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Physiology and Development |
Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Panama City, Republic of Panama
Received for publication June 27, 2000. Accepted for publication February 1, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Bromeliaceae • epicuticular wax • fluorographic dimensional imaging • SEM • trichomes • water repellency
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, who elaborated on the observation of Tietze (1906)
that life form and the function of leaf hairs was reflected in the taxonomic relationships of genera. Pittendrigh's scheme was further expanded by Benzing (2000)
into the five ecophysiological types summarized in Table 1.
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) are almost ubiquitous in Bromeliaceae (Benzing, 1976
) and are perhaps the most distinguishing vegetative feature of the family. It is well documented that the peltate trichomes belonging to species with Type 3, 4, and 5 life forms support epiphytism by endowing the shoot with the capacity to augment or replace the absorptive functions of roots (Schimper, 1888
; Billings, 1904
; Mez, 1904
; Benzing, 1970, 1976
; Benzing and Burt, 1970
; Benzing et al., 1976
; Nyman et al., 1987
; Smith, 1989
; see Benzing [1980]
for a detailed discussion of their mode of action). The trichomes of terrestrial Type 1 and many Type 2 bromeliads are incapable of this function (Benzing et al., 1976
; Lüttge et al., 1986
). Trichome function has therefore played a pivotal role in the adaptive radiation of Bromeliaceae via the operation of these different ecophysiological strategies.
However, the function(s) of the trichomes of Type 1 bromeliads remains enigmatic. Molecular phylogenetics indicates that the genera Ayensua and Brocchinia are basal to the rest of the family (Terry, Brown, and Olmstead, 1997
; Horres et al., 2000
; Crayn, Winter, and Smith, unpublished data). Although direct fossil evidence is negligible, mesic Type 1 Pitcairnioideae (e.g., Ayensua, some Brocchinia, Fosterella, Pitcairnia) are also considered to exhibit a primitive life form (i.e., ecophysiologically they most closely resemble a hypothetical ancestor of the family). This assessment is based not only on subfamilial characteristics such as the extensive root system (Tietze, 1906
), but also on the presence of less advanced nonsucculent C3 physiology (see Medina, 1974) and the simpler structure of the trichome (Benzing, 1980
). Indeed, within the genus Brocchinia advanced Type 4 species possess absorbing trichomes, while nonimpounding terrestrial species possess less highly organized trichomes and are more basal within the genus (N.B. the most primitive of these, B. prismatica, possesses stellate trichomes similar to those of Fosterella species; Givnish et al., 1997
). Thus, foliar trichomes of mesic Type 1 Pitcairnioideae mediate primitive functions.
Many roles other than water and nutrient absorption have been ascribed to bromeliad trichomes, but these functions often only apply to a small number of species (such as the attraction of pollinators or seed dispersers in the case of some Tillandsia and Billbergia species; Benzing, 2000
). More general hypotheses concerning the function of bromeliad trichomes include obstruction of predators and pathogens (Benzing, 2000
), reduction of transpiration (Billings, 1904
), and photoprotection (Benzing and Renfrow, 1971
; Lüttge et al., 1986
). The deterrence of predators and pathogens currently has no experimental support. Reduction of transpiration is a xeromorphic adaptation, and as such, it is unlikely that this would be an important selection pressure acting on ancestors in mesic habitats.
In high densities, bromeliad trichomes produce a whitish leaf surface that reflects light when dry. This has been quantified in Type 4 Tillandsia fasciculata (Benzing and Renfrow, 1971
) and semimesic Type 1 Pitcairnia integrifolia (Lüttge et al., 1986
) and is highly suggestive of a role in photoprotection. However, in the more relevant case of Type 1 P. integrifolia, trichomes are restricted to the abaxial surface of the leaf; had these trichomes developed primarily to serve a photoprotective role, then they would be expected to occur at least in equal densities on the glabrous adaxial surface. Lüttge et al. (1986)
note that the edges of the leaves of P. integrifolia roll inwards to expose the trichomed abaxial surface during the dry season, perhaps to promote reflectance, and propose this as a form of regulation of light reflectance. However, this behavior may occur simply as a consequence of drought in glabrous species (e.g., Pitcairnia valerii; personal observation), perhaps as a response to water loss and concomitant shrinkage of water-storage parenchyma in the hypodermis (see Billings, 1904
). More importantly, the trichomes of P. integrifolia and P. bifrons were not found to influence the heating of leaves (Lüttge et al., 1986
). Thus, a photoprotective role for trichomes remains without direct supporting evidence; an investigation of photoinhibition using fluorimetry techniques has yet to be undertaken.
