Palm Seed Storage Behaviour

A comprehensive guide to storing and germinating palm seeds

If you have read any article about germinating palm seeds then the general advise would have been that the fresher the seeds are, the better the results will be. This, on the most part is very good advice and is a good method to work to. However, you will undoubtedly find that you do not achieve 100% germination of your seeds. Some species will germinate better than others, but some species will repeatedly produce poor results for reasons possibly relating to the environmental conditions that you are germinating them in. However, you should consider the fact that a palm tree may produce a large quantity of non-viable seeds within any bunch, and some seeds within a bunch at the time of harvest may not be fully matured enough. Also some seeds amongst the bunch may have a parasitic insect infestation which is not visible to the outside of the seed. Damaged seeds can often be removed as floaters during the initial seed cleaning process by the collector and this is usually the case, as the collector tries to ensure the quality of the seeds he is supplying. (Note: not all palm seeds sink in water).

Consider this simple experiment:

Take 2 random samples of 100 seeds from a fresh batch of seeds. One sample is planted immediately in your preferred method for germinating palm seeds (the method that works best for you). The second sample is soaked over night in water and then allowed to become touch dry before placing in a sealed plastic bag for cool room storage. If the bag quickly mists up on the inside under normal room temperature then the seeds were not dried enough. Remove the seeds from the bag and dry some more before re-bagging. These, now soaked, then dried, then bagged seeds are now stored in a slightly cool room out of direct sunlight for a period of 3 months. After the elapse of 3 months, remove the seeds and soak overnight once again. The next day discard any seeds that are floating on the water, and discard any seeds that are obviously not looking good. You must now count exactly how many seeds out of your 100 you now have left. Now plant the second batch of seeds using the same method as the first batch. After a considerable time (perhaps one year) has elapsed, and you now have all the visible seedlings you are likely to get from the two batches you should now count how many you have in each batch. The results, if everything was done correctly will show about the same number of seedlings in each batch, though the germination percentage will be much higher in the second batch per number of seeds planted. For this reason germination percentages mean absolutely nothing at all without recognised benchmarks by which to calculate. A reputable seed supplier will know the expected shelf-life of his seeds, and will have made every effort to keep the germination percentage per seeds supplied to the optimum. Which explains why you often get higher germination percentages from older seed supplied.

Obviously this is an example and few species of palm have similar optimum seed storage requirements. But from a growers perspective a simple way of guessing is to consider the natural environment of the species. A species from a tropical jungle will have seeds that generally have a very short shelf-life, there will be losses if the seeds are fresh or if they are stored. If stored the seeds will become much more difficult to germinate and the losses will be higher, most tropical, low elevation palms’ seeds will lose viability if stored at temperatures below 15°C (60°F). If it is a species from arid deserts then a preliminary storage together with regular testing will usually produce a higher germination rate of supplied seed. Other considerations are that if the species comes from high altitude then they usually have a naturally low seed water content and these seeds can usually be stored under cool conditions for periods of time.

The traditional method for testing if a seed is good is to slice it in half and take a look at the embryo. If the embryo is malformed, discoloured, or smells bad then it is unlikely to germinate. But if it looks good then it is still not going to grow because you’ve just cut it open, and even good looking seeds (if not cut open) can still be inviable. Obviously many species are rare, and the few seeds which may be available can be expensive. Cutting open seeds to look at and sniff is not a good idea. Besides, many palm species produce what is known as a ruminate seed, where the seed coat is enfolded, creating dark, tangled streaks in the endosperm. Ruminate seeds cannot be inspected very easily. For these we can however use the Tetrazolium Chloride test to give us a much more accurate and quicker result. The method is to Mix a 1% (10 gm/l) aqueous solution of tetrazolium chloride. Cut a sample of the seeds in half to expose the embryo and place the half containing the embryo in the solution. Put the container in the dark for 2-24 hours. On inspection, if the embryo stains partially or completely red or pink, it is probably viable. If there is no stain, the seed is likely inviable.

It is a good idea to test sample seed lots for viability before purchasing large quantities. If the Tetrazolium Chloride test is beyond you try using the seed float test. The seeds are placed in water for 24 hours and those that float are discarded as inviable. However, some palm seeds naturally float because they are dispersed in nature by water. Furthermore, some growers have found that if the floating seeds are planted, a sizable number will in-fact germinate.

