In this article we will discuss about:- 1. Seed Drying 2. Seed Storage 3. Seed Maturation 4. Seed Collection.
Seed Drying:
After extracting the seeds from the fruit or pod, drying is essential before the seed can be safely stored until use in the nursery or packing for dispatch. The moisture content of most of the seeds for storage ranges between 10-12 per cent. Recalcitrant seeds lose viability seriously by drying, such seeds have to be stored in moist conditions.
Development of all type of storage fungi (Aspergillus spp. and Penicillium spp.) and storage insects (Internal feeders- Sitophilus oryzae, Rhizopertha dominica, Callosobruchus maculates, C. chinensis, C. piso- rum, Sitotroga cerealella, Caryedon serratus, Trogoderma granarium and External feeders, Tribolium castaneum, Oryzaephilus surinamensis, Cryptolestes ferrugineus, Plodia interpunctella, Ephestia cautella, Corcyra cephalonica, Liposcelis divintorius, L transvallensiss) are common in seeds stored in higher moisture content.
Increase in the moisture content from 15 to 24 per cent will increase in storage fungi and they cause reduction in germination and seedling vigour; discoloration, shrinkage and shriveling of seeds; production of heat and mycotoxin; reduction in nutritional value and bio-chemical constituents of seed; development of mustiness and caking and total decay of seeds.
The seed quality is affected on the basis of insect damage to endosperm or embryo during storage. If embryo portion is damaged, seed fails to germinate. Some seeds may germinate and develop into a seedling but the vigour of the seedling will depend up on the extent and intensity of the damage. The degree and type of insect infestation and insect species involved depend on the type of tree seed and prevailing ecological conditions.
To avoid the above said storage fungi and insects, proper precautions are necessary at harvest and during post-harvest operations. Removing the inert matter such as dried stick, leaves, soil clod, broken seeds, other tree seeds and weed seeds will reduce the attack from storage fungi and pests. Dried, sound, clean and undamaged seeds should be treated with insecticide and fungicide for better results.
The insecticides like Malathian 5 D @250g per quintal (or) carbaryl @200g per quintal) and fungicides captan (or) thiram @250 to 300g per quintal (or) Bavistin @200g per quintal are found effective. The treated seeds should be packed in moisture vapour pervious or moisture vapour resistant or moisture vapour proof containers and labeled properly. Extreme care is required to ensure that treated seeds are never consumed for human or animal food. Treated seeds should be clearly labeled as being dangerous, if consumed.
Seed Storage:
Seed storage is the preservation of viable seed until their sowing or requirement. It is essential to offset the uncertainty of seed production/availability during bad seed years. It delays deterioration, maintains viability and protects seed from rodent and insect damage. The longevity of seeds is a species specific characteristic. The seed of most of the species can be stored at low temperature and low moisture content in sealed containers.
It is important to dry the seed uniformly to prevent fluctuation in moisture content during storage. The moisture content of most of the seeds for storage ranges between 10 to 12 per cent. The respiration continues at low temperature, which is necessary to keep the embryo alive. Polythene bags make good containers because they are impermeable to water but less so to oxygen and carbon dioxide.
However, many species of moist tropical forests are so thoroughly adapted for germination that their seeds are almost impossible to store or even to transport.
On the basis of storage behaviour seeds can be divided into two broad categories:
1. Orthodox seed.
2. Recalcitrant seed.
1. Orthodox Seed:
It is a generally conceived notion that seeds endowed with natural longevity are hard- coated. They include a number of tropical leguminous species. Orthodox seeds acquire desiccation tolerance during development and may be stored in the dry state for predictable periods under defined conditions. Orthodox seeds maintain high vigour and viability at least from harvest until the next growing season or for many decades at -18°C.
Generally such seeds undergo a period of drying during their maturation and are shed at low water content which is equilibrium with the prevailing relative humidity. The equilibrium water content at any particular relative humidity is determined by seed composition, but all orthodox seeds can withstand dehydration to around 5 per cent (0.053 g H2O/ g dry material), even when maturation drying is not completed prior to shedding.
