Cyanogenic glucoside production in cassava: The comparable influences of varieties, soil moisture content and nutrient supply

Varieties and soil moisture content are the two agronomic factors mostly pointed out as influencers of cyanogenic glucoside production in cassava. The role of soil nutrient supply is however often overlooked or minimised, despite its known influence on cyanogenic glucoside production. A pot experiment was hence carried out to determine whether soil nutrient supply had an equal influence on cyanogenic glucoside production in cassava, as varieties and soil moisture content. The cassava varieties, Kiroba (a sweet cassava variety) and Salanga (a bitter cassava variety), were used in the experiment, together with three soil moisture treatments that respectively induced severe moisture stress, moderate moisture stress and no moisture stress (optimal soil moisture conditions where plants were kept well-watered). The soil nutrient treatments used depicted conditions of low (no fertiliser), moderate (25 N mg, 5 P mg, 25 K mg /kg) and high (25 N mg, 5 P mg, 25 K mg /kg) nutrient supply. A sole K treatment was also included (25 K mg/kg). Total hydrogen cyanide (HCN) levels in cassava leaves were used to indicate the effects of the three factors on cyanogenic glucoside production. The results of the study showed that nutrient supply had a significantly (p < 0.001) equal influence on cyanogenic glucoside production, as varieties (p < 0.001) and soil moisture content (p < 0.001). Cyanogenic glucoside production was however found to be differently influenced by soil moisture content (M) and nutrient supply (N) in both Salanga (MxN, p = 0.002) and Kiroba (MxN, p < 0.001). Leaf HCN levels of unfertilised Salanga and Kiroba were respectively increased by 1.8 times and 2.7 times their levels under optimal soil moisture conditions. Thus, under severe moisture stress, low soil fertility was found to have an increasing effect on leaf HCN levels in both varieties. A high supply of N, P and K, however also had an increasing effect on leaf HCN in both varieties regardless of soil moisture conditions. Leaf HCN levels in Salanga ranged from 95.5 mg/kg to 334.5 mg/kg and in Kiroba they ranged from 39.3 mg/kg to 161.5 mg/kg, on a fresh weight basis. The study managed to demonstrate that soil fertility had an equally important influence on cyanogenic glucoside production, just like varieties and soil moisture content. The study also showed that the effects of nutrient supply on cyanogenic glucoside production in various cassava varieties is dependent on changes in soil moisture content and vice versa.