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Seed treatment and inoculation

 

 

Agroforestry for the Pacific

Technologies

A pubilcation the the Agroforestry Information Services c/o Winrock International, Rt. 3, Box 376, Morrilton AR 72110, USA

To germinate and grow, seeds must absorb water. The seeds of many tree species have waxy or thick "seed coats" that inhibit water absorption and thus delay germination. Seedcoats dissolve slowly in several ways. Exposure to sun, rain and wind dissolves the seedcoat over time. The heat from natural fire cracks the seedcoat. Insects and animals may nibble holes in the seedcoat without damaging the seed embryo. When seeds are consumed by animals, stomach acid weakens the seedcoat. Once passed through the animal, seeds germinate. Under natural conditions the seed crop of one year germinates unevenly over a long period. This uneven germination strategy assures that at least some of the seedlings will survive environmental hazards such as drought, fire, frost or insect or animal attack. Seedlings that germinate early may be well enough established to survive such hazards late in the season, while seedlings that germinate late will avoid early-season problems. Some seeds will remain dormant in the soil for years.

Seed Treatment

In a nursery or plantation it is easiest to protect seedlings or trees when they are of uniform age and size. Uniform seedling size can be achieved by proper seed treatments before sowing. Seed treatments, which mimic nature, are designed to penetrate the protective seedcoat and allow water to be absorbed. Several seed treatment methods are common:

• Cool water - Seeds are soaked in cool, room temperature, water until they swell. The volume of water should be five times the volume of seeds. Soaking time may from 648 hours depending on species, provenance, age and quality of seed. This treatment is appropriate for seeds with a thin or soft seed coat, recently harvested seed, seed of small-size, and large quantities of seed.

• Hot water - Pour boiling water over the seeds at a volume five times the volume of seeds. The seeds must be stirred gently during the 2-5 minute soak. Hot water can kill the seed - it is important not to soak the seed for too long! Pour off the hot water, replace it with cool water and soak for 12 hours. This treatment is appropriate for seeds with hard or thick seed coats, old seed, and large quantities of seed.

 


Seed should be stirred for the duration of the hot water treatment.

 

• Acid - Cover seeds with sulfuric acid for 10-60 minutes. Seed should be completely submerged but just below the surface of the acid. Acid can kill the seed - do not soak the seed for too long! Gauge the length of acid treatment by the appearance of the seed. The waxy gloss of the seed coat should be replaced by a dull appearance. A pitted appearance indicates damage remove the seeds before this occurs. Remove seed from acid, rinse with water for 10 minutes and soak in cool water for 12 hours. Do not pour water into the acid or a violent reaction will occur! The acid can be used several times. This treatment is appropriate for seeds with hard and thick seedcoats. Acid treatment can be dangerous and expensive! In most circumstances it is not recommended!

• Nicking - Cut or scrape a small opening in the seedcoat. A knife, nail clipper, metal file, sand paper or sanding block can be used for this operation. To avoid damaging the seed embryo, cut or scrape the seedcoat only opposite the micropyle (the small ring where the seed was formerly attached to the fruit of its mother plant). Nicked seed is soaked in cool water for 12 hours. This treatment is appropriate for all types of seeds. with the exception of those that are very small or recalcitrant (having a soft seedcoat and short viability). However, being time consuming, this treatment is feasible only for small quantities of seed.

• No treatment - Some seeds germinate quickly without treatment. Application of the above methods may be impractical. make seed difficult to handle or decrease viability. No treatment is needed for tiny seed, i.e. Alnus sp. and Casuarina sp.: seeds with thin or incomplete seedcoats: and recalcitrant seed, i.e. Inga sp.

The length of the initial soak in cool water. hot water or acid will vary according to species. provenance, age and quality of the seed. If large numbers of seedlings will be produced, or nursery operations will last for several years. it is recommended that several soaking times be tested in order to determine the most suitable time length for local conditions. As noted, with all methods the last process is to soak seed in cool water for 12 hours. This final process allows seed to absorb water, results in visible swelling and further hastens germination. To improve this process, and thus germination, this period may be increased up to 48 hours. A summary of appropriate seed treatments for select species is listed in Table 1. Once removed from this final soaking the swollen seeds should be sown immediately - do not store treated seed! At this point, follow standard nursery or and plantation procedures (Evans 1982, Jackson 1987 Liegel and Venator 1987). When sowing seeds directly in the field, operations must coincide with the beginning of the rains. Treated seed germinates quickly, and a short dry period will result in high mortality. It is best to wait for the first rain before direct sowing treated seed.

 


After soaking the seed in acid for 10-60 minutes


Rinse with water for 10 minutes.

