The fact that the complete information for something as mighty as the oak is in a package as small as an acorn remains one of the great mysteries of biology. Developmental issues like dormancy, germination, and sprouting are poorly understood, especially in biochemical terms, and the seed remains an object of intense study. What follows is a discussion of the seed physiology of angiosperms. Although these plants do not represent the only ones with seeds, they are the most familiar, and if one wishes to learn more about gymnosperm seeds, a bibliography with additional references are available at the end of this page.
One of the most remarkable aspects of seed producing
plants is their ability to create embryos that are viable for lengthy periods
of time. Influenced by both environmental and genetic factors, this
capacity to contain living tissue in a state of dormancy until appropriate
conditions occur for germination is a mechanism that appears to have several
benefits. For one thing, dormancy allows seeds to be distributed
in time--either within a single season or over an extended period.
Seeds produced in some species have different degrees of dormancy, a condition
known as polymorphism. They germinate at slightly different times
during the same growing season, and the net effect of this strategy is
to reduce competition between seedlings. Other species produce seeds
that display a prolonged dormancy that requires exposure to an environmental
trigger such as prolonged chilling. By preventing seeds from breaking
dormancy during seasons that are not conducive to germination (such as
winter), plants whose seeds display this type of dormancy can endure unfavorable
conditions and emerge after they have passed.
Unfortunately for purposes of research in the classroom, measurable events which characterize seed germination or dormancy have yet to be discovered. In particular, because metabolic and other biochemical events must occur prior to radicle development to be considered part of the dormancy period and most observable differences are between germinating and non-germinating seeds, few useful indicators currently exist to study the change from dormancy to germination (Come & Carbineau, 1989). The concept of dormancy is therefore an area which provides a myriad of research possibilities.
Germination is defined as the inclusive period between
the time that a seed absorbs or imbibes water and the radicle emerges.
This period of transformation displays few changes in morphology, but physiologically,
it must be a time of great activity. There is a dramatic increase
in the overall metabolism of the plant, and the DNA transcription necessary
to cause the subsequent cell divisions and differentiation of tissues are
assumed outcomes of this process. Currently, though, the actual biochemical
sequence of events in germination has not been explored much, and the steps
are distinguished primarily by the rates of oxygen uptake for respiration.
With germination, the seed begins to consume oxygen at a tremendous rate that is probably due to the activation of the metabolic machinery within it. It is important to remember that a dormant seed is still capable of producing ATP but does not; hence, there is a great need for the cells to engage in a tremendous amount of respiration in order to provide the energy needed for the rapid cell divisions which are occurring. Gas exchange may be used to measure these events, and both the embryonic plant and the nutritive tissues demonstrate similar patterns of respiration at this time. Later, the rate of oxygen consumption slows down in some plants, and in those with significant amounts of storage tissue, this decrease is noticeable due to the depletion of resources (Come & Carbineau, 1989; Botha, Potegieter, & Botha, 1992).
C. Seedling Elongation and Development
With the emergence of the radicle, germination comes to an end, and the rate of oxygen consumption once more accelerates dramatically. Major storage reserves in the seed which had remained inactive until this time are mobilized to supply the developing plant with all its major nutrients until root absorption and photosynthesis can commence, and a dramatic change in respiration rates, particularly in the radicle, accompany this process. The oxygen requirements increase at this time for a variety of factors. The activity of respiratory enzymes escalates due to the differentiation that is taking place, and the number of actual mitochondria climbs in conjunction with this process. It is also suspected that the hormones abscisic acid and giberellin may play a role in the demand for more and more oxygen, and any or all of these factors may combine to cause an increase in the respiratory rate (Bewley & Black, 1994). What is significant and currently unexploited, though, is the fact that while the events of radicle elongation and shoot development have traditionally been measured by tissue growth rate, an examination of metabolic rates would be equally useful for studying these events.
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