ABSTRACT

The present work assessed variation in plant secondary metabolites in Cassava Green Mite (CGM) and Cassava Bacterial Blight (CBB) having resistant (M97/JW2 RL, M97/JW2 RSM, TMS 98/0581 RL, TMS 01/0040 RSM, TMS 01/ 0040 RL, TMS 98/0581 RSK) and non-resistant genotypes (M97/JW2 SL, M97/JW2 SSM, TMS 98/0581 SL, TMS 01/0040 SSM, TMS 01/0040 SL, TMS 98/0581 SSK). Eight enzymatic assays associated with secondary metabolite synthesis and antioxidant defence mechanisms namely. Polyphenol oxidase (PPO), Peroxidase (POD), Superoxide Dismutase (SOD), Catalase (CAT), Glutathione peroxidise (GPx), Ascobate Oxidase (A.O), Pheneylalnine ammonia Lyase (PAL) and Tyrosine ammonia lyase (TAL) were investigated using spectrophotometric method. Total phenolic compounds as possible agents conferring resistance to CGM and CBB were also evaluated using standard phytochemical methods. Results showed significant variations (p<0.05) in phenolic compounds and antioxidant enzymes in the resistant and non-resistant genotypes. Total phenolic content in resistant leaf genotypes ranged from 175.82±3.94 mg/lOOg (M97/JW2 RL) to 235.13±4.00 (TMS 98/0581 RSK) in the dried samples and from 251.40±0.78 mg/100 (TMS 01/0040 RL) to 476.07±1.19 mg/100 g (TMS 98/0581 RSK) in the fresh samples. Total Phenolic content in the non-resistant leaf genotype ranged from 20.40±0.52 mg/lOOg (TMS 98/0581 SSK) to 63.35±0.74 mg/lOOg (TMS 01/0040 SL) in the dried samples and from 49.78±4.97 mg/100 (TMS 01/0040 SL) to 84.22±0.54 mg/lOOg (TMS 01/0040 SSM) in the fresh samples. Antioxidant enzyme activities of PPO, POD, SOD, GPx, AO, CAT and secondary metabolite synthesis enzymes (PAL and TAL) were found to differ significantly (p<0.05) in resistant and non-resistant genotypes. Antioxidant enzyme activities were significantly (p<0.05) higher in resistant genotypes in comparison to nonresistant genotypes. The mean concentrations of phenol, flavonoids and anthocyanins were found to differ significantly (p<0.05) in resistant and non-resistant genotypes. PPO and total phenolic contents were significantly (p<0.05) higher in the genotypes that also exhibited high PAL and TAL activities (TMS 98/0581 RL, M97/JW2 RL and TMS 01/0040 RL). PPO activity for the resistant leaf genotypes ranged from 243.33±49.32 U/g (TMS 01/0040 RL) to 513.33±32.14 U/g (TMS 98/0581 RL). PPO activity in the non lesistant genotypes were significantly (p<0.05) lower than the activity in the resistant genotypes. GC-MS analyses showed that resistant genotypes had more content of phenolics than non-resistant genotypes. Resistant genotype TMS 98/0581 containing 16 secondary metabolites had 8 phenolic compounds while non-resistant genotype M97/JW2, containing 12 secondary metabolites had negligible phenolic compound. It could be concluded that genotypes with reasonable content of phenolics expressed lesistance trait while genotype with negligible amount of phenolic compounds as well as antioxidant enzyme activities of PPO, POD, SOD, GPx, AO, CAT and secondary metabolite synthesizing enzyme activities ofPAL and TAL were susceptible to CBB and CBM in the cassava genotypes studied. Upregulation oftheir genes may therefore yield XV CBB and CGM resistant Cassava genotypes leading to higher tuber yield and food security.