VIEW ARTICLE

Physiology and Biochemistry

The Effect of Some Pterocarpanoid Phytoalexins on Germ Tube Elongation of Stemphylium botryosum. Verna J. Higgins, Associate Professor, Department of Botany, University of Toronto, Toronto, Ontario, Canada M5S 1A1; Phytopathology 68:339-345. Accepted for publication 29 August 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-339.

Bioassays involving the continuous measurement of individual germ tubes of Stemphylium botryosum showed that maackiain (15 to 50 μg/ml) temporarily stopped elongation of the germ tubes and that the rate at which germ tubes resumed growth was dependent on the maackiain concentration. In up to 16% of treated germ tubes new growth began by branch formation just behind the apex. Medicarpin or a 1:1 mixture of maackiain and medicarpin had a similar effect on germ tube elongation, but phaseollin at a comparable concentration appeared to kill the germ tubes. Diffusates obtained from Helminthosporium carbonum-inoculated red clover leaflets and containing 20-30 μg/ml of maackiain plus medicarpin had the same effect on germ tube elongation as purified maackiain. The renewal of growth by maackiain-treated germ tubes of S. botryosum coincided with changes in the rates of glucose-¹⁴C uptake. The rate of loss of ¹⁴C-labeled compounds from germinated spores initially increased over threefold after maackiain treatment but by 8-12 hr was comparable to the rate of the control. That the resumption of growth was not the result of the previously described conversion of maackiain to noninhibitory compounds by S. botryosum was suggested by two methods. Firstly, germinated S. botryosum spores in which maackiain conversion was induced by small additions (2 μg/ml) of maackiain were as inhibited by later additions of maackiain (15 μg/ml) as were noninduced controls. Secondly, germ tube growth of H. carbonum, which cannot metabolize maackiain, was temporarily delayed by maackiain.