Everything you need to learn about the selection, extraction and processing of tree seeds.
How to Select Tree Seeds?
Basic material is the plant material from which the Forest Reproductive Material (FRM) is derived and includes seed stands, seed orchards, parent material held by tree breeders in archives, individual clones and mixtures of clones. Seedlings inherit the qualities of their parents, therefore seeds should be collected from trees with characteristics that are preferred by the eventual tree planters.
Useful guidelines for selection of suitable trees for seed collection are listed below:
i. All seed trees should be healthy, vigorous and should have good form.
ii. Trees intended for fodder should have abundant foliage and a bushy crown.
iii. Trees intended for fuel wood can have plenty of branches.
iv. Trees intended for timber should be straight and without too many thick branches.
v. If a species has multipurpose or more than one use, it will be necessary to compromise on these characters.
vi. Collect seeds from middle aged trees to matured trees, over matured trees should be avoided, since seeds from them may be of low viability.
vii. Seeds should be collected from dominant and co-dominant trees of the stand.
viii. Avoid collecting seeds from isolated trees.
ix. Tree stands containing numerous poorly formed individuals should not be considered for seed collection. Such trees are generally abnormal or diseased.
Provenance:
Provenance is defined as the geographic source or location to which plants are native and within which their genetic characteristics have been developed through natural selection. The term provenance is used to describe the location of the source from which the reproductive material is collected. Seeds from unknown origin are risky and those from inferior origin are disastrous.
Thus, use of seeds from known and genetically superior origin is desirable since while raising plantation, millions of inputs and even more millions of outputs are involved. Due to the considerable variation in the characteristics both between seed lots from different sources and within seed lots from the same source, many pretreatments are required for different tree species. It is better to use locally grown seed where possible, as this is usually cheaper, and trees growing in a particular locality tend to be better adapted to local conditions than those from distant areas, often with very different conditions of climate and soil.
Locally collected seed is performing better than those from distant areas in broad-leaved species like Eucalyptus spp., Grevillea robusta, Casuarina spp. etc. Teak drupes collected from moist tropical forest of Top Slip in Tamil Nadu showed superiority and gave maximum germination and seedling vigour than the drupes from far areas like Karnataka, Andhra Pradesh and Maharashtra. Dharmalingam and Masilamani (1997) reported that teak drupes from two provenances viz. Top Slip in Tamil Nadu, India and Lampang provenance of Thailand differed in size and weight within and between seed lots.
Seed Source:
Seed source covers all material from a single tree or collection of trees within a region of provenance or seed zone. In fact, different seed sources differ in germination and seedling vigour, they may require different type of pretreatments to enhance or activate the germination. Seed source variations are reported in several tropical tree species viz. Sivasamy (1991) in Azadirachta indica, Ailanthus excelsa and Pongamia pinnata; Masilamani (1992) in Cassia siamea, Hardwickia binata and Prosopis juliflora; Bahuguna et al (1989) in Albizia falcataria; Jambulingam (1990) in Casuarina equisetifolia and Masilamnai et al (1999a) in Tectona grandis. Location effects on seed quality of tree species are well studied and clearly indicate the influence of seed source on physical and physiological quality of seeds.
Age of Mother Tree:
Seed production depends mainly on climatic conditions, but is also affected by the age of the tree and manner of growth. Few trees seed after two to three years, others may take 20- 30 years. Seeds are borne by trees that have sufficient maturity and it is believed that, in most species, there is an optimum age of seed bearing. Seeds from trees just beginning to bear are often of poor quality.
Tree scientists therefore should know the optimum age of the tree for seed collection and effective seedling production. Swaminathan et al (1994) reported that in Ceiba pentandra, seeds collected from 7 year old stand showed superior physical and physiological parameters and gave maximum germination and seedling vigour compared to seeds collected from 4, 5 and 6 year old stand.
Similarly, an increase in germination percentage, seedling dry matter and seedling vigour of neem seeds from 20 year old trees compared to neem seeds from lower ages (5, 10 and 15 years) and higher ages (25 and 30 years) was reported by Masilamani et al (2000).
Suresh (1994) reported maximum germination (90 per cent) in the seeds of Acacia nilotica collected from 9 and 11 year old trees, which was drastically reduced viz. 56 and 34 per cent in 7 and 3 year old trees. Prasad and Kandya (1992) recommended that the seeds should be collected from mature or nearly mature trees. Over mature or immature trees should be avoided, since seeds from them may be of low viability. Various research results suggest that tree age may significantly influence seed quality characteristics.
How to Extract Tree Seeds?
Extraction denotes the procedure of physically releasing and separating the seeds from the enclosing fruit structure. Extraction of seeds in tropical species is generally done close to the site of collection. After the collection of fruit, seeds should be extracted in the shortest possible time to avoid damage to seeds, as well as to facilitate storage and sowing.
