Abstract:
Nutrients such as boron, nitrogen and calcium stimulate the production of phenols. This research focuses
on the role of calcium in increasing phenol metabolism in potato peels and the ensuing tuber resistance
to soft rot pathogens was investigated. Two field experiments were conducted at the University of
Zimbabwe campus plots in 2008 and 2009 summer seasons. Sprouted tubers of cv. BP1 were planted in
plots treated with different fertilizer combinations. The treatments were: 1) compound S (7N: 21P:
8K) þ ammonium nitrate (34 %N) 2) compound D (7N: 14P: 7K) þ calcium nitrate (19 Ca: 15.5N) 3)
compound S þ calcium nitrate and 4) compound D þ ammonium nitrate. The harvested progeny tubers
were inoculated with Pectobacterium carotovorum subsp. brasiliense. Calcium amendment increased the
activities of enzymes (phenylalanine ammonia-lyase, polyphenol oxidase and peroxidases) involved in
the metabolism of phenolics and total soluble phenols. Calcium amendment significantly reduced
(P < 0.05) the maceration effect of P. carotovorum subsp. brasiliense in tuber tissues. Chlorogenic acid,
caffeic acid and calcium, but not ferulic acid content were significantly higher (P < 0.05) in plants grown
in calcium treated plots. Calcium positively and significantly correlated with polyphenol oxidase (PPO),
phenylalanine (PAL) and peroxidase (POD), while chlorogenic and caffeic acid showed a positive relationship
with POD and PPO. Calcium amendment significantly reduced maceration symptom caused by
the bacteria (P < 0.05), resulting in smaller decayed zone diameters on inoculated tubers from calciumtreated
plots. This study shows that soil amendments of calcium increase concentration of calcium,
caffeic and chlorogenic acid in tuber peels and also reduces maceration effect of pectinolytic pathogens.
Reduced maceration could be due to increased levels of caffeic and chlorogenic acid which have antimicrobial
properties.