Evidence for a further general hypothesis concerning the role of the trichome in terrestrial Bromeliaceae is present in the literature, but has apparently been overlooked. Krauss (1948–1949)
working on Ananas comosus noted that "the trichomes on the lower surface of the leaf blade proper appear unwettable. Drops of water placed on this surface do not spread, but remain unabsorbed for experimental periods of 3 to 6 h."
Krauss (1948–1949)
also went on to observe that, whereas the absorbent trichomes of Tillandsia usneoides lost their pale whitish color when wetted (Billings, 1904
), those on the abaxial surface of A. comosus did not, as a consequence of air trapped beneath the trichomes. This implies that the trichomes on the abaxial surface of A. comosus repel water. Also, the abaxial surfaces of Pitcairnia integrifolia and P. macrochlamys leaf blades appear to be unwettable (Benzing, Seemann, and Renfrow, 1978
; Lüttge et al., 1986
), and in the case of P. integrifolia, "water repellent." Indeed, Benzing (1970)
discovered that after 12 h of exposure the abaxial surface of P. macrochlamys had absorbed 
3.5 times less zinc65 than the glabrous cuticle of the adaxial surface, perhaps suggesting that the trichome layer hindered absorption. Widespread occurrence of repellent trichome layers on the abaxial leaf blade surfaces of mesic Type 1 bromeliads would therefore suggest that hydrophobia was an important property of the foliar trichome in ancestral Bromeliaceae.
Also relevant to this study are the hydrophobic waxy surfaces of Brocchinia reducta and Catopsis berteroniana. Tomlinson (1969)
suggests that in the case of C. berteroniana these promote the run-off of water from the leaf blades into the tank and attraction and entrapment of insect prey by these carnivorous species have also been suggested (Fish, 1976
; Frank and O'Meara, 1984
). These species also share advanced Type 4 life forms, which usually possess hydrophilic trichomes at least lining the tank. Determinations of the occurrence of hydrophobic surfaces in Tillandsioideae and Brocchinia could shed additional light on the evolution of the Type 4 life form.
The present study employs a novel technique, fluorographic dimensional imaging (FDI), to assess the interactions between aqueous droplets and the leaf blade surfaces of 86 ecologically diverse bromeliad species representing 25 genera and all three subfamilies. Fluorographic dimensional imaging is used in conjunction with scanning electron microscopy (SEM) and spectroradiometry to reveal the mechanism by which certain trichomes and epicuticular wax powders repel water. Fluorimetry is used to investigate the hypothesized role of trichomes and wax layers in photoprotection. Nomenclature follows that of Luther and Sieff (1998)
, with the exception of the recently rejected genus Pepinia (Taylor and Robinson, 1999
), which is recognized as a subgenus of Pitcairnia (sensu Smith and Downs, 1974
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Repellency was denoted by the depth of aqueous droplets on adaxial and abaxial leaf blade surfaces. For FDI of aqueous droplets, calibration standards were prepared using glass coverslips (2 cm wide), one-half being coated with a flat film of paraplast wax (Sigma Chemical, St. Louis, Missouri, USA), and the other half remaining as an exposed glass surface. The thickness of these wax and glass standards was measured by micrometer, and these standards were lightly fixed along one edge of a strong glass plate of 40 x 40 cm.
