Characterization of a glabrate Encelia farinosa mutant: morphology,ecophysiology, and field observations

An individual Encelia farinosa (Asteraceae) from Death Valley, California is described that is completely lacking in the dense leaf pubescence covering characteristic of the species. While leaf absorptances to solar radiation of the mutant E. farinosa are much greater than those of the wild type, other morphological aspects and photosynthetic characteristics appear to be similar to those typical for E. farinosa. Leaf temperatures of mutant and wild type E. farinosa in the field are similar, but only because of steeper leaf angles and higher leaf conductances to water vapor in the mutant form. As a consequence of a greater water expenditure for transpirational cooling, the mutant E. farinosa becomes deciduous much earlier in the drought period than do the normal pubescent leaved E. farinosa.     

Characteristics of Encelia species differing in leaf reflectance and transpiration rate under common garden conditions

Summary The performance of coastal and desert species of Encelia (Asteraceae) were evaluated through common garden growth observations. The obectives of the study were to evaluate the roles of leaf features, thought to be of adaptive value (increased leaf reflectance and/or transpirational cooling), on plant growth in the hot, arid, desert garden versus their impact on growth under cooler, relatively more moist coastal garden conditions. E. californica native to the coast of southern California and E. farinosa, and E. frutescens, interior desert species, were grown in common gardens at coastal (Irvine, California) and interior (Phoenix, Arizona) sites under both irrigated and natural conditions. Although all species survived in both gardens during the two and a half year study period, there were large differences in their sizes. In the desert garden, leaf conductance and leaf water potential were both lower than at the coastal site. E. californica shrubs were leafless much of the time under natural conditions in the desert garden and had the smallest size there as well. Under natural conditions, E. farinosa, with its reflective leaf surface, was able to maintain lower leaf temperatures and attained a large size than the other two species in the desert garden. The green-leaved species (E. californica and E. frutescens) were not able to maintain leaves into the drought periods in the desert garden, with the exception of the irrigated E. frutescens which did maintain its leaf area if provided with supplemental watering to maintain transpirational leaf cooling. In the coastal garden, all species survived and there were few clear differences in the physiological characteristics among the three species. E. californica, the coastal native, attained a larger size in the coastal garden when compared with either of the two desert species.     

Intraspecific variation of drought adaptation in brittlebush: leaf pubescence and timing of leaf loss vary with rainfall

Reflective leaf pubescence of the desert shrub Encelia farinosa (brittlebrush) reduces leaf temperature and plant water loss, and is considered adaptive in xeric environments. Yet, little is known about intraspecific variation in this trait. Among three populations in the northern range of E. farinosa, which span a very broad precipitation gradient, both leaf absorptance variation and differences in the timing of drought-induced leaf loss were broadly associated with climatic variability. Where mean annual rainfall was greatest, drought-induced leaf loss was earliest, but these plants also had higher population-level mean leaf absorptance values. Higher absorptance increases the relative dependence on latent heat transfer (transpirational cooling), but it also provides greater instantaneous carbon assimilation. Plants at the driest site reached lower leaf absorptance values and maintained leaves longer into the drought period. Lower leaf absorptance reduces water consumption, and extended leaf longevity may buffer against the unpredictability of growing conditions experienced in the driest site. These observations are consistent with a trade-off scenario in which plants from wetter regions might trade off water conservation for higher instantaneous carbon gain, whereas plants from drier regions reduce water consumption and extend leaf longevity to maintain photosynthetic activity in the face of unpredictable growing conditions. Received: 2 April 1997 / Accepted: 11 August 1997     

