Sexual Propagation

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. Sexual propagation is so termed “sexual” 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, seed propagation 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 incereals 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).

Refered to: http://www.cropsreview.com/sexual-propagation.html

By,
M Anem
Senior Agronomists
Tangkak Agriculture Station,
Ledang, Johore,
Malaysia.
(4 Shawal 1434H)