Time-Course Studies of Phytoalexins and Glucosinolates in UV-Irradiated Turnip Tissue

Sliced turnip root (Brassica campestris L. ssp rapa) was irradiated for a total of 20 min with a 15 W germicidal lamp and the tissue incubated at 25 °C. The effects of such treatment on indole phytoalexins (methoxybrassinin (I); brassinin (II); cyclobrassinin (III); spirobrassinin (IV) and glucosinolates were determined using high performance liquid chromatography procedures. Accumulation of phytoalexins I - III was evident within 8 h of irradiation, whilst formation of spirobrassinin was evident only after 24 h. Maximal levels of III and IV (> 100 μg g freeze dried tissue) were greater than those of I and II (27 and 17 μg g , respectively). The individual glucosinolate levels were affected in a complex manner; whilst most glucoinolates decreased on storage, the levels of indole glucosinolates, glucobrassicin (XI) and 1-methoxyglucobrassicin (XIII), increased until 5 to 6 days after irradiation and thereafter declined. Whilst structural features of I - IV , XI and XIII suggest close biosynthetic relationships between these classes of biologically-active indoles, further studies are needed to establish this point unambiguously.