Hormonal regulation of tuberization of cassava (Manihot esculenta Crantz)
Melis, Robertus Johannes Maria.
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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.