Quantitation of a bioactive metabolite in undisturbed rhizosphere—Benzyl isothiocyanate fromCarica papaya L.

A “continuous root exudate trapping system” was recently developed in this laboratory by which the allelopathic metabolites from the undisturbed rhizosphere ofHemarthria altissima (Poir.) Stapf. et Hubb. were collected. Twelve phenolic compounds were identified by capillary GC-MS (Tang and Young, 1982). In this report, we further describe its use for the quantitation of benzyl isothiocyanate (BITC), a highly bioactive volatile compound, in the rhizosphere of growing papaya plants. Samples were collected from 64 individual papaya (Carica papaya L.) trees of Waimanalo and Higgins cultivars. Hydrophobie compounds collected on XAD-4 columns were eluted with acetone and the levels of BITC were determined by a gas Chromatograph equipped with a sulfur-specific flame photometric detector. For 2-month-old trees, the rate of BITC released from the root system of Waimanalo was 2.03 ± 0.85μg/tree/day. From Higgins, it was 2.36 ± 1.06 μg/tree/day. It has been reported that Waimanalo is resistant whereas Higgins is susceptible toPhytophthora palmivora Butl., the major root rot fungal pathogen of papaya trees in Hawaii. Our results lead us to conclude that the rate of BITC released alone cannot account for differences in the resistance of these two cultivars to the pathogen.