How Do Plants Live in Salt Water? Handling What Kills Everything Else

Salt harms plants in two distinct ways that require different solutions. The first is osmotic, since water moves from where dissolved substances are dilute towards where they are concentrated, so a root sitting in salty water faces water leaving the plant rather than entering it, which produces drought conditions in the middle of abundant water. The second is toxic, since sodium and chloride entering the plant interfere with enzymes, disrupt the balance of other ions the plant needs, particularly potassium, and damage the machinery of photosynthesis as they accumulate in leaves. A plant surviving in saline conditions therefore has to solve a water acquisition problem and an ion management problem simultaneously, and different species weight the two differently.

Salt-tolerant plants use several approaches, frequently in combination:

• •Excluding salt at the root, filtering it out so little enters the plant at all
• •Accumulating compatible solutes in cells, which restores water balance without poisoning the enzymes
• •Compartmenting salt in the vacuole of each cell, where it is out of contact with the machinery
• •Excreting salt through specialised glands on the leaves, visible as crystals
• •Succulence, diluting the salt taken up in a large volume of stored water
• •Sacrificing leaves, loading old leaves with salt and dropping them

The habitats involved are distinctive and several are among the most productive ecosystems anywhere. Salt marshes on temperate coasts are dominated by a small number of highly tolerant species arranged in zones according to how long each is submerged, and that zonation is a textbook illustration of species sorting along an environmental gradient. Mangroves occupy the tropical equivalent, with trees that filter salt at the root, excrete it from leaves or both, and that support enormous quantities of marine life and protect coasts from storms. Inland salt flats and desert basins hold their own specialised floras. Saline soils inland arise where evaporation exceeds rainfall and salts accumulate, which occurs naturally and is enormously accelerated by irrigation.

Several salt-tolerant plants are already food and their histories are worth knowing. Samphire, a fleshy coastal plant, has been gathered from salt marshes for centuries and is now sold commercially, with its salinity being the point rather than a drawback. Sea kale and sea beet grow on shingle and cliff and are the wild ancestors of cultivated beet and chard, which retain some tolerance as a result. Saltwort was harvested and burned across the Mediterranean to produce soda ash for glass and soap, which was a substantial industry before synthetic production replaced it. Quinoa tolerates moderate salinity and is grown on saline soils in the Andes. Mangrove fruits are eaten in several regions after processing to remove tannins. These are the practical starting points for anyone trying to domesticate tolerance rather than breed it in.

Salinisation of agricultural land is a large and growing problem and these plants are being studied as part of the response. Irrigation water carries dissolved salts, and where drainage is poor and evaporation high those salts concentrate in the soil, which has degraded a substantial share of irrigated land worldwide and has ended agriculture in regions where it was practised for millennia, a process documented in ancient Mesopotamia and repeating in several modern irrigation schemes. Rising seas and reduced river flow push saline water into coastal aquifers and deltas. The responses being examined include breeding salt tolerance into conventional crops, which has proved difficult because tolerance involves many genes, and domesticating tolerant wild species directly, with several coastal plants under development as grain, forage and oilseed crops irrigated with seawater.

Salt draws water out of roots and poisons cells, so tolerance requires solving a water problem and an ion problem at once. Exclusion at the root, compatible solutes, compartmenting in vacuoles and excretion through leaf glands are the main strategies. Irrigation concentrates salts where drainage is poor, which has degraded a large share of irrigated land and is driving work on tolerant crops.