Poikilohydry is the lack of ability (structural or functional mechanism) to maintain and/or regulate water content to achieve homeostasis of cells and tissue connected with quick equilibration of cell/tissue water content to that of the environment. The term is derived from Ancient Greek ποικίλος (poikílos, “spotted or variegate”).[1]
Tolerance to desiccation has been utilized in the Archaea, Bacteria, and Eukaryote kingdoms[1] to take advantage of ecological niches.[2] The tolerance to desiccation is often combined with other abiotic stress factors such as temperature extremes, malnutrition, vitamin imbalances, salinity content, and ultraviolet radiation. Many plants control desiccation tolerance through non-specialized structures such as vegetative tissues or specialized structures such as spores, seeds, and tubers. Desiccation tolerance is distributed among Bryophytes that have no cuticle or stomata, nine Pteridophyte families and ten Angiosperm families, vascular plants that do have a cuticle and stomata.[1][3]
Selaginella lepidophylla is a vascular lycophyte native to the Chihuahuan Desert in New Mexico, Texas and Mexico. It occurs in north-facing rock crevices and in open habitats. The notable leaf curling attributed to S. lepidophylla, tested by Lebkeucher and Eickmeier in 1991, occurs to prevent photoinhibition in the microphylls in response to UV radiation and gradual leaf uncurling when rehydrated, protects the plant from the same photoinhibition until photon fluxes are fully processed.[3]
Hymenophyllaceae are poikilohydrous ferns that grow in high-humidity, high-shade areas like the rainforests of Trinidad, Venezuela and New Zealand. These ferns face the unique challenges of having a high-resistance for gas exchange and limited water loss. This provided strong selective evolutionary pressure towards individuals that were the most energy efficient. Two major components of efficiency have been focused on by these ferns: reducing wasteful structures and increasing light absorption. Regulating water levels takes energy and it is not critical to survival in these environments, so structures such as the stomata are no longer present in Hymenophyllaceae. Thick mesophyll is inefficient in environments with low-sunlight and makes gas exchange more difficult, so Hymenophyllaceae have adapted thin mesophyll. When removed from their natural habitat, Hymenophyllaceae are vulnerable. Compared to other ferns they are more susceptible to desiccation, which is a tradeoff they had to make to adapt to their environment.[2]