Jeffrey D. Weidenhamer

Just how insoluble are monoterpenes

Prior generalizations about the ecological roles of monoterpenes may be misleading if based on the presumed insolubility of monoterpenes in water. We determined the aqueous solubility of 31 biologically active monoterpenes by gas chromatography. While hydrocarbons were of low solubility (< 35 ppm), oxygenated monoterpenes exhibited solubilities one or two orders of magnitude higher, with ranges of 155–6990 ppm for ketones and of 183–1360 ppm for alcohols. Many monoterpenes are phytotoxic in concentrations under 100 ppm, well below the saturated aqueous concentrations of oxygenated monoterpenes. Therefore, even dilute, unsaturated solutions of monoterpenes, occurring naturally in plant tissues and soil solutions, may act as potent biological inhibitors.

Stimulation of witchweed germination by sesquiterpene lactones: a structure-activity study

Abstract Twenty-four natural and synthetic sesquiterpene lactones were tested as witchweed germination stimulants to determine the essential structural requirements for the stimulation of witchweed germination by dihydroparthenolide. All of the 10 germacranolides tested induced 40–65% germination of witchweed across a broad concentration range, from 10 −5 to 10 −9 M. The highest activity was observed with a mixture of the two eudesmanolides, santamarin and reynosin. These findings indicate that it is the spatial arrangement of the germacrane-eudesmane skeleton in conjunction with the five-membered lactone ring which is important in the promotion of witchweed germination, rather than the presence of any specific functional group on the molecule. The structure of these compounds is similar to portions of the active molecule (+)-strigol.

Dihydroparthenolide and other sesquiterpene lactones stimulate witchweed germination

Abstract Four sesquiterpene lactones which share structural features of the lactone rings of strigol were tested as witchweed germination stimulants. Confertiflorin and parthenin significantly increased witchweed germination at 10 −4 M, and parthenin and desacetylconfertiflorin increased germination at 10 −5 M. Dihydroparthenolide induced 70% germination of witchweed across a concentration range of 10 −7 to 10 −9 M. This activity is comparable to the activity of strigol and its synthetic analogues.

Inhibition ofSchizachyrium scoparium (poaceae) by the allelochemical hydrocinnamic acid

Bare zones around shrubs in the Florida scrub indicate the possibility of allelopathy by shrubs controlling the distribution of grasses invading from adjacent sandhills. The allelochemical, hydrocinnamic acid, has been identified as a breakdown product of ceratiolin, which is released from the shrubCeratiola ericoides. Here, hydrocinnamic acid (HCA) was shown to have a strongly inhibitory effect on shoot and root biomass of the grassSchizachyrium scoparium in greenhouse bioassays lasting 4.5 months. Linear increases in the concentration of HCA from 0 to 200 ppm, applied biweekly, resulted in exponential decreases in root and shoot biomass at harvest. Plants grown at 200 ppm HCA had root and shoot biomasses 13% and 17% of controls, respectively. Concurrent investigation of reduced nutrient levels indicated greater inhibition by HCA in a reduced nitrogen (N) treatment and in a reduced potassium (K) treatment relative to HCA inhibition in the full nutrient treatment. The negative slopes of the regressions of log of biomass on HCA concentration were steepest in the reduced N and reduced K treatments. Root and shoot biomasses in reduced N treatments were 20–43% and 24–34% less than the respective biomasses in the full nutrient treatment. Comparable reductions in the reduced K treatment were as much as 19% and 10% for root and shoot biomasses, respectively. The effects of HCA in a reduced phosphorus (P) treatment and in a reduced P and K treatment were not significantly different from the effects of HCA in the full nutrient treatment. Extraction of the soils at harvest indicated no buildup of HCA at the end of the experiment. The sensitivity ofSchizachyrium scoparium to HCA in general and increased sensitivity under low N and low K solutions may be important in the Florida scrub community where levels of N and K are known to be low.

Inhibition and promotion of germination by several sesquiterpenes

The sesquiterpene lactones, burrodin, confertiflorin. desacetyl-confertiflorin, dihydroparthenolide, parthenin, and 7α-hydroxy-3-desoxyzaluzanin C, and the sesquiterpene ester guayulin A were assayed at concentrations of 1, 10, and 100 μM for effects on seed germination of 16 dicot and nine monocot species. Six of the dicot and two of the monocot species were affected by one or more of these compounds. Germination was both inhibited and promoted, depending on the compound and the specific species or cultivars, at concentrations as low as 1 μM. For example, guayulin A, which promoted the germination of lettuce at all concentrations tested, inhibited the germination of tomato. Confertiflorin stimulated germination of the lettuce cultivar Grand Rapids at 1 μM, but inhibited germination of a light-sensitive cultivar at all concentrations tested.

