It is caused by the oxidation of phenolic substances, mainly tyrosine to melanin. Potassium, magnesium and boron can all have effects on discolouration and blackening.
Potassium affects enzymatic blackening and after-cooking blackening and low levels of potassium can increase the incidence of after-cooking blackening.
Potassium influences the concentration of organic anions such as citric acid or ascorbic acid (i.e. vitamin C within the tuber). These 2 molecules have an antioxidative function, which decreases the incidence of enzymatic and non-enzymatic discolouration by slowing down the oxidation processes.
This trial from the USA shows the influence of potassium on enzymatic discolouration on two different varieties.
Low magnesium levels can increase susceptibility to enzymatic discolouration. This trial from the USA shows the influence of magnesium on enzymatic discolouration.
Boron reduces the oxidation of phenols that are responsible for enzymatic discolouration. Thus, a good supply of boron reduces the occurrence of enzymatic discolouration in potato tubers.