Sporobolus stapfianus is a member of one of the largest and most important plant families, the Poaceae, but, alas, as Gaff et al. have put it, it is “obscure”. In this case, the desiccation tolerance is not apparent in all tissues, but is restricted to immature leaves of intact plants, and an as yet uncertain portion of the root system. Young leaves can tolerate drying to a comparable to the dryness of seeds for at least a year. Additional stress tolerances are to ionizing radiation, extreme temperatures and salinity to at least 215 mM NaCl.
Two species of Xerophyta – X. viscosa and X. humilis – have been studied at the molecular level. Both are native to southern Africa. Like other monocots, Xerophyta spp. lose their chlorophyll during desiccation. Seedlings also lose desiccation tolerance briefly upon germination and recover it gradually during seedling development. X. viscosa, the only species with a sequenced genome, is a chasmophyte. It is also self-incompatible and thus shows a high degree of heterozygosity. In contrast, X. humilis, for which transcriptome resources are available, is a non-chasmophyte.
The eXtreme plant bona fides of Arabidopsis lyrata stem from its preference for sandy, gravelly, rocky or chasmophytic “soil”. At the molecular level, its value has been greatest for its contribution to understanding of mechanisms of mutation, selection and genome-size transformations in plants.
Schrenkiella parvula – a 7 chromosome member of the Brassicaceae – has an eXtreme ability, in the natural world, to function in the hypersaline conditions surrounding Lake Tuz in central Anatolia, Turkey. It is also notable for its tolerance of high levels of other cations, especially Li+ and Mg2+ and of Boron. These extreme adaptations were central to the initial decision to sequence the genome of this species.