Agriculture Reference
In-Depth Information
Temperature
The ability of a seed to germinate rapidly is dependent on temperature along with many other factors.
For this reason, temperature guidelines are precise and must be maintained by specialized germination
equipment.
In the AOSA and ISTA rules, temperatures are presented as either constant temperatures expressed as
a single temperature for 24 hours (e.g., 20°C) or alternating expressed by two temperatures (e.g., 15-25°C)
separated by a dash. For alternating temperatures, the irst temperature is for approximately 16 hours and
the second for
approximately 8 hours per day. The lower temperature should coincide with darkness and the
higher with light to simulate the day-night cycle, which is believed to promote germination. In some cases,
alternating temperatures may not be possible because of the need to hand transfer samples over weekends.
In these instances, test samples should be kept at the lower temperature for the 24 hour period. In other
instances, more than one temperature is recommended by the rules. These temperatures are shown in order
of increasing temperature, and no temperature is considered preferred over any other.
The question of why alternating temperatures increase seed germination over constant temperatures is
often asked. While the answer is not precisely known, at least three reasons can be suggested. First, alternat-
ing temperatures more closely mimic the natural luctuations in day and night temperatures that temperate
region species encounter. Second, seed physiologists now believe that unique changes in hormonal balances
occur in seeds during low temperature exposures such as the lower temperature of the alternating cycle.
These changes in hormonal balance are physiologically expressed during the warm temperature exposure
of the alternating cycle. Third, alternating temperatures may change cell membrane permeability to allow
some compounds easier entry or exit from cells leading to higher rates of growth and development. A
change in cell membrane permeability may lead to changes in hormonal balance discussed earlier.
Maintenance of temperatures within the recommended ±1°C AOSA guidelines is crucial for germina-
tion testing, particularly when comparing results for the same seed lot. The general recommendations for
a germination chamber (Justice, 1972) are: (a) be located near the area where interpretations will be made,
(b) have water-resistant walls and ceilings and waterproof loor, (c) have automatically controlled cooling
and heating systems that will maintain the desired constant or alternating temperatures, (d) possess a reli-
able spray system or other germination chamber system to maintain high relative humidity, (e) provide slow
circulation of the warm and cool air, and (f) be equipped with daylight luorescent light.
Light
Light is useful for breaking dormancy of some seeds and improving seedling development in others. Both
the intensity and quality of light inluence these responses. Both aspects of light can be better controlled by
luorescent light, which is recommended for agricultural, vegetable, herb, and lower seeds. The AOSA has
speciied the use of a cool, white luorescent light source with an intensity of 75 to 125 foot candles (750 to
1250 lux) evenly distributed over the test. When light is required, seeds should be illuminated for at least 8
hours in the 24 hour cycle. When light and alternating temperatures occur simultaneously, the light period
should coincide with the high temperature cycle. All tests requiring light should be germinated on top of the
speciied substratum. For tree and shrub seeds, light is provided as described above with the following pro-
visions: (a) during germination, illuminance should remain at 75 to 125 foot candles (750 to 1250 lux) and
(b) although 16 hours of light may be beneicial in some tests, continuous light should not be used unless it
is known not to inhibit germination.
durATIon of TEST
The duration of germination tests (expressed in days) is divided into two counts (evaluations) for most spe-
cies. The
irst count
is approximate and can deviate one to three days from the speciied time. It serves a
