Tree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plant

Nepenthes pitcher plants are typically carnivorous, producing pitchers with varying combinations of epicuticular wax crystals, viscoelastic fluids and slippery peristomes to trap arthropod prey, especially ants. However, ant densities are low in tropical montane habitats, thereby limiting the potential benefits of the carnivorous syndrome. Nepenthes lowii, a montane species from Borneo, produces two types of pitchers that differ greatly in form and function. Pitchers produced by immature plants conform to the ‘typical’ Nepenthes pattern, catching arthropod prey. However, pitchers produced by mature N. lowii plants lack the features associated with carnivory and are instead visited by tree shrews, which defaecate into them after feeding on exudates that accumulate on the pitcher lid. We tested the hypothesis that tree shrew faeces represent a significant nitrogen (N) source for N. lowii, finding that it accounts for between 57 and 100 per cent of foliar N in mature N. lowii plants. Thus, N. lowii employs a diversified N sequestration strategy, gaining access to a N source that is not available to sympatric congeners. The interaction between N. lowii and tree shrews appears to be a mutualism based on the exchange of food sources that are scarce in their montane habitat.     

The use of light in prey capture by the tropical pitcher plant Nepenthes aristolochioides

Nepenthes pitcher plants deploy tube-shaped pitchers to catch invertebrate prey; those of Nepenthes aristolochioides possess an unusual translucent dome. The hypothesis was tested that N. aristolochioides pitchers operate as light traps, by quantifying prey capture under three shade treatments. Flies are red-blind, with visual sensitivity maxima in the UV, blue, and green wavebands. Red celluloid filters were used to reduce the transmission of these wavebands into the interior of the pitchers. Those that were shaded at the rear showed a 3-fold reduction in Drosophila caught, relative to either unshaded control pitchers, or pitchers that were shaded at the front. Thus, light transmitted through the translucent dome is a fundamental component of N. aristolochioides' trapping mechanism.     

Mutualism between tree shrews and pitcher plants: Perspectives and avenues for future research