For some species of palm namely Butia and Jubaea it appears that 3 months warm dry storage of seeds is required prior to planting. More research is needed to identify why this is. However, it may be that the embryo requires a period of time to fully mature. We know this to be true for many Cycad species. Cycad seeds must be dry stored for a period of 3-12 months before planting, whereas fresh seeds can quickly rot when planted. The seeds of the Cycad genus: Macrozamia can be stored for six years and still produce 100% germination.

With few exceptions palm seed should be collected when the fruit is completely ripe (showing full colour), or as soon as it falls from the tree. A few exceptions have been noted. Seed from green fruits of queen palm Syagrus romanzoffiana germinate better than seed from half-ripe or ripe seed, perhaps due to inhibitors in the fruit.

For seed banking purposes palm tree seeds are generally considered to be ‘non-orthodox’ Most plant species have seeds that stay viable when dried to the low moisture contents necessary for long-term storage. These are termed ‘orthodox’ species. Palms fit into the categories of recalcitrant and intermediate species, where a medium to high water content must remain in the seed, and seed cannot be conserved long-term in a seed bank via pre-drying and freezing.

The water content of the developing seed is similar to that of any actively growing tissue – about 70 to 80 percent. As the seed reaches maturity and the stage at which it is shed from the plant, its moisture content drops rapidly. The amount of water that remains at maturity depends on the species of the plant and the environment in which the seed matures. The seeds from tropical rain forest palms generally contain over 50% moisture, whereas palm species’ seeds from arid environments can have very low, almost orthodox levels of water content. Similarly, palm species which grow at high elevation have seeds which contain around 18% less moisture than related species at low elevation. This lower water-holding capacity of seeds from higher elevations suggests that plant tissues that must survive severe cold usually contain less water than those of warmer climates. The mechanism by which the living seed is protected under such varying conditions of development can be related to their chemical composition.

When water and heat are applied to a seed the embryo within the seed draws on its endosperm for the nutrients it needs to germinate and grow. A delicate balance of internal conditions regulates its life processes. The living seed is able to incorporate small molecules and simple substances such as glucose, phosphorus, and sulphur into complex chemical units of a cell. These organised parts are the cell wall and the protoplasm, which contains the cell nucleus. Enzymes act as the go-between in these conversions and building processes. The energy for this work comes from the breakdown, or catabolism, of some of the cell’s chemical components, usually by combining them with oxygen in the process called respiration. Most of the seed components from which the enzymes of the protoplasm and cell walls form new cells can be classed as proteins, fats, carbohydrates, organic acids, and amino acids. Thus the seed lives as long as its outside and internal environments maintain active enzymes and a good balance of chemical substances. Only under these conditions can the embryo, the result of the fusion of the sperm and egg nuclei, produce new cells and a healthy plant.

Moisture, temperature, and gases, particularly carbon dioxide and oxygen, can affect markedly the enzymes and chemical components of the living seed. Fungi, insects, bacteria, chemicals, or light can diminish or destroy the seed’s power to germinate. Many of the same factors, in the right concentration or combination, can enhance the life processes of the seed. Although enzymes are present in dry seed, they are activated only on movement of water into the seed. As the temperature increases, the rate of metabolism enzyme activity also increases. One measurable product of this metabolism is the amount of carbon dioxide given off and oxygen taken up. A rapidly metabolising seed has a higher gas exchange rate than a quiescent seed. If the energy made available by respiration is not used in growth, it is liberated as heat, and the temperature of the stored seed goes up. Water content, one of the most important factors in seed viability, therefore cannot be considered alone. If the water content within a seed is too high, large amounts of the chemicals required for growth will be used up. The seeds will then be unable to germinate when they are placed under proper conditions. Removal of too much water from the seed also causes death.

