Plant Propagation PDF

COLLEGE OF AGRICULTURE AND NATURAL RESOURCES FACULTY OF AGRICULTURE DEPARTMENT OF HORTICULTURE HORT 252: PLANT PROPAGATION BY P.D. KALEDZI Sexual Propagation and Factors Affecting Sexual Propagation Basically refers to any...

COLLEGE OF AGRICULTURE AND NATURAL RESOURCES FACULTY OF AGRICULTURE DEPARTMENT OF HORTICULTURE HORT 252: PLANT PROPAGATION BY P.D. KALEDZI Sexual Propagation and Factors Affecting Sexual Propagation Basically refers to any form of propagation where seed or the sex cells are employed There are a number of advantages and disadvantages associated with this form of propagation process. Crops from seed Crops from seed  Advantages of Sexual Propagation Common means of propagation for cultivated plants, - (E.g. annuals, biennials, grains, vegetables and large proportion of ornamental plants, - some herbaceous as well as woody perennial plants can be grown commercially by seeds (e.g. coconut, cocoa, etc. The germination of seeds give rise to seedlings extensively used to provide rootstocks for grafting and budding methods  Very beneficial in the work of plant breeders because they largely depend on seedlings to develop new cultivars Using seedlings usually help to overcome continued propagation virus infected material It is the natural means by which most plants maintain their continued existence in the environment  For some plants that cannot be propagated by any vegetative means, seed propagation remains the only option by which they can be multiplied or propagated Several quantities of seeds can be transported from one place to the other much easier than would be for vegetative propagated materials.  Disadvantages of Sexual Propagation Sexual propagation gives rise to offspring that are not true-to-type For most fruit trees, they take a much longer time to come into bearing when they are propagated using sexual means If seeds are recalcitrant, sexual propagation becomes particularly difficult Additionally, most recalcitrant seeds cannot store longer Different characteristics of castor bean seeds Maintenance of Seed Purity Variability in seed reproduction which is the main disadvantage associated with seed propagation must be avoided since - it tends to reduce the value of the cultivar in cultivated plants. - There is therefore an urgent need to develop pure seed of a variety - capable of producing plants typical of that variety under the conditions in which the plants are raised Pure seed of a variety can be maintained using three main methods, namely, - Isolation, - Rouging and - Seed Certification  Maintenance of Seed Purity Three main methods are used to maintain seed purity: Isolation: - Time, - Space, - Emasculation & - Enclosing plants/ Female flowers Rouging and Seed Certification Isolation by Time: In this method, seeds are sown or grown at different times to ensure that they do not flower at the same time. This is to prevent them from being pollinated at the same time. J F M A M (flower) a a (flower) b b (flower) c c Isolation by Space: The method of separation by space demands that plots from which seeds are collected are some distance apart from one another This varies from crop to crop and it depends on how far pollen can travel. Also, collection of seeds must be done in the middle portions of the plot because the outside rows being the guard rows may be affected or contaminated by insects Guard row 360m 360m Seeds collected from middle portion ❖ Isolation by Emasculation: This is the removal of the male flowers or flower part It is done to prevent any form of natural crossing to ensure artificial pollination This is commonly done in tomato ❖ Isolation by Enclosing plants or Female flowers: In this method, the female flower is covered with transparent polythene sheet, bag or paper bag before pollination and fertilization. ❖ Rouging: It is the elimination of off-type plants or other varieties or weeds or volunteer plants Off-type plants may result from recessive genes as a result of mutation Remove them as soon as possible Volunteer plants from accidental seeds or from seeds left during previous season should all be rouged Seed Certification: Government or co-operative bodies institute strong regulatory procedures for maintaining seed purity The objectives of these bodies are to maintain and make available pure crop seeds, tubers or bulbs and sometimes turf grasses to interested individuals or bodies The seed certifying agencies set up production standards for isolation, presence of off type plants, and quality of harvested seeds and they make regular inspection of production fields to see that the standards are very rigidly maintained Certified seeds come from Breeder Seeds - Foundation Seeds - Registered Seeds - Certified Seeds Breeder’s seed: The breeder’s seed is a limited amount of seed usually controlled by the plant breeder and produced under the direct supervision of the breeder. It is not for sale to the general public. It is used for the production of foundation seed. A white tag is used to identify it. Foundation seed: It is also available in limited quantities only and it’s produced under direct supervision of an agronomist or foundation seedstock organization. It should be handled in order to maintain the genetic purity and identity of the seed. Foundation seed is also labelled with a white tag and issued by an official certified agent. Registered and certified seeds are produced from foundation seed. In some countries the foundation seed is referred to as pre-basic seed. Registered seed: The registered seed is usually a non-commercial seed class and is produced from foundation seed for the purpose of increasing certified seed. It should also be handled properly to maintain satisfactory genetic identity and purity. It is identified by a purple tag and issued by a proper certified agency. In some countries the registered seed is referred to as basic seed. Certified seed: Certified seed is produced from foundation seed and/or registered seed. It represents the final product of the certification. It should be handled to maintain satisfactory genetic identity and purity as approved by official seed certified agency. It is identified by a blue tag or labels given by official seed certifying agency.  