Chemical variation and defense of Encelia farinosa

Chemical analysis of leaves from 12 different localities of Encelia farinosa (including var. phenicodonta and var. radians) collected on the peninsula of Baja California (Mexico) revealed the presence of various chemotypes that differed with regard to the concentrations of chromenes and sesquiterpene lactones. Localities of E. farinosa collected in the northern part of Baja California were characterized by high concentrations of the chromene encecalin (up to 252 μmol g⁻¹ dry wt.), whereas the sesquiterpene lactone farinosin was not detected. Localities of E. farinosa collected at the southern tip of the peninsula lacked encecalin, but were shown to accumulate farinosin (up to 85 μmol g⁻¹ dry wt.) instead. On the mainland of Mexico, as well as in Arizona (U.S.A.), farinosin concentrations varied from 18 to 44 μmol g⁻¹ dry wt. for 10 different localities analyzed. Chromenes were not detected or present only in minor amounts (up to 13 μmol g⁻¹ dry wt.), when compared to the samples from northern Baja California. Chemical variation within localities was small when compared to variation between different localities. Accumulation of encecalin and aridity seem to coincide at least on the peninsula of Baja California, as localities of E. farinosa that receive the least amount of rainfall contained the largest amounts of encecalin in their leaves. Leaves of E. farinosa that contained sufficiently large amounts of either encecalin or farinosin were both detrimental to neonate larvae of the polyphagous pest insect Spodoptera littoralis as shown by addition of the respective crude leaf extracts to artificial diet. Possible advantages of the observed intraspecific chemical variability of E. farinosa with regard to adaptation by generalist insect herbivores are discussed.     

INFLUENCE OF IRRADIATION,SOIL WATER POTENTIAL,AND LEAF TEMPERATURE ON LEAF MORPHOLOGY OF A DESERT BROADLEAF,ENCELIA FARINOSA GRAY (COMPOSITAE)

Laboratory experiments were performed to evaluate observed seasonal changes in leaf morphology of the desert perennial shrub, Encelia farinosa Gray. Plants were grown under low or high conditions of photosynthetically active irradiation, soil water potential (Ψ^soil), and leaf temperature (8 different experimental regimes). The relative growth rate, leaf water vapor conductance, leaf water potential, and leaf length were all greater for the high Ψ^soil regimes, the largest leaves occurring at low irradiation. High irradiation during growth led to thicker leaves with a higher internal to external leaf area ratio (A^mes/A); low Ψ^soil tended to increase A^mes/A somewhat. High irradiation also led to decreased absorptance to solar irradiation caused by increased pubescence. High leaf temperature during development resulted in slightly smaller, thicker leaves with higher A^mes/A. Thus, irradiation appeared to have its major influence on leaf thickness, A^mes/A, and absorptance, with a secondary effect on leaf length; Ψ^soil affected primarily leaf length, growth rate, and water status, and secondarily A^mes/A. Results are discussed with regard to recent ecophysiological studies on the observed seasonal changes in leaf morphology of E. farinosa.     

Pubescence and leaf spectral characteristics in a desert shrub,Encelia farinosa

Summary The effects of leaf hairs (pubescence) on leaf spectral characteristics were measured for the drought-deciduous desert shrub Encelia farinosa. Leaf absorptance to solar radiation is diminished by the presence of pubescence. The pubescence appears to be reflective only after the hairs have dried out. There are seasonal changes in leaf absorptance; leaves produced at the beginning of a growing season have high absorptances, whereas leaves produced during the growing season are more pubescent and have lower absorptances. The decrease in leaf absorptance is the result of an increase in pubescence density and thickness. Between 400 and 700 nm (visible wavelengths), pubescence serves as a blanket reflector. However, over the entire solar spectrum (400–3000 nm), the pubescence preferentially reflects near infrared radiation (700–3000 nm) over photosynthetically useful solar radiation (400–700 nm). Leaf absorptance to solar radiation (400–3000 nm) varies between 46 and 16%, depending on pubescence; whereas leaf absorptance to photosynthetically useful radiation (400–700 nm) may vary from 81 to 29%.C.I.W.-D.P.B. Publication No. 612     

Intraspecific competitive effects on water relations,growth and reproduction in Encelia farinosa

Summary An experiment was conducted to assess the importance of intraspecific competition on water relations, growth and reproductive output in Encelia farinosa, a common deciduous-leaved shrub of the Sonoran Desert. Nearest neighbor analyses in monospecific stands indicated that plants exhibited a clumped distribution. Plant size and nearest neighbor distance were positively correlated, inferring intraspecific competition. Removal experiments monitored for two years indicated that plants now without neighbors had higher leaf water potentials, higher leaf conductances, and a greater leaf area than control plants. As a consequence, growth rates and reproductive output were significantly higher in plants without neighbors. These data strongly support the notion that warm desert plants with a contagious spatial distribution compete for water.     

