وبلاگ تخصصی مهندسین مرتع و آبخیزداری

(1)Palm Seed Germination

Palms are unique among woody ornamental plants because, with relatively few exceptions, palm species can only be propagated from seed. Palms are also notorious in the nursery trade for slow and uneven seed germination. It has been estimated that over 25% of all palm species require over 100 days to germinate and have less than 20% total germination (Tomlinson, 1990). The reasons for this remain obscure, as very little investigative work has been accomplished on seed dormancy conditions in palms. Nonetheless, the palm grower can maximize success with germinating palm seeds by paying careful attention to a number of basic guidelines. The purpose of this publication is to outline and discuss the various aspects of handling and germinating palm seeds in the most cost-effective and reliable manner possible.

The Palm Seed

Palm seeds vary tremendously with respect to size. Many palms have seeds no larger than 1/4 inch in diameter, while the largest seed of any flowering plant in the world is that of a palm (the double coconut, Lodoicea maldivica). The bulk of a palm seed is taken up by nutritive tissue called endosperm that provides food for the germinating seedling for a longer period of time than most flowering plants (Figure 1). The "milk" and white meat of a coconut are liquid and solid endosperm, respectively. The palm embryo is very small, either cylindrical or top-shaped. The seeds themselves may be either round or variously elongated. Their surfaces may be smooth or intricately sculptured. Some are surrounded by a hard, water- and air-impermeable coat. Fibers from the fruit wall frequently remain attached to the seed, even after cleaning.

Figure 1. Longitudinal and cross section through a coconut fruit.

Types of Palm Seed Germination

The way palm seeds germinate falls into one of two categories. In palms with remote germination (Figure 2a-c), the seedling axis develops at some distance from the actual seed. The first structure to emerge from the seed is called the "cotyledonary petiole." It resembles, and many people mistake it for, the first seedling root. The cotyledonary petiole grows downward into the soil (sometimes very deeply) and swells at its base. From this swelling emerges the first seedling root (radicle) and seedling shoot (plumule). The actual cotyledon or seed leaf remains inside the seed, functioning as an absorptive organ called the "haustorium." The haustorium transfers nutrients from the endosperm to the young seedling. In palm seeds with remote germination, the radicle persists for some time and produces lateral roots. The seeds of Mediterranean fan palms (Chamaerops humilis), Chinese fan palms (Livistona chinensis), date palms (Phoenix spp.) and Mexican fan palms (Washingtonia robusta) have remote germination.

The other main class of palm seed germination is called adjacent germination (Figure 2d-f). In these seeds, only a small portion of the cotyledon emerges from the seed. It appears as a swollen body abutting the seed surface and is called the "button." The radicle and plumule emerge from the bottom and top of the button. In palms with adjacent germination, the first seedling root or radicle is usually narrow and very short lived and is quickly replaced by roots formed at the seedling stem base (adventitious roots). As with remote germination, a haustorium remains inside the seed absorbing food from the endosperm. Some common palms with adjacent germination include areca (Dypsis lutescens), King Alexander palm (Archontophoenix alexandrae) and coconut (Cocos nucifera). In coconut, however, the first stages of germination occur in the fibrous fruit wall that adheres to the seed. They cannot be observed without dehusking the nut.

A number of palm species (Bismarckia, for example) bury the seedling axis deep in the soil. These species require some special handling that will be discussed later in this circular.

Figure 2. Main classes of palm seed germination. A-B. Remote germination, date palm (Phoenix dactylifera). A. Early germination with cotyledonary petiole emerged and seedling root (radicle) beginning growth. B. Seedling stem (plumule) emerging from cotyledonary sheath. C. First leaf (eophyll) emerged, radicle continuing to elongate, and haustorium inside seed absorbing nutrients from endosperm. D-F. Adjacent germination, piccabean palm (Archontophoenix cunninghamiana). D. Early germination with button emerged. E. Seedling root (radicle) and stem (plumule) emerging from button. F. First leaves (eophylls) emerging, first adventitious root formed and supplanting radicle, and haustorium inside seed absorbing nutrients from endosperm.

Sources of Seed

Seeds may be collected from local sources (trees in the landscape) or purchased from commercial dealers. Local collection has certain advantages; the freshness, degree of maturity and parentage of the seed is usually known by the collector. If the seed will be stored, the collector will be controlling the methods used and the duration of storage. Commercial dealers can usually offer larger quantities and a greater diversity of species. Commercial dealers will also be handling the time consuming chores of seed cleaning, which may require special equipment. However, the age and ultimate germination percentage of purchased seed is frequently unknown. Testing a sample of commercially available seed before purchase is a wise precaution (see next section).