Changes in organic compounds also occur with the uptake of oxygen and release of carbon dioxide in living, but non-growing, seeds. If these seeds are germinated, the rate of respiration increases, and the chemical changes, uptake of oxygen, and release of carbon dioxide are easy to detect. The gas atmosphere surrounding mature seeds can determine if the seeds remain alive. If a container of seeds is evacuated and the oxygen pressure reduced, the seeds keep better than in air. Lack of oxygen retards respiration. Palm seeds planted too deeply in the soil, where little oxygen is present, will not live. As the depth of planting increases, the available oxygen and seed survival decrease. Wet or poorly drained soils also lack oxygen and inhibit the living processes of the seed. Ravenea musicalis is remarkable in being the only truly aquatic palm species. It grows in a single fast flowing river in Madagascar. It’s seeds germinate while still on the parent tree and then drop into the water, sink to the bottom, and commence growth under water. Most seeds immersed in water for lengths of time will die unless air is bubbled through the water. Also a shortage of oxygen usually kills a seed when the temperature or respiration is high. This happens because enzymes need oxygen to produce energy for growth of the embryo. The energy is released when the enzymes combine oxygen with various cell compounds. Good air circulation is therefore an essential ingredient for germination of palm seeds under their high temperature environmental requirements and is one of the primary reasons why seed soaked in stagnant water, or seed germinated using the polly-bag (Baggie) method so often fail. Direct sowing of palm seed into deep pots of a well drained soil produces a much better yield and a much safer option to the polly-bag method of germination.
Sometimes, however, high levels of oxygen are not required by the living cell to obtain energy from its chemical compounds. Some seeds have an abundance of the anaerobic enzymes, which function without oxygen. These enzymes produce enough energy for certain life processes. The Nypa fruticans palm is an unusual and primitive example that grows in tidal mud banks, in mangroves and even in salt water in tropical Asia. Its seeds do not require much oxygen to function. The cells of the embryo and seedling have a system of anaerobic enzymes and a special kind of respiration that requires little oxygen. Its seeds can remain viable and germinate under water which contains too little oxygen for the survival of most seeds.

Carbon dioxide, the end product of respiration, also has marked effects on seed viability. If it accumulates inside the seed or in the soil environment surrounding the seed, injury may result. The role of carbon dioxide is difficult to study, because gas concentrations inside and outside the seed may differ widely and the effects vary with the temperature. Accumulation of an enzyme product, such as carbon dioxide, in the living cell slows down the enzyme that produces the product. When seeds are stored for a long time, factors that increase the carbon dioxide around them frequently must be controlled to assure maximum viability. Fungi and bacteria can produce large amounts of carbon dioxide. These micro-organisms commonly occur on and in seeds. They, too, require water to grow, and drying the seed to low moisture inhibits their activity. Bacteria and fungi also contain enzymes, which metabolise and convert chemical compounds. They usually affect the chemicals on the seedcoat or in the seed. Some fungi or bacteria produce chemicals that harm the embryo. Some may produce compounds or excrete enzymes that soften the seed-coat, so that air and water diffuse into the seed and hasten its metabolism and loss of viability. Others may metabolise and exhaust the seed’s storage compounds. Old seeds and seeds that are stored under unsuitable conditions of moisture and temperature are particularly susceptible to attack by micro-organisms, usually to the detriment of the seed. The seedcoat therefore often is treated with a chemical disinfectant before it is stored.

Old seeds, which show marked decreases in nonsoluble carbohydrate or protein, germinate poorly. The breakdown or coagulation of protein in old seeds may advance so far as to modify the protein present in the nucleus. When that occurs, the seeds usually die. If such seeds do germinate, they frequently produce mutant plants. Some success has been achieved by attempting to replenish the carbohydrate or protein food source for the embryo by soaking the seed in seaweed extract fertiliser prior to sowing. Preliminary studies on ancient lotus seeds report the presence of an enzyme called L-isoaspartyl methyltransferase which may play a role in anti-ageing through their repair of proteins. Research such as this may one day provide a rejuvenation product for use with old palm seed.

Inhibitors, including many compounds that occur naturally in living seeds, keep the seeds dormant until conditions are favourable for germination. Some seeds contain compounds that enforce dormancy until there is enough water in the soil to leach the inhibitors out of the seed. The concentration of such inhibitors diffusing out of seeds or roots may be great enough to prevent germination if seeds are sown too close together or too close to other plants. But these same inhibitors, reduced to sufficiently low concentration, may stimulate germination. Many of these inhibitors are lactones; parasorbic acid and coumarin are two examples. They apparently prevent germination by inactivating certain enzymes necessary for elongation of the radicle. Palm seed sown in a well drained soil soon degrades any inhibitors there may be in the seed, even if the fruit is still on the seed.

Conclusion
Keeping seeds alive requires the consideration of many important physical and chemical factors. We now know enough about the manipulation of storage environments to at least minimise undesirable changes in most palm seeds for up to several years for some species. Yet many so-called short-lived seeds do not retain their viability even under the best known procedures and ongoing study is required.