Three species of Nepenthes pitcher plants from Borneo engage in a mutualistic interaction with mountain tree shrews, the basis of which is the exchange of nutritional resources. The plants produce modified “toilet pitchers” that produce copious amounts of exudates, the latter serving as a food source for tree shrews. The exudates are only accessible to the tree shrews when they position their hindquarters over the pitcher orifice. Tree shrews mark valuable resources with feces and regularly defecate into the pitchers when they visit them to feed. Feces represent a valuable source of nitrogen for these Nepenthes species, but there are many facets of the mutualism that are yet to be investigated. These include, but are not limited to, seasonal variation in exudate production rates by the plants, behavioral ecology of visiting tree shrews and the mechanism by which the plants signal to tree shrews that their pitchers represent a food source. Further research into this extraordinary animal-plant interaction is required to gain a better understanding of the benefits to the participating species.Key words: Nepenthes, tree shrew, nitrogen sequestration, mutualism, animal-plant interactionsThe pitcher plant genus Nepenthes comprises approximately 120 species, with the centre of diversity lying in the perhumid tropics of Southeast Asia. All species are vines or subscandent shrubs that produce highly modified leaf organs (“pitchers”) which typically attract, trap, retain and digest arthropods for nutritional benefit. The pitchers of almost all Nepenthes species share the same physical components,¹ including the pitcher cup, the peristome and the lid. The pitcher cup usually consists of two main sections: an upper zone which is often covered with wax crystals and anisotropically-oriented semilunate cells^2,3 that assist in the capture and retention of prey; and a lower portion, which contains fluid and is lined with digestive glands.^2,3 The peristome is a ridge of hardened tissue that lines the orifice. Its anisotropic, wettable surface plays a key role in prey capture.^4,5 In most species, the lid is a broad, flat structure which overhangs the orifice and prevents the entry of rainwater which, if unimpeded, can cause the pitchers to overflow, thereby losing digestive enzymes and the products of their activities. The lid is often brightly coloured, has many nectar glands on its surfaces and plays an important role in prey attraction.²The degree of development and/or modification of each pitcher component varies substantially among (and even within) Nepenthes species^2,6,7 and recent research has demonstrated that unique modifications to pitcher structure possessed by several species play important roles in specialized nutrient acquisition strategies.^8–12 One such species, Nepenthes lowii, demonstrates a remarkable nitrogen sequestration strategy, in which mountain tree shrews (Tupaia montana) defecate into its pitchers while feeding on exudates secreted by glands on the inner surface of the pitcher lid. Feces accounts for 57–100% of foliar nitrogen in this species¹³ and N. lowii “toilet pitchers” are ineffective arthropod traps. The large orifices and reflexed, concave lids of N. lowii pitchers induce T. montana to sit astride the pitcher whilst feeding, facilitating fecal deposition.Chin et al.¹⁴ found that two other montane species from Borneo, Nepenthes rajah and Nepenthes macrophylla, also trap tree shrew feces. Detailed analysis of trap geometry revealed that these two species and N. lowii share a unique arrangement of trap characteristics that was not detected by earlier studies on the genus. This involves the production of pitchers with very large orifices, large, concave lids that are reflexed approximately 90° away from the orifice and lid glands that produce copious exudates.¹⁴ The distance from the front of the pitcher orifice to the inner surface of the lid precisely matches the head + body length of T. montana, resulting in the tree shrews'' food source being positioned behind the pitcher orifice and ensuring that the animals'' hindquarters are positioned over the orifice while they feed on the lid gland exudates.Thus, N. lowii, N. macrophylla and N. rajah are all engaged in a mutualism with T. montana, the basis of which is the exchange of nutritional resources that are scarce in these species'' habitats. The interaction with T. montana is facilitated by trap geometry, but all three Nepenthes species produce pitchers that differ substantially in structure, apart from the shared characteristics outlined above.¹⁴ Through a series of modifications to trap structure and geometry—none of which appears to have compromised their ability to trap arthropod prey—N. rajah and N. macrophylla benefit from a highly specialised nitrogen sequestration strategy that is not available to congeners other than N. lowii.Although Clarke et al.¹³ demonstrated that N. lowii derives nutritional benefit from T. montana feces, there are many facets of the association that have yet to be investigated and the discoveries of Chin et al.¹⁴ give rise to a number avenues for further research, several of which are discussed below.The behavioral ecology of T. montana with respect to Nepenthes has not been studied in detail. We do not know whether individual tree shrews defend valuable pitchers against other animals or whether such resources are shared. However, video footage, showing T. montana scent-marking a toilet pitcher of N. lowii after feeding from it, supports the former scenario (Clarke et al.