Salt vines in use on Uoagranyu, removing sodium and magnesium salts from ocean water during the terraformation process
Salt vines are essentially a solar-powered, biological desalination system based on wall-climbing vines. Most consist of a single, fairly straight trunk from which a flat, roughly rectangular array of branches and leaves grow, reflecting their origins as a trellis-grown vine that were typically arranged in much the same way as photovoltaic panels to maximize their light exposure.
The desalination process occurs primarily along the single, fairly straight trunk as it transports water and nutrients to the stems and leaves. The desalination process is gradual and most forms of the plant need about two to three meters of height to convert water as saline as Earth's oceans into freshwater. Originally this was exuded along the trunk and in sacrificial leaves with salt concentrating on the lower portion of the plant. Modern species of salt vines address the salt by many methods, depending on the intended application. Industrial-scale desalination vines tend to exude from their trunk so it falls around the base of the plant with the expectation operators or some mechanism will collect it. Others will concentrate it in pseudo-fruits, sometimes even produce large salt crystals in a thin gourd. Those crossed with deliplants use it to simulate salt-cured meats and cheeses. Some still collect it in sacrificial leaves, which may be dried to produce flavored salts (garlic, hot peppers, cilantro, and others are known).
Diversification of salt vines is mostly based on application. They are rarely used in large-scale industrial desalination anymore because of low energy efficiencies compared to nanotechnological, algal, or even Information Age reverse osmosis desalination techniques. However, such salt vines are the most productive, delivering hundreds of liters of distilled water per day per square meter of vine, but have the expectation of bountiful hydroponic nutrients, such as sewage, sun-tracking trellises, and constant attention from agrimonkeys, synsects, etcetera.
Other high capacity vines are designed for independent operation, such as nurturing non-halophilic plants and ecosystems in desolate environments and may be used to create oases or verdant oceanic islands. These tend to feature deep-growing roots that can seek out saline aquifers and cleverly balanced tropisms to that throttle the vine's operation to avoid depleting the soil of nutrients, burying itself in excess salt, and so on.
Another common application is to provide safety for travelers in isolated areas. These low-output vines may store water in their trunks, where it may be squeezed out or eaten by travelers, or in durable gourds. These salt "vines" are often engineered to stand on their own as shrubs or small trees so they aren't trampled. Some may be planted at exit ramps of road roots.
The final notable application for salt vines is gardening: a few low-output vines can easily meet the needs of an isolated home while growing various savory deliplant fruits, like salt-cured pork or feta cheese.
Salt vines are a plant that originated in the Second Century AT as humanity rushed to address its chronic shortages of freshwater. In that era, extant aquifers were almost exhausted and rising seas were endangering coastal freshwater supplies. "Salt vines" were one solution, essentially a solar-powered, biological desalination system.
The original vines were intended for controlled hydroponic conditions owing to contemporary, pervasive fears of genetically modified organisms. They were designed to grow on trellises, and keeping their roots in hydroponic piping allowed the trellises to track the sun without injuring the roots as would have happened had they been planted in soil.
Initially, output was low and limited by the speed of capillary action. A Second Century trellis of salt vines produced about two liters of freshwater per square meter where the vines could intercept five kilowatt-hours or more of sunlight daily. The engineering improved rapidly, though, and well-fed salt vines could produce hundreds of liters per square meter daily by the mid-300s. This was a shadow of non-biological desalination, which needed somewhat less than one kilowatt-hour per cubic meter of desalinized seawater by the same era. However, such desalination systems required energy from a separate power plant, and required a substantial investment in the system itself. The early salt vines were a low-cost, self-growing addition to sewer treatment plants. This gave the vines a viable economic niche.
For a couple of centuries they were quite common on Earth. Variants were engineered to help bring water to parts of Earth dried out by climate change. For example, "mangrove salt vines" could shelter drying ocean islands and coasts from waves and supply water inland. Others were used where farmland had grown salty from centuries of irrigation, or where depletion of aquifers had led to saltwater intrusion. They were mostly obsolete by the Technocalypse and hard hit by a number of plagues, but revived during The Recovery for those same restorative purposes.
A heavily engineered variant found use on Mars before the Technocalypse wiped those out. They were able to process water and permafrost contaminated by Mars' pervasive perchlorate salts to supply habitats and terraforming oases with freshwater. Similar salt vines found work in some interstellar colonies during the First Federation as tools for terraforming, ecopoesis and habitat life support.
In the 11th Millennium AT, salt vines have largely transitioned to a completely different niche: low maintenance desalination. Nanotechnological and engineered algal solutions are much more space- and energy-efficient, and better suited for reliable, controllable utilities. Modern salt vines have been re-engineered to survive in low nutrient, desolate environments to support travelers and hermits, or create oases. Others have been hybridized with deliplants to produce assorted salty foods and are popular with gardeners.
Environmental Engineering - Text by M. Alan Kazlev, from the original by Robert J. Hall Ensuring environments remain favourable to bionts. Includes both environmental protection (pollution control, waste recycling or treatment) and habitat biosphere optimization, biont hygiene and health issues and standards, and biosphere engineering in general (biospherics).