Allelopathic potential of menthofuran monoterpenes fromCalamintha ashei

A reversed-phase HPLC analysis was used to separate and quantify five menthofuran monoterpenes inCalamintha ashei leaf soaks and washes. (+)-Evodone and desacetylcalaminthone were the major constituents of both soaks and washes. Concentrations of (+)-evodone and desacetylcalaminthone were as high as 0.66 and 0.74 mM, respectively, in leaf soaks. The highest concentration of monoterpenes in leaf washes obtained by misting was 0.021 mM. Aqueous solubilities of the menthofurans were determined to exceed concentrations required for growth inhibition. Bioassays of individualCalamintha monoterpenes demonstrated effects on germination as low as 0.05 mM for (+)-evodone. An equimolar mixture of desacetylcalaminthone and (+)-evodone reducedRudbeckia hirta germination by 17% at a combined concentration of 0.025 mM forLeptochloa dubia. Confirmation of allelopathic effects byCalamintha ashei will require long-term bioassays ofCalamintha menthofurans on the growth of native sandhill species under conditions comparable to the harsh environment of the Florida scrub.

Diterpenes from Chrysoma pauciflosculosa: Effects on Florida sandhill species

Abstract The aerial parts of Chrysoma pauciflosculosa (syn. Solidago pauciflosculosa ) gave the known diterpenes, 17-oxygrindelic acid, 17-oxogrindelic acid and 17-carboxygrindelic acid, as well as the new diterpene, 17-hydroxy-7α,8α-epoxygrindelic acid. 17-Oxogrindelic acid decomposes in the presence of air giving three products. The structural data of the diterpenes as well as those of degradation products and derivatives were elucidated by chemical and spectroscopic methods. 17-Oxygrindelic acid and 17-oxogrindelic acid were tested for their effect on the germination and radicle growth of three Florida sandhill species and Lactuca sativa . At concentrations of 12 to 48 ppm, 17-oxogrindelic acid reduced the germination and radicle growth of Schizachyrium scoparium and Leptochloa dubia , two native sandhill grasses, but had no significant effects on germination and only a slight stimulatory effect on radicle growth of Rudbeckia hirta and Lactuca sativa . Mixtures of 17-oxogrindelic acid with an equimolar mixture of three C. pauciflosculosa sesquiterpenes, (+)-curlone, (+)-sesquiphellandrene and (−)-α- trans -bergamontene, did not enhance activity. 17-Oxygrindelic acid was in general less active in reducing germination and growth than 17-oxogrindelic acid.

Evaluation of high-performance liquid and capillary gas chromatography for analysis of sesquiterpene lactones of the Melampodiinae

Abstract The capacity of reversed-phase high-performance liquid chromatography (HPLC) and capillary gas chromatography (GC) to separate 37 sesquiterpene lactones of the Melampodiinae has been investigated. The HPLC system employed a Hypersil ODS column and methanol-water as eluent for isocratic and gradient separations. The GC system utilized a RSL-150 bonded phase capillary column with flame ionization detection. The previously characterized melampolide melnerin B was determined by both HPLC and GC to be a mixture of two isomers, containing a 2-methylbutanoate (melnerin B) or a 3-methylbutanoate (melnerin B′) substituent at C-8. The HPLC and GC methods developed have been applied to the analysis of a crude extract ofMelampodium cinereum.

Allelopathic actions in the Florida scrub community

The hypothesis that allelochemicals released from members of the Florida scrub community deter the invasion of fire-prone sandhill grasses was investigated. Constituents of the endemic scrub members, Ceratiola ericoides, Conradina canescens and Calamintha ashei, were examined for their phytotoxic activity. Effects of the plant natural products on the germination and radicle growth of lettuce (Lactuca sativa), as well as little bluestem (Schizachyrium scoparium) and green sprangletop (Leptochloa dubia), two native grasses of the Florida sandhill community, were tested.The data suggest that ursolic acid and other natural detergents which are released from a source plant enhance the solubilization of allelopathic lipids via micellization. In general, natural surfactants seem to play a significant role in the water transport of lipophilic phytotoxins to target seeds or seedlings.

SESQUITERPENES FROM CHRYSOMA PAUCIFLOSCULOSA

Abstract The aerial parts of Chrysoma pauciflosculosa (syn. Solidago pauciflosculosa) gave two new sesquiterpenes, (+)-β-turmerone and a bisabolane endoperoxide, together with the known (-)-α-trans-bergamotene and (+)-β-sesquiphellandrene. When exposed to air and light, (+)-β-turmerone and (+)-β-sesquiphellandrene gave oxidative degradation products, involving hydroperoxide intermediates. Photosensitized singlet oxygen reactions of (+)-β-turmerone provided a series of bisabolane-type endoperoxides. The structures of the natural compounds as well as those of the degradation products and derivatives were elucidated by chemical and spectroscopic methods, mainly NMR and MS. Aqueous solutions of (+)-β-turmerone,(+)-β-sesquiphellandrene and (-)-α-trans-bergamotene were tested for their effects on the germination and radicle growth of three Florida sandhill species, Rudbeckia hirta, Schizachyrium scoparium, Leptochloa dubia, as well as Lactuca sativa. (+)-β-Turmerone significantly inhibited germination of L. sat...