Seasonal variation in glucosinolate accumulation in turnip cultivars grown with colored plastic mulches

Glucosinolates (GSLs) are thioglucosides with important properties for plant defense and human health. The objective of this study was to quantify yield and GSL concentration in turnip (Brassica rapa subsp. rapa) roots and shoots as influenced by colored plastic mulches. Four turnip cultivars ('Just Right', 'Purple Top', 'Royal Crown', and 'Scarlet Queen') were grown over five mulch treatments: white, yellow, silver, red, blue, and a bare soil control in both a May and an August planting in 2006 and 2007. Yield varied by variety; however, there was no consistent relationship between mulch treatment and yield. Glucosinolate concentrations and profiles varied with tissue type, genotype, and environmental factors, including temperature and planting date. Mulch-dependent increases in GSL concentrations were not consistent across tissue types, cultivars, planting dates, and years of the study, possibly as a result of differences in climatic factors and mulch-dependent changes in soil temperature between planting dates and years of the study.