The following points highlight the two types of seeds according to freezing. The types are: 1. Orthodox Seeds and 2. Recalcitrant Seeds (Unorthodox Seeds).
Seeds Type # 1. Orthodox Seeds:
Orthodox seeds are long-lived seeds and can be successfully dried to moisture contents as low as 5% without injury and are able to tolerate freezing. Orthodox seeds are therefore, also termed as desiccation tolerant seeds. In-fact, the life span of orthodox seeds can be prolonged with low moisture content and freezing temperatures. Ex-situ conservation of orthodox seeds is therefore, not problematic.
Orthodox seeds are exemplified by most annual and biennial crops and Agroforestry species which are relatively small-seeded (in comparison to unorthodox seeds). Orthodox seeds include for example, Citrus aurantifolia, Capsicum annum, Hamelia patens, Lantana camera, guava (Psidium guajava), Cashew (Anacardium occidentale) and most grains and legume types.
The longevity or life span of orthodox seeds may vary from over a year to many hundred years depending upon the particular species and storage conditions. A notable example of a long-lived orthodox seed which survived accidental storage followed by controlled germination as mentioned earlier, is the case of 2000 years old Judean date palm seed which was successfully sprouted in 2005. However, the upper survival time limit of properly stored orthodox seeds remains unknown.
Seeds Type # 2. Recalcitrant Seeds (Unorthodox Seeds):
Recalcitrant seeds are remarkably short-lived which cannot be dried to moisture content below 20-30% without injury and are unable to tolerate freezing. Recalcitrant seeds are therefore, also termed as desiccation sensitive seeds. Recalcitrant seeds are difficult to be successfully stored and their ex-situ conservation is problematic.
It is because of their high moisture content that encourages microbial contamination and results in more rapid seed deterioration. Secondly, storage of recalcitrant seeds at freezing temperatures causes the formation of ice-crystals which disrupt cell membranes and causes freezing injury. Therefore, the plants that produce recalcitrant seeds must be stored in growing phase (i.e., as growing plants) rather than as seeds and propagated vegetatively.
Recalcitrant species belong to trees and shrubs of mostly tropics and also of temperate areas which are moist and some plants which grow in aquatic environment. Some common examples of plants that produce recalcitrant seeds (which are generally larger than orthodox seeds) include, avocado, cacao, coconut, jackfruit, lychee, mango, rubber, tea, some horticultural trees, and several plants used in traditional medicine.
The longevity or life span of recalcitrant seeds is remarkably very short. Seeds of Acer saccharinum, Zizania aquatica, Salix japonica and S. pierotti lose their viability within a week if kept in air. Seeds of several other species remain viable only for a few weeks and months to less than a year.
It has been observed by many scientists that certain seeds do not conform fully either to orthodox or recalcitrant category. Some of these seeds may have a limited desiccation tolerance but are sensitive to freezing temperatures. For these seeds, an intermediate category has been suggested by Ellis (1991). Citrus and Coffee seeds may fit this intermediate classification.