The biology of the blackening phenomenon in cut carrots

A financially crippling blackening phenomenon of unknown cause has appeared in Scottish cut carrots in recent years. Full grown carrots are stored underground by the producers. They are covered in black plastic tarpaulin and straw throughout the harvesting year to prevent sprouting. The harvested carrots are then cut into batons and distributed to supermarkets and other outlets. Thereafter, blackening can occur within hours to days. Blackening is observed only in mature carrots that are over 1 year old and that have been stored underground until the end of the annual harvest period. This observation suggests that organ aging and/or length of storage underground are important factors contributing to the blackening. However, the environmental, metabolic and molecular triggers for blackening remain poorly characterised. An analysis of environmental conditions of carrot fields revealed that some fields yield carrots that show a higher level of blackening than others. The geographical locations of the fields or the local environmental conditions may enhance susceptibility to blackening. Metabolite profiling analysis revealed that the levels of amino acids and sugars were decreased in the blackened regions of the carrot batons, while fatty acids and phenolic compounds were increased. Immunofluorescence microscopy and a carbohydrate analysis revealed that pectin was less abundant in the cell walls in the blackened regions, which show high autofluorescence suggesting increased polyphenol accumulation. Moreover, lignin levels were higher in blackened regions. Transcript profiling analysis revealed that transcripts encoding proteins involved in phytohormone signalling were more abundant in the blacked regions. In particular, transcripts associated with auxin signalling and ethylene-responsive transcription factors were higher in the blackened regions. In contrast, the levels of transcripts encoding proteins associated with secondary metabolism were decreased in the blackened regions. Taken together, these findings suggest that hormonal and metabolic changes that occur during long periods of storage underground prior to harvest and that are associated with a loss of bud dormancy may predispose the carrots to wound-induced blackening discolouration. These studies provide new insights into the molecular and metabolic mechanisms that underpin the blackening process and will facilitate more rapid progress to solutions to address the problem.