The main rationale of extraction is to reduce bulk, ease handling, enhance germination and improve storability of seeds and lower the risk of transfer of diseases or pests between countries and this is achieved by removing material that may harbour disease and pests. Different methods of extraction are available for both dry fruits and fleshy fruits (Table 17.5).
I. Dry Fruits:
This is very diverse group and examples include follicles that split open down one side, pods from legumes (e.g. Acacia spp., Albizia spp.) that spilt down two sides and capsules from Eucalyptus that split down three (or) more sides. From extraction point of view, dry fruits can be divided into two groups: truly dehiscent fruit that will split open and expel the seed when dried e.g. Leucaena leucocephala and Cassia siamea etc. and indehiscent fruit that will remain closed after drying e.g. Albizia lebbeck, Cassia fistula, Cassia grandis, Delonix regia, Peltophorum pterocarpum and Prosopis juliflora etc.
In case of dehiscent fruits, the fruit should be picked before split open and spread on the cement floor or on trays and exposed to the sun (or) artificial drier. These fruits dry too fast, and they will have to be extracted or threshed after drying to extract seed e.g. Albizia lebbeck. The most common seed extraction methods for dry fruits include beating with stick, splitting by hand, using paddy thresher or dehuller machines.
The major advantages for threshing of leguminous fruits or pod through thresher or dehuller are:
1. Complete removal of the seeds from the fruit or pod is achieved.
2. Minimum labour requirement.
3. Increase the efficiency of operation.
4. Maximum seed extraction with minimum cost.
5. A beneficial scarification of seeds may be made due to rubbing action.
6. Since the thresher winnows the threshed seeds, the chaff and other lighter impurities are separated. Hence the quality of the seed is improved.
Disadvantages are:
1. It may damage the seeds causing loss of viability and vigour, if the machine is not adjusted properly
2. Effective and economical only if large quantity of seeds is to be threshed.
Some seeds may be extracted by light rubbing or tumbling, whereas others may need a series of hard mechanical, chemical or acid treatments e.g. Prosopis juliflora seed extraction is a challenging task. The pods have tough pericarp, waxy exocarp, spongy mesocarp and a cartilaginous endocarp, enclosing the seeds in tomentaceous segments. As the cartilaginous inner lining of the pod does not allow the seed to escape easily, due to that the natural regeneration through seeds in the normal way is time consuming.
The different treatments for seed extraction are passing through steam, hot water treatment (90°C for 10-20 minutes), cold water soaking (24 hrs), treatment with commercial H2SO4 (20-30 minutes), termite feeding, scarification while passing goat or other animal digestive system, electric scarification etc. Soaking of pods in diluted NaOH for 30 minutes followed by washing in water and rubbing with a cloth is the best treatment for removing seed from the septa.
Casuarina equisetifolia seeds are normally extracted by spreading the cones on canvas or plastic sheets in the sun or oven drying in 40°C until the bracteoles are open. Shaking and sieving can then separate the seeds. In some cases, the only option may be extracting each seed by hand e.g. Hardwickia binata and Pongamia pinnata.
In case of Tectona grandis, artificial regeneration are achieved only through sowing of fruits. The hard fruit is irregularly round consisting of an outer thin epicarp, the middle thick felty mesocarp and the inner strong impervious endocarp encircling the tetracarpellary ovary. The true seed inside the cavity is oval in shape, having a thin seed coat.
The delayed germination of teak fruits has been ascribed to the strong endocarp acting as a physical barrier to imbibition or imposing mechanical resistance to embryo enlargement, the presence of water soluble germination inhibitors in the felty mesocarp has also been thought to be one of the reasons for poor germination.
Besides the poor germination of fruit, it occupies more space for storage (31 to 50 gram for 100 fruit weight). Methodologies to be developed to extract the seeds from fruit, are really challenging task to early and enhanced germination with lesser space for storing of true teak seed.
Maharashtra Van Sanshodhan Sanstha has developed a prototype machine for teak fruit scarification for removing the mesocarp from the fruits. The scarified fruit were 100 per cent devoid of mesocarp under 2-3 hours of scarification at 50-55 rpm. Teak fruit treatment mainline prototype – II machine completely removes the mesocarp besides exposing the seeds, when the drupes are subjected to scarification.
Similarly mechanical scarification of trees seeds in a mechanical scarifier (Westrup LA-H type) is also practiced. The role of some fungi on weathering teak fruits is also found useful e.g. Scytalidium spp. decomposes the epicarp and mesocarps of teak seeds and seeds extracted from such decomposed drupes exhibit early and high germination.
II. Fleshy Fruits:
Species with juicy and pulpy fruits are potentially troublesome. Soon after collection, the pulp begins to soften and various fermentation processes may start. After soaking or decomposition the fruit pulp and fruit skin are separated from the seed. Several methods, often used in combination, are applicable. Selection of the most appropriate method depends on fruit type, quantity of fruits to be processed and equipment available (Table 17.6).