¹³ and Suppl. video). It is not known whether or how, the plants signal to tree shrews that their pitchers provide a nutritional resource (or even how valuable that resource is—the composition and nutritional value of the lid gland exudates has not been determined). When newly-formed pitchers first open, their tissues generally remain soft for several days while they undergo rapid expansion during the final stages of development.¹ During this period, the pitchers are incapable of supporting a tree shrew without suffering significant damage, yet few pitchers of N. lowii, N. macrophylla or N. rajah that we observed exhibited signs of such damage. One possible explanation for this is that the plants signal the tree shrews to indicate whether or not individual pitchers are “open for business.” This might be achieved using variations in color: Tupaia spp. are dichromatic, with sensitivity maxima at ca. 440 and 550–560 nm¹⁵ and the pitchers of all three feces-trapping Nepenthes species utilize combinations of green, red, yellow, orange and purple pigments, which change as individual pitchers age.¹ In N. lowii, the inner surfaces of the feces-trapping pitchers are uniformly dark purple when mature, but when they first open, they are unevenly covered with purple, pink and green patches. The production of copious lid gland exudates in N. lowii appears to commence after the pitchers have hardened and the uniform dark purple color has developed on the inner surfaces.The study by Chin et al.¹⁴ was based on a series of three field trips to northern Borneo that were conducted in March, April and May 2009. The first of these two visits took place during the wet season and heavy rain fell on most days throughout these months. In contrast, May was unusually dry. During this period, many N. rajah plants exhibited signs of stress due to lack of water, including wilting or senescence of developing pitchers and inflorescences. This coincided with an apparent (but unquantified) decline in the number of pitchers that received tree shrew feces: whereas such pitchers were relatively easy to locate during our visits in March and April, they were rare during May. Furthermore, most pitchers that received copious amounts of feces in March and April received none in May. The reasons for this are unknown and may involve changes in the foraging behaviour of T. montana or perhaps a reduction in the quantity and/or quality of the lid gland secretions. Through video recordings, we found that T. montana still visited pitchers of N. rajah during May (Chin et al.¹⁴), but very few fecal pellets were deposited inside them. Tree shrews mark valuable resources using feces,¹⁶ so it is feasible that during periods of decreased nectar production, T. montana alters its foraging behavior to utilize alternative food resources, resulting in decreased rates of defecation into N. rajah pitchers.N. lowii, N. rajah and N. macrophylla are virtually confined to montane habitats above 1,800 m altitude, but the geographical range of T. montana extends well beyond that of the “toilet pitchers” and includes a number of sites that are substantially lower than 1,800 m.¹⁷ Given this, why are the toilet pitchers not found at lower altitudes? Large, fleshy fruits with small seeds (such as figs) comprise a major component of the diet of T. montana,¹⁸ but plants that produce these are relatively scarce in alpine and upper montane equatorial habitats.¹⁹ This could limit the distribution of toilet pitchers in two ways. First, the lack of fleshy fruits at high altitudes might make toilet pitchers a valuable resource for T. montana in upper montane habitats. Furthermore, the density of arthropods at high altitudes is considerably less than in the lowlands.²⁰ This exerts selective pressure on Nepenthes to adopt non-carnivorous nutrient acquisition strategies.¹³ Accordingly, the production of very large, specialized pitchers that receive a steady input of feces may provide a net benefit for the plants, but only at high altitudes. Second, at lower altitudes, fleshy fruits (and arthropods) are more abundant and at these sites the benefits of producing toilet pitchers may be reduced or even negated, hence their absence from smaller mountains.Through their unique pitcher characteristics and trap geometries, a number of Nepenthes species derive supplementary nutrition from a wide variety of arthropod groups, leaf litter and animal feces.^{6,10,13,14,21,22} It is arguable that no other plant family has such a complex and diverse array of interactions with animals. Recent discoveries add to a growing body of evidence to suggest that Nepenthes demonstrate adaptive radiation with regard to nutrient sequestration strategies (see Chin et al.¹⁴ for a more detailed discussion). The findings of Chin et al.¹⁴ provide the strongest support for this hypothesis to date; in addition, they provide the first plausible explanation for the extraordinary size of N. rajah pitchers. This iconic species is the world''s largest carnivorous plant and was first described 150 years ago. The population studied by Chin et al.¹⁴ grows at a site on Mount Kinabalu that has been visited by tourists since 2001 and has been regularly examined by scientists and Sabah Parks staff for more than 30 years. Despite this, the association between N. rajah and T. montana remained undetected until we employed remote survey methods to record pitcher visitors. To date, this technique has been used on just five species of Nepenthes^4,5,13,14 and in each case, remarkable insights into the interactions between animals and Nepenthes have been gained. The potential for further discoveries using this method is therefore high and through new and innovative experimental methodologies now being employed, we anticipate many more exciting discoveries in the near future.     