Leaf discs were cut from intact and surface denuded midleaf portions of leaf blade (from two-thirds of the way along the blade). In many species denudation was achieved using sticky tape, although some species such as Ananas comosus required careful scraping with a scalpel blade. In the case of apparently glabrous leaves, the procedure of denudation with sticky tape was conducted for the sake of consistency. Leaf discs from replicate leaves (where possible from separate individuals) were then fixed in rows onto the glass plate, with intact and denuded examples of both surfaces presented uppermost.
Droplets (10-µL each) of 0.05% (mass by volume in distilled H2O) fluorescein sodium solution were quickly pipetted onto the surface of the leaf discs and calibration standards and left to stand for 40 min in a darkened room. In these darkened conditions, the leaf discs and standards were then illuminated with an ultraviolet (UV) transilluminator (Fotodyne, Hartland, Wisconsin, USA), and the resulting fluorescence from the excited fluorochrome was photographed using a level camera mounted directly above the leaf discs. Initial tests determined that the following camera settings provided the greatest depth of field and contrast, with well-exposed fluorescence and a darkened background: an aperture of f/22, aperture priority (or a 9-sec exposure with a cable release), using ISO 100/DIN 21° color-reversal film (Kodak Elite). The depth of droplets on wax and glass standards was determined by micrometer immediately after the fluorograph was taken.
After processing, fluorographs were digitally scanned (LS-2000, Nikon, Shinagawa-Ku, Tokyo, Japan) and the luminosity of fluorescein droplets was determined using Corel PHOTO-PAINT7 (Corel, Ottawa, Ontario, Canada) imaging software (selecting each particular region of the image with the "eyedropper" tool, and recording the luminosity (L) of the "paint" color). To compensate for possible uneven lighting, eight measurements were taken from each droplet, and the measurements were averaged. Luminosity and depth data from the glass and wax standards were then regressed (Excel, Microsoft, Seattle, Washington, USA) to create a calibration equation, from which the depth of droplets on leaf discs was calculated using respective luminosity values. This technique allowed rapid, inexpensive, mass screening of samples. The difference in droplet depth (
D) due to surface features can be summarized by the following equation:
 | (1) |
In order to examine the effect of water surface tension on the interaction between trichomes and water, the above FDI technique was also used on the leaves of Ananas comosus, using droplets (10-µL each) of fluorescein sodium solution (5 mL of 0.05% fluorescein and 0.5 mL distilled H2O); with further replicates on which 10-µL droplets of a solution of fluorescein and household detergent (5 mL of 0.05% fluorescein and 0.5 mL neat detergent) were used.
Reflectance of light by leaves was measured using an LI-1800 portable spectroradiometer (LI-COR, Lincoln, Nebraska, USA), via an 1800–12s external integrating sphere (LI-COR). Ranges of reflectance values were normalized to 100% using barium sulfate (BaSO4) as a standard; this compound has an absolute reflectivity of 99.3% in the wavelength range 300–800 nm (Munsell Color, New Windsor, New York, USA). Measurements were taken of intact, water-inundated, and denuded leaf surfaces (both adaxial and abaxial). Species with water repellent trichome layers were inundated by soaking in water for 1 h or until a surface film of water could be sustained on their removal from the water. Once again, in the case of surfaces that appeared to have no trichomes, the denudation process was carried out with sticky tape for consistency's sake. Average reflectance values of photosynthetically active radiation (PAR) were calculated as a mean across the wavelength range 400–700 nm. The reflectance conferred by trichomes or wax powders is defined as the difference in mean reflection between intact and denuded surfaces.
Photoinhibition of photosystem II was investigated using a PAM-2000 portable modulated fluorimeter (H. Walz, Effeltrich, Germany). Aechmea dactylina, Ananas comosus cv. Cayenne Lisse, Catopsis micrantha, Pitcairnia integrifolia, Tillandsia flexuosa, and Werauhia sanguinolenta were maintained in seminatural conditions in an open-sided greenhouse at the main Smithsonian Tropical Research Institute facility in Panama. Excluding the cultivar of Ananas comosus, these species grow in semi-exposed to exposed microhabitats and may experience several hours of direct sunlight each day (Lüttge et al., 1986
; personal observations). A treatment of excessive excitation therefore consisted of transferring plants grown in moderate sunlight (450 µmol photon·m–2·sec–1 at midday) to direct sunlight at midday (PPFD 
1700 µmol photon·m–2·sec–1) for 1 h. The degree of photoinhibition was denoted by the decline in the dark-adapted ratio of variable to maximum chlorophyll fluorescence (Fv/Fm) following this treatment, with intact and denuded surfaces being compared.
