The dynamics of the accumulation of cyanogenic glucosides in cassava during its growth cycle.

Pages 711-719

Y. Chukwumah and M. Bokanga

ABSTRACT

The accumulation of cyanogenic glucosides in the cassava plant throughout its growth cycle was investigated. Results show that total plant cyanogenic glucosides content increases throughout the plant growth cycle. Cyanogenic glucosides content in the aerial parts (especially leaves and stem) were the major source of whole plant cyanogenic glucosides content up to 25 weeks after planting (WAP). The petioles maintained a relatively low amount of cyanogenic glucosides throughout the growth cycle. From 25 WAP to maturity, the roots had higher cyanogenic glucosides content than the aerial parts. From 16 weeks after planting (WAP) to 48 WAP, cyanogenic glucosides content increased in the root cortex and to a lesser extent, in the root parenchyma. At 48 WAP, cyanogenic glucosides content in root cortex increased to 4 times the amount at 16 WAP while that of the root parenchyma doubled. The ratio of cyanogenic glucosides content in the underground parts to that in the aerial parts of the cassava plant increases during its growth cycle up to 36 WAP which is the driest month of the growth cycle and when cassava drops almost all of its leaves. It gradually decreases between 40WAP and 52 when the plant starts producing new leaves. The aerial parts accumulate more cyanogenic glucosides than the underground parts from 1 WAP to 25 WAP. Between 25 WAP and 50 WAP, the situation is reversed. This indicates the sink role played by the underground organs of the plants. At 52 WAP, the rams had resumed, leaf growth intensified and the balance of cyanogenic glucosides shifted to the aerial part of the plant indicating a greater source activity. Cyanogenic glucoside concentration in the plant increases rapidly after germination and reaches a peak at 9 WAP. Thereafter, the concentration decreases with age and remains relatively stable from 16 WAP to maturity. Cyanogenic glucosides concentrations were higher throughout the growth cycle in leaves, herbaceous stem, hardwood stem (rind) and root cortex (1.09-5.83 mg.g-1 tissue) than in other parts of the plant (0.18-2.34 mg.g-1 tissue). Root cortex cyanogenic glucosides concentration was found to be 2-10 fold greater than that in the parenchyma throughout the plant growth cycle.We conclude that cassava produces cyanogenic glucosides throughout its growth cycle. The root cortex, the rind of hardwood stems, the leaves and the root parenchyma hold most of the plant cyanogenic glucosides. The mechanism controlling the accumulation of cyanogenic glucosides in specific plant tissues needs to be investigated.

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