A textbook truism about the poor ferns being held back by a weak link in their life cycle may not be so true after all.
The upright bursts of fronds that we think of as ferns produce unferny offspring in the form of bits of free-living, filmy, green tissue. In a pattern of alternating generations, these scraps of green create gametes that give rise to new fronds.
Gametophytes, typically the size of a fingernail and only one cell thick, get portrayed as “wimpy little delicate things,” says James E. Watkins Jr. of Harvard University. Their need for wet environments has supposedly hampered fern species’ spread into dry habitats.
When Watkins and his colleagues tested gametophytes of 12 species of tropical ferns, however, most showed some ability to bounce back after drying out. Seven species recovered at least 70 percent of their chlorophyll function after desiccation, Watkins and his colleagues report in the November New Phytologist.
“It was a surprise to me,” comments Melvin Oliver, a U.S. Department of Agriculture researcher based in Columbia, Mo., who has studied desiccation tolerance. Watkins’ finding “makes me want to go back and rethink,” he says.
Oliver emphasizes the distinction between dehydration and desiccation. Botanists describe a plant as desiccated when its water concentration has dropped to that of the air around it, usually a state too dry for normal cell operations. Plants that survive such an ordeal need special physiological mechanisms to endure the shutdown and then repair the damage.
Only a few flowering plants can desiccate and then go back into business when they rehydrate, Oliver explains. That ability is mostly the province of ancient, nonflowering lineages, such as mosses and liverworts.
Several researchers in the early 20th century described desiccation tolerance in two species of fern gametophytes, notes Watkins. But the last survey of desiccation tolerance dates from the 1930s.
“I guess a lot of fern people haven’t really thought about it,” says Michael C.F. Proctor of the University of Exeter in England.
Watkins started thinking about gametophytes as he contemplated what determines which species of ferns can live where. Some ferns do live in deserts or in other parched places, such as the wind-blasted canopies of tropical trees. Yet the prevailing wisdom had been that bursts of rain allowed the gametophytes to live out their lives before drought set in again.
Watkins collected fern spores from a range of tropical habitats, including canopies, shadowy ground, and swamps. He germinated the spores in the lab and subjected the small frills of gametophytes to a series of desiccation tests.
For example, he put the gametophytes in chambers at 50 percent humidity, until the plants desiccated. After 24 hours, he rehydrated them.
To check for recovery, he measured the fluorescence of the gametophytes’ chlorophyll. When damage disables a plant cell, the remnants of chlorophyll fluoresce more strongly since the cell can no longer process the energy captured by the pigment. Watkins found that some fern gametophytes, particularly those of canopy species, recovered much of their normal chlorophyll.
The research “definitely shows desiccation tolerance,” says Oliver. Now he says that he’d like to know how long the gametophytes can tolerate desiccation and how the lab conditions relate to those in the wild.