Effects of transplanting time and plant density on yield, quality and antioxidant content of onion (Allium cepa L.) in southern Italy

• Onions were grown under a range of transplanting times and plant densities.
• The crops planted in February gave the highest production.
• Some of the bulb quality traits improved with late plantings.
• Pungency, as pyruvic acid, decreased with late planting.
• Vitamin C and polyphenol content were unaffected by transplanting time.

The aim of this research was to evaluate the effects of four transplanting times (1 February, 16 February, 3 March, 18 March) and three plant densities (21.2, 27.2 and 38.1 pt m−2) on the growth, productivity and bulb quality indicators (size, dry weight, soluble solids content, sugars, acids, amino acids, antioxidants, mineral composition) of long-day onion crops, cultivar “Ramata di Montoro”.Crop yield, bulb mean weight and calibre decreased significantly from the earliest to the latest transplanting time. Plant density did not affect production significantly but the most spaced crop (21.2 pt m−2) produced bulbs of the biggest size.The bulb quality indicators responded differently to transplanting time: dry weight, glucose, fructose, raffinose, citric and malic acids and vitamin C content did not change; soluble solids content, sucrose, and oxalic acid increased from the 1 February to the 18 March transplanting time, while pyruvic acid, calcium, chlorides and nitrates content decreased. The sequence of transplanting times resulted in complex pattern of variations of the mineral composition and of the free amino acid content of the bulbs. The bulb total polyphenol content did not vary with the transplanting time and it was about 7 mg g−1 dry weight, with the outer skins containing more than double polyphenols compared to the inner fresh scales.This research on “Ramata di Montoro” onion has shown that early transplanting increases bulb yield and size, whereas some aspects of bulb quality improve with late transplants.