With a flick of the lid: a novel trapping mechanism in Nepenthes gracilis pitcher plants

Carnivorous pitcher plants capture prey with modified leaves (pitchers), using diverse mechanisms such as 'insect aquaplaning' on the wet pitcher rim, slippery wax crystals on the inner pitcher wall, and viscoelastic retentive fluids. Here we describe a new trapping mechanism for Nepenthes gracilis which has evolved a unique, semi-slippery wax crystal surface on the underside of the pitcher lid and utilises the impact of rain drops to 'flick' insects into the trap. Depending on the experimental conditions (simulated 'rain', wet after 'rain', or dry), insects were captured mainly by the lid, the peristome, or the inner pitcher wall, respectively. The application of an anti-slip coating to the lower lid surface reduced prey capture in the field. Compared to sympatric N. rafflesiana, N. gracilis pitchers secreted more nectar under the lid and less on the peristome, thereby directing prey mainly towards the lid. The direct contribution to prey capture represents a novel function of the pitcher lid.     

A unique resource mutualism between the giant Bornean pitcher plant, Nepenthes rajah, and members of a small mammal community

The carnivorous pitcher plant genus Nepenthes grows in nutrient-deficient substrates and produce jug-shaped leaf organs (pitchers) that trap arthropods as a source of N and P. A number of Bornean Nepenthes demonstrate novel nutrient acquisition strategies. Notably, three giant montane species are engaged in a mutualistic association with the mountain treeshrew, Tupaia montana, in which the treeshrew defecates into the pitchers while visiting them to feed on nectar secretions on the pitchers' lids.Although the basis of this resource mutualism has been elucidated, many aspects are yet to be investigated. We sought to provide insights into the value of the mutualism to each participant. During initial observations we discovered that the summit rat, R. baluensis, also feeds on sugary exudates of N. rajah pitchers and defecates into them, and that this behavior appears to be habitual. The scope of the study was therefore expanded to assess to what degree N. rajah interacts with the small mammal community.We found that both T. montana and R. baluensis are engaged in a mutualistic interaction with N. rajah. T .montana visit pitchers more frequently than R. baluensis, but daily scat deposition rates within pitchers do not differ, suggesting that the mutualistic relationships are of a similar strength. This study is the first to demonstrate that a mutualism exists between a carnivorous plant species and multiple members of a small mammal community. Further, the newly discovered mutualism between R. baluensis and N. rajah represents only the second ever example of a multidirectional resource-based mutualism between a mammal and a carnivorous plant.     

Bacterial communities associated with the pitcher fluids of three Nepenthes (Nepenthaceae) pitcher plant species growing in the wild

Nepenthes pitcher plants produce modified jug-shaped leaves to attract, trap and digest insect prey. We used 16S rDNA cloning and sequencing to compare bacterial communities in pitcher fluids of each of three species, namely Nepenthes ampullaria, Nepenthes gracilis and Nepenthes mirabilis, growing in the wild. In contrast to previous greenhouse-based studies, we found that both opened and unopened pitchers harbored bacterial DNA. Pitchers of N. mirabilis had higher bacterial diversity as compared to other Nepenthes species. The composition of the bacterial communities could be different between pitcher types for N. mirabilis (ANOSIM: R = 0.340, p < 0.05). Other Nepenthes species had similar bacterial composition between pitcher types. SIMPER showed that more than 50 % of the bacterial taxa identified from the open pitchers of N. mirabilis were not found in other groups. Our study suggests that bacteria in N. mirabilis are divided into native and nonnative groups.     

The carnivorous syndrome in Nepenthes pitcher plants: Current state of knowledge and potential future directions

Nepenthes is the largest genus of pitcher plants, with its center of diversity in SE Asia. The plants grow in substrates that are deficient in N and offset this deficiency by trapping animal prey, primarily arthropods. Recent research has provided new insights into the function of the pitchers, particularly with regard to prey tapping and retention. Species examined to date use combinations of wettable peristomes, wax layers and viscoelastic fluid to trap and retain prey. In many respects, this has redefined our understanding of the functioning of Nepenthes pitchers. In addition, recent research has shown that several Nepenthes species target specific groups of prey animals, or are even evolving away from a strictly carnivorous mode of operation. Future research into nutrient sequestration strategies and mechanisms of prey attraction would no doubt further enhance our knowledge of the ecology of this remarkable genus.Key words: carnivory, mutualism, Nepenthes, pitcher plants     