Some of the most outstanding examples of maintaining viability, after lengthy periods of storage, are mentioned in a work emanating from FRI as far back as in the year 1948, by T.V. Dent. Recent research has provided more precise information on the conditions of storage, such as low moisture content, low temperature and low oxygen pressure, as the three most important constituents of the storage conditions which must be provided to maintain viability of orthodox seeds in long term storage.
The orthodox seeds can further be subdivided into two classes viz.:
i. Orthodox seeds with hard coats.
ii. Orthodox seeds without hard coats.
i. Orthodox Seed with Hard Seed Coats:
Most of the seeds which have been recorded to fall in this category are hard coated. More specifically, this category refers to those in which the seed coat is impermeable to water, and do not swell even after keeping in a moist medium for weeks or even months. They include a number of tropical leguminous species.
Since most seeds have pretty long natural viability span, therefore, little well-planned work has been done on their storage in controlled environmental conditions. However, for the purpose of conservation of genetic resources, where the objective is to store seed for a couple of decades, a long term storage will have to be planned, e.g., for species like Acacia, Prosopis, Albizia, Cassia, Bauhinia etc., which are being planted on a massive scale in social forestry plantations.
ii. Orthodox Seeds without Hard Seed Coats:
Many species belonging to important genera of forest trees, fall into this group, e.g. Pinus, Picea, Eucalyptus and several other genera. Kandya (1987) listed seeds of some tree species according to their probable period of longevity. The list includes mostly the probable orthodox seeds without hard seed coats.
a. Two Weeks:
Michelia champaca, Syzygium cumini, Soymida febrifuga, Aegle marmelos, Anogeissus latifolia, A. pendula, etc.
b. Two Months:
Chloroxylon, Chukrassia, Podocarpus
c. One Year:
Artocarpus heterophyllus, Toona ciliata (Syn. Cedrella toona), Mesua ferrea, Terminalia tomentosa, T. bellerica, Bambusa arundinacea, Semecarpus anacardium, Pongamia pinnata, Pterocarpus marsupium, Lagerstroemia parviflora, Lannea coromandelica, Diospyros melanoxylon, Garuga pinnata, B. lanzan (Syn. Buchanania latifolia).
d. Two Years or More:
Dalbergia sissoo, Eucalyptus tereticornis, Anthocephalus cadamba, Tectona grandis. Some of the short viability seeds include Dendrocalamus strictus, Holoptelia integrifolia, Terminalia myriocarpa, Pongamia pinnata, Alnus nepalensis and Alnus nitida, Bambusa tulda etc.
2. Recalcitrant Seeds:
These seeds are characterized mostly by large size and high moisture content, which cannot be dried without causing injury. Recalcitrant seeds cannot be stored for a longer period. Recalcitrant seeds are those that undergo little or no maturation drying and remain desiccation sensitive both during development and after they are shed. Wide range of variability is observed among recalcitrant seeds of different species and of individual species under different conditions.
Recalcitrant seeds are shed from the tree in hydrated condition and water content can generally be in the range from 0.43 to 4.0 gram per gram of dry material which is 30 to 80 per cent on a wet mass basis. Most of the recalcitrant seeds belong to woody species. Most of the species belong to the family Dipterocarpaceae and Lauraceae. Quercus, Castanea, Aesculus indica, Shorea robusta, Azadirachta indica, Bassia latifolia (Syn. Madhuca indica), Mangifera indica, Dipterocarpus, Hopea, etc. fall into this category.
Seed Maturation:
The maturation of a seed is a positive growth process, which includes the following physical changes: increase in size and weight; accumulation of dry matter; development of essential structures of the embryonic axis; an increase in viability and vigour and finally, a loss of moisture.
Changes that occur in seeds as they mature are summarized below:
1. Moisture content decreases rather uniformly from 70-80 to 15-20 per cent.
2. Seed size increases to a maximum then decreases somewhat as the seed dry.
3. Dry weight increases to a maximum then may decrease slightly.
4. A few seeds become capable of germinating within a few days after fertilization, maximum germination is reached at a somewhat later date.
