Effect of calcium soil amendments on phenolic compounds and soft rot resistance in potato tubers

• Calcium soil amendments increase resistance of potato tubers to soft rot pathogens.
• Calcium reduces the maceration effect of soft rot pathogens on tubers in storage.
• Calcium soil amendments increase production of caffeic and chlorogenic acid.
• Calcium soil amendments increase the concentration of PAL, PPO and POD enzymes.

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) + ammonium nitrate (34 %N) 2) compound D (7N: 14P: 7K) + calcium nitrate (19 Ca: 15.5N) 3) compound S + calcium nitrate and 4) compound D + 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 calcium-treated 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.