Resin acids as the potential growth-affecting component of pine oleoresin

The nonvolatile fraction of the oleoresin of <i>Pinus sihestris</i> L. was found to contain substances which inhibit growth of wheat ceoleoptile and oat mesocotyl sections in standard bioassays. The inhibition is mainly confined to the fraction of resin acids. Among the seven authentic resin acids tested, the effects of dehydroabietic and abietic acids were most sifgnificant. Palustric, pimaric and isopimaric acids were not effective in the wheat coleoptile section straight growth test. None of the substances, in the amounts tested, except for extremely high concentration, exerted an inhibitory effect on natural or IAA-induced elongation of pine hypocotyl sections. Neither was an inhibitory effect discovered in the microbiological test with the <i>Aspergillus niger</i> van Tiegh. The results obtained with pine hypocotyl sections, allow the conclusion that resin acids interfering with the results of standard bioassays are probably not effective as inhibitory factors in the regulation of pine tissue growth.

Xanthoxin, a recently discovered plant growth inhibitor

The naturally occurring plant growth inhibitor xanthoxin which was discovered in these laboratories has been prepared in vitro by the oxidation of the pigment violaxanthin with neutral zinc permanganate solution. By the use of chemical and spectroscopic techniques, xanthoxin has been characterized as a mixture of the 2- cis , 4- trans - and 2- trans , 4- trans -isomers of 5-(1', 2', -epoxy-4'-hydroxy-2', 6', 6'-tri-methyl-1'-cyclohexy)-3-methyl-pentadienal. These are also obtained by similar oxidations of neoxanthin, antheraxanthin and lutein epoxide. Cis, trans -xanthoxin probably arises from the corresponding cis -xanthophyll and its conversion to ( + )-ABA by simple chemical procedures is reported. The biological activity of cis, trans -xanthoxin has been shown to be considerably greater than that of the trans, trans -isomer. It is comparable with that of ( ± )-ABA in the wheat coleoptile section, the lettuce hypocotyl and bean petiole abscission tests, but greater than that of ( ± )-ABA in the cress seed germination test. Xanthoxin is shown to antagonize the growth-promoting effects of the plant hormones 3-indolylacetic acid, gibberellic acid and kinetin. Other xanthoxin derivatives have been prepared and their activities in several tests are also reported. The uptake of xanthoxin by tomato shoots leads to a subsequent large increase in the ABA content of the tissue. Evidence is presented for a possible biogenetic conversion of cis, trans -xanthoxin into ( + )-ABA by plants.

Some Properties of a Plant Growth Inhibitor Present in Xylem Sap of Woody Species

An ether-soluble, non-toxic inhibitor of plant growth was shown to be present in varying amounts in xylem sap from shoots of various woody species. Some chromatographic properties of the inhibitor are described. The inhibitor reduced growth of wheat coleoptile sections and lettuce hypocotyls, and inhibited germination of lett,uce and cress seeds. The inhibitor did not interact competitively with either auxin in the wheat coleoptile elongation test or with gibberellic acid in the lettuce hypocotyl test. Gibberellic acid did not overcome the inhibitory effect on seed germination. The inhibition of coleoptile section growth was reversed on transfer from inhibitor solution to sucrose solution.