Contribution of pitcher fragrance and fluid viscosity to high prey diversity in a Nepenthes carnivorous plant from borneo

Mechanisms that improve prey richness in carnivorous plants may involve three crucial phases of trapping:attraction, capture and retention.Nepenthes rafflesiana var. typica is an insectivorous pitcher plant that is widespread in northern Borneo.It exhibits ontogenetic pitcher dimorphism with the upper pitchers trapping more flying prey than the lower pitchers.While this difference in prey composition has been ascribed to differences in attraction,the contribution of capture and retention has been overlooked.This study focused on distinguishing between the prey trapping mechanisms, and assessing their relative contribution to prey diversity.Arthropod richness and diversity of both visitors and prey in the two types of pitchers were analysed to quantify the relative contribution of attraction to prey trapping.Rate of insect visits to the different pitcher parts and the presence or absence of a sweet fragrance was recorded to clarify the origin and mechanism of attraction.The mechanism of retention was studied by insect bioassays and measurements of fluid viscosity. Nepenthes rafflesiana was found to trap a broader prey spectrum than that previously described for any Nepenthes species,with the upper pitchers attracting and trapping a greater quantity and diversity of prey items than the lower pitchers.Capture efficiency was low compared with attraction or retention efficiency.Fragrance of the peristome,or nectar rim,accounted mainly for the observed non-specific, better prey attraction by the upper pitchers, while the retentive properties of the viscous fluid in these upper pitchers arguably explains the species richness of their flying prey.The pitchers of N. rafflesiana are therefore more than simple pitfall traps and the digestive fluid plays an important yet unsuspected role in the ecological success of the species.     

Why are Aedes mosquitoes rare colonisers of Nepenthes pitcher plants?

1. Nepenthes pitcher plants produce fluid‐containing animal traps that are colonised by a variety of specialised arthropods, especially dipterans. However, container‐breeding vector mosquitoes, such as Aedes albopictus Skuse have rarely been recorded from pitchers. Increasing overlap in the geographical ranges of Nepenthes and Ae. albopictus in urban parts of Southeast Asia owing to urbanisation highlights a growing need to investigate the potential role of pitchers as larval habitats for vector mosquitoes. 2. The ability of Ae. albopictus larvae to survive in three common lowland Nepenthes in Peninsular Malaysia that are most likely to co‐occur with Ae. albopictus [viz., Nepenthes ampullaria Jack, Nepenthes gracilis Korth., and Nepenthes mirabilis (Lour.) Druce] was investigated. 3. The larval survival rates of Ae. albopictus in pitcher fluids of the three Nepenthes species were determined, then the effects of low pH, larvicidal agents (such as microbes, predators, and chemical compounds) through manipulative experiments were investigated. 4. It was found that pitchers represent a hostile environment to Ae. albopictus, but that the principal cause of larval mortality varies among Nepenthes species (i.e. low fluid pH in N. gracilis, predation by Toxorhynchites acaudatus Leicester larvae in N. ampullaria, and microbial activity in N. mirabilis). It was concluded that Nepenthes pitchers are generally not suitable larval habitats for Ae. albopictus. However, the pitcher environment of N. ampullaria is worthy of further study, as pitchers that lack predators are nevertheless rarely colonised by Ae. albopictus, indicating that other aspects of the host pitcher environment inhibit oviposition or larval survivorship.     