 

Nitrogen Fixation. Many tree species can transform or "fix" atmospheric nitrogen into a form the tree can use for growth. This allows the "nitrogen fixing tree" (NET) to grow on degraded soils that are poor in nitrogen. NFTs are common components in farming systems, land reclamation schemes, pasture systems, fallow improvement, home gardens and ornamental plantings. These trees provide valuable timber, fuelwood, poles, fodder, green manure, honey, fruit, vegetables, shade and other products. For further details concerning NET management systems and products see Macklin ( 1990).


Nicking the seedcoat opposite the micropyle

(along the dotted line) avoids damage to the seed embryo.

 

 

 

 

Table 1. Appropriate seed treatments for select species (Revised from Macklin et al. 1989).

Species

Treatments

Seeds / kg

Acacia acuminata

C; D

60,000-80,000

Acacia aneura

A;C

75,000-95,000

Acacia nagustissima

C: D

90,000-100,000

Acacia auriciformis

A for 30 sec. B for 15 min A C

30,000-90,000

Acacia crassicarpa

A for 30 sec ;C

40,000-60,000

Acacia holosericea

A for 1 min: C

70,000-80,000

Acacia mangum

A for 30 sec ;C

80,000-100,000

Acacia meamsit

A;C

48,000-85,000

Acacia melanoxylon

A B for 15 min. C

60,000-100,000

Acacia nilonca

A; C; D

7,000-11,000

Acacia polyacantha

D

10,000-25,000

Acacia salinga

A C

14,000-25,000

Acacia senegal

C; D

10,000-30,000

Acacia tortilis

A; C; D

12,000-18,000

Albizia lebbeck

A; C; D

6,000-16,000

Albizia procera

A; C

20,000-24,000

Albizia saman

(syn. Samanea saman,)

A; C

6,000-8,000

Alnus sp

E

200,000-2,300,000

Cajanus cajan

D; E

5,000- 12,000

Calliandra caiolhyrsus

A; C; D

18,000-20,000

Casuanna sp.

E

150,000-1,500,000

Chamaecyasus palmensis

A for 4 min

38,000-42,000

Dalbergia sp

D

10,000-30,000

Desmodium sp

E

500,000-600,000

Enterolobium cvolocan

C; D

800-2,000

Erythnna poeppigiana

D

3,000-5,000

Faidherbia albida syn Acacia albida)

A; B for 20 min. C; D

20,000-40,000

Flemingia macrophylla

A B for 15 min D

50,000-80,000

Glincidia septum

C; D; E

7,000-12,000

Inga .

E - recalcitrant

50-150

Leucaena sp.

A; B for 5-15 min C

22,000-35,000

Mimosa scabrella

C: D

60,00-90,000

Parasertathes falcataria

(*syn Albizia falcatana)

A B for 10 min C

40,000-50,000

Pithecellobum dulce

C; E

9,000-25,000

prosopis sp

A; C

20,000-50,000

Pterocarpus indicus

E

1,5000-2,000

Robina pseudoacocia

A; B for 20-60 min C

35,000-50,000

Sesbania grandflora

C; D

20,000-30,000

Sesbana sesban

A; C; D

85,000-100,000

Tipuana tipu

E

1,600-2,500

 

Key: A-Hot water: B-Acid: C-Nichng; D- Cold water: E - No treatment.

Nitrogen fixing trees fall into two categories based on the symbiotic microorganisms which enable nitrogen fixation to occur. Trees in the families Leguminosae and Ulmaceae fix nitrogen with bacteria from the genus Rhizobium. Species from several families - including Betulaceae Casuarinaceae. Elaeagnaceae. Rhamnaceae. Rosaceae fix nitrogen with actinomycete bactena from the genus Frankia (Postgate 1987, NFTA 1989). The most important NFT genera include the legumes

Acacia. Albizia. Calliandra. Chamaecytisus, Dalbergia,

Erythrina. Flemingia, Gliricidia. Leucaena. Rrosopis and Sesbania. as well as. the non-legumes Alnus. Casuarina. Elaeagnus. Hippophae and Robinia. The bactena form nodules on the roots of the tree. and nitrogen fixation occurs inside these nodules. Healthy nodules that are fixing nitrogen are easy to identify. Cut them open and check their color. Pink or red nodules are fixing nitrogen, while green black or brown nodules are not.

There are many strains of Rhizobium and Frankia. These bactena and the NFTs exhibit an exclusive preference for one another. Some Rhizobium will form nodules with some legumes but not others. The same type of preference is shown between Frankia and the non legumes. A successful match must be made if functioning nodules are to be formed.

In areas where a tree is native or naturalized the soil will likely contain bacteria of appropriate bacteria stranns. If a tree is not common in an area or the site is degraded. there may not be enough of the correct bacteria in the soil to stimulate nodulation and fixation. It should be noted that Frankia survives in the soil without the presence of the host plant for longer periods than Rhizobium. In any case, it is best to treat seeds or seedlings with the appropriate strain of bacteria to stimulate maximum nitrnogen fixation This process is called inoculation.

