Poikilohydry

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”)..

Poikilohydry is coupled with the capacity to tolerate dehydration to low cell or tissue water content and to recover from it without physiological damage. This condition occurs in lichens and some bryophytes that lack mechanisms such as a waterproofing cuticle or stomata that can help resist desiccation.[1] Poikilohydry also occurs in many forms of algae, which may be able to survive desiccation between successive high tides, or during occasional stranding due to the drying of a lake or pond. Similarly, it often occurs in land plants that must survive environmental conditions when water supplies are seasonal or intermittent, as in the liverwort genus Targionia, which lives in Mediterranean habitats with hot dry summers.

Poikilohydry may be regarded as one end of the spectrum of desiccation tolerance, with the other end being homoiohydry, a suite of morphological adaptations and strategies largely seen in vascular plants that enable organisms to regulate or achieve homeostasis of cell and tissue water content.[2] Although the two conditions are often regarded as absolutes, some organisms exhibit characteristics of both. For example, the widespread moss Polytrichum commune has internal water-conducting tissues and regulates water loss during gas exchange by altering its leaf shape, but it lacks the true stomata necessary for water-efficient gas exchange.[3] Likewise, while seeds and spores are often desiccation tolerant, only about 300 species of vascular plants have desiccation-tolerant mature sporophytes;[1] these include resurrection plants (such as Selaginella lepidophylla) and aerophytes (including some species of Tillandsia)..