The biochemical and cytokinin changes in the developing and germinating seeds of Podocarpus henkelli stapf.
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Date
1981
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Abstract
A review of the literature revealed that there is a
lack of depth in our knowledge of gymnospermous seeds with
regard to the development and germination processes. The
phytohormones, particularly the cytokinins have been implicated
in these processes. The seeds of Podocarpus henkelii
were thus selected as experimental material for studying the
biochemical and cytokinin-like changes associated with development
and germination.
The development of these seeds was also followed at
the ultrastructural level. These studies revealed that cellular
detail within the female gametophyte only began .to form
in December (early summer), approximately six weeks after
fertilization had taken place. At this time some reserve
protein was evident and the embryo sporophyte consisted of
only a few pro-suspensor and pro-embryo cells. Concurrently,
the cytokinin levels were fairly high in the female gametophyte
but low in the epimatium. In both seed components two cytokinin-
like compounds predominated which co-chromatographed with
the free base cytokinin zeatin and its ribonucleoside.
The second sample was taken in late January (mid-summer)
by which time the embryo sporophyte had developed rapidly into a
readily distinguishable seed component. The cellular detail
indicated that much cell division had recently taken place
and that the cells were currently increasing in size and accumulating
starch and lipid. In the female gametophyte the
soluble sugars were at the maximum level recorded during these experiments and the level of starch was increasing. The extractable
cytokinin content of the seed was high at this time,
particularly in the embryo sporophyte. In all three seed components
cytokinin-like compounds which co-chromatographed with
zeatin and ribosylzeatin were present. These high levels of
cytokinin coincided with the rapid increase in both fresh and
dry mass of the embryo sporophyte and female gametophyte.
Ultrastructural studies of the third sample collected
in mid-March (early autumn) showed that cellular changes were
associated mainly with increases in cell size and the accumulation
of food reserves, particularly starch. The cytokinin
levels had decreased in all three seed components at this
time. There was an increase in the cytokinin which co-chromatographed
with glucosylzeatin in the female gametophyte. The
seeds matured in late April (autumn) and had the unusual
features of not drying out during maturation. Fresh seeds
collected from the ground had a moisture content of ca. 62
per cent. The main food reserve was starch with relatively
small amounts of protein and lipid also present.
The seeds of Podocarpus henkelii germinated readily
after scarification in the absence of water provided that
their moisture content remained ca. 60 per cent. Seeds in
which the moisture content had fallen below ca. 54 per cent
required additional water for germination. The moisture
content of the seeds fell rapidly under natural conditions
and viability was lost below a moisture content of ca. 34
per cent. Unscarified seeds of 52 per cent moisture content placed under moist conditions at a constant 25°C took 23
weeks to achieve 68 per cent germination. These experiments
showed that although the epimatium limited water uptake by
the seeds it did not prevent moisture loss to the atmosphere.
This appears to be the main factor contributing to the seed's
inefficiency as a propagule. A small degree of after-ripening
was recorded with the embryo sporophyte increasing in
size with storage. This appeared to contribute to the increased
rate of germination of the scarified seeds. An interesting
feature of the seeds of Podocarpus henkelii is that
they have the ability to fix atmospheric carbon, which is
subsequently translocated from the epimatium to the female
gametophyte and embryo sporophyte. The mature seeds were
stored at 4°C for six weeks during which time little change
had occurred at the ultrastructural level. Protein vacuoles
in the embryo sporophyte had disappeared and in all three
seed components cytokinin levels were low. Three days after
scarification and the start of incubation, little change in
cellular detail was apparent as limited rehydration was
necessary due to the high moisture content. The cytokinin
levels in the embryo sporophyte and epimatium had increased,
whilst the levels in the female gametophyte had decreased at
this time. In the embryo sporophyte lipid mobilization had
commenced with these reserves apparently being metabolized
within vacuoles. The rate of respiration measured in terms
of increases in CO[2] evolution, increased 60 hours after the
start of the incubation period, just 12 hours before ten per
cent germination was recorded. Germination was accompanied by a large increase in
the levels of cytokinins in the female gametophyte and embryo
sporophyte. The cytokinins detected co-chromatographed with
the free base cytokinin zeatin and its riboside, ribosylzeatin.
Concurrently, marked ultrastructural changes were
recorded with increases in the amounts of dictyosomes, endoplasmic
reticulum and the formation of polyribosomes, all of
which are indicative of increased metabolic activity. Similar
increases in the female gametophyte were ofa lower order and
occurred only after nine days of incubation. By this time
the levels of cytokinins had decreased considerably.
After 12 days of incubation 65 per cent of the seeds
had germinated. As much of the food reserves in the female
gametophyte and embryo sporophyte remained, it is suggested
that these reserves are utilized for subsequent seedling
establishment rather than for germination. The actual role
that cytokinins play in the development and germination of
these seeds is not clear. High levels of this phytohormone
coincide with periods of food deposition and mobilization
suggesting that they play an important part in these processes.
The results of the biochemical, cytokinin and ultrastructural
studies are discussed in relation to the developmental
and germination processes and are compared to the data
of other seeds.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1981.
Keywords
Conifers--Seeds., Cytokinins., Germination., Gymnosperms--Development., Podocarpaceae., Seeds., Theses--Botany.