The relative distribution of dry matter over shoot and
tuberous roots is an important factor in determining the yield of
cassava. Under sub-tropical conditions the dry matter distribution
pattern is not always efficient. During the summer the vegetative
growth is generally excessive due to long days and high temperatures.
Furthermore, it was found that a reduction of tuberous root mass
occurs shortly after the winter period when the new canopy is formed.
The role of endogenous hormones, in particular abscisic acid and
cytokinins, in dry matter distribution and tuberization was studied.
Furthermore, experiments were conducted on the effect of exogenous
applied plant growth regulators on the growth of cassava.
Inhibitor activity present in plant extracts, was tentatively
identified as cis-abscisic acid. A high level of inhibitor
activity, co-eluting with abscisic acid, was found throughout the
tuberous roots. The highest level of inhibitor activity was
detected under conditions which caused a high rate of tuberous root
growth, ego with a low level of nitrogen fertilization. Gibberellic
acid application to the leaves promoted shoot growth and led to a
decrease in inhibitor activity of the tuberous roots. Shoot
removal, however, stopped tuberous root growth without leading to
changes in inhibitor activity. No clear correlation was found
between inhibitor activity of tuberous roots and the rate of tuberous root growth of plants harvested from the field tentatively identified as the major cytokinins in tuberous root
extracts. Cytokinin activity was concentrated in the meristematic
region of the xylem. The level of cytokinin activity in the roots
was much higher than in primary roots of the same plants.
Gibberellic acid application to the leaves and shoot removal
resulted in a reduction of the cytokinin level of tuberous roots.
The nitrogen application to the plants had no clear effect on the
cytokinin levels. In field-grown plants the highest level of
cytokinin activity was found shortly after tuber initiation.
Applications of Alar caused satisfactory reduction of shoot
growth of young cassava plants grown in the greenhouse. The
internodes were shortened, the leaf area generally reduced, while a
relatively larger part of the dry matter was allocated to storage
roots. The effect of Alar was further studied in field experiments.
A growth analysis showed that a reduction of tuberous root mass
occured shortly after the winter period, in September. Later in the
second growing season, shoot and tuberous root mass increased at a
relatively constant rate. Alar application (up to 4,5 grammes per
Iitre) early in the second season failed to bring about major
changes in dry matter distribution. Shoot removal at the end of the
winter period was followed by excessive vegetative growth. Alar
application reduced the internode length but the reduction of shoot
growth was insufficient to cause a significant increase in yield. A
nitrogen topdressing at the start of the second season increased the
vegetative growth. However, Alar application, later in the season,
did not inhibit shoot growth at any of the nitrogen levels applied.
RSW 0411 which caused good reduction of shoot growth in the
greenhouse, was not effective in the field. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1984.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/11025 |
| Date | January 1984 |
| Creators | Melis, Robertus Johannes Maria. |
| Contributors | Van Staden, Johannes. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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