Nutritional benefit from leaf litter utilization in the pitcher plant Nepenthes ampullaria

The pitcher plant Nepenthes ampullaria has an unusual growth pattern, which differs markedly from other species in the carnivorous genus Nepenthes. Its pitchers have a reflexed lid and sit above the soil surface in a tighly packed 'carpet'. They contain a significant amount of plant-derived materials, suggesting that this species is partially herbivorous. We tested the hypothesis that the plant benefits from leaf litter utilization by increased photosynthetic efficiency sensu stricto cost/benefit model. Stable nitrogen isotope abundance indicated that N. ampullaria derived around 41.7 ± 5.5% of lamina and 54.8 ± 7.0% of pitcher nitrogen from leaf litter. The concentrations of nitrogen and assimilation pigments, and the rate of net photosynthesis (A(N)), increased in the lamina as a result of feeding, but did not increase in the trap. However, maximal (F(v) /F(m)) and effective photochemical quantum yield of photosystem II (Φ(PSII)) were unaffected. Our data indicate that N. ampullaria benefits from leaf litter utilization and our study provides the first experimental evidence that the unique nitrogen sequestration strategy of N. ampullaria provides benefits in term of photosynthesis and growth.     

Structure and development of the pitchers from the carnivorous plantNepenthes alata (Nepenthaceae)

The pitchers of the tropical carnivorous plant Nepenthes alata are highly specialized organs for the attraction and capture of insects and absorption of nutrients from them. This study examined the structure and development of these pitchers, with particular focus on the nectaries and digestive glands. Immature pitchers developed at the tips of tendrils and were tightly sealed by a lid structure that opened during the end of pitcher elongation. Opened pitchers exposed a ridged peristome containing large nectaries. Like other members of the genus, a thick coating of epicuticular waxy scales covered the upper one-third of the pitcher. Scattered within this zone were cells resembling a stomatal complex with a protruding ridge. Cross sections showed that this ridge was formed by asymmetric divisions of the epidermal cells and lacked an underlying pore. The basal region of the trap had large multicellular glands that developed from single epidermal cells. These glands were closely associated with underlying vascular traces and provided a mechanism for supplying fluid to closed immature pitchers.     

Local and Regional-Scale Food Web Structure in Nepenthes alata Pitchers1

Tropical Nepenthes pitcher plants provide small, isolated aquatic habitats. We examined inter-pitcher variation in the community structure of the inhabitants of Nepenthes alata Blanco in West Sumatra, focusing on the conditions of the pitchers, bacterial density in the pitcher fluid, density and biomass of metazoan inhabitants, and the frequencies of interspecific encounters. Older pitchers contained more insect carcasses. The bacterial density increased with the age of the pitchers, but decreased in withered pitchers that contained finely decomposed detritus. In live pitchers, the bacterial density, the density, mass and species richness of metazoa, and the number of trophic levels per pitcher were positively correlated with detrital mass, which was correlated with volume of pitcher fluid. The metazoan fauna from N. alata consisted of 4 predators and 12 saprophages, among the richest known for Nepenthes species. However, each individual pitcher harbored a limited numbers of species, owing to (1) the low incidence of many species, and (2) the aggregated distribution and different temporal colonization pattern of major species. Six dipteran taxa (one predator and five saprophages) accounted for the bulk of metazoan inhabitant biomass. Of 48 combinations of predator-prey encountered, only four occurred frequently (in > 30% of pitchers), which included two predators and three saprophages. Thus, individual pitchers harbored relatively simple communities despite the regional species richness, and only limited kinds of predator-prey encounters seemed to occur frequently in the regional food web. The local-scale properties of the subdivided communities presented here provide the basic information for understanding the maintenance of regional species richness and food web complexity.     

A novel resource-service mutualism between bats and pitcher plants

Mutualistic relationships between vertebrates and plants apart from the pollen and seed-dispersal syndromes are rare. At first view, carnivorous pitcher plants of the genus Nepenthes seem to be highly unlikely candidates for mutualistic interactions with animals, as they form dimorphic terrestrial and aerial pitchers that trap arthropods and small vertebrates. Surprisingly, however, the aerial pitchers of Nepenthes rafflesiana variety elongata are poor insect traps, with low amounts of insect-attractive volatile compounds and low amounts of digestive fluid. Here, we show that N. rafflesiana elongata gains an estimated 33.8 per cent of the total foliar nitrogen from the faeces of Hardwicke's woolly bats (Kerivoula hardwickii hardwickii) that exclusively roost in its aerial pitchers. This is the first case in which the faeces-trapping syndrome has been documented in a pitcher plant that attracts bats and only the second case of a mutualistic association between a carnivorous plant and a mammal to date.     

