may help keep numbers of vermin down.  In the literature there’s also some suggestion that salty environments can help prevent disease, although I didn’t examine this fully.

Examples of Halophytes

So what plants are halophytes?  It partly depends on your definition, but below are some examples.

In the grass family Poaceae, Marram grass and Cord Grass grow on salty sand dunes.

English Cord-grass Spartina anglica

The Amaranthaceae family includes the obligate halophyte Glasswort.  It also includes Saltwort Salsola kali.

Saltwort

Other members of this family are Pig-weeds, Goose-foot, and Beet.

Sea Beet Beta vulgaris maritima

In the Plumbaginaceae family there’s Sea Lavender

In the Legumes we have the Sea pea, Lathyrus japonicus

Sea pea Lathyrus japonicas

There are databases of halophytic plants, including the Halophyte Database and a list of salt-tolerant plants from the Biosalinity Awareness Project

Why are Halophytes so important in 2020?

Halophytes aren’t just fascinating plants.  They could be vital to us humans, in our rapidly changing world.  Most crops are glycophytes, and are salt-sensitive.  With many places at increasing risk from rising sea levels, crops which are resistant to salty water could have an important role to play.  Research is being done to see if cross-breeding and genetic modification could help develop new salt-resistant crop plants.

(Sea Sandwort, below, is edible.  However, I found no evidence it was being trialled as a salt-resistant crop…as yet!)

Sea Sandwort Hockenya peploides

Salt-affected and land made toxic with heavy metals areas could be cleaned with the help of halophytes.  Some halophytes are able to regulate the ions entering their xylem stream.  These ions include sodium and other elements.    Scientists such as Lutts & Lefevre are researching their potential role as a way to clean heavy metals from the soil. (Lutts & Lefevre 2015  How can we take advantage of halophyte properties to cope with heavy metal toxicity in salt-affected areas?  Annals of Botany 2015).  Halophytes may prove vital in these processes of phytoremediation.

Ecologically, halophytes have an important role to play with land reclamation.  Their networks of tough roots and ability to withstand tidal flooding make them perfect candidates to help re-colonise saline lands.

Sea buckthorn Hippophae rhamnoides

With halophytes helping humanity reclaim salty land, and produce salt-tolerant crops for a rising population in an environmentally changing world; I think it would be hard to over-estimate their importance to our future.

Conclusion

With their ingenious adaptations and ability to colonise salty habitats, halophytes are fascinating.  Couple this with their potential as an important aid to humanity, and they become ever more deserving of our attention.

Below is a list of further reading.  There are many nuances to current research which hasn’t been covered in this blog; issues relating to biochemistry and seed viability amongst them.  Hopefully the bibliography below will allow an interested reader to pursue the topic further.

(Many of these original illustrations are available to buy, just search for them in by name in the “Original Illustrations for Sale” section of my website).

Bibliography

Colmer & Flowers, 2008  Salinity tolerance in halophytes  New Phytologist  179

Dassanayake & Larkin, 2017 Making Plants Break a Sweat: The structure, function, & evolution of plant Salt glands Frontiers of Plant Science 2017

Flowers & Colmer,  2015 Plant Salt Tolerance: Adaptations in Halophytes   Annals of Botany, February 2015

Flowers et al, 1990   Salt tolerance in the halophytic wild rice, Porteresia coarctata Tateoka  New Phytologist  1990 

Gonzalez, 2019 Adaptation of Halophytes to Different Habitats  DOI: 10.5772/intechopen 87056 link

Gupta, Halophyte Plants Biology Discussion 

Lutts & Lefevre 2015  How can we take advantage of halophyte properties to cope with heavy metal toxicity in salt-affected areas?  Annals of Botany 2015

Nikita, Halophytes: Classification and Characters of Halophytes Biology Discussion

Reddy, Halophytes: Meaning and Types Biology Discussion

Ungar, I. A. & Binet, P., Factors influencing seed dormancy in Spergularia media, Aquatic Botany, 1, 45, 1975.

Ventura & Sagi, 2015 The Development of Halophyte-based agriculture: past & present  Annals of Botany 2015