Ultrastructure of the germinating Welwitschia mirabilis seed.
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Date
1975
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Abstract
The structure and chemical composition of quiescent Welwitschia
mirabilis (Hooker fil.) seeds and resultant changes occurring
during the first 7 days of germination were studied. Research
was focussed on the megagametophyte and its interrelationship
with the non-vascularised outgrowth produced by the embryo.
This outgrowth was given the name "feeder" by Bower in 1881
who postulated that it functions as an absorptive organ. However
the possibility existed that it merely fulfilled the
mechanical role of anchorage.
Following hydration activation of embryonic collar cells precedes
that of gametophyte cells whose rate of activation is
governed by relative distance from the embryo. This sequence
of activation is suggestive of a stimulatory factor diffusing
from the embryo into the gametophyte. Starch, protein and
lipid reserves in the collar and developing feeder are consumed
within 36 to 48 h. As a consequence the rapidly developing
seedling is probably largely dependent on nutrient material in
the gametophyte until the plumule emerges, after approximately
5 to 6 days germination. Ventral feeder cells in contact with
gametophyte tissue apparently act as transfer cells, developing
numerous small wall projections invested with plasmalemma which
result in a much greater absorptive surface area. The large
numbers of mitochondria occurring in these cells might suggest
active uptake of nutrients. At the 3- to 4-day-stage the
feeder and gametophyte adhere firmly. While this adherance probably facilitates translocation of nutrients it could also
have the secondary function of anchoring the feeder in the
gametophyte, thus providing the emerging plumule with a firm
base. The apparent root cap origin of ventral feeder cells
might explain the positive geotropism of the feeder, as
recent work inter alia by Wilkins and Wain (1975) has shown
that root cap cells may be geoperceptive.
Cytochemical methods used at the light and electron microscope
level suggest that reserve material within protein
bodies of the embryo and gametophyte might exist as a proteincarbohydrate
complex and that globoid origin might be cytoplasmic.
The immediate digestion of protein body reserves in
the embryo and gametophyte interface zone argues the presence
of pre-existing hydrolytic enzymes laid down within the protein
bodies prior to quiescence. However the enzymes responsible
for reserve breakdown in deep gametophyte tissue seem to be
synthesised de nova. Protein hydrolysis precedes lipid digestion
which possibly indicates that some of the resulting free
amino acids might be used in the de nova synthesis of lipases.
Lipid bodies, microbodies, mitochondria and amyloplasts encircled
with ER seem to form a complex. Fatty acids resulting
from lipase action in the lipid bodies (Ching 1968) are probably
converted by microbodies (glyoxysomes) to succunate
(Breidenbach and Beevers 1967) which is converted to sucrose
by the action of mitochondria (Cooper and Beevers 1969a, b).
Excess sucrose is probably converted to starch and stored in
the amyloplasts. In 5 days the mean dry mass of the gametophyte decreases by approximately 47% during which time the
total amount of lipid decreases by 76.5% and protein by 14%.
Although some of the hydrolysed fatty acids and amino acids
are no doubt utilised in the gametophyte it is suggested that
the majority of fatty acids are probably converted to sugarsĀ·
which, together with free amino acids (and possibly simple
peptides) are transported to and absorbed by the embryo via
the feeder whence they are utilised for seedling growth.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1975.
Keywords
Gnetales., Theses--Botany.