For scanning electron microscopy, the majority of leaf samples were dehydrated through an alcohol series, critical point dried (CPD) in CO2, and then sputter-coated with gold-palladium (Hummer VI-A, Anatech, Springfield, Virginia, USA) before examination in the scanning electron microscope (Jeol JSM-5300LV, Jeol, Tokyo, Japan). However, samples of Catopsis were not dehydrated in this manner, as the solvents used in CPD may destroy the structure of wax surfaces (Juniper and Jeffree, 1983
); samples were placed in the scanning electron microscope without preparation.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Leaf blade interactions with water
A typical fluorograph for a single species (Catopsis micrantha) is shown in Fig. 17. Fluorographic dimensional imaging determined that droplet depth had diminished after 40 min on the intact leaf blade surfaces of Type 5 species when compared with surfaces denuded of trichomes (D). For example, on leaf blades of Tillandsia nana, Dd = –732 and Db = –876 µm; confirming these leaves to be highly hydrophilic.
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0.05; Table 3). However, there were some notable exceptions; for example the hydrophobic abaxial surface of Vriesea monstrum (Dd = 214 µm; Table 3) and both hydrophilic surfaces of Tillandsia elongata (Dd = –210 µm and Db = –190 µm). Many Type 4 taxa possessed hydrophobic waxy surfaces, e.g., Catopsis micrantha (Dd = 800 µm and Db = 960 µm), Guzmania macropoda (Db = 216 µm), and Werauhia capitata (Db = 350 µm).
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Dd = –220 µm and Db = –130 µm; Type 3 A. fendleri, Dd = –130 µm and Db = –110 µm), and the hydrophobic abaxial surfaces of species such as Type 2 Ananas comosus (Db = 160 µm; Fig. 8) and Type 1 Ronnbergia explodens (Db = 100 µm). A number of bromelioid species possessed both hydrophilic adaxial surfaces and hydrophobic abaxial surfaces (e.g., Type 3 Aechmea nudicaulis, Dd = –267 µm and Db = 226 µm; Type 1 Cryptanthus whitmanii, Dd = –205 µm and Db = 407 µm; Type 1 Orthophytum benzingii, Dd = –477 µm and Db = 474 µm).
Of the mesic Type 1 pitcairnioids, genera such as Fosterella and Pitcairnia either possessed hydrophobic abaxial surfaces, due solely to trichome cover (e.g., Pitcairnia integrifolia, Db = 230 µm), or were entirely glabrous and noninteractive (e.g., Pitcairnia patentiflora), with a small number possessing hydrophobic adaxial surfaces (Pitcairnia arcuata, Dd = 310 µm). The more xeromorphic pitcairnioid genera showed a range of trichome-mediated interactions with surface water, including species that possessed both hydrophilic and hydrophobic leaf blade surfaces (e.g., Dyckia marnier-lapostollei, Dd = –457 µm and Db = 740 µm; Table 3).
Of the 16 species examined from the elfin cloud forest at Cerro Jefe in central Panama, six possessed water-repellent leaf surfaces (Table 3). These were either Type 1 species with repellent trichomes (Pitcairnia arcuata, Ronnbergia explodens) or Type 4 species with relatively upright leaves that used trichomes (Vriesea monstrum) or epicuticular wax powders (Catopsis micrantha, Guzmania macropoda, Werauhia capitata) to shed water. A further six were Type 4 species equipped with hypostomatous and horizontally orientated leaves.
