Cytokinins in Lupinis albus L.
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Date
1978
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Abstract
A review of the literature pertaining to the sites
of synthesis, distribution and metabolism of cytokinins,
established that there was a need to investigate the
quantitative and qualitative changes in cytokinin levels
during the growth cycle of a whole plant. An annual
legume, Lupinus albus L. (the white lupin), was selected
as the experimental plant for such a study.
The levels of cytokinin activity in the root exudate,
leaves and terminal (primary) apices of mature plants were
examined between 5 and 17 weeks after planting. The
results indicated that the total level of activity in the
root exudate of a single plant is much greater than that
which accumulates in the leaves and apices up to the time
of flowering. This was considered to be indicative of
rapid utilization of cytokinins in the actively growing
shoot. It was also established that, after flowering,
the level of activity in the terminal apices increases
markedly, while that in the root exudate decreases.
The latter finding was examined more closely in the
next experiment, during the course of which cytokinin
levels in the root exudate, leaves, fruits and fruit sap
were determined at 2 weekly intervals after flowering.
As in the previous experiment, the results showed that,
although there may be a slight increase in the amount of
activity in the root exudate, this activity was insufficiently high to account for the levels of cytokinin
which accumulate in the fruiting apices. Cytokinin
activity was however detected in the sap passing into the
fruits (largely phloem sap), thereby indicating that at
least some of the cytokinins detected in both pod wall and
seed extracts could have originated in other parts of the
plant.
Embryonic development was followed at the gross
morphological, anatomical and ultrastructural levels, and
estimates were made of the cytokinin levels in the embryos
and adjacent tissues. The results indicated that the
cytokinin levels in the flowers, and young fruits, were low,
but gradually increased with time. At 2 weeks after
anthesis the level of activity in the pod wall was higher
than that in the individual seeds. By 6 weeks, however,
the activity in each seed was in excess of that recorded
in the pod wall. Decreasing levels of activity in the
seeds, and increasing levels in the pod wall, reversed
this trend at later developmental stages. The level of
activity recorded in the maturing embryo was low, while in
the adjacent tissues it was high. This was especially
noticeable at 6 weeks after anthesis when the cytokinin
activity in the endosperm reached a high level. It would
appear that these cytokinins facilitate nutrient
mobilization to the tissues surrounding the embryo, and
that they are possibly utilized in order to promote certain
growth processes taking place in the embryonic cells during
development. The cytokinin activity in the mature embryo was low, but increased slightly in the cotyledons, radicles
and young shoots during germination and seedling
establishment.
The cytokinins present in all white lupin extracts
had chromatographic properties, on paper and/or Sephadex
LH-20, similar to the naturally occurring cytokinins
zeatin and zeatin riboside and their glucosylated or
phosphorylated derivatives. Mass spectrometric-analysis
of the compound co-eluting with zeatin showed that this
cytokinin was present in white lupin extracts, while
analysis of the peak co-eluting with zeatin riboside
indicated that the cytokinin at this elution volume was an
adenine derivative. The possible presence of dihydroderivatives
of these cytokinins was also indicated. The
results did not exclude the possible presence of other
cytokinin-like compounds. The significance of qualitative
as well as quantitative changes in cytokinin activity were
assessed.
It is concluded thatcytokinins do not accumulate at
excessively high levels in actively growing tissues, unless
these tissues function as a transitory store for nutrients,
as for example the endosperm, testa and pod wall.
Cytokinins are probably involved in the control of a number
of growth processes of which nutrient mobilization may be
of considerable importance to the plant.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1978.
Keywords
Cytokinins., Growth (Plants), Theses--Botany.