Leaves Accumulate Excess Salt

light green leaves with small salt crystals on them, appearing as snowflakes

Biological Strategy

Mangroves

Natalie Chen

Image: Florida Sea Grant / Flickr / Some rights reserved

Capture, Absorb, or Filter Chemical Entities

Living systems often require chemical elements and chemical compounds, including complex sugars, proteins, and odor-making compounds, to perform critical activities. These compounds exist in various states–solid, liquid, and gas–and are ubiquitous in soil, water, and air. This requires that living systems not only have ways to capture, absorb, or filter them, but also ways to differentiate among them, selecting those that are valuable or harmful. For example, mangrove trees live with their roots in salty water and sediments. Various mangrove species have different strategies for removing salt from the water they take in so that their tissues can use the fresh water.

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Expel Solids

Living systems must often discharge solids–such as seeds, eggs, pollen, and mineral salts–for reproduction, protection, or other reasons. Solids can be easily moved when a living system applies force to them. But creating that force requires energy, so living systems must either have efficient strategies worth the energy investment or use an outside force (such as gravity). For example, a tropical mistletoe positions its pollen so that the weight of a bird landing nearby triggers the pollen’s release onto the bird’s forehead. The bird then fertilizes other mistletoes that it visits.

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Maintain Homeostasis

When a living system is in homeostasis, it means that internal conditions are stable and relatively constant. For example, a human’s internal temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit) unless there’s an illness. The human body maintains this temperature despite external ambient temperature. However, as with all physiological processes, maintaining homeostasis requires communication and coordination. So living systems have ways to detect changes from the norm, mechanisms to cause an adjustment, and negative feedback connections between the two. A desert lizard called the Gila monster offers a good example of maintaining homeostasis. The lizard goes from eating large meals to fasting for extended time periods. To maintain its blood sugar levels at a steady level, when food is scarce, its endocrine system releases a hormone that raises its blood sugar levels.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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The leaves of some mangroves regulate salt concentrations by accumulating excess salt in special compartments within their cells.

Mangroves are small shrubs or trees that grow in the presence of salt water along coastlines. Constant exposure to salt water can lead to an imbalance of ions within the plant’s cells, which can have toxic effects on enzymes or interfere with water absorption. To address this, the cells control their internal environment by keeping sodium, potassium, and chloride ions at a stable concentration. Mangroves can survive in especially salty environmental conditions using various strategies that regulate how much salt ends up in their tissues. For example, certain species exclude salt by preventing salt from entering the roots, some excrete excess salt through glands in their leaves, while others accumulate ions in special compartments in their leaves.

Ion sequestration is one mechanism used to relieve the negative effects of salt stress. Mangroves reduce the amount of sodium ions, for example, in the cell’s main compartment by moving excess sodium into a special membrane-enclosed compartment called a vacuole. By sequestering excess ions into vacuoles, the cell’s main compartment maintains a balanced concentration of ions.

These ions are transported into the vacuole with special proteins embedded in the vacuole membrane, such as proton (hydrogen ion) pumps and sodium-hydrogen antiporters. These membrane proteins work together to build a high concentration of sodium ions in the vacuole. Because the ions are moving from an area of low concentration to a high concentration (against its concentration gradient), this requires energy. First, proton pumps use the energy-rich molecule, ATP, to move protons into the vacuole and establish a proton gradient. This proton gradient functions as stored energy. With the help of the sodium-hydrogen antiporters, the protons flow down their concentration gradient out of the vacuole and into the cell’s main compartment. This releases the stored energy, which is then used to transport sodium ions into the vacuole. The sodium ion concentration in the vacuole builds and the ions remain there until the leaf matures and falls.

This strategy was contributed by Natalie Chen and edited by Dimitri Smirnoff.

Last Updated January 23, 2018

References

“One distinctive structural feature of plant cells is the presence of large membrane-bound compartments, the vacuoles. Early biochemical and tonoplast transport analyses led to the model that the sequestration of excessive Na⁺ under salt stress is mediated by Na⁺/ H⁺ antiporters localized in the vacuolar membrane (Blumwald and Poole, 1985, 1987). These Na⁺/H⁺ antiporters mediate Na⁺ uptake into vacuoles, which is driven by the vacuolar proton gradient established by the vacuolar (V-type) proton ATPase that acidifies the vacuolar lumen (Fig. 1). The resulting vacuolar Na⁺ sequestration protects essential enzymatic reactions in the cytoplasm from excess Na⁺ levels while maintaining turgor (Glenn et al., 1999).” (Horie and Schroeder 2004: 2457)

Journal article

Sodium Transporters in Plants. Diverse Genes and Physiological Functions.

Plant Physiology, 136: 2456-2462 | 01/09/2004 | Horie T; Schroeder JI

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Reference

“Vacuolar sequestration of Na⁺ is an important and cost-effective strategy for osmotic adjustment that also reduces the Na⁺ concentration in the cytosol. Na⁺ sequestration into the vacuole depends on expression and activity of Na⁺/H⁺ antiporters as well as V-type H⁺– ATPase and H⁺– PPase. These phosphatases generate the necessary proton gradient required for activity of Na⁺/H⁺ antiporters. Salt accumulation in mangroves occurs with the sequestration of Na⁺ and Cl^–into the vacuoles of the hypodermal storage tissue of the leaves (Werner and Stelzer 1990; Aziz and Khan 2001a; Kura-Hotta et al. 2001; Mimura et al. 2003).” (Parida 2010: 203)

Human kidneys filter waste from the blood and recycle beneficial compounds back into it using osmosis, variations in membrane permeability, and protein pumps.

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