The wax powder layer of Catopsis micrantha was less pronounced towards the tip of the leaf blade, where it still promoted beading up of water (Figs. 18, 19). This layer was continuous over the leaf blade surface (Figs. 13, 15, 20), but was not present on the adaxial surface of the leaf sheath within the tank of the plant. This surface is densely covered with peltate trichomes (Fig. 21). Powdery epicuticular wax was also present on hydrophobic surfaces of Alcantarea odorata, Brocchinia reducta, and Werauhia capitata (Figs. 22–25).
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0.05; Table 3)—this species possesses two types of trichome, one being highly modified with an elongate wing that spirals around itself to form a hair-like structure (Fig. 33).
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On the hydrophilic adaxial surface and hydrophobic abaxial surface of Cryptanthus whitmanii the trichomes appeared no different, although the lower densities on the adaxial surface revealed the leaf epidermis proper to SEM (Figs. 38, 39). Aechmea nudicaulis also has low densities of thin, filmy trichomes on the hydrophilic adaxial surface (Fig. 40) and a typical hydrophobic abaxial surface (Fig. 41). No species in any subfamily possessed a hydrophobic adaxial surface combined with a hydrophilic abaxial surface. Water repellent epicuticular wax powders or confluent layers of large ring-peltate trichomes occurred exclusively on surfaces that possessed stomata in the species studied.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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45 and 55% photoprotect certain desert plants (Ehleringer and Björkman, 1976
; Robinson, Lovelock, and Osmond, 1993
). Also, the present study indicated that the reflectance conferred was correlated with the mode of interaction between surfaces and water. Hydrophobic surfaces did not lose reflectivity when wet, whereas hydrophilic trichomes did (see also Billings, 1904
; Krauss, 1948–1949
; Benzing, Seemann, and Renfrow, 1978
), and higher reflectivities on abaxial surfaces were correlated with the presence of dense hydrophobic indumenta (e.g., Ananas comosus, Pitcairnia integrifolia). Thus, the data indicate that hydrophobic and dry hydrophilic trichome layers inherently scatter light, but are unlikely to have evolved primarily for the purpose of photoprotection in Bromeliaceae.
The highly unusual, woolly trichomes of Puya laxa (Fig. 33) did not interact with water droplets on the leaf surface (Table 3). These trichomes probably act as protection against frost damage as exhibited by a number of Puya species growing in high altitude habitats (Miller, 1994
). As this example illustrates, distinct taxa produce trichomes that represent a more specific adaptation to local environmental conditions. Thus, dense indumenta could yet prove to furnish photoprotection in the case of more extreme xerophytes (Type 5 species). A thorough investigation of the fluorescence characteristics of this life form was beyond the scope of the present study.
The mechanism of water repellency
Brewer, Smith, and Vogelmann (1991)
noted three kinds of interaction between water and the trichomes of flowering plants: (1) low trichome densities that do not influence droplet retention or the location of surface moisture, (2) low densities of trichomes that induce surface water to aggregate into patches, and (3) high densities of trichomes that lift water off the leaf surface. The leaf blade surfaces of many Type 4 bromeliads exhibit low trichome densities (Benzing, 1980
) and did not interact detectably with surface water in the present study (Table 3). Bromeliads that have low densities of attenuated stellate trichomes, such as Type 1 Pitcairnia arcuata, appear to interact with water as described by situation 2, loosely aggregating surface droplets. Consistent with the third, ‘lifting,’ mechanism of repellency, continuous layers of powdery wax or ring-peltate trichomes produce an irregular hydrophobic surface that prevents water from coming into contact with the epidermis proper. A summary of the principal interactions between leaf blade trichome layers and water within each ecophysiological type is presented in Table 5.