GERMINATION AND FACTORS AFFECTING IT This is the resumption of active growth by the embryo, - causing rupture of the seed covering and - emergence of a new seedling - capable of independent existence The process of germination would be enhanced if - seed is viable and also if - environmental conditions such as water, temperature, oxygen and light are favourable. Germination Germination  Factors that affect Germination Hard seed coat Dormant embryos Internal Mechanical resistance to embryo expansion Factors Restrictions of gaseous exchange Chemical inhibitors Water External Temperature Oxygen Factors Light  Internal Factors affecting Germination Hard seed coat: Reduced moisture content of seeds at maturation coupled with deposition of particular substances in the seed covering makes the seed covering impermeable to water - This can be a genetic trait or can be - induced by environmental conditions and - methods of handling Impermeability can also be induced by - hard outer cover or seed coat or by - part of the pericarp Seed coat conditions for example, hardened seed coat can be modified by certain treatments during harvest and storage Different types of seed coats  Hard seed coats Date stones (seeds) Noni Velvet Tamarind(Yoryi) ❖ Dormant embryos: Seeds with dormant embryos may need - certain treatments like stratification (moist chilling) for the embryo to be activated in order to effect germination On the other hand certain dormant seeds need - high temperatures followed by - low temperatures to enable them germinate ❖Mechanical resistance to embryo expansion: In some seeds the rapture of the seed coat is impossible because - the pressure of the germinating embryo is insufficient to break the seed covering or - to separate the hard endosperm This prevents the embryo from expanding In some cases, moist warm storage helps improve the situation ❖Restriction of gaseous exchange: Lack of oxygen to - the embryo, - the inner membranes of the seed coat or - the enclosing nucellus or - endosperm layer has been cited as a factor in seed dormancy. Thus increase in atmospheric oxygen surrounding the seed can sometimes stimulate germination ❖Chemical Inhibitors: Certain chemical or substance that inhibit germination such as - ethylene, - ammonia, - cyanide, - essential oils, - unsaturated organic acid and - alkaloids (nicotine, cocaine, caffeine and codeine) can be extracted from plant parts including seeds. Sometimes the presence of such substances slightly inhibits germination. ❖ Pre-conditioning seeds to stimulate germination The essence of pre-conditioning seeds is to - break seed dormancy, - soften seed coats and other covering to enhance germination One way of breaking such seed dormancy is scarification Scarification is any process of breaking, scratching, mechanically altering or softening the seed covering to make them permeable to water and gases. Some scarification methods are: 1. Mechanical scarification 2. Hot water scarification 3. Acid scarification ❖ Mechanical scarification: involves chipping hard seed coats by - rubbing with sand paper, - cutting with a file, or - cracking with a hammer without causing injury to the seed. ❖ Hot water scarification: Seeds are dropped into four to five times their volume of hot water with temperature range 77 to 100°C and then - the heat source is immediately removed. - Seeds are left soaked in the gradually cooling water for 12 to 24 hours. - Swollen seeds should be planted immediately after the treatment and - non-swollen seeds could be retreated or discarded. Scarification Mechanical Mechanical Hot water ❖ Acid scarification: Dry seeds are placed in containers and covered with concentrated sulphuric acid (conc. H2SO4) in a ratio of about one part seed to two parts acid Treatment time could range between 10 minutes and 6 hours depending on the type of seed Containers used for such treatment should be glass, earth ware or wood but not metal or plastic Stirring of the mixture should be done at short intervals to help produce uniform results.  