Obviously a seed must be viable in order to germinate. A full seed can be aborted because it contains no embryo. To germinate, a seed must also be subjected to the right environmental conditions.
Seeds are broadly divided into two groups: dormant and non-quiescent. Overwhelmingly, plants produce dormant seeds. For some plants this might be a period of a few days, or some years or the occurrence of particular conditions. For instance, after wintering, after a drought, after the parent tree dies or after a fire.
Hard Seeds have a protective coating which is impermeable to water until at least part of this coating has been rubbed off. Most seeds cannot germinate until after leaving the fruit. Some of these seeds also contain chemical germination inhibitors in the fleshy parts; this adaptation is used by most fruits. For this reason all the fruit flesh must be removed from a seed prior to planting. Palms frequently have a fleshy coat and a fibrous layer.
Physiological inhibition is common, particularly among pasture plants and usually causes a one year delay prior to germination. This is frequently environmentally induced, for instance, some seeds require freezing prior to germination.
Some seeds are double-coded. This means, they have more then one requirement before germination will occur and in some cases these requirements must follow a definite sequence.
Many people like to grow plants; some allegedly succeed because of their "green fingers." Perhaps it is more likely that they are aware of some of the following facts:
Hard seeds can be most easily scarred on concrete, sandpaper or with a file for easy germination. Similar results can be achieved by covering seeds for three minutes with boiling water. Then leave the seeds in cold water for 24 hours prior to planting.
Germination of many seeds will improve simply after soaking them for 12 hours in water. However, some seeds need to pass through the gut of a bird. This harsh treatment can be simulated by placing a volume of seeds in two volumes of concentrated sulphuric acid for 10 minutes. Wash the remainder in a plastic sieve under running water and the seeds are ready for planting. Take care with concentrated acid. Always add the acid to the water.
Inland plants frequently require moist chilling prior to germination. This can be simulated by initially soaking the seeds for 12 hours in water. The seeds then are packed in a moisture-retaining mixture (peat) and refrigerated for one day prior to planting.
Temperature is an important factor in the germination of tropical plants. The night temperatures are of crucial importance and these must not fall below 24-28°C. About 95% of tropical fruits fit into this category. Tropical, high-altitude plants are fairly critical in their requirement for 15-18°C.
Chemical stimulants are effective in increasing the germination rate of 'difficult' seeds. Seeds soaked for 24 hours in Giberellic Acid #3 at a concentration of between 100-10,000 parts per million will overcome most germination barriers.
Cheaper and more easily available is a 0.2% solution of Potassium Nitrate and this will aid germination considerably. Place the seeds in a dish covered with the solution until the first signs of germination and then plant the seeds.
Thiourea at a concentration of between 0.5-3% is reputed to aid the germination of old seeds. Soak the seeds for one day and rinse in water prior to planting.
Damping off is caused by Pythium, Rhizoctonia and Phytophthora species. These organisms cause: non-emergence, death post emergence (seedlings appear desiccated when 5 cm high) and collar rot. Wirestem, yellow base, or die-off is a common problem in North Queensland.
Controls to minimize these problems are: clean, sterile pots (pouring boiling water into pots is a good means for the home gardener). Fresh tap water is superior for watering of all seeds and young plants. The organisms causing damping off are most active during daytime temperatures between 20-30°.
Salt assists the reproduction of Pythium and Rhizoctonia and must be avoided.
There are several chemicals available which have proven effective against damping off. Benomyl (Benlate) is a fungicide [No longer produced], Dexon is particularly useful against Phytophthora, Previcur is another fungicide, it does not stop bacterial infection. Perhaps most useful at present is Terrazole, this covers a broad spectrum, is long lasting and effective in low concentrations.
Seeds are still by far the most important way of plant reproduction and the scientific literature contains thousands of references. Interestingly, about 99% of these refer to temperate regions and appropriate seeds. Research into tropical growth conditions is not well documented. Interesting phenomena can be discovered by amateur growers, if they keep good records and pass the information along.
From a talk by Mr. Bob Reid to the Townsville Branch of the Rare Fruit Council.
DATE: September 1983
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