 


Seed can be "nicked " by cutting or scraping through the seedcoat.

Inoculation

The material used to inoculate seeds or seedlings is produced by isolating bacteria the root nodules on healthy trees, groowing them in a laboratory and mixing them with peat. The peat-based inoculant contains 1000 times the number of bacteria normally found in the soil. The bacteria in inoculants are alive. They are sensitive to heat drying out and direct sunlight. The inoculant should be tightly sealed and stored in a moist, cool, dark place when not in use. It should not be frozen, as freezing kills the bacteria. The inoculant should be applied to tree seed directly after seed treatment and immediately before sowing. Application of inoculant is most effective when seeds are first covered with a sticker solution. Place seeds in a plastic bag or bucket and cover them with the solution made of gum arable, sugar or vegetable oil. Either dissolve the 40 g of gum arabic in 100 ml of hot water and allow to cool, or dissolve I part sugar in 9 parts water. Combine 2 ml of one of these mixtures or 2 ml of vegetable oil. with 100 g of seeds and shake or stir until the seeds are well covered. Then add 5 mg of inoculant and shake or stir until the seeds again are well covered. Allow the inoculated seeds to dry for 10 minutes to eliminate any stickiness and sow immediately (Keyser 1990). Do not store inoculated seed the bacteria will die.

Seedlings can also be inoculated in the nursery after germination. Mix inoculant in cool water and irrigate the seedlings with the suspension. Keep the mixture well shaken and irrigate until the inoculant is washed into the root zone. A 50 g bag of inoculant is sufficient to inoculate 10,000 seedlings (Keyser 1990). For effective inoculation of seeds or seedlings to occur the bacteria in the inoculant must be appropriate for the tree species involved. For information on inoculum availability contact:

Nitrogen Fixation in Tropical Agricultural

Legumes (NifTAL) Center

1000 Holomua Road

Paia, Hawaii 96779 USA

Phone: 1 (80-8) 579-9568

Fax: 1 (808) 579-8516

Lipha Tech (Nitrogen Inoculants)

3101 West Custer Avenue

Milwaukee,Wisconsin 53217 USA

Phone: 1 (414)462-7600

Fax: 1 (414)462-7186

AgroForester

Tropical Seeds

PO Box 428

Holualoa, Hawaii 96725 USA

Phone/Fax 1 (808) 3264670

It may not always be possible to obtain laboratory produced inoculant. At such times, soil containing the appropriate bacteria can be gathered from under trees of the appropriate species. Choose healthy trees that are growing well and that have abundant and robust red or pink nodules. Some of this soil can be mixed with nursery potting mix or added to planting pits. Inoculation by this method ensures that the bacteria will be appropriate for the tree species and the local environment. However, this approach is generally not as effective as using a correct laboratory-produced inoculant.

Further Reading:

Evans, J. 1982. Plantation forestry in the tropics. New

York, NY USA): Oxford University Press, 472 pp.

Jackson, J.K. 1989. Manual of afforestation in Nepal.

Kathmandu: Nepal-UK Forestry Research Project,

Forest Research Division, Department of Forestry and Plant Research, 402 pp.

Keyser, H. 1990. Inoculating tree legume seed and seedlings with Rhizobia. Paia, HI (USA): Nitrogen

Fixation in Tropical Agricultural Legumes (NifTAL)

Center, 2 pp.

Liegel, L.H. and C.R. Venator. 1987. A technical guide for forest nursery management in the Caribbean and

Latin America. General Technical Report S0-67. New

Orleans, LA (USA): USDA Forest Service, Southern

Forest Experiment Station, 156 pp.

Macklin, B. 1990. An overview of agroforestry systems:

A classification developed for extension training. In E.

Moore, ed. Agroforestry land-use systems. Morrilton,

AR (USA): Nitrogen Fixing Tree Association

(NFTA), 112 pp.

Macklin, B., N. Glover, J. Chamberlain, and M. Treacy.

1989. NFTA Cooperative Planting Program establishment guide. Morrilton, AR: Nitrogen Fixing

Tree Association (NFTA), 36 pp.

NFTA. 1989. Why nitrogen fixing trees? NFT Highlight

89-03. Morrilton, AR (USA): Nitrogen Fixing Tree

Association (NFTA), 2 pp.

NFTA. 1990. The production. management and use of nitrogen fixing trees: A manual for field trainers.

Morrilton. AR (USA): Nitrogen Fixing Tree

Association (NFTA), 51 pp.

Postgate, J.R. 1987. Nitrogen fixation. Second Edition

The Institute of Biology's studies in biology. London.

(UK): Edward Arnold LTD, 73 pp.

Somasegaran. P and H.J. Hoben. 1985. Methods in

Legume-Rhizobium technology. Paia, HI (USA):

Nitrogen Fixation in Tropical Agricultural Legumes

(NifTAL) Center, 367 pp.