At the end of the treatment period the acid should be poured off carefully and seeds washed with water. Acid-treated seeds can either be planted immediately when wet or dried and stored for a later planting. ❖ Other methods of pre-conditioning seeds to stimulate germination are; 1. Moist chilling or stratification 2. Use of chemical stimulants eg. Potassium nitrate(KNO3), Gibberellic acid(GA3) and sodium hypochlorite Acid Moist chilling- Stratification External Factors affecting Germination ❖ Water: Water is necessary for the imbibitions process, but uptake of water depend on - the nature of the seed, - the seed covering and - amount of water available in the surrounding area The amount of water and the rate at which it is absorbed depends on - the type of seeds and also on - temperature and humidity Higher temperatures favour increased rate of water uptake  Maintaining adequate supply of water during germination is very essential but care must be taken to prevent water-logging conditions Thus good drainage must be provided in the media. ❖ Temperature: Temperature is another important requirement for germination Generally, seeds when planted or sown at optimum temperatures perform very well, thus temperature largely determines when to plant in the field Minimum temperatures for cool season crop will be about 4.4°C and a maximum of about 30-40°C Lower and/or higher temperatures than these are likely to kill most seeds ❖ Oxygen: Oxygen is very essential in respiration The low rate of respiration in non-germinating seeds increases during germination It is involved in starting germination In some seeds with plenty of fat or oils, these are converted into sugars before they are translocated But fatty acid molecules contain less oxygen than sugars therefore extra oxygen supply is needed Energy to drive cellular mechanism and to transform specific materials into growth process is provided through the oxidation ❖ Light: It is important in the initiation of germination as well as on seedling growth yet not all plants need light to germinate Germination control is exerted through photo-chemically reversible reaction involving the response of a pigment known as phytochrome in a particular wavelength Phytochrome is a protein pigment that mediates in photoperiodic responses and certain other photoreactions Red light can penetrate to about an inch in a porous loam but deep layers can only be reached by infrared light hence the importance of depth of sowing seeds Qualities of a Good Seed Genetically true to the species, variety or cultivar Must have a high germination capacity Must be free from diseases and pests Must be clean or pure thus free from all types of seeds whether of weeds or cultivated plants Must be well-developed  Seed Testing Seed testing - provides information to meet legal standards, - determines seed quality and - establishes the rate of sowing for a given number of seedlings. It is desirable to retest seeds that have been in storage for a prolonged period. Sampling: The first step in seed testing is to obtain a uniform sample representing the entire lot under consideration.  Germination Test A germination test usually runs from one to four weeks but could continue for three months for some slow-germinating tree seeds with dormancy. It can be carried out in seed boxes, seed pans, perlite, vermiculite and sandy soils, petri dishes, etc. There are basically 3 kinds of tests that can be carried out: 1. Excised-embryo test 2. Tetrazolium test 3. X-ray analysis ❖Excised-embryo test The excised-embryo test is used to test seed viability of woody shrubs and trees whose dormant embryos require long treatment periods - to relieve dormancy before true germination will take place Under the excised-embryo test the following processes are carried out: 1. Soak seeds in water for one to four days until they are completely swollen. 2. If seeds are to be soaked beyond one day, water must be changed daily. 2. Seed coats of the swollen seeds must be cut with a sharp scalpel, razor blade or knife under clean conditions with sterilized instruments. 3. The embryo must be carefully removed to avoid injury to the embryo. 4. The excised embryo is then germinated in petri dishes lined with moisture filter paper - The time required for the test varies from three days to three weeks. 5. Viable embryos remain firm and show some indication of viability, depending upon the species - Types of response that occur include spreading of the cotyledons, - development of chlorophyll and - growth of the radical and plumule. - The rapidity and degree of development may give an indication of the vigour of the seed. 6. Non viable embryos become soft, brown and mouldy within 2 to 10 days Excised-embryo test ❖ Tetrazolium test: The tetrazolium test is a biochemical method of viability determined by the red colour appearing when seeds are soaked in a 2,3,5-triphenyltetrazolium chloride (TTC) solution for 2 -5 days The test does not test germination itself but the power or vitality with which the seeds germinate. The chemical is absorbed within the living tissues and when the tissues changes to red or scarlet, the seed is said to be viable Non-living tissues will not change colour Tetrazolium test Tetrazolium test Barley ❖ X – ray analysis: X – ray photographs of seeds can be used as a rapid test for seed soundness Seeds are soaked in water for 16 hours and transferred to concentrated solution of barium chloride for 1 -2 hours to remove all materials and then dried Soft x-ray photographs are then taken using standard x-ray equipment to detect mechanical disturbances, presence or absence of embryo and/or endosperm, number of embryos, insect infestation, shrinkage of interior tissues, etc. Soybean seed images obtained by X- ray test and related seedling: a), seed without mechanical or stink bug damages, resulting in a normal seedling (b); c), seed with a severe mechanical damage on the embryonic axis, resulting in an abnormal seedling (d); e), seed with severe stink bug damages on the embryonic axis and cotyledons, resulting in an abnormal seedling (f) Stink bug NEXT WEEK Asexual propagation