Phytotoxicity of quassinoids: Physiological responses and structural requirements

Quassinoids are naturally occurring compounds with phytotoxic and allelopat hic activities isolated from several plant species of the Simaroubaceae fam ily. There is relatively little information about the structural characteri stics imparting biological activity to these diterpene lactones or their ef fects on plants. We studied the effects of the oxymethylene ring substituti on on the biological activity and molecular conformation of several quassin oids. The presence of this functional group had a great effect on the three -dimensional conformation and biological activity of these natural products . In the absence of the oxymethylene ring, the quassinoids were more planar and had little phytotoxicity. In addition, this bridging function introduc ed a new reactive center that caused the terpene backbone to bend. Molecule s with such conformation were highly phytotoxic, reducing root growth of le ttuce (Lactuca sativa) and affecting all stages of mitosis in onion (Allium cepa) root tips.