The effect of light and temperature on glucosinolate concentration in turnip (Brassica rapa)

Abstract

Glucosinolates (GSLs) are thioglucosides produced by turnip (Brassica rapa subsp. rapa) and other cruciferous vegetables with important properties for plant defense and human health, particularly cancer prevention. We compared yield and GSL concentrations in the roots and shoots of turnip cultivars grown using plastic colored mulch and photoselective netting treatments in both a May and August planting. Turnip yield and GSL concentrations were dependent on tissue type, genotype and environmental factors. ‘Just Right’ roots and shoots consistently had the highest yield and gluconapin concentration of the cultivars examined in field experiments. In most instances, ‘Scarlet Queen’ roots had the highest total GSL concentrations while ‘Just Right’ roots had the least. Although colored plastic mulches significantly influenced both total and individual GSL concentrations, mulch-dependent increases in GSL concentrations were not consistent across tissue types, cultivars, planting dates and years of the study. Photoselective nettings did not consistently affect shoot or root yield or GSL concentration in root tissues. Netting was only a significant factor for glucobrassicanapin (GBN) concentration in shoots with the no netting treatment resulting in the highest GBN concentration. Planting date and year interactions were significant for total and individual GSL concentrations and proportions in both plastic mulch and photoselective netting experiments. These interactions are partially explained by differences in air temperatures and solar radiation prior to harvest. To assess the specific role of temperature on GSL concentration and biosynthetic regulation in roots and shoots, turnips were grown under three different temperature regimes in a controlled environment. Gene expression analyses indicate that some BrMYB transcription factor transcript levels are associated with temperature-dependent changes in GSL concentration, however this association varies between cultivar and tissue type. When compared to low temperature treatments, high temperature treatments increased total, aliphatic and indolic GSLs in a tissue and genotype specific manner. Gluconasturtiin (GNS), an aromatic GSL, concentration was inversely correlated with temperature with high temperature treatments resulting in 20% and 48% less GNS than low temperature treatments in JR and SQ roots, respectively. The indolic GSL, 1-methoxyglucobrassicin (1MGB) was the root GSL most elevated by increased temperature resulting in a nine-fold increase on average in both cultivars between the low and high temperature treatments. These results show promise for the use of temperature to enhance the health promoting properties of turnip as 1MGB has potent chemopreventive effects.