Seed Maturity

With few exceptions seed should be collected when the fruit is completely ripe (showing full color), or as soon as it falls from the tree (Figure 3). 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 (Broschat & Donselman, 1987), perhaps due to inhibitors in the fruit. Seed of royal palm (Roystonea regia) from ripe fruits germinated more slowly than seed from half-ripe or green fruits, but fewer of the unripe seed ultimately germinated (Broschat & Donselman, 1987).

Figure 3. Queen palm (Syagrus romanzoffiana) fruit harvested from the tree and ready for processing.

Viability of Palm Seed

Viability of palm seeds can vary among trees of the same species, and even from year to year from the same tree. Age of the seed and/or the storage methods used (see next section) can directly influence the ultimate germination percentage. Seeds of some palms generally remain viable for only 2-3 weeks (e.g., latan palms, Latania spp.), while others may retain viability for over a year (areca, Dypsis lutescens) if stored properly (Broschat & Donselman, 1986). It is a good idea to test sample seed lots for viability before purchasing large quantities. Some growers advocate using a seed float test. The seeds are placed in water 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 germinate. There are two recognized ways to quickly test seed viability on a random sample of the entire lot:

Observation

Cut open a sample of the seeds. The endosperm should be firm and the tiny embryo should fill its chamber (located at one end of the seed). If the endosperm is soft and spongy; or the embryo shriveled, discolored, or absent; or if the seed coat appears to have detiorated, then the seed is probably inviable (Figure 4).

Figure 4. Cross section through viable (right) and inviable (left) seed of Areca palm (Dypsis lutescens). Note shrunken endosperm and embryo in seed on left.

Tetrazolium Chloride Test

Mix a 1% (10 gm/l) aqueous solution of tetrazolium chloride (available from any chemical supplier). Cut a sample of the seeds in half to expose the embryo and place the half containg the embryo in the solution. Put the container in the dark for at least 2 hours (a full day is sometimes required). If the embryo stains partially or completely red or pink, it is probably viable. If there is no stain, the seed is likely inviable.

Cleaning Palm Seed

Figure 5. Cleaned (left) and uncleaned (right) seed of queen palm (Syagrus romanzoffiana).

Palm seeds are enclosed by a fleshy or fibrous fruit wall (mesocarp) that, with few exceptions, must be removed prior to storage or planting (Figure 5). Uncleaned seeds of areca palms (Dypsis lutescens) have been known to germinate if planted immediately upon harvest. Coconut seed does not require husking before planting (in fact, the seedling germinates within the fruit before emerging).

If only a small number of seeds are to be processed, they can be cleaned by hand, using a knife to cut away the fruit tissue. For large quantities, machine cleaning is advisable. Species (Areca, for example) having relatively thin fruit walls can be cleaned easily by rubbing seeds across a strong, large mesh screen while rinsing with a hose to remove the mesocarp, or by rubbing off the fruit wall by hand in a bucket of water and rinsing.

A number of palm species have an irritant in the fruit pulp (calcium oxalate crystals) that can make cleaning by hand a painful experience. Gloves should be worn when handling fruits of the following: all Caryota (fishtail palms), most Chamaedorea, Carpenteria acuminata, Roystonea species (royal palms), and Arenga (sugar palms).

Figure 6. Palm fruit soaking in water to soften the mesocarp.

Most palm seeds require a soak in water to first soften (ferment) the fleshy fruit wall (Figure 6). The water should be changed each day if possible. The fruits are ready for processing when the mesocarp yields easily to finger pressure. The fruits are then placed in a commercial seed cleaning machine which abrades the fermented mesocarp from around the seed. The pulp is washed from the cleaning chamber and collected below. A small cement mixing machine partially filled with coarse sand or gravel works fairly well (Figure 7). In both cases, water is continuously rinsing the seeds as the machines operate. Hard or very fibrous palm fruits can be cleaned by mixing the fruits with gravel or rock and repeatedly stepping on them. Similarly, some palm species with very hard seeds have been cleaned by driving a truck or other vehicle over burlap bags of the softened fruits. Cleaned seeds should be air dried before storage.

Figure 7. Cement-mixing machine used to clean palm seeds. Gravel inside the machine abrades the fruit pulp from the seed.

Storage of Palm Seed

With few exceptions, it is best to plant palm seed shortly after cleaning. If this is not possible, the best general storage procedure is to dust cleaned and air-dried seed with thiram (Thylate^®) or captan, seal the seed in plastic bags, and store at 65-75°F. There is some evidence that maintaining this covering of fungicide when the seeds are sown may http://flrec.ifas.ufl.edu/pdfs/tropicLine/TropicLine_07-2.pdf (Meerow, 1994). Seeds of most tropical palms will lose viability if stored at temperatures below 60°F. Broschat and Donselman (1986, 1987, 1988) found that cleaned seed of Areca (Dypsis lutescens) could be stored at 73°F for over 1 year without significant loss of viability, royal palm (Roystonea regia) for 9 months, queen palm (Syagrus romanzoffiana) for 4 months, and pygmy date (Phoenix robelenii) for 8 months. In the case of royal palm, up to 9 months of storage actually increased germination relative to planting fresh seed immediately.

Seeds of pindo palm (Butia capitata) actually require a period of dry storage for optimum germination (Carpenter, 1988b). The duration of the period increases with decreased temperature: 90 days at 77°F, 120 days at 59°F, and 150 days at 41°F. Generally, palms from seasonal climates (versus uniformly tropical) may have greater tolerance for low temperature storage. Seeds of the native silver palm (Coccothrinax argentata) and thatch palm (Thrinax morrisii) have withstood -4°F and 15°F respectively for one week without loss of viability (Carpenter, 1988a; Carpenter & Gilman, 1988). Seeds of more tropical species (areca, Dypsis lutescens, for example) may be killed after storage for 24 hours at 40°F (Broschat & Donselman, 1986).

Pretreatment Before Planting

Due to the often slow and uneven germination of palm seeds, there has been a great deal of interest in any preplant treatments that might speed germination or result in more even rates of germination. For the commercial palm grower, the value of seed pretreatments must be weighed against the additional labor costs involved.

Water Soak

A fairly universal recommendation has been to soak palm seed in water for 1 to 7 days. It is advisable to change the water daily. Such a pretreatment is useful only after dormancy requirements (if any) have been met, though few palm species have been tested for indications of seed dormancy. The seed must be planted immediately after the treatment, as storage following water imbibation may induce a secondary dormancy. One researcher germinated seeds of fifteen species of the genus Copernicia entirely in water (Kitzke, 1958). Not all species respond positively to a water soak treatment (Broschat & Donselman, 1987, 1988; Carpenter, 1987, 1988; Doughty et al. 1986; Odetola, 1987), and experiments documented in the literature have rarely tried varying the duration of the presoak period on seeds of the same species. However, unlike some of the other pretreatments described below, a water soak poses little danger to the seed.

Soak in Gibberelic Acid (GA3)

A number of investigators have reported a hastening affect on germination by soaking seed in 10 to 2000 parts-per-million (ppm) concentration of GA3 for 1 to 3 days (Doughty et al., 1986; Nagao & Sakai, 1979; Nagao et al., 1980; Odetola, 1987). One study found 10-25 ppm worked well for a wide variety of species (Odetola, 1987). However, treatment with this growth regulator causes excessive elongation of the seedling (Figure 8), in some cases even preventing the seedling from supporting itself (Broschat & Donselman, 1987, 1988). Consequently, it is not advisable to use a GA3 presoak despite any positive effects on germination rate.

Figure 8. Excessive elongation of Areca palm (Dypsis lutescens) seeds on left was caused by pretreating seed with a presoak of gibberelic acid (GA3).

Scarification

Scarification of palm seed involves thinning the bony endocarp of palm seeds that may impede imbibition of water. It may be accomplished mechanically, by abrading the surface of the seed until the endosperm becomes visible, or by soaking the seed in dilute to concentrated sulfuric acid (H₂SO₄) for 10 to 30 minutes. Scarification has increased the rate of germination of a number of palm species with hard, water-impermeable seed coats (Holmquist & Popenoe, 1967; Nagao et al., 1980; Odetola, 1987). The danger in mechanical or acid scarification is damage to the embryo during the process. The practice should be reserved for seeds with hard and impermeable seed coats. Species that have slow or uneven germination without scarification should have seed scarified on a trial basis before the entire lot of seed is treated.