A SCHMITT¹, KH GANSBAUER², O VOSTROWSKI², J HUBER¹ and HJ BESTMANN²

¹Biological Research Centre for Agriculture and Forestry, Institute for Biological Control, 64287 Darmstadt, Germany; ²FA-University Eriangen-Nornberg, Institute for Organic Chemistry, 91054 Eriangen, Germany

Background and objectives
Extracts from Reynoutria sachalinensis are known for their resistance-inducing properties in several plant-pathogen interactions. The plant extracts are used against powdery mildew fungi or grey mould in vegetables and ornamentals. In greenhouse-grown cucumbers the efficacy against Sphaerotheca fuliginea reached more than 90%. Former investigations on active compounds in the plant extract excluded proteins and salts [1]. However, the nature of the resistance-inducing compounds in R. sachalinensis was not elucidated. The objective of this study therefore was to isolate and identify relevant compounds in this plant.

Results and conclusions
An aliquot of dried plant powder from R. sachalinensis was extracted with organic solvents. The sequential extraction was performed with hexane, chloroform, ether and methanol. The methanol extract was finally washed with a non-polar solvent for further purification. All extracts that resulted from soxhiet extraction underwent efficacy trials in the cucumber-S. fuliginea system. From these the methanol extract showed highest efficacy. For further studies this extract was hydrolysed, since from former work it was known that acid hydrolysis enhances the activity of R. sachalinensis extracts.

The HPLC profile of the two resulting fractions showed that after hydrolysis the number and height of peaks were enhanced to a large extent. Also, on thin layer chromatograms (TLC) the unhydrolysed methanol fraction could not be separated in the given solvent system, whereas after hydrolysis two yellow bands could be seen. These bands and the rest of the TLC plate were re-eluted and bioassayed against S. fuliginea on cucumber. Band 1 reached an efficacy of 83%, while band 2 and the rest of the TLC plate had no influence on the development of the pathogen. Investigations by GC-MS and comparison with known compounds led to the identification of the two anthrachinones physcion (band 1) and emodin (band 2). Bioassays on cucumber with synthetically produced physcion and emodin supported these findings: only physcion reduced the development of powdery mildew on cucumbers, while emodin did not. Both compounds could be identified in the HPLC profile of the hydrolysed, but not in the unhydrolysed methanol extract, indicating that the aglyca have been released by acid hydrolysis from a bonded form. In literature there are reports on the occurrence of physcion and emodin and their glycosides in R. sachalinensis. Therefore the activity in this fraction could either be due to the physcion glycoside or to other compounds not yet determined. Further investigation will be necessary to clarify this question.

References
1. Kowalewski AK, Herger G, 1992. Mededelingen de Faculteit van de Landbouwwetenschappen Universiteit Gent 57, 449-456.