Developmental transition of the flavonoid contents in safflower leaves during stress-loaded cultivation

We have previously reported that strong visible light with limited water caused a significant increase in the polyphenol contents of safflower seedlings (Carthamus tinctrius L.), suggesting that the appropriate stress loading could be applied to effectively cultivate flavonoid-rich plants. In this present study, we investigated in detail the time-dependent transition in the flavonoid contents of safflower leaves during the stress-loaded cultivation. In the cotyledons, the light/water stress continuously increased the content of luteolin 7-O-glucoside, which is a strong antioxidant, whereas the content of acacetin 7-O-glucuronide, a weak antioxidant, generally remained unchanged. In the foliage leaves under the stress condition, the contents of the flavonoid glucosides (luteolin 7-O-glucoside and quercetin 7-O-glucoside) markedly increased on the 2nd day and then decreased to the level before stress loading on the 5th day. These results indicate that appropriate selection of the time for stress loading could provide more flavonoid-rich plants during the practical cultivation of vegetables.