The fruit of many species such as Azadirachta indica, Cinnamomum camphora and Gmelina arborea are fleshy and pulpy. It is necessary to remove the pulp immediately at maturity to prevent loss of germination capacity. In neem, several methods are available for remain firm and fruits with relatively hard seed coats or stones.
Depulper mechanically abrades the fruit pulp by rupturing and squeezing against or between its mechanical parts and the pure seeds are separated e.g. Azadirachta indica and Gmelina arborea extracting the seeds from the fruits viz., the fruits were depulped in a coffee depulper, fruits soaked in water for six hours, then depulped by rubbing them over a soft wire mesh and the pure seeds were separated. All the afore said methods are laborious and time consuming.
The seeds soaked in concentrated sulphuric acid (50 ml /kg) for 3 minutes are found to have higher percentage of mesocarp removal influencing germination and initial seedling vigour attributes of neem, with lesser time and cost. Similarly pulping machines are found to be effective in removing mesocarp.
In amla (Emblica officinalis), alternate soaking and drying of fruits in 20 per cent sodium chloride solution up to three cycles could extract seeds. The fruit pulp after treatment is easily extractable and the nuts on sundrying expel seeds. In jamun (Syzygium cumini), heaping fruits for a day make the separation of seed easier while longer duration of heaping damages the seed. In ber (Ziziphus mauritiana), removal of flesh with a sharp knife and thorough washing with water followed by soaking in lime solution for 30 minutes, make the extraction easier than drying under sun or water washing of fruits.
These extraction techniques not only make the extraction easier but also make the edible part economically useful (amla and ber). The extracted seeds have higher seed quality in terms of germination, seedling vigour and storability. In Tamarindus indica, seed extraction from fruit is quickest using the thresher while knife is also used which is slower.
The thresher results in the highest percentage of damaged seeds and manual maceration give the lowest percentage of damaged seeds. Fleshy fruits are also adapted to being ingested by animals. Both ants and termites may readily and efficiently clean fruits and seeds of sweet material, both moist and dry.
How to Process Tree Seeds?
Seed processing encompasses all the steps involved in the preparation of harvested seed for marketing. It is done to narrow down the level of heterogeneity of the harvested produce with the sole objective of improving the quality of the seed that is produced carefully in the field, using suitable processing methods.
The harvested seed lot can be heterogeneous due to the following reasons:
i. Heterogeneous soil, which vary in the fertility due to the availability of nutrients, physical, chemical and biological properties.
ii. Due to time of pollination and fertilization over a period of time.
iii. Due to the position of seed on the plant or fruit.
iv. Due the interference of biotic factors and their variability in infestation.
v. Due to the management practices like water, land preparation, leveling, staggered sowing, uneven distribution of fertilizer and irrigation water, uneven plant protection sprays and uneven maturity at harvest.
This heterogeneity can be narrowed down in the processing of seeds by eliminating the undersized, shriveled, immature, ill filled seeds using appropriate sieve size and the physical purity of the seed lot can be improved. The germinability and vigour of the seed lot can be improved by grading the seeds based on size, specific gravity, length and density of the seeds.
Researches in seed processing reveal that the inherent qualities such as germinability and vigour are exemplified by certain physical characteristics of the seed i.e., large size, denser seed, optimum length etc. Hence on managing of seed with a particular range of size, shape, length and density, the quality of the total seed lot can be improved.
Sequence in Seed Processing:
Seed processing steps can be broken down into several definite steps that follow in a specific sequence:
i. Receiving:
This is the first step in seed processing. Seed arrive at the processing plant in bags, pallet boxes or in bulk.
ii. Bulk Storage:
From the receiving station the seed go into bulk storage to be held for later processing, or directly into the processing line for cleaning.
iii. Conditioning and Pre-Cleaning:
The air-screen machine is probably the most common basic cleaner. It makes size separations and separates the seeds. Seed lots may come from the field in good condition with few contaminated seeds and require cleaning only on the air- screen machine.
iv. Separating or Upgrading:
Most often, however, it is necessary to send the seed through one or more special separating or upgrading machines to remove a specific contaminant. These special machines separate crop and weed seeds by differences in their specific physical characteristics. For effective and efficient separation, the crop seeds must differ from the weed seed sufficiently in some characteristics so that the machine can differentiate them efficiently.
v. Bagging:
When all possible inert material and weed or other crop seeds have been removed, the seeds are ready for bagging.
vi. Treatment:
Sometimes a fungicide or insecticide treatment is applied directly to seed lots.
vii. Storage:
After proper fungicide and insecticide treatment the seeds are held in storage until they are needed.
viii. Shipping:
The seed lot is transported to the destinations where they are needed.
No comments yet.