Canopy dynamics and carbon gain in response to soil water availability in Encelia frutescens gray,a drought-deciduous shrub

Summary The production and longevity of leaves of Encelia frutescens Gray, a drought-deciduous subshrub of the Mohave and Sonoran Deserts, were followed during the summer and fall of 1983 in an experimental field garden. The relationships between seasonally changing plant water status, extent of canopy development, and photosynthetic capacity per unit leaf area were determined. Maximum leaf life spans during a summer activity period were between 3 and 4 months, with the great majority living between 1 and 3 months. Leaf production occurred synchronously in well defined cohorts triggered by precipitation events. Extensive leaf turnover occurred during the summer period even though the plants remained in continuous leaf. Turnover was most pronounced when precipitation triggered the production of new leaf cohorts.Five weeks were required for plants to reach maximum canopy development when renewed soil-water availability followed a prolonged drought. Photosynthetic capacity per unit leaf area recovered much sooner than total leaf area, and submaximal leaf area development was the major factor limiting whole-plant carbon gain during a leaf-flushing period lasting several weeks. As the soil began to dry out, physiological capacity declined more rapidly than leaf area, and became the primary limiting factor to whole plant carbon gain.     

Leaf hairs: Effects on physiological activity and adaptive value to a desert shrub

Summary The effects of leaf hairs on photosynthesis, transpiration, and leaf energy balance were measured on the desert shrub Encelia farinosa in order to determine the adaptive significance of the hairs. The pubescence reduces leaf absorptance resulting in a reduced heat load, and as a consequence lower leaf temperatures and lower transpiration rates. In its native habitat where air temperatures often exceed 40° C, the optimum temperature for photosynthesis in E. farinosa occurs at 25° C, and at leaf temperatures above 35° C net photosynthesis declines precipitously. An advantage of leaf pubescence is that it allows a leaf temperature much lower than air temperature. As a result, leaf temperatures are near the temperature optimum for photosynthesis and high, potentially lethal leaf temperatures are avoided. However, there is a disadvantage associated with leaf pubescence. By reflecting quanta that might otherwise be used in photosynthesis, the presence of leaf hairs reduces the rate of photosynthesis. A tradeoff model was used to assess the overall advantage of possessing leaf hairs. In terms of the carbon gaining capacity of the leaf, the model predicted that for different environmental conditions different levels of leaf pubescence were optimal. In other words, under aird conditions and/or high air temperatures, leaves of E. farinosa would have a higher rate of photosynthesis by being pubescent than by not being pubescent. The predictions from this model agreed closely with observed patterns of leaf pubescence in the field.C.I.W.-D.P.B. Publication No. 613     

A comparison of photosynthetic characteristics of encelia species possessing glabrous and pubescent leaves

Measurements of the dependence of photosynthesis on light, CO₂, and temperature are reported for two species of Encelia (Compositae) which differ in leaf pubescence and in geographical distribution. Encelia californica is glabrous and occurs in relatively mild, but arid habitats and Encelia farinosa is heavily pubescent and occurs in hot, arid habitats. Both species possess the C₃ photosynthetic pathway. Under high irradiances and normal atmospheric conditions the two species have high photosynthetic rates, exceeding 3 nanomoles of CO₂ per square centimeter per second (48 milligrams of CO₂ per square decimeter per hour) and complete light saturation does not occur by full noon sunlight. The high photosynthetic capacity is related to a high efficiency of utilization of intercellular CO₂ combined with high stomatal conductance. Leaf estimates of total soluble protein and fraction I protein are higher in these species than in most plants, although the proportion of fraction I protein is not higher. Both E. californica and E. farinosa attain a maximum rate of photosynthesis between 25 and 30 C, despite the fact that the two species grow in very different thermal habitats. Neither E. californica nor E. farinosa shows significant acclimation in the temperature dependence of photosynthesis when grown under different temperature regimes. The presence of leaf hairs which reduce leaf absorptance and consequently leaf temperature plays an important part in the ability of E. farinosa to survive in its native high temperature environment. When the effects of pubescence are taken into account, there are few if any significant differences in the photosynthetic characteristics of the two species.     

