A. Dormancy
Seed dormancy provides a number of benefits to plants,
and one of the most significant is the way in which it helps plants deal
with changing conditions, specifically how it provides embryonic plants
with the means to wait for an optimal growth environment. Plants
need access to sunlight, water, and other resources to grow and survive,
and seeds that do not have these conditions cannot develop. Many
plants, therefore, produce seeds that only break dormancy when they have
been dispersed far enough to reduce competition from surrounding flora.
The environmental factor that most often triggers these types of seeds
is light. For example, some seeds only germinate at certain wavelengths
of red light because any surrounding canopy of leaves would normally block
the transmission of this light. Thus, by remaining dormant until
they sense the presence of red light, these seeds ensure that they will
grow in an area free of the the canopy and out of competition with other
plants (Bewley and Black, 1994).
In addition to spatial distribution, dormancy also
allows plants which have already initiated germination to revert to the
dormant state. This type of condition is called secondary dormancy,
and it is distinguished from "normal" dormancy by when it occurs and what
triggers it. "Normal" or primary dormancy is initiated during seed
development and is probably the result of the genotype of interacting with
environmental factors as the zygote matures into the embryo. With
this type of dormancy, seeds are released from the parent plant in a pre-existing
resting state which can only be triggered by the imbibition of water.
Secondary dormancy, on the other hand, refers to an induced state in which
the seed is made dormant again after the embryo has started to develop.
Seeds that are non-dormant and have not yet fully germinated can be induced
into this state by a variety of environmental triggers, including anoxia,
darkness, prolonged light, temperature, and water stress. The entire
process is part of a synergistic interaction between environmental changes
and the plant's genotype and is probably part of a survival mechanism designed
to deal with false Springs or other sudden mid-winter warm-ups.
B. Germination
One of the reasons cellular respiration in germinating plants is not well understood or documented is because the biochemical changes that occur within the seed during this time are not themselves well understood. Scientists have tried to study the physiological changes taking place during germination using techniques like labeled metabolites to study carbon dioxide release, but this work has largely been unsuccessful since the impermability of the seed coat prevents current research chemicals from reaching the inner layers of the embryonic plant where the main metabolic activity is. Furthermore, the fact that the embryo and nutritive layers of seeds also display dramatically different physiologies makes studying germination at the molecular level difficult. With the different affinities for carbon dioxide and oxygen that various seed structures exhibit (e.g. phosphoenol pyruvate (PEP) carboxylase), it becomes almost impossible to know what is going on where with regard to cellular respiration in germinating seeds (Botha, Potegieter, & Botha, 1992), and researchers are left trying to find manifestations of metabolic activity at the gross morphological level.
C. Seedling Elongation and Development
Two general patterns of seedling development commonly occur
among angiosperms. Epigeal seedling development is characterized
by the emergence of the hypocotyl, and the epicotyl as a continuous structure
with the radicle. (See Figure 4.)
The epicotyl forms a hook which raises the seed coat and the cotyledons
above the soil line. The other pattern of seedling development is
hypogeal. (See Figure 5.)
In this pattern, the epicotyl emerges separately from the radicle, and
the cotyledon remains beneath the soil surface (Hopkins, 1995).
While seedling development of angiosperm seeds is
typically characterized by such large scale anatomical changes and tissue
differentiations, there are also corresponding changes that occur within
the cell at the ultra structural level. One of the most remarkable
is the modification of the nucleolus. There is a distinct loss of
the ribonucleoproteins during early radicle elongation (Deltour & De
Barsy, 1985), and researchers have demonstrated this trend in both monocot
and dicot seeds. They examined the developing radicle using ultra-cytological
techniques and found a consistent pattern between the changes in the nucleolus
of radicle cells and radicle elongation, and although the function of these
changes is not known, it is clear that they are involved in the changing
metabolism of the differentiating tissue.
It is widely believed that phytohormones are the
mediators of the biochemical events that regulate germination and seedling
elongation. Application of abscisic acid (ABA), for example, almost
always prevents seed germination, and it has been shown that following
the activation of the embryo, large amounts of gibberellic acid (GA) are
produced as cells begin divide and differentiate. The gibberellins
stimulate the aleurone cells to produce alpha-amylase and the metabolism
of carbohydrates.
The mechanisms of plant hormone action, though,
are the topic of much investigation because virtually nothing is known
about how these unique biochemicals actually work (Come & Carbineau,
1989) and a number of such mechanisms have recently been suggested (Palme
& Schell, 1993). One assumption is that plant hormones, like
some animal ones, are involved in the regulation of the DNA transcription
and translation process. However, while ABA is thought to inhibit
seed germination, virtually nothing is known about the action of such inhibitors,
and there is now evidence that ABA and GA (which actually counteract one
another's effects at the empirical level) share a common precursor molecule,
melavonic acid. How plants shift the biochemical pathway between
ABA and GA might tell us much about how plant hormone not only initiate
germination and seedling elongation but govern the life-long growth patterns
of adult plants as well (Come & Carbineau, 1989).
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