such as Betullaspp. (birch), respond to long photoperiods and are categorized as “long-day” seeds. Small-
seeded species, including numerous weeds, that are favored by light for germination need only low-in-
tensity light. Photosensitive seeds should be sown upon or near the surface and shaded to prevent exces-
sive moisture loss from the germinating surface. Reaction to light (photosensitivity) is mediated through
phytochrome, a pigment that absorbs either red or far-red light [13].
Phytochrome. The phytochrome pigment system plays a key role in the photosensitivity of seeds
and is of particular significance to many small-seeded species. Phytochrome is converted to Pfr (far-red
light–absorbing form) by 660-nm (red) light and, in turn, can be reconverted to Pr (red light–absorbing
form) by 730-nm (infrared) light.
Much of the phytochrome in quiescent seeds is in the Pfr form. However, within several hours after
seeds are fully hydrated, conversion of Pfr to Pr can occur in the absence of light [14]. Because Pfr ac-
tively promotes germination and lack of Pfr inhibits germination, photosensitive seeds germinate in re-
sponse to exposure to the Pfr-forming 660-nm light.
The ratio of 655–665 nm light to 725–735 nm light varies significantly in nature. Sunlight filtered
through foliage has a low ratio of 655–665 nm light to 725–735 nm light because chlorophyll selectively
absorbs 655–665 nm light while transmitting 725–735 nm light [15].
The ecological significance of the seed phytochrome system involves allowing shaded, light-sensi-
tive seeds on or near the soil surface to remain dormant until the leaf canopy above the seed disappears
[16]. Of course, even though fully hydrated, buried light-sensitive seeds of weedy species, for example,
will also remain dormant until returned to the soil surface by tillage or other soil disturbances.
Interactions between light and temperature are known for some kinds of seed. For example, photo-
sensitivity may be overcome by alternating high and low temperatures. Externally applied chemicals can
also interact with light and temperature. Many nitrogenous compounds, including cyanide, nitric acid,
ammonium salts, urea, thiourea, and particularly potassium nitrate (10–20 mM solutions), have been
found to stimulate the germination of photosensitive seeds [17].
V. ADAPTIVE FACTORS
A. Life Cycle
In the life cycle of every sexually reproduced plant, the seed germinates and the plant makes its vegeta-
tive growth, flowers and bears seeds (physiological maturity), and sooner or later dies. The duration of
the cycle determines the three broad categories of plants: annuals, biennials, and perennials.
In its natural habitat, an annual plant usually lives for only 1 year or one season, a biennial for 2 years,
and a perennial for more than 2 years. A perennial will continue to grow more or less indefinitely and,
once physiologically mature, will annually produce flowers, fruits, and seeds under suitable condition.
The distinction between annual and the other categories is not absolute. A biennial that seed prema-
turely within the first growing season is considered to be an annual by this outcome. An annual or bien-
nial grown year-round in a greenhouse, or outdoors in a warmer climate, becomes a perennial. A peren-
nial that would normally grow indefinitely in a warmer climate may be killed by frost in a colder climate.
Perennials are either herbaceous(having annual tops but perennial roots, crowns, or related under-
ground structures) or woody(having biennial or perennial tops and perennial roots). Woody perennials
consist mostly of trees, shrubs, and vines. These are readily distinguishable from herbaceous perennials,
biennials, and annuals having nonwoody tops and /or roots, which are typically killed by frost in colder
climates. Because of their longer juvenile periods, most woody species grown from seed do not start pro-
ducing flowers and seeds until many years later [18]. These categories describe the pattern of adaptation,
cultural requirements of plants, and, to a great extent also, their seed germination requirements. In gen-
eral, but not always, seeds of woody perennials are more difficult to germinate or may possess more com-
plex germination constraints than those of herbaceous types. Seeds of species within the same plant fam-
ily or genera tend to have similar requirements for germination [19].
B. Dormancy
Germination may be immediate or delayed. Seeds of many flowering garden annuals and trees, such as
Acer saccharinum(silver maple), will germinate within a month after maturing and some with almost no
delay when removed from their protective fruits or seed coverings. Occasionally, germination in some
GERMINATION AND EMERGENCE 61