Rapid germination of seeds is very desirable because the shorter the time required the less opportunity there is for injury by insects, fungi, or unfavourable weather conditions or for seeds to be eaten by birds or rodents. Among the most important environmental factors controlling seed germination are water, temperature, light, oxygen and various chemicals.
Non dormant seeds must imbibe water before they resume the physiological processes involved in germination. For example, seed respiration increase greatly with an increase in hydration above some critical level. The absolute amounts of water required to initiate germination are relatively small, usually not more than 2 to 3 times the weight of the seed.
After seed dormancy is broken by low temperatures, much higher temperatures are needed to induce rapid germination. Non dormant seeds can also germinate at low temperatures but much longer times are required. Seeds of many species will germinate equally well over a rather wide temperature range. Seeds of lodgpole pine germinated at about the same rate at 20 0C as at 30 0C and germination of kack pine seed does not vary appreciably at 15 0C, 21 0C or 270C under continuous light. However, at temperatures below or above 27 0C, Azadiracta indica germinates poorly. Other seeds require special germination conditions such as alternating temperature for Terminalia ivorensis.
Most seeds appear to be insensitive to light and germinate as well in the dark as in the light. However, seeds of some species require light for germination. Bothwick et al (1954) obtained a detailed action spectum for the breaking of dormancy in the Grand Rapids cultivar of lettuce. It revealed that the major activitty is at 660m. Nwankiti (1982) in his experiment on seed germination and flower colour inheritance in Talinum triangulare emphasized that germination trials, carried out on its seed showed that they require light for germination. The promotive action of light on germination operates by increasing the growth potential of the embryo.
For seeds of the majority of light – sensitive species of woody plants the most rapid and greatest total germiation occur in daily light periods of 8 to 12 hour. Interrupting the dark period with a short light flash or increasing temperature usually has the same effect as extending the duration of exposure of light.Eucalyptus seeds germinated well in 8 hour days and those of birch in 20 hour days. Seed of Doughlas fir, however, germinated in continuous light or 16 hr days, but not 8 hr days.
Germination of seeds of a number of species of herbaceous and woody plants are sensitive to wavelength. The germination responses to wavelength controlled by the red and infra-red phytochrome pigment system. Red light promotes germination and infra-red light inhibits it. The red-light requirement for promoting germination often varies with temperature or duration of water uptake. Toole et al., (1961), noted for example, that germination of virginia pine seeds occurred faster for seeds promoted with red light after a 20 day period of imbibition of water at 5 0C, than in seeds given a 1-day period of imbibition.
As stimulation of respiration is an essential early phase if seed germination, it is not surprising that oxygen supply affects germination. Seeds usually require higher oxygen concentrations for germination than seedlings require for subsequent growth. Removal of seed coats, followed by exposure of the de-coated seeds to high oxygen concentration, accelerates respiration even more Oxygen plays a primary role as the electron acceptor in respiration. Soaking of seeds for a few hours hasten germination, but prolonged soaking induces injury and loss in viability of many seeds, presumably because of the reduced concentration and availability of dissolved oxygen in comparison with that of the water. Soaking seeds of several upland species for 3 to 5 days did not decrease germination, but soaking for 10 days reduced germination considerably and soaking for 30 days killed the seeds.
Effects of Seedbed
Because of wide differences in physical characteristics, temperatures, and availability of water and mineral nutrients establishment of plants varies greatly in different natural seedbeds. Mineral soil is a good seedbed because of its high infiltration capacity, adequate aeration, and close contact between soil particles and seeds. Decayed wood also is an excellent natural seedbed for seeds of forest trees, probably because of its capacity for water retention.
Effect of Chemicals
Several applied chemicals, including insecticides, fungicides, herbicides, and fertilizers sometimes check plant establishment by direct suppression of seed germination, toxicity to young seedlings, or both. Other herbicides at compariable dosages, that is N-1-naphthylphthalamic acid (naptatam), 2-chlorallyl dietyl dithiocarbamate (CDEC), S-ethl dipropythiocarbmate EPTC), N, N-daillyl – 2-chloraecetamide (CDAA), 2,4 – dichlorophenozylacetic acid (2,4-D) variously inhibited both seed germination and really seedling growth. Both 2,4 – D and CDAA greatly suppressed seed germination. Seed germination and growth of young seedlings are inhibited not only by applied chemicals but also by a variety of naturally-occurring compounds in plants that are released to the soil. Naturally occurring compounds that have inhibitory effects on the seed germination and growth of neighboring plants include phenolic acids, coumarins and quinones, terpense, essential oil, alkaloids and organic cyanides.
Linhares (1980) reported that heavy seeds of wheat exhibited greater vigour than lighter ones. The explanation was that larger seeds had large food reserves for the growing seedlings. However, increase in seed size led to diminishing returns in seedlings size, explaining that seedlings from small seeds assimilated much more carbon per unit weight that seedlings from large seeds. Thus although other Environmental factors may obscure the effects of size, it is one of the most important factors that influence seed and seedling characteristic.
Sowing methods can be divided into two namely: broadcast sowing and drill sowing, but seed sowing time varies with the species, the plant seedling size and the time to attain this size. In broadcast sowing the seeds are distributed uniformly over the seed bed or germination bed, while in drill sowing they are arranged in parallel uniformly spaced bands that run length or breadth of the beds and spaced as narrowly as possible. Broadcast sowing has the advantage of permitting the production of a greater number of plants per unit area of germination bed and quicker formation of a dense soil cover as well as rapid suppression. The transplanting operation is however more difficult in broadcast sown beds.
Large seeds are particularly suitable for drill or row sowing method. The drills are made by hand, using small hoes or by machine, leaving farrows of predetermined depth on recently pulverised seed beds for seeds to be sown manually or mechanically. Another technique is sowing directly into individual plant containers which will ultimately be planted with seedlings. It eliminates the need for germination beds, pricking out operation and the associated problems.
In most cases it is unsuitable to sow tree seeds direct in the plantation because they are small in size, their viability is uncertain, and they are under the influence of dormancy. Trees whose seeds are scarce and expensive, and those whose seedlings have slow initial growth rate, as well as predominance of harsh environmental conditions have made the use of nurseries essential. Seedlings or transplants have to be raised in a nursery which has the facilities to protect and care for seeds and young seedlings unit they are strong enough to withstand the more harsh and difficult field conditions and later transferred to plantation.
The nursery is a seedbed specially prepared to provide a moist, weed free, fertile soil of good tilth, free from pathogens, free from predatory insects, and free from weed seeds, while ensuring the presence of mychorrhiza and nodule bacteria where necessary. The aim of a nursery is to obtain the highest quality seedling in the minimum period of time.
The choice of site for nurseries which can be grouped into temporary or shifting, and permanent or central nurseries is dictated by the availability of an accessible good land located on a suitable topography close to the planting sites, free from pollution, pests and diseases, and with adequate water and labour supply.
Temporary nurseries, as the name implies, are used for a few years only, to grow seedling for planting in a limited area. They are relatively small in size and have the advantages of being cheaper to establish while proximity to the planting site reduces the cost of transportation, handling risk, and the time interval between lifting the seedling and planting them on the field.
Permanent or central nurseries are usually large and more intensively managed. They are used for raising seedling for many years on the same site that is different from the planting site. Centralization of their location, labour, management and use of resources yield higher economic benefits in the long run. However, seedlings have to be transported over longer distances at higher cost and risks. Fertility of the nursery also has to be maintained while ensuring that pests and diseases do not build up.
Source National Open University of Nigeria