|Seeds and Plant Propagation. Agroforestry Technology Information Kit (IIRR, 1992, 105 p.)|
The seed or any form of planting material (generally termed propagule) is a basic requirement in all development programs, i.e., agricultural crop production, agroforestry, plantation and reforestation projects. Unfortunately, its supply has always been a problem. The production, multiplication and handling technologies of these are also not well-known or established. The desire to meet the pressing need for propagules has often led to the sacrifice in quality, suitability and overall sustainability in favor of assured supply. K must be remembered that the form, type and quality (such as viability, germinability, vigor, health, purity and authenticity, moisture content and genetic uniformity) of the propagule contribute greatly to the success of a development undertaking.
The form of propagule, i.e., whether they are seed or clone (asexually or vegetatively propagated materials such as cuttings, marcots, grafts, tubers, corms, suckers, slips, tissue culture seedlings) could spell the degree of genetic uniformity of the plants or trees in a population. Clones are genetically uniform having come from the mother tissue. They did not undergo sexual reproduction or fertilization to produce a true seed and a different individual. They may be the more feasible form of propagation for some difficult-to-seed species, where the same characteristics as the mother is desired and/or when shorter time is desired to bear fruits. However, they have generally shorter life span than plants from seeds.
Some seeds, referred to as apomicts as opposed to true seeds, are produced without fertilization (e.g., many forage grasses). Others, such as mango and citrus species, have polyembryonic seeds producing several embryos one of which is sexual and the others, clones.
Genetic uniformity of true seeds varies depending on the type, i.e., whether they are hybrids or open-pollinated. Hybrids are seeds or plants produced through highly controlled pollination. They are highly uniform and generally expensive. Unlike clones, they cannot be authentically recycled, although may be asexually propagated. This is because plants from their seeds or the succeeding generations are highly variable and different from the original hybrid variety. Open pollinated plants or seeds, on the other hand, are produced through natural crossing and composed of more variable plants in a population. In nature, many species, such as cabbage, carrots, cucurbits, onions, corn and many fruit and forest species, are of this type. Such plants could be recycled, unlike hybrids.
Naturally self-pollinating species, such as rice, many legumes, tomato, pepper and lettuce, have varieties with genetically uniform plants (purelines or inbreds). Unlike hybrids and as in clones and open-pollinated varieties, purelines could be recycled for several generations. They could be made more genetically diverse in the field by planting together different varieties or populations of the species.
The way seeds are collected also affects the genetic uniformity of succeeding generations of plants. Collecting and planting seeds from only one or a few plants, especially of cross-pollinating species could lead not only to genetic uniformity but also to loss of the original characteristics of the variety. This is because the genetic traits of the variety may be carried by different plants in the population. Some genes may be left out and eventually disappear due to limited sampling. The widespread use of a single species of variety, especially when they are genetically uniform, has led to crop failure due to pest epidemics end environmental stresses. It has also led to the loss of indigenous species and varieties (genetic erosion) and biological diversity of many ecosystems.
Some known technologies applied to propagules are useful and effective, but only in meeting short-term needs. In many cases, their appropriateness especially in the long nun is a question. This is with particular reference to their effect on the farmer's self-reliance, the environment (as with the use of chemical inputs, non recycling of nutrients and continuous monocropping) and the economics of the farm or the project itself. Development and adoption of sound alternative technologies in propagation and handling, therefore, need to be actively pursued.