In conventional methods, sexual propagation is with the use of seed or spore. Seeds are used in the spermatophytes or seed-bearing plants while spores are used in the seedless, spore-producing ferns and allies and the bryophytes.
The descriptive word “sexual” is attached to this type of propagation because the union of the male and female sexual gametes (the process is called fertilization) is a requisite in the production of the seed or in the development of a new plant from a spore. The certainty of sex in plants was established by Camerarius in 1694 (Poehlman, 1977).
Compared to vegetative or asexual methods, sexual propagation with the use of seed has the advantage of massive production of seedlings and rootstocks. However, the seedlings may not be true-to-type because both the maternal and paternal factors contribute, through the process of fertilization, to the genotype of the embryo in the resulting seed. It is the embryo which gives rise to a new plant. This is similar to humans in which the offspring (child) exhibits physical traits which are variants of either parent.
The methods employed in sexual propagation using seeds are simple, but a nurseryman needs to be familiar with the characteristics of different seeds. S/He ought to know also seed dormancy and the different methods of seed pretreatment to break dormancy or to hasten seed germination. Another important consideration is seed longevity which affects viability.
It is in relation to seed longevity and the effects of drying and storage temperature on germination that seeds are classified as orthodox seeds and recalcitrant seeds. These classifications are in accordance with their sensitivity to drying and temperature. A third group is the intermediate seeds which exhibit the drying tolerance characteristic of the orthodox seeds but are sensitive to low temperature storage like the recalcitrant seeds.
Familiarization with these seed types can mean the difference between success or total failure in sexual propagation. As a general rule, the propagation of orthodox seeds can be delayed by drying. But the recalcitrant seeds must be planted immediately.
Orthodox or drying-tolerant seeds are “exemplified by most annual and biennial crops and agroforestry species which are relatively small-seeded. As in cereals and grain legumes, these seeds can tolerate drying to as low as 5% moisture content under common conditions and low storage temperatures. Their life span is, in fact, prolonged with low seed moisture and temperature.”
In contrast, recalcitrant or drying-sensitive seeds are “readily killed by drying, most especially if their moisture content falls below the critical value ranging from 12-30%. Unlike orthodox seeds they generally cannot withstand temperatures lower than 20 C, partly because of the high moisture content which renders the seed prone to chilling or freezing injury. Some can maintain viability at slightly lower temperatures but vigor of the seedling may be affected.
The seed of a number of species, especially those of temperate origin, can tolerate much lower temperatures and may even require such conditions for germination. Even if kept moist, recalcitrant seeds are relatively short-lived with viabilities maintained from only a few weeks to a few months, depending on the species.
Recalcitrant species belong to trees and shrubs of the tropics and temperate areas which are moist, and some plants which grow in aquatic environments. These include some aquatic species, large-seeded tropical fruits and perennial plantation crops and timber species.” (Agroforestry Seeds Circular, March 1993).
The following plants have recalcitrant seeds (selected from Fernandez 1993):
Avocado (Persea americana)
Banaba (Lagerstroemia speciosa)
Black plum, duhat (Syzygium cumini)
Chestnut (Castanea spp. )
Cacao (Theobroma cacao)
Canistel, tiesa (Pouteria campechiana)
Coconut, niyog (Cocos nucifera)
Sweet palm, kaong (Arenga pinnata)
Durian (Durio zibethinus)
Jackfruit, (Artocarpus heterophyllus)
Lanzones (Lansium domesticum)
Lychee (Litchi chinensis)
Macademia nut (Macadamia
Mango (Mangifera indica)
Mangosteen (Garcinia mangostana)
Rambutan (Nepheleum lappaceum)
Rubber (Hevea brasiliensis)
Santol (Sandoricum koetjape)
Tea (Camellia sinensis)
Zapote (Diospyros digyna)
Arecaceae / Palmae
Arecaceae / Palmae
Clusiaceae / Guttiferae
With a few exception, the information that one crop species belonging to a family is recalcitrant indicates that other members of the same family are likewise recalcitrant. One exception is found in the family Anacardiaceae. Mango is recalcitrant, but cashew (Anacardium occidentale) is orthodox.
Guided by knowledge of the characteristics of seeds, a protocol can be formulated for the sexual propagation of plants. To ensure high germination, the general rule is to plant recalcitrant seeds immediately after seed extraction. If drying is necessary, care should be exercised to prevent overdrying. Better still, it may be done by air drying. If immediate planting is not possible, the seeds can still be stored, but only for a limited time duration and with the employment of special techniques.
For mango (Mangifera
spp.), the fleshy, edible portion (the mesocarp) of the fruit is first removed. The seeds (like those discarded after eating ripe mango) can be exposed to direct sun until the remaining
flesh, which is slippery, dries. To hasten germination, the hard husk
(endocarp) which covers the true seed is removed with the use of a
sharp tool like bolo.That outermost, filamentous covering is then removed. The seed is now ready for sowing.
Fernandez PG. (ed.). 1993. Recalcitrant seeds and intermediates. Agroforestry Seeds Circular No, 3. (March 1993). p. 22-26.
Poehlman JM. 1977. Breeding Field Crops. Connecticut: AVI Publishing Co., Inc. p. 4.