Photosynthesis in Encelia farinosa Gray in Response to Decreasing Leaf Water Potential

Photosynthetic responses of intact leaves of the desert shrub Encelia farinosa were measured during a long term drought cycle in order to understand the responses of stomatal and nonstomatal components to water stress. Photosynthetic rate at high irradiance and leaf conductance to water vapor both decreased linearly with declining leaf water potential. The intercellular CO₂ concentration (c_i) remained fairly constant as a function of leaf water potential in plants subjected to a slow drought cycle of 25 days, but decreased in plants exposed to a 12-day drought cycle. With increasing water stress, the slope of the dependence of photosynthesis on c_i (carboxylation efficiency) decreased, the maximum photosynthetic rates at high c_i became saturated at lower values, and water use efficiency increased. Both the carboxylation efficiency and photosynthetic rates were positively correlated with leaf nitrogen content. Associated with lower leaf conductances, the calculated stomatal limitation to photosynthesis increased with water stress. However, because of simultaneous changes in the dependence of photosynthesis on c_i with water stress, increased leaf conductance alone in water-stressed leaves would not result in an increase in photosynthetic rates to prestressed levels. Both active osmotic adjustment and changes in specific leaf mass occurred during the drought cycle. In response to increased water stress, leaf specific mass increased. However, the increases in specific leaf mass were associated with the production of a reflective pubescence and there were no changes in specific mass of the photosynthetic tissues. The significance of these responses for carbon gain and water loss under arid conditions are discussed.     

Temperature influences on root growth for Encelia farinosa (Asteraceae), Pleuraphis rigida (Poaceae), and Agave deserti (Agavaceae) under current and doubled CO2 concentrations

To help evaluate root distribution patterns, elongation rates of individual roots were measured as a function of soil temperature for Encelia farinosa (a C₃ species), Pleuraphis rigida (C₄), and Agave deserti (CAM), sympatric codominants in the northwestern Sonoran Desert. Measurements were made at current and doubled CO₂ concentrations under winter and summer conditions of air temperature (day/night temperatures of 17 C/10 C and 33 C/22 C, respectively). The three species had different optimal temperatures for root elongation (T_opt) under winter conditions (25 C for E. farinosa, 35 C for P. rigida, and 30 C for A. deserti); T_opt increased by 2-3 C under summer conditions for all three species. The limiting temperatures for elongation also acclimated from winter to summer conditions. The rate of root elongation at T_opt was higher under summer than winter conditions for E. farinosa (9 vs. 6 mm d⁻¹) and P. rigida (20 vs. 14 mm d⁻¹), reflecting conditions for maximum photosynthesis; no difference occurred for A. deserti (9 vs. 10 mm d⁻¹). Decreased elongation rates at extreme temperatures were associated with less cell division and reduced cell extension. The doubled CO₂ concentration increased average daily root elongation rates for A. deserti under both winter (7%) and summer (12%) conditions, reflecting increased cell extension, but had no effect for the other two species. Simulations of root elongation as a function of soil temperatures showed that maximum elongation would occur at different depths (16-20 cm for E. farinosa, 4-8 cm for P. rigida, and 0-4 cm for A. deserti) and during different seasons (winter to spring for E. farinosa, spring to summer for P. rigida, and all year for A. deserti), contributing to their niche separation. Shading of the soil surface moderated daily variations in soil temperature, reducing seasonal root elongation for winter and spring and increasing elongation for summer. Shading also altered root distribution patterns, e.g., optimal rooting depth for A. deserti and especially P. rigida increased for a hot summer day.     

Photosynthetic responses to slowly decreasing leaf water potentials in Encelia frutescens

The importance of reduced leaf conductance (stomatal and boundary layer) in limiting photosynthetic rates during water stress was studied in Encelia frutescens, a drought-deciduous leaved subshrub of the Mohave and Sonoran Deserts. Light-saturated CO₂ assimilation rates of greenhouse grown plants decreased from 42.6±1.6mol CO₂ m^-2 s^-1 (x±s.e.) to 1.7±1.7 mol CO₂ m^-2s^-1 as leaf water potential decreased from-1.5 MPa to-4.0 MPa. The dependence of light saturated, CO₂ assimilation rate on leaf intercellular CO₂ concentrations between 60 and 335 l l^-1 was also determined as leaf water potential decline. This enabled us to compare the effects of leaf water potentials on limitations to carbon assimilation imposed by leaf conductance and by intrinsic photosynthetic capacity. Both leaf conductance and intrinsic photosynthetic capacity decreased with decreasing leaf water potential, but the decrease in leaf conductance was proportionately greater. The relative stomatal limitation, defined as the percent limitation in photosynthetic rate due to the presence of gas-phase diffusional barriers, increased from (x±s.e.) to 41±3% as water potentials became more negative. Since both leaf conductance and intrinsic photosynthetic capacity were severely reduced in an absolute sense, however, high photosynthetic rates could not have been restored at low leaf water potentials without simultaneous increases in both components.     

