Flavonoid aglycones from the leaf surfaces of someLabiatae species

The distribution of excreted flavonoid aglycones within the familyLabiatae was studied and differences were found, especially in the A-ring substitution patterns. Thus, 5,7-dihydroxy-6-methoxyflavones with substituted B-rings are characteristic of species ofSalvia (sect.Salvia),Rosmarinus andOcimum; 5,7-dihydroxy-6,8-dimethoxyflavones occur only inOcimum and 5,6-dihydroxy-7,8-dimethoxyflavones inThymus and related species. Members of the two subfamiliesLamioideae andNepetoideae produce exudate flavonoids, but some genera are devoid of these compounds. There is a correlation between the habitat where the plant grows and production of these compounds, the species from (semi-)arid habitats being those which generally accumulate external flavonoids.     

Ultrastructure and histochemistry of Citrus limon (L.) stigma

Citrus limon has a wet stigma which can be divided in two zones: a glandular superficial one formed by papillae, and a non-glandular one formed by parenchymatic cells. The stigmatic exudate is produced by the papillae after the latter have reached their ultimate size. The papillae of the mature pistil are of varying size and composition. Both the unicellular and multicellular ones are present. The cells at the base of the papillae are rich in cytoplasm, whereas the tip cells are vacuolated. Histochemical analysis has shown that the exudate of Citrus is composed of lipids, polysaccharides, and proteins. Our results indicate that the lipidic component is produced and secreted first, followed by production and secretion of the polysaccharidic component. The lipidic component of the exudate is produced in the basal papillae cells and accumulates as droplets in dilated parts of the smooth endoplasmic reticulum (SER). Subsequently the lipid droplets are transported to the plasma membrane, and transferred by the latter into the cell walls. Then the exudate component is accumulated in the intercellular spaces and in the middle lamellar regions of the walls. Subsequently, the polysaccharidic component of the exudate is produced and secreted by the tip cells of the papillae.Abbreviations RER rough endoplasmic reticulum - SER smooth endoplasmic reticulum     

Guttation droplets of <Emphasis Type="Italic">Penicillium nordicum</Emphasis> and <Emphasis Type="Italic">Penicillium verrucosum</Emphasis> contain high concentrations of the mycotoxins ochratoxin A and B

Eight of eleven ochratoxigenic isolates of Penicillium nordicum and Penicillium verrucosum produced guttation droplets when grown on Czapek yeast extract (CYA) agar for 10–14 days at 25°C. Parallel cultivation of one strain each of P. nordicum and P. verrucosum on malt extract agar demonstrated that higher volumes of exudate are produced on this agar. However, HPLC analyses revealed higher concentrations of ochratoxin A (OTA) and B (OTB) in droplets originating from cultures on CYA. For quantitative determination of the mycotoxin contents, triplicates of three isolates each of P. nordicum and P. verrucosum were grown as single spot cultures on CYA for up to 14 days at 25°C. Guttation droplets were carefully collected between day 11 and 14 with a microliter syringe from each culture. Extracts from exudates and corresponding mycelia as well as fungal free agar were analyzed by HPLC for the occurrence of ochratoxin A (OTA) and B (OTB). Mean concentrations ranging between 92.7–8667.0 ng OTA and 159.7–2943.3 ng OTB per ml were detected in the guttation fluids. Considerably lower toxin levels were found in corresponding samples of the underlying mycelia (9.0–819.3 ng OTA and 4.5–409.7 ng OTB/g) and fungal free agar (15.3–417.0 ng OTA and 12.7–151.3 ng OTB/g). This is the first report which shows that high amounts of mycotoxins could be excreted from toxigenic Penicillium isolates into guttation droplets.     

Phytotoxic polyacetylenes from roots of Russian knapweed (Acroptilon repens (L.) DC.)

There are several factors thought to assist invasive weeds in colonization of ecosystems. One of these factors is allelopathy, the negative effect of chemicals produced by one plant on neighboring plants, frequently mediated through root exudates and other plant leachates. Acroptilon repens (Asteraceae) is one of the most invasive and ecologically threatening weed species in western North America. A bioassay-guided fractionation of the root extracts of this plant led to the isolation of five polyacetylenic compounds, of which one [5′-methoxy-1′-(5-prop-1-yn-1-yl-2-thienyl)-hexa-2′,4′-diyin-6′-yl acetate] was hitherto unknown. The structures of these compounds were elucidated on the basis of spectroscopic analysis (IR, ESIMS, ¹H, ¹³C NMR and 2D NMR). All of the compounds obtained, except 1-chloro-4-(5-penta-1,3-diyn-1-yl-2-thienyl)but-3-yn-2-ol, showed phytotoxic activity against Arabidopsis thaliana seedlings. The presence of 4′-chloro-1′-(5-penta-1,3-diyn-1-yl-2-thienyl)-but-2′-yn-3′-ol was detected in the root exudates of aeroponically grown A. repens plants. None of the polyacetylenes isolated in this study were found in Colorado soils collected between September 2006 and July 2007 in an A. repens colonized site. However, polyacetylene 5 in A. repens infested soil from Washington was found in June, 2007. Contrary to our previous report, the compound 7,8-benzoflavone (6) was not detected in root exudates, nor was it encountered in extracts of roots, aerial parts or infested soil. Since we could not repeat this work, the original report has been retracted [Stermitz, F.R., Bais, H.P., Foderaro, T.A., Vivanco, J.M., 2003. 7,8-Benzoflavone: a phytotoxin from root exudates of invasive Russian knapweed [A retraction]. Phytochemistry 64, 493-497.].     