Morphological correlates of necromass accumulation in the traps of an Eastern tropical pitcher plant, Nepenthes ampullaria Jack, and observations on the pitcher infauna and its reconstitution following experimental removal

I studied the trap morphology, necromass accumulation rates and pitcher infauna of an eastern tropical pitcher plant, Nepenthes ampullaria, that grew in `kerangas' heath forest in the Sungei Ingei Conservation Area, Brunei. I surveyed 164 pitchers distributed among 35 plants and extracted the necromass and larval infauna from the pitchers and then resampled the pitcher contents after 14 days. Plants varied significantly in the morphology of their pitchers, in their rate of necromass accumulation per pitcher and in the abundance and composition of the pitcher infaunas. On average, pitchers accumulated 11.5 mg dry weight over 14 days, but larger pitchers accumulated more necromass than smaller ones. Pitcher morphology explained 45% of the variation in necromass accumulation among plants. On average, pitchers initially contained 26.3 individual larval inquilines. Collectively, the larval infauna was composed of nine taxa of dipteran larvae and infrequent anuran tadpoles. These ten taxa were never found together in a single pitcher and the mean species richness per pitcher was 4.0. Of the six taxa that could be assessed, all except Toxorhynchites spp. had a contagious distribution among the pitchers. Pitcher morphology and necromass accumulation explained only 15% of the variation in inquiline abundance among plants. I found little evidence for the existence of density-dependent interactions between inquiline species: a partial correlation analysis detected only one statistically significant pairwise relationship between the abundances of inquiline taxa, which was a positive association. Fourteen days after being emptied, pitchers contained an average of 9.6 inquilines. There was no evidence that the species composition of the infauna recolonising each pitcher was related to that of its pre-removal infauna. Received: 2 June 1997 / Accepted: 9 September 1997     

Distribution of naphthoquinones, plumbagin, droserone, and 5-O-methyl droserone in chitin-induced and uninduced Nepenthes khasiana: molecular events in prey capture

Prey capture and digestion in Nepenthes spp. through their leaf-evolved biological traps involve a sequence of exciting events. Sugar-rich nectar, aroma chemicals, narcotic alkaloid secretions, slippery wax crystals, and other biochemicals take part in attracting, capturing, and digesting preys in Nepenthes pitchers. Here we report the distribution of three potent naphthoquinones in Nepenthes khasiana and their roles in prey capture. Plumbagin was first detected in N. khasiana, and its content (root: 1.33 ± 0.02%, dry wt.) was the highest found in any natural source. Chitin induction enhanced plumbagin levels in N. khasiana (root: 2.17 ± 0.02%, dry wt.). Potted N. khasiana plants with limited growth of roots and aerial parts, showed higher levels of plumbagin accumulation (root: 1.92 ± 0.02%; root, chitin induction: 3.30 ± 0.21%, dry wt.) compared with field plants. Plumbagin, a known toxin, insect ecdysis inhibitor, and antimicrobial, was also found embedded in the waxy layers at the top prey capture region of N. khasiana pitchers. Chitin induction, mimicking prey capture, produced droserone and 5-O-methyl droserone in N. khasiana pitcher fluid. Both these naphthoquinone derivatives provide antimicrobial protection to the pitcher fluid from visiting preys. A two-way barrier was found between plumbagin and its two derivatives. Plumbagin was never detected in the pitcher fluid whereas both its derivatives were only found in the pitcher fluid on chitin induction or prey capture. The three naphthoquinones, plumbagin, droserone, and 5-O-methyl droserone, act as molecular triggers in prey capture and digestion in the carnivorous plant, N. khasiana.     

