Date Published: April 18, 2019
Publisher: Public Library of Science
Author(s): Matema L. E. Imakumbili, Ernest Semu, Johnson M. R. Semoka, Adebayo Abass, Geoffrey Mkamilo, Tunira Bhadauria.
In areas where konzo (a cassava cyanide related paralytic disorder) persists, the agronomic factors causing increased cyanogenic glucoside levels in cassava, during periods without water stress, are hardly known. However, through their assessment of cassava root toxicity, using its bitter taste, farmers may have noticed factors unrelated to water stress that additionally influence the cyanogenic glucoside content of cassava cultivated in these areas. Increased cassava root bitterness is often associated with an increase in cyanogenic glucoside levels, making it a good indicator of changes in root cyanogenic glucoside content. Bitter cassava varieties that are preferentially planted by people living in most konzo-affected areas, are an additional known contributor to high cyanogenic glucosides. It is water stress that further increases the inherent toxicity of the planted bitter cassava varieties. Using konzo-affected Mtwara region in Tanzania as a case study, a household survey was carried out to identify the overlooked agronomic factors that additionally influence cyanogenic glucoside levels in cassava cultivated in konzo-affected areas. A total of 120 farmers were interviewed and they mentioned a number of factors unrelated to water stress, as agronomic factors that influenced cassava root bitterness and hence cyanogenic glucoside production in cassava. The mentioned factors included; certain soil characteristics (14.2%), plant age at harvest (7.5%), poor weeding (0.8%), piecemeal harvesting (0.8%), and branch pruning (0.8%). The revealed factors constitute permanent environmental characteristics and crop management practices commonly used by farmers living in konzo-affected Mtwara region in Tanzania. The revealed factors could be contributing to increased cyanogenic glucoside levels in cassava, during periods without water stress in areas where konzo persists.
Cassava (Manihot esculenta Crantz) is one of the world’s most important food crops. Despite its importance, cassava unfortunately contains potentially toxic cyanogenic glucosides, which release poisonous hydrogen cyanide upon hydrolysis. Without access to foods containing sulphur amino acids, continuous ingestion of cyanogenic glucosides from improperly processed cassava products, often results in cases of cassava cyanide intoxication . Rural poor cassava dependent communities are particularly at risk. Cases of cassava cyanide intoxication have been reported in a number of countries in sub-Saharan Africa, such as, the Democratic Republic of Congo (DRC), Mozambique, Tanzania, Cameroon, Central African Republic and Angola [2–6]. The reports consisted of cases of acute cyanide intoxication, but more commonly of a cassava cyanide health disorder called konzo (spastic paraparesis), which results in an irreversible paralysis of legs [7–10].
Table 3 shows the factors perceived by farmers as contributors of cassava root bitterness. The percentage of farmers that attributed root bitterness to each factor is also shown. The factors can be broadly categorised into genotype (variety), environment and crop management factors. It is important to note that while some factors were mentioned more than others, this does not mean that they are less significant contributors of cassava root bitter; this can only be proved by research.
As revealed by the perceptions of farmers, using cassava root taste, the agronomic factors that possibly contribute to increased cyanogenic glucoside levels in cassava during periods without water stress, include: certain soil characteristics (or types of soils) that induce nutrient stress and water stress, and agronomic practices used by farmers like, the age at which they harvest cassava, poor weeding practices, piecemeal harvesting and branch pruning. The mentioned factors could have contributed to the reported persistent episodes of konzo in Tanzania and could also be contributing to newly occurring cases of konzo in these areas, although not being reported. The revealed agronomic factors could also be responsible for causing increased cyanogenic glucoside levels outside periods of water stress in other areas where konzo persists. Research is however needed to validate the effects of the revealed agronomic factors on root cyanogenic glucoside production, in order to understand their significance.