Seasonal variation in glucosinolate accumulation in turnips grown under photoselective nettings

Glucosinolates (GSLs) accumulation is greatly influenced by environmental conditions including planting date and light quality and quantity. Photoselective nettings influence both the quality and quantity of light transmitted onto the crop canopy. The objective of this study was to determine yield and GSL concentrations in turnip (Brassica rapa subsp. rapa L.) roots and shoots grown under different photoselective nettings at different planting dates. Field studies were conducted with red and white turnip cultivars at May and August planting dates for two years using blue, red, and yellow photoselective nettings and a no netting control. Netting did not significantly affect the shoot yield and had an inconsistent effect on root yield. Cultivars differed in GSL concentrations with JR, the white turnip cultivar, having the highest gluconapin (3-butenyl GSL) concentrations in root and shoot tissues and the red turnip cultivar having the highest total GSL concentration in root tissues. Netting did not significantly influence total or individual GSLs in root tissues. Netting was only a significant factor for glucobrassicanapin (GBN; 4-pentenyl GSL) concentration in shoots with no netting treatments resulting in the highest GBN concentrations. May plantings resulted in 50% higher total GSL concentrations than August plantings. Planting date × year interactions were significant for total and individual GSL concentrations. These interactions may be due to differences in mean air temperatures and solar radiation prior to harvest.