Atriplex

Description

The species in genus Atriplex are annual or perennial herbs, subshrubs, or shrubs. The plants are often covered with bladderlike hairs, that later collapse and form a silvery, scurfy or mealy surface, rarely with elongate trichomes. The alternate or rarely opposite leaves are petiolate or sessile, often persistent or tardily deciduous. The flat or slightly fleshy leaf blades are either entire, or serrate, or lobed and very variable in shape.

The inflorescences consist of axillary or terminal spikes or spicate panicles, or axillary clusters of glomeruled flowers. The flowers are unisexual, some species are monoecious, others dioecious. Male flowers consist of 3-5 perianth lobes and 3-5 stamens. Female flowers are usually lacking a perianth, but are enclosed by 2 foliaceous bracteoles, and contain an ovary with a short style and 2 stigmas.

In fruit, the bracteoles can enlarge, thicken or become appendaged. They enclose the fruit tightly, without becoming connate to it. The pericarp is adnate to the vertically orientated, flattened seed. The seed coat is thick, leathery or hardening. The annular embryo surrounds the perisperm.

The chromosome base number is x = 9, except for Atriplex lanfrancoi, which is x=10.

A few Atriplex species are C₃-plants, but most species are C₄-plants, with a characteristic leaf anatomy (kranz anatomy).

Distribution and evolution

The genus Atriplex is distributed nearly worldwide from subtropical to temperate and to subarctic regions. Most species-rich are Australia, North America, South America and Eurasia. Many species are halophytes and are adapted to dry environments with salty soils.

The genus evolved in Middle Miocene, the C₄-photosynthesis pathway developed about 14.1-10.9 million years ago, when the climate became increasingly dry. The genus diversified rapidly and spread over the continents. The C₄Atriplex colonized North America probably from Eurasia during the Middle/Late Miocene, about 9.8-8.8 million years ago, and later spread to South America. Australia was colonized twice by two C₄ lineages, one from Eurasia or America about 9.8-7.8 million years ago, and one from Central Asia about 6.3-4.8 million years ago. The last lineage diversified rapidly, and became the ancestor of most Australian Atriplex species.

Ecology

Atriplex species are used as food plants by the larvae of some Lepidoptera species; see the list of Lepidoptera which feed on Atriplex. For spiders such as Phidippus californicus and other arthropods, saltbush plants offer opportunities to hide and hunt in habitat that is otherwise often quite barren.

Use by humans

The favored species for human consumption is now usually Garden Orache (A. hortensis), but many species are edible and the use of Atriplex as food is known since at least the late Epipaleolithic (Mesolithic).

Common Orache (A. patula) is attested as an archaeophyte in northern Europe, and the Ertebølle culture is presumed to have used it as a food. Its seed has been found among apparent evidence of cereal preparation and cooking at Late Iron Age villages in Britain. In the biblical Book of Job, mallûaḥ (מַלּ֣וּחַ, probably Mediterranean Saltbush, A. halimus, the major culinary saltbush in the region) is mentioned as food eaten by social outcasts (Job 30:4). Grey Saltbush (A. cinerea) is used as bushfood in Australia since prehistoric times. Chamiso (A. canescens) and Shadscale (A. confertifolia) were eaten by Native Americans, and Spearscale (A. hastata) was a food in rural Eurasia.

Meat from sheep which have grazed on saltbush has surprisingly high levels of vitamin E, is leaner and more hydrated than regular lamb and has consumer appeal equal to grain-fed lamb. The vitamin E levels could have animal health benefits while extending the shelf-life and maintaining the fresh red colour of saltbush lamb. This effect has been demonstrated for Old Man Saltbush (A. nummularia) and River Saltbush (A. amnicola). For reasons unknown, sheep seem to prefer the more fibrous, less nutritious River Saltbush.

Saltbushes are also used as an ornamental plant in landscaping and can be used to prevent soil erosion in coastal areas. Old Man Saltbush has also been successfully used to rehabilitate old mining sites around Lightning Ridge (Australia).

Systematics

The genus Atriplex was first described in 1753 by Carl Linnaeus. The type species (lectotype) is Atriplex hortensis.

Atriplex is an extremely species rich genus and comprises about 250-300 species. After phylogenetical research, Kadereit et al. (2010) excluded Halimione as a distinct sister genus. The remaining Atripex species were grouped into several clades, that mostly are not identical with the traditional sections:

Selected species not yet investigated phylogenetically:

Excluded species: