Agriculture Reference
In-Depth Information
sidered a “natural” compound that provides results more indicative of ield performance. As a component
of the soil following fertilizer applications, KNO 3 logically fulills this criterion.
A number of other “natural” dormancy breaking compounds have been also discovered. While their
effectiveness is unquestioned, their acceptance by the seed testing community has been slow.
Gibberellic Acid (GA 3 ). Gibberellic acid is a natural growth hormone found in plants and seeds which
is essential for germination. When dormancy exists, either the synthesis or release of GA 3 to sites of its
physiological activity may be retarded. Exogenous applications appear to circumvent the endogenous dei-
ciency by providing GA 3 to the seed, enabling it to germinate. Gibberellic acid breaks dormancy in a wide
array of seeds that have prechill, predry, and light requirements. The germination substratum should be
moistened with 200-500 ppm GA 3 (200-500 mg GA 3 in one liter of water) and standard germination proce-
dures followed.
Ethylene. Ethylene exists as a gas capable of breaking dormancy of a number of species, including
peanut. The AOSA rules recommend that 5 ml of ethylene gas per cubic ft be injected into the germinator in
which rolled towel tests are placed. Following injection, the germinator door should remain closed until the
irst count. If the germinator door is inadvertently opened, the volatile gas will escape and another injection
will be required. Because of dificulties in working with a gas, ethephon [(2-chlorethyl)phosphonic acid] is
suggested as an alternative to ethylene in the rules.
Ethephon is a liquid which slowly releases ethylene gas. A 0.0029% solution composed of 0.6 ml of
ethephon containing 2 lb of active ingredient per gallon in a propylene glycol base with 5 liters of distilled
water is added directly to the germination substratum. Regardless of the formulation, ethylene, like GA 3 ,
stimulates the germination of many species in addition to its inluence on fruit ripening, bud dormancy, leaf
abscission, and other growth processes. Its physiological role in the breaking of seed dormancy is not fully
understood.
Modification of the Seed Coat and Associated Structures
Seed coats of many species have been reported to inhibit germination by preventing the entry of water and
gases, by their content of germination inhibitors or by physically restraining tissue expansion, therefore lim-
iting the seeds' ability to germinate. Consequently, their removal can alleviate these dormancy mechanisms.
The process of seed coat modiication is called “scariication” and can be accomplished either mechani-
cally or chemically. Mechanical scariication includes clipping, iling, piercing opposite the radicle end, or
rubbing the seed coat with an abrasive material such as sandpaper. Chemical scariication is accomplished
by placing dry seeds in concentrated sulfuric acid (H 2 S0 4 ) for prescribed periods in the rules followed by
thorough rinsing in running tap water with appropriate safety precautions. Following scariication, the seeds
are exposed to the speciied germination conditions.
Most scariication treatments are extremely harsh and often harmful to seeds. They degrade the seed
coat and are not uniform. While the seed coat may be altered, other tissues are also inadvertently damaged.
Thus, it is not uncommon to cause seed damage during scariication that can cause abnormal seedlings
during germination. Analysts should be alert to this kind of injury when interpreting germination of scari-
ied seeds.
Embryo Excision
The complexity of dormancy and its exact cause can often be shown by removing the embryo and growing
it independently of the seed. Flemion (1936) irst reported that excised embryos would readily grow and
turn green if placed on ilter paper under favorable germination conditions (Fig. 5.11). The excised embryo
test is particularly useful for tree and shrub seeds (not only as an effective dormancy-breaking method, but
also because it can signiicantly reduce germination time of many species). Its principles and procedures are
described further by Flemion (1948), Heit (1955), AOSA (2010) and ISTA (2010).
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