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 o f 20 min with a 15 W germicidal lamp and the tissue incubated at 25 °C. The effects o f such treatment on indole phytoalexins (methoxybrassinin (I); brassinin (II); cyclobrassinin (III); spirobrassinin (IV) and glucosinolates were determined using high performance liquid chromatography procedures. Accumulation o f phytoalexins I I I I was evident within 8 h o f irradiation, whilst formation o f spirobrassinin was evident only after 24 h. Maximal levels o f III and IV (> 100 |ig g“1 freeze dried tissue) were greater than those o f I and II (27 and 17 jag g ~ \ respec­ tively). The individual glucosinolate levels were affected in a complex manner; whilst most glucoinolates decreased on storage, the levels o f indole glucosinolates, glucobrassicin (XI) and 1-methoxyglucobrassicin (XIII), increased until 5 to 6 days after irradiation and thereafter declined. Whilst structural features o f I I V , XI and XIII suggest close biosynthetic relation­ ships between these classes o f biologically-active indoles, further studies are needed to estab­ lish this point unambiguously.