5. Some seeds may become dormant as they ripen, and require special treatments to obtain maximum germination per cent.
6. Seedling vigour increases as seed dry weight increases and reaches a maximum at the time of maximum dry weight.
Maturity is defined as the point of maximum dry weight of seed. This point is reached when the seeds are relatively high in moisture content (35 to 45 per cent). The sooner the seeds are harvested after maturity, the higher the quality.
Maturity stage is differentiated into the following:
i. Physiological maturity.
ii. Harvestable maturity.
i. Physiological Maturity:
This is the stage at which the, maximum dry matter accumulates, where the moisture content of the seed will be 26 to 50 per cent. If the seed is devoid of dormancy, it registers highest seed germinability and vigour index at that stage. The seeds have to be effectively sandy dried to safe moisture contents.
ii. Harvestable Maturity:
This is the stage at which the seed first time dries to a moisture level of 14 per cent or less, which allow commercial harvesting and the seeds can be stored without supplemental drying.
Characteristics features are:
i. Loss of the fruit chlorophyll content and photosynthetic activity as they ripen. Loss or gain of a distinctive colour.
ii. Softening in fleshy fruits or achievement of brittleness as in many conifer seeds.
iii. A change in flavour or odour.
iv. In most seeds, complex carbohydrates, fats, oils and proteins usually accumulate and lipids form the major food reserve in many tree species. In general, nitrogenous constituents and organic acids are found in increased quantity.
v. Respiration reaches a peak as maturation is attained and then declines as senescence begins.
vi. Developing seeds of both angiosperms and gymnosperms are rich source of growth regulating substances and changes in the levels of these compounds have also been associated with ripening. Normal levels of growth regulating substances are mostly associated with meristamatic activity and the highest levels are found in immature seeds.
The planting value of seed and its storability is directly related to the level of maturation of the seed at the time of collection. Seed collection in bulk quantities in the field can only be economical and feasible, if exact stage and time of seed maturation are known.
Seed collectors must be aware of the physical (length, breadth and thickness of pod, number of seeds per pod, moisture content of pod and seed, specific gravity of pod and seed and seed colour), physiological (germination, seedling length, dry matter accumulation and seedling vigour) and bio-chemical indices (crude fat, reducing and non-reducing sugars, starch, soluble nitrogen, protein nitrogen and isoenzymes) of the developing fruit and seed.
Change in fruit colour (physical indices) and germination (physiological indices) of developing seed is by far the best available criteria for judging the maturation of different tropical forest species.
It is important to know the time requirement between anthesis (flowering) and physiological maturity for each species. Seed harvested before reaching the physiological maturity, being high in moisture, will shrivel when dry and probably will not germinate. On the other hand, delaying harvest excessively beyond physiological maturity generally will result in increase in field deterioration attack by insects and disease causing organisms and harvest loss.
Most of the leguminous species seed must be collected at physiological maturity stage to achieve higher germination and seedling vigour. Beyond this stage, germination and seedling vigour of these species start declining due to the formation of hard seed coat.
Seed Collection:
Time and Season of Seed Collection:
The time of seed collection depends on the local weather conditions through the natural ranges of each species. Progress of ripening and dispersal time of seeds need to be closely monitored. In a number of species, for example teak, lagerstroemia and most legumes, seeds ripen in a fairly definite time. In such cases, dry seed or fruits must be collected as soon as possible.
Bhardwaj and Chakraborty (1994) reported that the Terminalia bellirica and T. chebula, when the seeds were collected during first fortnight of January and sowing was done at last week and third week of March gave better germination of 52.2 percent and 44.4 per cent, respectively. Dalbergia sissoo is profusely seeding almost every year and collection time of seed is different from place to place (Table 17.3).
The fruiting season of teak in India is between November and March or April. Collection of fruits during the later part of the season gives better results. Studies conducted on the effect of time of collection on germination of teak fruits from Top slip source in Tamil Nadu revealed that fruits collected in March-April gave better germination per cent than those collected in other months. The natural fall of teak fruits take place in March-April.