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; Juniper and Jeffree, 1983
), obviating adhesion (Brewer, Smith, and Vogelmann, 1991
; Watanabe and Yamaguchi, 1993
). The physics of these surface–water interactions are outlined by Barthlott and Neinhuis (1997)
. This hydrophobic mechanism is readily demonstrable in Bromeliaceae. For example, a wetted pineapple leaf (Ananas comosus) will lose the pale coloration of the abaxial surface only if detergent is first added to the water. Species with absorbent trichomes, on the other hand, lose this pale coloration and reflectivity immediately on wetting (Billings, 1904
; Benzing and Renfrow, 1971
; Benzing, 1980
; Fig. 3). Also, droplets of water will only spread on the abaxial leaf surface of pineapple if detergent is added (demonstrated quantitatively in Table 4). Pineapple leaves soaked overnight in a detergent solution or 100% acetone will regain their repellency if subsequently rinsed and dried, suggesting a physical rather than chemical mechanism (personal observations). Additionally, if a pineapple leaf is partially dipped into a detergent solution rather than pure water, then liquid will be drawn or "wicked" up out of the solution along the trichome layer, i.e., once the surface tension of the water is broken the leaf surface becomes strongly hydrophilic. Thus, the physical properties of water are central to the mechanism of repellency. This mechanism also demonstrates, at least in part, how Type 4 species prevent water loss from the tank via capillary action. Trichomes that characterize hydrophilic and hydrophobic surfaces usually share the same structure, with trichome density differing (e.g., the adaxial and abaxial surfaces of Cryptanthus whitmanii and hydrophilic Aechmea dactylina compared with hydrophobic Ronnbergia explodens; Figs. 34–39). The lower densities of peltate trichomes of Aechmea dactylina and the adaxial surface of Cryptanthus whitmanii would allow water to come into contact with the epidermis proper, with the interaction between the two presumably allowing water to spread and envelop trichomes. In addition, the adaxial trichomes of Aechmea nudicaulis differ structurally—lacking the irregular surface characteristic of the hydrophobic abaxial indumentum (Figs. 40, 41). The chemical composition of hydrophilic and hydrophobic surfaces in Bromeliaceae has not been investigated and the degree to which chemical vs. morphological interactions contribute to repellency remains undetermined. Nevertheless, the physical characteristics of hydrophobic trichome layers in Bromeliaceae are typical of water-repellent surfaces in other families, and the qualitative tests above suggest that surface morphology is paramount to the operation of hydrophobia.
Ecophysiological consequences of a hydrophobic indumentum
It may be significant that the majority of bromeliads are hypostomatous (Tomlinson, 1969
; Benzing and Burt, 1970
), with stomata and hydrophobic trichome layers occurring together. Intriguingly, Barthlott and Neinhuis (1997)
demonstrate that particulate matter will adhere more readily to water droplets than to hydrophobic leaf surfaces, lending such leaves a "self-cleaning" capability when wetted. In concert with a possible function as a physical barrier to pathogens (Benzing, 2000
), this self-cleaning effect could remove pathogens and prevent the physical blockage of stomata by particulates. A continuous trichome layer could also deter herbivores from the softer underside of the leaf, although to date this protective role is only evident in two species possessing glandular trichomes (see Benzing, 2000
).
Benzing, Seemann, and Renfrow (1978)
determined that photosynthetic gas exchange was not inhibited by wetting the leaf blades of six species on the surfaces of which water did not spread (including Pitcairnia macrochlamys). Conversely, the wetted trichomes of Type 5 bromeliads hold films of water that slow the exchange of gases between the air and the leaf (Benzing, Seemann, and Renfrow, 1978
; Schmitt, Martin, and Lüttge, 1989
). Clearly, most Type 5 bromeliads must reconcile both gas exchange and water acquisition through the same surface, relying on temporal separation of these two processes by performing gas exchange when the leaf is dry. In contrast, Type 1 and Type 2 bromeliads separate the processes of gas exchange and water acquisition spatially between roots and leaves and tank-forming species between the leaf sheath and blade. Thus, these latter life forms do not need to compromise carbon gain to acquire water. In this respect, wettable trichomes on the leaf blade would not only be an unnecessary investment but would be disadvantageous in mesic habitats, whereas repellent trichomes would favor gas exchange, as perhaps demonstrated by Pitcairnia macrochlamys (Benzing, Seemann, and Renfrow, 1978
).