Architecture of coastal and desert Encelia farinosa (Asteraceae): consequences of plastic and heritable variation in leaf characters

The shrub Encelia farinosa (Asteraceae) exhibits geographic variation in aboveground architecture and leaf traits in parallel with environmental variation in temperature and moisture. Measurements of plants occurring across a natural gradient demonstrated that plants in desert populations produce smaller, more pubescent leaves and are more compact and branched than plants in more mesic coastal environments. This phenotypic variation is interpreted in part as adaptive genetic differentiation; small size and pubescence reduce leaf temperature and thus increase water-use efficiency but at the cost of lower photosynthetic rate, which results in slower growth and more compact growth form. We explored the basis of phenotypic variation by planting seed offspring from coastal and desert populations in common gardens in both environments. Phenotypic differences among populations persisted in both common gardens, suggesting a genetic basis for trait variation. Desert offspring outperformed coastal offspring in the desert garden, suggesting superior adaptation to hot, dry conditions. Herbivore damage was greater for all offspring in the coastal garden. Phenotypic characters also showed plastic responses; all offspring had smaller, more pubescent leaves and more compact growth form in the desert garden. Our results confirm that leaf size and pubescence are heritable characters associated with pronounced variation in plant architecture.     

THE INFLUENCE OF WATER STRESS AND TEMPERATURE ON LEAF PUBESCENCE DEVELOPMENT IN ENCELIA FARINOSA

The degree of leaf pubescence development in the arid land shrub Encelia farinosa Gray is affected by air temperature, leaf water potential, and previous history of the apical meristem during the current growing season. Changes in leaf pubescence levels change leaf spectral characteristics and affect both leaf temperature and photosynthesis. Decreasing leaf water potentials and increasing air temperatures both independently increase pubescence development as measured by decreased leaf absorptances. During any one growing season leaf absorptance may change reversibly coincident with air temperature changes, but with respect to water stress leaf absorptance only decreases as the season progresses. The ecological significance of regulation of the leaf spectral characteristics is discussed.     

Evolutionary history and phylogeography of Encelia farinosa (Asteraceae) from the Sonoran,Mojave, and Peninsular Deserts

Pleistocene glaciations have had a profound influence on the genetic structure of plant species throughout the Northern Hemisphere because of range contractions, fragmentations, and expansions. Phylogeographic studies have contributed to our knowledge of this influence in several geographic regions of North America, however, very few phylogeographic studies have examined plant species in the Sonoran, Mojave, and Peninsular deserts. In this study, we used sequence data from the chloroplast DNA psbA–trnH intergenic spacer to obtain information on phylogeographic patterns among 310 individuals from 21 populations of Encelia farinosa (“brittlebush”; Asteraceae) across its range. We applied several population and spatial genetic analyses that allowed us to interpret our data with respect to Pleistocene climate change. These analyses indicate that E. farinosa displays patterns of genetic differentiation and geographic structuring consistent with postglacial range expansion. Populations of E. farinosa are characterized by distinct haplotype lineages significantly associated with geography. Centers of genetic diversity for the species occur in southwestern Arizona, the plains of Sonora, and Baja California Sur, all of which are putative sites of glacial refugia as predicted by analyses of macrofossil and pollen data. Nested clade analysis suggests that genetic structure in E. farinosa has been affected by past fragmentation followed by range expansion. Range expansion in several locations is further supported by significant departures from neutrality for values of Fu’s F_S and Tajima’s D, and mismatch analyses.     