Aspects of Pitcher Morphology and Spectral Characteristics of Six BorneanNepenthesPitcher Plant Species: Implications for Prey Capture

Pitcher plants of the genusNepenthesattract and trap invertebrateprey using nectar-secreting pitchers. Pitcher morphology andspectral reflectance characteristics were investigated for sixNepenthesspeciesfrom northwest Borneo (N. albomarginata, N. ampullaria, N. bicalcarata,N. gracilis, N. mirabilisvar.echinostomaandN. rafflesiana).Morphological measurements focused on the size of the pitcherrim (or peristome, the site of the major nectaries) in relationto pitcher length. The results show considerable interspecificvariation in morphology. Spectral reflectance measurements quantifiedthe degree of colour contrast between the peristome and pitcherbody, from ultraviolet (UV) to red wavelengths. The contrastmaxima for each species were compared with insect visual sensitivitymaxima. The six species showed a wide range of reflectance patterns,with pitchers ofN. rafflesianapossessing the greatest degreeof ‘fit’ between contrast maxima and insect sensitivitymaxima, in the UV, blue and green regions of the spectrum. Basedon the morphological and reflectance analyses, we hypothesizedthat pitchers ofN. rafflesianawould be more attractive to anthophilous(flower-visiting) invertebrates than the sympatricN. gracilis.Analysis of prey contents generally supported the hypothesis,suggesting possible interspecific resource partitioning. Morphologicaland spectral characteristics of the other species are discussedin relation to published studies on prey capture by those species.Copyright1999 Annals of Botany Company Borneo, carnivory, colour,Nepenthesspp., pitcher plants, prey attraction.     

Carnivorous syndrome in Asian pitcher plants of the genus Nepenthes

BACKGROUND AND AIMS: Pitcher plants Nepenthes alata and N. mirabilis are carnivorous species with leaves composed of a photosynthetic part (lamina) and a pitcher trap. This characteristic permitted direct physiological and anatomical comparison between these two distinct parts of the leaves to determine those features involved in the 'carnivorous syndrome', which include low net photosynthetic assimilation rate (A(N)) and low photosynthetic nitrogen use efficiency (PNUE). METHODS: Photosynthetic rate (A(N)) and respiration rate (R(d)) were measured gasometrically, chlorophyll concentration was determined spectrophotometrically and nitrogen concentration was determined using a CHN elemental analyser in lamina and trap separately. Anatomy of N. alata was observed using light, fluorescence and transmission electron microscopy. A(N), foliar nitrogen and chlorophyll concentration were also compared with values for other carnivorous plant species (genera Sarracenia, Drosera) that combine both autotrophic and carnivorous functions into the same physical organ. KEY RESULTS: It was found that the A(N) in Nepenthes lamina was low and PNUE was only slightly higher or similar in comparison with other carnivorous plants. It was not observed that the pitcher had a higher R(d) than the lamina, but A(N) in the pitcher was significantly lower than in the lamina. Nepenthes possesses a cluster of characters that could result in reduced photosynthesis in the pitcher and be responsible for carnivorous function of the leaf: replacement of chlorophyll-containing cells with digestive glands, low chlorophyll and nitrogen concentration, compact mesophyll with a small portion of intercellular spaces, absence of palisade parenchyma and low stomatal density. CONCLUSION: Low photosynthetic capacity, nitrogen efficiency, chlorophyll and nitrogen concentration of Nepenthes pitchers was found, together with a set of features that characterized the carnivorous syndrome. Dual use of leaves for photosynthesis and nutrient gain can decrease photosynthetic efficiency in carnivorous plants in general.     

猪笼草消化液成分及其应用价值研究进展

猪笼草是一类食虫植物,通过捕虫囊内消化液分解猎物,为自身生长提供营养。猪笼草消化液中含天冬氨酸蛋白酶、几丁质酶等水解酶类,还有萘醌、自由基及一些无机离子。猪笼草消化液具有抗真菌,治疗创伤、头痛等药用功能,并有抗肿瘤、降血压、抗疟疾等潜在药用开发价值。对猪笼草消化液的成分及活性进行归纳,为其药用开发提供思路。