PLSC 368: Chapter 8

TECHNIQUES OF PROPAGATION BY SEED

I. SEED TESTING

    - Federal Seed Act (1939) mandates labeling for commercial seed
            Label:  Name, cultivar, origin, % germination, purity

    - Tests involve:
            Sampling- for uniformity
            Purity-  check contaminants (broken chaff, stones, dust, etc.)
            Moisture-  % moisture (by oven drying at 130° C)
            Viability- % germination

II. SEED VIABILITY DETERMINATION

    A. Direct Germination Test
            -usually 100-seed samples
            -favorable environment (>90% RH, blotting paper, varying temperature)
            -rolled paper towel method commonly used

            -time period: most seeds 1-4 weeks
                                 some         several months
            -determine numbers of germinated, hard (non-imbibed), dormant, decaying seeds

    B. Excised - Embryo Test
            -woody plant seeds
            -for embryos requiring long, after-ripening period
            -excised embryos are germinated in petridish to determine vigor and growth


    C. Tetrazolium Test
            -Triphenyl tetrazolium chloride (TTC) 0.1-1.0% solution (pH 6-7)
            -works for both dormant and non-dormant seeds

            -living tissues stained red

    D. X-ray Analysis

            -not usually for viability test
            -presence of embryo, insect infestation, broken seed coats, etc.

III. OVERCOMING SEED DORMANCY

    A. Scarification

                -overcomes physical dormancy (seed coat)

            1) Mechanical scarification
                -chipping (sand paper), cutting (file), cracking (hammer)
                -commercial scarifier machines

            2) Hot water scarification
                -drop seeds in 77-100° C (170-212° F) water
                -gradually cooled and soaked

            3) Acid scarification
                -soak seeds in concentrated sulfuric acid (H2SO4, specific gravity 1.84)
                -time period---10 min to a few hours, depending on seed coat thickness
                    (length determined with a small lot, terminated when seed coat is paper thin)
                -rinse in water before germination
                    some legume seeds, Rosaceous species, Hamamelis, Tilia, etc.

            4) High temperature scarification
                -resins in some pine seeds are melted away

    B. Stratification

                -overcomes physiological deep dormancy (embro dormancy)
                -moist chilling 4-8 weeks
            1) Refrigerated stratification
                -imbibe seeds before storage (12-24 hrs soak)
                        seed soaking must be accomanied by aeration, if extended
                -mixed with moisture-retaining medium (peat, sawdust, vermiculite)
                        'naked chilling' -------- store seeds in plastic bags w/o peat
                -temperature 0-10° C (32-50 F)
                        high temp- sprout problem
                -storage time 1-4 months depending on species
                        seeds taken out if they start sprouting

     
                -post-stratification storage possible for some species
                        cherry, fir (Abies)--- reduce % moisture, store for 1 year at 0° C
            2) Outdoor stratification
                -same procedures except for storage
                -store in outdoor pits, coldframe
                -protect seeds from freezing, drying damage by rodents
            3) Outdoor planting
                -seeds planted directly in seed beds and let the nature take care of stratification requirement
                        some woody plants
                        wild flower seeds -i.e. Applewood Seed Co.
                -Prunus (cherries, peaches, plums)
                        plant 1-2 months before onset of chilling -better germination
                -Nature's way of germination
                        (less practiced commercially)

    C. Other Methods

            1) Leaching in running water
                    12-24 hours, removes inhibitores

            2) Combinations of Treatments
                    Scarification (for seed coat dormancy)-----> stratification (for embyo dormancy)------->germinate

IV. ENHANCING GERMINATION BY CHEMICAL TREATMENT

    A. Growth Regulators and Chemical Stimulents

            1) Gibberellins
                -soak seeds overnight
                -100-500 ppm GA3 (better if buffered)
                        K-salt formulation  ----- excellent solubility
                        Pro-Gib---------commercial product
                -problem with seedling elongation in some crops


            2) Cytokinins ( Kinetin, BAP, Zeatin)
                -soak seeds in 100 mg/l (100 ppm)
                -overcome high-temp dormancy in lettuce

            3) Ethylene (Ethephon, Ethrel)
                -enhances germination in some seeds (i.e. cocklebur)
            4) Sodium Hypochlorite (Clorox)
                -removes water-soluble inhibitor in rice hull
                        1% solution, soak

    B. Seed Priming (Osmoconditioning)

              Controlling seed hydration to trigger pre-germination metabolic activities while preventing radicle emergence
              -seeds are primed and then dried

              -treat seeds in aerated solution at 15- 20° C for 7-21 days
              -enhances uniform germination under suboptimum environmental conditions
              -requirement varies by species, cultivar
              -problem -short shelf life
                             -difficulty in bulk treatment
            1) Infusion
                -incorporation of growth regulators, fungicides, antibiotics, herbicidal antidotes into seeds by organic solvent
                            seeds soaked 1-4 hours in acetone-dichloromethane solution containing these chemicals
            2) Fluid Drilling
                -priming and pregermination --------> sowing suspended in gel-----> plant in the field
                                      (sort pregermed seeds)
                -enhances uniform, vigorous seedling stands in the field

V. SEED PROTECTION AGAINST PATHOGENS

    A. Disifestants
            -Clorox 5-10%

    B. Disinfectants
            -Hot water treatments
                treat dry seed immersed in hot water 120-135° F for 15-30 min
                i.e. Alternaria (vegetable seeds)
                      loose smut of barley
            -Aerated steam
                Aeration at 105-143° F for 10-30 min

    C. Protectants
            -Protect seeds from pathogenic fungi in soil
            -insecticides
             fungicides       }  all are treated as dry, liquid, slurry materials
             nematocides

VI. SEED PROPAGATION SYSTEMS

    A. Field Seedling of Herbaceous Plants
            -Direct field seeding
            -Indoor seeding, then transplanting

    B. Mechanized Seedling Production
            -Plug production                                    200-700 plugs/tray

                                     4 stages             Stage I:   Radicle emergence
                                                              Stage II:  Cotyledon spread
                                                              Stage III: Unfolding 3-4 true leaves
                                                              Stage IV: More than 4 leaves
            -Transplanting

    C. Woody Plant Seedling Production
            -container seedling
            -seedbeds