Anthocyanin Biosynthesis of White Asparagus Spears in Relation to the Preharvest Exposure to Infrared and Sunlight Radiation

White asparagus spears were grown in the dark under a low tunnel (100 cm wide and 60 cm high) for 7 days before being exposed to sunlight for 0, 2, 3, 4, 5 or 7 h. Subsequently, the spears were either harvested and kept in the dark or continued to grow in the dark for 0, 24 or 48 h. At the end of the treatments, the color and the anthocyanins’ and total phenols’ content of the top 7 cm of the spears were determined. The results showed that the infrared radiation that was trapped below the tunnel during the 7-day growing period of white asparagus spears induced anthocyanin biosynthesis only in the lower part (base) of the spears. Exposure to sunlight for at least 3 h was critical for significant anthocyanin biosynthesis in the epidermal cells of the top 7 cm part of white asparagus spears. The amount of anthocyanins 24 h later was proportional to the exposure time and was greater in the spears harvested and kept in the dark than in spears that continued to grow in dark conditions.