Sources of water that may moisten the underside of the leaf may include dew and, perhaps more importantly in cloud forests, wind-borne mist. These factors in conjunction with the terrestrial lifestyle (i.e., the close proximity of vegetation and/or the ground surface from which rainwater can splash upwards onto the underside of the leaf) may help explain the evolution of hydrophobic trichome layers in Bromeliaceae. Indeed, in the family as a whole, rosulate habits typical of genera such as Fosterella and Cryptanthus tend to have hydrophobic abaxial surfaces (Table 3). Also, terrestrial Orthophytum benzingii has basal leaves close to the substrate that possess a repellent trichome layer on the abaxial surface, but on cauline leaves this layer is less apparent (personal observation).
Trichome evolution
The mechanism of water repellency outlined above accords with the scheme of trichome structural evolution detailed by Benzing (1980)
. In this scheme, the hypothetical ancestral morphology is stellate (the simple filamentous trichomes of some Navia species appear to be derived; Benzing, 1980
; Terry, Brown, and Olmstead, 1997
). Low densities of stellate trichomes provide only discontinuous, patchy repellency (e.g., extant Pitcairnia arcuata), increased densities of which would maintain a greater proportion of the moistened leaf surface dry. Following this proposed early increase in trichome density, stellate trichomes may then have undergone an increase in the number of ring cells, becoming truly peltate. This would increase the area covered by each trichome and thereby foster the "lifting" mechanism of repellency (high densities of intermediate stellate/ring-peltate trichomes occur in Pitcairnia corallina and P. integrifolia [Figs. 10, 31] and P. macrochlamys; Benzing, Seemann, and Renfrow, 1978
). Additionally, the extrusive ring cells of such peltate trichomes appear to lend the overall surface an extremely irregular small-scale texture (e.g., Figs. 29–31).
Hydrophilic trichome layers among extant Bromelioideae feature lower trichome densities, suggesting a decline in trichome density from ancestors with dense hydrophobic layers. This perhaps reflects adaptive radiation into less crowded or relatively xeric niches. Indeed, Type 1 Ronnbergia explodens has dense hydrophobic trichome layers and grows in the understory of cloud forest habitats (Figs. 36, 37; Table 3). More xeromorphic terrestrial species (CAM equipped and succulent) such as Cryptanthus warasii and C. whitmanii may possess hydrophilic surfaces characterized by fewer trichomes (Fig. 38; Table 3; unpublished data), as do many Type 3 species (Aechmea dactylina, A. nudicaulis; Figs. 1, 2, 34, 40; Table 3).
Dense trichome layers in Tillandsioideae are usually hydrophilic, unlike those of Bromelioideae and Pitcairnioideae. Indeed, Billings (1904)
points out that one of the most unusual features of Tillandsia usneoides is that "unlike most similar appendages of the epidermis, the scales do not hinder the leaf from becoming wet." Dense hydrophilic trichome layers in Tillandsioideae must possess a difference that can account for their lack of water repellency. At present, differences in the chemical composition of these surfaces cannot be ruled out. However, a striking structural difference between the trichomes of Tillandsioideae and those of the other subfamilies is apparent, which could also explain the different interaction with water. From scanning electron micrographs published in other sources (Benzing, Seemann, and Renfrow, 1978
; Benzing, 1980
; Adams and Martin, 1986
), it is possible to see that the parts of adjacent tillandsioid trichomes that overlap one another are the flexible wings, which overlap when flattened (wet). Thus, when the leaf is dry and the wings are flexed upwards, underlying epidermis cells are exposed (Benzing, Seemann, and Renfrow, 1978
). When the leaf is wetted, surface tension forces acting on the epidermis and/or the underside of the trichome wing may permit water to spread. Thus, dense trichome layers in most Tillandsioideae have different configurations when wet and dry and will only form a confluent layer after wetting. The moveable trichome wing of the Type 5 life form may therefore be regarded as a device allowing the presence of high densities of trichomes while avoiding repellency.