Seeds with the highest germination capacity and with the least damage to the seed are found at the end of the season. In Cauvery delta zone of Tamil Nadu, neem seeds collected during June-July (normal bearing season) gave maximum germination and seedling vigour as compared to seeds collected during off season (December-January).
Methods of Seed Collection:
There are two general methods of collecting the fruits and seeds of forest trees in India. First method is the collection of naturally fallen fruits or seeds from the ground surface, as in the case of Tectona grandis, Gmelina arborea and several Dipterocarpus and most other heavy seeds or fruits.
Second is the direct collection of fruits from felled or standing trees, as in the case of Cassia fistula, Casuarina equisetifolia, Simarouba glauca, Emblica officinalis, Shorea robusta, Eucalyptus spp., Cassia siamea and many other conifers.
There are certain disadvantages in ground collection viz.:
i. Seed if not collected immediately after shedding from the trees, may be attacked by insects, fungi and animals e.g. Acacia spp.
ii. Seeds having short viability lose germinability quickly. Once the seeds have been shed, any delay in collection may lead to collection of non-viable seed e.g. Bassia latifolia, Pongamia pinnata etc.
Collection of seeds from standing trees also poses some practical problems. Specially designed extension ladders, tree cycles etc., have been designed for use in collecting seed from difficult trees. Whereas the ladders cannot be safely used in inhospitable terrain, the use of tree cycle needs exceptional energy, skill and training.
Acacia catechu pods must be picked by hand directly from the trees before the pods open and scatter their seeds, pods laying on the ground might already damaged by Bruchus bilincatopygus or other insects. Masilamani et al (1995) reported that in Prosopis juliflora, pods collected from natural forest floor showed heavy microbial population (fungi, bacteria and actinomycetes) and seeds extracted from infected pods exhibited lesser germination and vigour than seeds of uninfected pods. Masilamani et al (1999b) observed that teak seeds collected from the crown of the tree (straight from the tree) gave maximum germination compared to seeds collected on the ground.
Effect of Pest and Disease on Seed Production:
Damages done by insect pest and diseases, are one of the major constrains for seed productivity. Regeneration failures in most of the tropical broad-leaved species are attributed to the heavy insect attack in the natural stands, at the time of inflorescence, seed formation and seed shedding.
The problem of seed destruction is increasing due to our increasing reliance on seed sources for the production of seedlings or for trees of known genetic characteristics in reforestation and reclamation programs. The flowers, fruits, seeds and cones, being rich food sources, are vulnerable to insect pests and diseases. Insects that feed on them often cause seed crop failure and thus, exert an adverse impact on natural and artificial reforestation.
Regeneration failure, especially in teak and Sal in India and conifers in other countries can be attributed to two factors, firstly, the heavy insect attack in the natural stands at the time of inflorescence and seed formation, and secondly, insect attack on the ground after seed shedding.
Some seeds, such as those of Acacia catechu, Albizzia lebbeck and Cassia species may be attacked or destroyed by insects or disease, if left on the parent trees. Other tree species such as Cassia siamea and Santalum album, do not have a single, well defined period at which their fruit ripen and they may have two or more seed crop each year.
At the time of seed collection, care must be taken to avoid collection of insect or disease infested seeds. It may cause impairing germination and seedling vigour. Mittal and Sharma (1981) have reported seed borne mycoflora of Cassia fistula which includes common fungi Rhizopus nigricans, Aspergillus flavus, A. niger and Penicillum species. These fungal species were also frequently associated with abnormal seedlings, in which either roots were altogether suppressed or their growth was retarded.
Randhawa et al (1986) reported that Rhizopus nigerians was associated with Cassia fistula seed, it causes inhibition of root and collar rot and also caused partial or complete withering of one or both the cotyledons during germination. Conway (1975) observed that bruchid attack on the pods of Acacia tortilis takes place between 30-100 days after pod initiation.
Pods and seeds of Acacia nilotica were destroyed by Caryedon serratus and in Callosobruchus chinensis and the incidence of attack was 100 per cent. Insect attacked fruit or seed should be avoided during seed collection, otherwise those insect will continue their activity during seed storage.
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