Do Extended Cultivation Periods and Reduced Nitrogen Supply Increase Root Yield and Anthocyanin Content of Purple Carrots?

Purple carrots are rich in anthocyanins which are interesting as natural dyes in food and beverages. It is, thus, relevant to increase the concentration of anthocyanins by agricultural practices. We tested whether the combination of reduced nitrogen (N) supply and extended harvesting periods maximized the anthocyanin concentration of purple carrot roots, ideally without reducing their yield. The carrot variety ‘Deep Purple’ was grown with total N supplies of 220 kg N ha−1 (controls) and 73 kg N ha−1 (reduced N), respectively. Upon harvests in September, October and November, root yield and quality were assessed. Concentrations of chlorophylls (leaves) and anthocyanins (roots and leaves) were determined by spectroscopic and chemical analyses, and carbon and N content were quantified. Reduced N supply neither affected leaf or root biomass nor their chemical composition. Later harvests did not impact the yield of roots, but increased their diameter by 8.5–20%. Additionally, the anthocyanin concentrations of the roots increased by 40–50% in the controls, but not in N-limited plants, at late harvests. Consequently, extending the harvesting period might increase the anthocyanin concentration in roots of ‘Deep Purple’. Moreover, N supply might be reduced for this carrot variety without negative effects on root yield