Allelopathic activity of Stratiotes aloides on phytoplankton—towards identification of allelopathic substances

The allelopathic activity of the aquatic macrophyte, Stratiotes aloides, was determined with laboratory experiments. Active compounds exuded in the medium or present in plant tissue were extracted using standard procedures and solid phase extraction (SPE). The activity towards various cyanobacteria and chlorophytes was tested in two different bioassay systems using agar plates and liquid cultures of phytoplankton. Extracts and exudates of S. aloides affected phytoplankton growth. SPE-enriched exudates and enriched water from a natural Stratiotes stand caused inhibition of target species, however, also some controls were active. Phytoplankton species exhibited differential sensitivity to extracts of S. aloides. We observed inhibitory and stimulatory effects on phytoplankton. In general, more cyanobacteria than other phytoplankton species were inhibited, and the inhibition of cyanobacteria was stronger. In most cases, nutrient (P or K) limitation of Synechococcus elongatus and Scenedesmus obliquus decreased the sensitivity of these species towards allelochemicals from Stratiotes aloides, except for P-limited cultures of Scenedesmus. The allelopathically active compound(s) from Stratiotes are moderately lipophilic and most likely no phenolic compounds. Our results indicate that allelopathy (besides nutrient interference and shading) might also account for the low phytoplankton and filamentous algae densities in the vicinity of Stratiotes plants, at least during certain phases of the life-cycle of Stratiotes.     

Free amino acids in the seed and root exudates in relation to the nitrogen requirements of rhizosphere soil Fusaria

Summary The amounts of amino acids in seed exudates were generally higher than in root exudates of the same plant. The spectra and relative abundance of amino acids in both plants were similar but they were generally more abundant in cowpea exudates than in sorghum. Glutamic acid and alanine were the most abundant amino acids in the seed and root exudates of both plants. The proportions of the amino acids in the seed exudates were comparable to that stored in the seeds. Many of the major amino acids identified in the exudates were also found to support thein vitro growth ofFusarium spp. isolated from the rhizosphere and rhizoplane. This suggests that the amino acids exuded might contribute signficantly to Fusaria nutrition and its consequent predominance around the root. The significance of this pathogenesis is also discussed.     

Plant exudates trigger leaf-trenching by cabbage loopers, Trichoplusia ni (Noctuidae)

Cabbage loopers, Trichoplusia ni, cut a narrow trench across leaves of plants that release exudate, then feed distal to the trench in an area of reduced exudation. The larvae do not normally trench plant species such as plantain, Plantago lanceolata, that lack exudate. To determine what cues elicit trenching, I reared larvae to the final instar on plantain, then applied test solutions to their mouthparts during feeding. Loopers that received latex from Lactuca serriola (Asteraceae) or phloem exudate from watermelon, Citrullus vulgaris (Cucurbitaceae), often responded by cutting a trench in plantain, even though these larvae had not previously encountered exudate nor previously trenched. Loopers that were allowed to trench and feed on L. serriola for 1 day prior to the assay subsequently cut trenches in plantain more frequently and in response to more fluids, including a viscous solution of polyethylene glycol and latex from a non-host, poinsettia (Euphorbia pulcherrima). Subsequent bioassays with larvae reared entirely on plantain tested whether bitter cucurbitacins or gelation are essential cues for trenching. Sap from non-bitter cucumber plants (Cucumis sativus) caused larvae to trench, showing that cucurbitacins are not required to induce trenching. Loopers also trenched after receiving cucumber sap that did not gel due to the addition of mercaptoethanol. An extract of sap lacking the proteins that cause gelation likewise triggered trenching. Further fractionation revealed that cucumber sap and also butternut squash sap (Cucurbita moschata) contain trenching stimulants that are small (molecular weight < 3,000) water-soluble molecules. Received: 13 January 1997 / Accepted: 6 June 1997     

Current perspectives on the economic botany of the genus Aloe L. (Xanthorrhoeaceae)

Aloe L. (Xanthorrhoeaceae) is a genus of over 500 species found on the African continent, Arabian Peninsula, Madagascar and eastern Indian Ocean Islands. It is valued by people at many economic scales but verifiable data, with which to quantify the role of Aloe in local livelihoods and commercial trade, are scarce. For a speciose genus of appreciable ethnological value, few taxa have been known in formal and international trade. Leaf mesophyll and exudate from certain Aloe species support sizeable commercial industries that differ markedly in geographical focus, supply chain and taxa used. Leaf mesophyll is primarily sourced from plantations of the well-known A. vera in Mexico, the United States and parts of South America, and is generally used in products made in the same region. By contrast, leaf exudate is principally wild-harvested from A. ferox in South Africa and A. secundiflora in Kenya for export to Europe and Asia. To a much lesser extent, Aloe spp. are used commercially in foods and to produce honey. It is unclear to what degree Aloe spp. are traded as medicinal plants for traditional use throughout their range. Their popularity in horticulture as decorative and/or collectable ornamentals sustains a considerable international trade. Besides habitat loss, wild populations of many species of Aloe are threatened by exploitation for the succulent plant trade and a few species by over-utilisation for natural products, making rare, endemic and utility taxa an obvious priority for conservation. With the exception of Aloe vera, all species of Aloe are protected by the Convention on International Trade in Endangered Species (CITES). There is, nevertheless, considerable potential for commercially-valuable species of Aloe to be employed for rural development and poverty alleviation. The success of the expanded Aloe-based industries will depend on sustainable harvesting and other practices compliant with the Convention on Biological Diversity.