Orthodoxy and Recalcitrancy: 

 

Not all seeds process the same metabolic and biological properties, which affect the duration over which various kinds of seeds can be stored. 

 

A seed’s physical and physiological states determine whether it can be stored by conventional means or not. Seed water content, environmental relative humidity and ambient temperature are key interacting factors affecting longevity. In order for a seed to be able to survive sub-zero storage temperatures, it needs to be capable of surviving desiccation or removal of most of the moisture in their cell walls. When under freezing temperature any extra moisture in the cells will rupture cell tissues, effectively killing the seed. 

 

This is where orthodox and recalcitrant seeds are distinguished: 

 

Recalcitrant seeds: are seeds that can tolerate some water loss but not the extreme drying and desiccation survived by orthodox seeds. When fresh recalcitrant seeds begin to dry, viability is first slightly reduced as moisture is lost, but then begins to decline considerably at a certain moisture content termed the “critical moisture content”. If drying continues further, viability is eventually reduced to zero. Notable species include oaks (Quercus spp.)

There is no satisfactory method for maintaining the viability of intact recalcitrant seeds over the long term. The longevity of recalcitrant seeds is short, ranging from a few weeks to a few months. The key to successful storage of recalcitrant seeds depends on maintaining the moisture level just above the point at which seeds start losing viability (rule of thumb of 5%). Conventional cold storage is not suitable for these seeds as the remaining moisture in the cells forms lethal ice crystals that puncture cell tissues. Typically, the optimal temperature range for these seeds varies between -3°C and 5°C.

Orthodox seeds: can be dried without damage to low moisture content and, over a wide range of environments, their longevity increases with decreases in seed storage moisture content and temperature in a quantifiable and predictable way. Mature seeds survive desiccation to low moisture contents, at least to 2-6%,  depending on the species. Above this value (but within the air-dry range) there is a negative relation between seed moisture content and longevity.

Cold moist storage (stratification) methods have commonly been practiced as a method for short-term storage when dealing with very dormant seeds of trees and shrubs. For example, the benefit of moist storage at low temperatures of 3-5°C for orthodox seeds of temperate species (e.g. Fagus, Fraxinus, Liriodendron, Magnolia, Prunus)

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