Frequencies, Microclimate and Root Properties for Three Codominant Perennials in the Northwestern Sonoran Desert on North-vs.South-facing Slopes

At a site in the northwestern Sonoran Desert the percent groundcover for the C₃subshrubEncelia farinosawas eight-times higheron more arid 20° south-facing slopes than on 20° north-facingslopes at 820 m elevation, and was six-times higher on north-facingslopes at a 300-m-lower elevation, also the more arid condition.The ground cover of the C₄bunchgrassPleuraphis rigidadecreasedover 50% from 20° north-facing slopes to the more arid conditionsof a 36° north-facing slope, a 20° south-facing slopeand a 20° north-facing slope at a 300-m-lower elevation.The CAM leaf succulentAgave desertialso had greater ground coverfor the 20° north-facing slopes at 820 m compared with 520m. For these three codominants that averaged 58% of the totalground cover, the key for the relative frequency ofE. farinosawasapparently its greater root growth on the warmer slopes duringthe winter. The key for the other two species was most likelysoil water availability, especially during the seedling stageforA. deserti. The wetter soil conditions on 20° north-facingslopes at 820 m apparently led to individual plants ofP. rigidathatwere twice as large as on south-facing slopes. Thus root propertiesmay exert the primary influence on relative plant frequencyin this desert ecosystem for which soil temperature and wateravailability are crucial.Copyright 1997 Annals of Botany Company Agave deserti; Encelia farinosa; Pleuraphis rigida; rooting patterns; soil temperature; Sonoran Desert; water availability     

Plant transpiration at high elevations: Theory,field measurements,and comparisons with desert plants

Summary The influence of elevational changes on plant transpiration was evaluated using leaf energy balance equations and well-known elevational changes in the physical parameters that influence water vapor diffusion. Simulated transpirational fluxes for large leaves with low and high stomatal resistances to water vapor diffusion were compared to small leaves with identical stomatal resistances at elevations ranging from sea level to 4 km. The specific influence of various air temperature lapse rates was also tested. Validation of the simulated results was accomplished by comparing actual field measurements taken at a low elevation (300 m) desert site with similar measurements for a high elevation (2,560 m) mountain research site. Close agreement was observed between predicted and measured values of transpiration for the environmental and leaf parameters tested.Substantial increases in solar irradiation and the diffusion coefficient for water vapor in air (D _wv) occurred with increasing elevation, while air and leaf temperatures, the water vapor concentration difference between the leaf and air, longwave irradiation, and the thermal conductivity coefficient for heat in air decreased with increasing elevation. These changes resulted in temperatures for sunlit leaves that were further above air temperature at higher elevations, especially for large leaves. For large leaves with low stomatal resistances, transpirational fluxes for low-elevation desert plants were close to those predicted for high-elevation plants even though the sunlit leaf temperatures of these mountain plants were over 10°C cooler. Simulating conditions with a low air temperature lapse rate (0.003° C m^-1 and 0.004° C m^-1) resulted in predicted transpirational fluxes that were greater than those calculated for the desert site. Transpiration for smaller leaves decreased with elevation for all lapse rates tested (0.003° C m^-1 to 0.010° C m^-1). However, transpirational fluxes at higher elevations were considerably greater than expected for all leaves, especially larger leaves, due to the strong influence of increased solar heating and a greater D _wv. These results are discussed in terms of similarities in leaf structure and plant habit observed among low-elevation desert plants and high-elevation alpine and subalpine plants.     

Leaf expansion,net CO2 uptake,Rubisco activity,and efficiency of long-term biomass gain for the common desert subshrub Encelia farinosa

Encelia farinosa is one of the most abundant and highly studied species of the Sonoran Desert, yet characteristics of its leaf development and long-term photosynthetic capacity are relatively unknown. The net CO₂ uptake rate and the Rubisco activity per unit leaf area for E. farinosa in a glasshouse increased in parallel for about 18 days after leaf emergence (leaf area was then 5 cm²), after which both were constant, suggesting that Rubisco levels controlled net CO₂ uptake. Instantaneous net CO₂ uptake rates at noon for well-watered E. farinosa in the glasshouse at different temperatures and light levels correctly predicted differences in daily net CO₂ uptake at four seasonally diverse times for transplanted plants under irrigated conditions in the field but overpredicted the daily means by 13%. After this correction, seasonally adjusted net CO₂ uptake per unit leaf area multiplied by the estimated monthly leaf area predicted that 42% of the net carbon gain was incorporated into plant dry weight over a 17-month period. The ecological success of E. farinosa apparently reflects an inherently high daily net CO₂ uptake and retention of a substantial fraction of its leaf carbon gain.     

Benzofuran derivatives from two Encelia species

From Encelia actoni and E. virginensis two new benzofuran esters could be isolated. The unusual structure of the skeleton comprises an acetyl group at C-2 of the furan ring.