Indeed, dense layers of peltate trichomes that lack wings in Tillandsioideae are hydrophobic (e.g., Vriesea monstrum; Fig. 29; Table 3). Also, the immobile trichomes of Type 3 bromeliads demonstrate that a moveable wing is not essential for absorption (Benzing, Givnish, and Bermudes, 1985
). The moveable wing is generally associated with higher trichome densities and effective water and nutrient absorption by the leaf surface (Benzing and Burt, 1970
).
Epicuticular wax powders
Benzing, Givnish, and Bermudes (1985)
suggest that Tillandsioideae and Brocchinia—both of which include advanced Type 4 tank forms equipped with absorbent trichomes—are derived from a common ancestor. Indeed, in the present study only Tillandsioideae and Brocchinia provided examples of species in which epicuticular wax powders are produced. Waxy Catopsis species have been shown to use wing-peltate trichomes to take up mineral ions and amino acids (Benzing et al., 1976
; Benzing, 1980
; Benzing and Pridgeon, 1983
), and this probably also applies to C. micrantha. Both leaf surfaces bear a powdery layer of epicuticular wax, and this is also one of the few taxa reported to be amphistomatous (see Tomlinson, 1969
; Figs. 13, 14). Thus, extensive epicuticular wax powders appear to have evolved only in taxa containing Type 4 life forms, which use trichomes to acquire water and minerals from tanks.
It is likely that in many Type 4 species a combination of the horizontal orientation of the leaf and the hypostomatous condition are sufficient to keep stomata unobstructed by water; in the present study, predominantly those species that possessed upright leaves (e.g., Brocchinia reducta, Guzmania macropoda, Werauhia capitata), and/or stomata on the adaxial surface (Catopsis micrantha) possessed hydrophobic wax powders on the leaf blade. Possibly the upright funnelform habit increases the utility of the tank as an impoundment, and tank formers face a trade-off between gas exchange and impoundment capacity, wax powders being a method of maximizing both. Reflective epicuticular wax powders have also been implicated in the attraction and entrapment of insects in a small number of Type 4 bromeliads—Catopsis berteroniana, Brocchinia hechtioides, and B. reducta (Fish, 1976
; Frank and O'Meara, 1984
; Givnish et al., 1984
; Owen, Benzing, and Thomson, 1988
; Owen and Thomson, 1991
; Benzing, 2000
). It is possible that a slippery and reflective epicuticular wax powder helped predispose these lineages to carnivory.
Conclusions
Hydrophobic leaf surfaces of Bromeliaceae possess a highly irregular microrelief, thereby reducing the adhesion and spread of water on the leaf blade. Hydrophobic trichome layers occur on the abaxial leaf blade surfaces of many mesic Type 1 pitcairnioids and, as these species exhibit the putative primitive ecological condition, water repellency appears to have been an important condition in early Bromeliaceae. The trichomes of Type 4 species are specialized for the alternative function of water and nutrient absorption from a water-filled tank, with epicuticular wax powders employed by some species to shed water from the leaf blades. Hydrophobic trichome layers and wax powders could potentially obstruct pathogens and particulates, aid in self-cleaning, and/or maintain gas exchange during wet weather.
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FOOTNOTES |
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2 University of Cambridge, Department of Plant Sciences, Downing Street,
Cambridge, CB2 3EA, UK.
3 Author for reprint requests.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; 1994) and references therein, or determined via carbon isotope discrimination by Crayn, Winter, and Smith (
; unpublished data). Plant material originated from the following countries: AR = Argentina, BO = Bolivia, BR = Brazil, CO = Colombia, CR = Costa Rica, CU = Cuba, DO = Dominica, EC = Ecuador, GT = Guatemala, GY = Guyana, HO = Honduras, JA = Jamaica, ME = Mexico, PA = Panama, PE = Peru, PG = Paraguay, TR = Trinidad, VE = Venezuela. Life forms or ecophysiological types follow the classification of Benzing (2000)
