This week’s species extraordinaire is the ice plant, Mesembryanthemum crystallinum, a member of the fig-marigold family (Aizoaceae). The plant’s common name ‘ice plant’ is in reference to the plants leaves, which look like they are covered in glistening, pebbly ice. However, the ice plant lives in far from icy habitats. It is a native of Africa but widely distributed through the coastal United States, Australia, and the Mediterranean, where it is an invasive species. In the United States, the ice plant was introduced along the coast of California in the 1960’s to stabilize sand dunes, although the practice was quickly discontinued after the plant naturalized and began to outcompete native species and choke out every other plant in its path.
The glistening, ice-like surface of its leaves is key to Mesembryanthemum’s competitive success. Many plants have leaves that are covered in hair-like cells called trichomes. In ice plants these trichomes are modified for water storage and are fittingly called bladder cells. The bladder cells balloon outwards with stored water, making the surface of the leaf seem pebbly and shiny. These cells actively accumulate salts and/or oxalate, which
causes water to flow into them, inflating them like balloons and squishing the metabolically active cytoplasm of the cell into a thin layer against the cell wall. The majority of the cell is occupied by a massive water-filled vacuole, and a waxy covering on the surface of the bladder cells prevents water loss from evaporation. The bladder cells can hold an astounding 25% of the volume of the plant in water, giving Mesembryanthemum a competitive edge in arid environments where it is able to efficiently suck up moisture from the soil and store it for periods of drought. All that salt accumulation doesn’t just give the ice plant a water storage advantage. When an ice plant dies, its cells rupture, releasing these concentrated salts into the soil and making it uninhabitable for most plants. However, Mesembryanthemum can tolerate soils with high salinity even as seedlings, so the ice plant is free to germinate and grow in these saline patches with little or no competition.
The ice plant is not just adapted for water storage, but for water conservation too. The majority of plants photosynthesize via a C3 mechanism that uses light energy to fix carbon dioxide molecules into three carbon molecules that are eventually joined together to form simple sugars. It is an energetically efficient process when light conditions are normal and water is freely available, but the plant’s air-exchanging stomata remain open throughout the hottest parts of the day, meaning the plant loses significant amounts of water through evapotranspiration. Young ice plants use C3 photosynthesis, but they rapidly develop what is known as inducible CAM photosynthesis. When faced with drought conditions, the ice plant is able to shift to a more energetically costly two-step CAM photosynthesis process that allows it to conserve water. During the night, the plant keeps its air-exchanging stomata open and collects carbon dioxide, which it stores in its leaf cells as four-carbon malate molecules. Come daybreak, the plant closes its stomata, preventing water from escaping from its leaves during the heat of the day. It then transports all of the malate it made over night into its chloroplasts and sets about using light energy to convert it into sugars. Inducible CAM photosynthesis allows the ice plant to prioritize its energy expenditure and water loss according to environmental conditions, giving it a competitive advantage over other plants that are less physiologically flexible. Given its flexible physiology, water storage capacity, and dirty tricks to make the neighbourhood less friendly, it is little wonder that the ice plant has become such a successful invasive species in dry environments on multiple continents.