The Effect of Some Pterocarpanoid Phytoalexins on Germ Tube Elongation of Stemphylium botryosum

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Abstract
Bioassays involving the continuous measurement of individual germ tubes of S. botryosum showed that maackiain (15-50 .mu.g/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 phaseolin at a comparable concentration appeared to kill the germ tubes. Diffusates obtained from Helminthosporium carbonum-inoculated red clover [Trifolium pratense] leaflets and containing 20-30 .mu.g/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 3-fold after maackiain treatment but by 8-12 h 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 2 methods. Firstly, germinated S. botryosum spores in which maackiain conversion was induced by small additions (2 .mu.g/ml) of maackiain were as inhibited by later additions of maackiain (15 .mu.g/ml) as were noninduced controls. Secondly, germ tube growth of H. carbonum, which cannot metabolize maackiain, was temporarily delayed by maackiain.