Recalcitrant seeds are

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

s that do not survive drying and freezing during ''ex situ'' conservation. By and large, these seeds cannot resist the effects of drying or temperatures less than 10 °C (50 °F); thus, they cannot be stored for long periods like orthodox seeds because they can lose their viability. Plants that produce recalcitrant seeds include

avocado The avocado, alligator pear or avocado pear (''Persea americana'') is an evergreen tree in the laurel family (Lauraceae). It is native to Americas, the Americas and was first domesticated in Mesoamerica more than 5,000 years ago. It was priz ...

mango A mango is an edible stone fruit produced by the tropical tree '' Mangifera indica''. It originated from the region between northwestern Myanmar, Bangladesh, and northeastern India. ''M. indica'' has been cultivated in South and Southeast As ...

mangosteen Mangosteen (''Garcinia mangostana''), also known as the purple mangosteen, is a tropical evergreen tree with edible fruit native to Island Southeast Asia, from the Malay Peninsula to Borneo. It has been cultivated extensively in tropical Asia ...

lychee Lychee ( , ; ''Litchi chinensis''; ) is a monotypic taxon and the sole member in the genus ''Litchi'' in the soapberry family, Sapindaceae. There are three distinct subspecies of lychee. The most common is the Indochinese lychee found in So ...

, cocoa,

rubber tree ''Hevea brasiliensis'', the Pará rubber tree, ''sharinga'' tree, seringueira, or most commonly, rubber tree or rubber plant, is a flowering plant belonging to the spurge family, Euphorbiaceae, originally native to the Amazon basin, but is now p ...

, some horticultural trees, aquatic plants such as ''

Nymphaea caerulea ''Nymphaea nouchali'' var. ''caerulea'', is a water lily in the genus '' Nymphaea'', a botanical variety of '' Nymphaea nouchali''. It is an aquatic plant of freshwater lakes, pools and rivers, naturally found throughout most of the eastern h ...

'''','' and several plants used in traditional medicine, such as species of ''

Virola ''Virola'' is a genus of flowering plants in the nutmeg family, Myristicaceae. It includes medium-sized trees native to rainforests of the tropical Americas, ranging from southern Mexico to Bolivia and southern Brazil. Species are known commonl ...

'' and ''

Pentaclethra ''Pentaclethra'' is a small genus of trees from the tropics. They are flowering plants in the family Fabaceae. They belong to the mimosoid clade of the subfamily Caesalpinioideae. The name ''Pentaclethra'' is derived from Ancient Greek, ''penta ...

''. Generally speaking, most tropical

pioneer species Pioneer species are resilient species that are the first to colonize barren environments, or to repopulate disrupted biodiverse steady-state ecosystems as part of ecological succession. Various kinds of events can create good conditions for pi ...

have orthodox seeds but many

climax species Climax species, also called late seral, late-successional, K-selected or equilibrium species, are plant species that can germinate and grow with limited resources; e.g., they need heat exposure or low water availability. They are the species wi ...

have recalcitrant or intermediate seeds.

Mechanisms of damage

The two main mechanisms causing damage to recalcitrant seeds are

desiccation Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...

effects on the intracellular structures and metabolic damage from the formation of toxic chemicals such as

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...

s. An example of the first type of damage would be found in some recalcitrant nontropical

hardwood Hardwood is wood from Flowering plant, angiosperm trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal ecosystem, boreal latitudes they are mostly deciduous, but in tropics and subtropics mostl ...

seeds, specifically the

acorn The acorn is the nut (fruit), nut of the oaks and their close relatives (genera ''Quercus'', ''Notholithocarpus'' and ''Lithocarpus'', in the family Fagaceae). It usually contains a seedling surrounded by two cotyledons (seedling leaves), en ...

s of recalcitrant

oak An oak is a hardwood tree or shrub in the genus ''Quercus'' of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisp ...

s, which can be stored in a non-frozen state for up to two years provided that precautions are taken against drying. These seeds show deterioration of cell membrane lipids and proteins after as few as 3–4 days of drying. Other recalcitrant seeds, such as those of the sweet chestnut (''

Castanea sativa The sweet chestnut (''Castanea sativa''), also known as the Spanish chestnut or European chestnut, is a species of tree in the family Fagaceae, native to Southern Europe and Asia Minor, and widely cultivated throughout the Temperate climate, te ...

''), show oxidative damage resulting from uncontrolled metabolism occurring during the drying process.

Storage

Preservation of recalcitrant seeds remains experimental. Some approaches include: * Removal of the embryo inside for

cryopreservation Cryopreservation or cryoconservation is a process where biological material - cells, tissues, or organs - are frozen to preserve the material for an extended period of time. At low temperatures (typically or using liquid nitrogen) any cell ...

(liquid nitrogen). * Cryopreservation of the whole seed in an anti-freezing solution. Intermediate seeds are between orthodox and recalcitrant seeds in their survivability. They are initially identified by their inability to survive conventional dry-freezing storage while being able to survive cryopreservation as a whole. The storage guideline is to put them in refrigeration at 45–65% RH, for a maximum of five years.

References

{{reflist, 25em Plant reproduction Seeds

Mechanisms of damage

Storage

See also

References