Horticultural Reviews Volume 2

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Synopsis About this title Janick, Jules "synopsis" may belong to another edition of this title. Book Description : Activities surrounding plant germplasm have increased in recent years, and changes in federal programs have prompted questions from people within and outside the government. Buy New Learn more about this copy. Other Popular Editions of the Same Title.

Wiley, Hardcover. Search for all books with this author and title. Customers who bought this item also bought. Stock Image. Plant Breeding Reviews J. New Quantity Available: 4. Seller Rating:. Plant Breeding Reviews Janick J. Majestic Books London, ,, United Kingdom. New Quantity Available: 2. New Quantity Available: 1. Bookshub Karol Bagh, India. Reddy and coworkers reported no association between the density of wax crystals and susceptibility of Rosa taxa to black spot Diplocarpon rosae WolD. They evaluated a collection of species and cultivars, which had diverse wax crystal structure, and found no association among resistant and susceptible types, suggesting that factors other than wax crystal structure and density limited blackspot disease susceptibility.

It has been suggested that the cutin meshwork within the cuticle proper likely plays a more physical role, whereas the epi- and sub-cuticular waxes playa more chemical role, in inhibiting fungal penetration Kolattukudy ; Kolattukudy et al. There have been several reports that specific chemical constituents of epicuticular waxes inhibit the development of fungal pathogens. For instance, Yang and Ellingboe reported that the powdery mildew fungus Erysiphe graminis DC produced more malformed appressoria on leaves of barley mutants exhibiting altered wax crystallization patterns than on wildtype leaves.

Malformed appressoria also developed on recrystallized waxes from wildtype barley. Since these recrystallized waxes presumably had the same chemical composition as wildtype waxes in situ, but had different crystallization patterns, wax crystallization pattern alone was thought responsible for altered fungal development. However, E. Thus, important questions about the Yang and Ellingboe and Carver and Thomas studies still remain.

It is still unclear what effect wax removal had on the exact proportions of wax chemical constituents at the interface between the plant surface and fungal infection structures. Moreover, it is unclear whether the exact chemical profile of wax constituents was the same on recrystallized surfaces as wildtype barley leaves, or how the chemical profiles of wax constituents on mutants differed from wildtype. Without this information, it is difficult to distinguish whether wax crystal structure or chemical composition was the primary feature mediating E.

Erysiphe graminis germlings develop normally on the adaxial leaf surface of ryegrass Lolium spp. Adaxial surface waxes have plate-like crystalline patterns, whereas abaxial surface waxes have amorphous sheet morphologies. Interestingly, normal E. Since normal appressoria of these E. Chemical analysis of waxes on the abaxial and adaxial leaf surfaces showed that the abaxial leaf surfaces contained significantly higher levels of long-chain aldehydes, alkyl esters, and primary-alcohols than the adaxial surfaces. It has also been suggested that acidic wax constituents on plant surfaces may have antifungal activity.

For example, the unsaturated fatty acids, linoleic acid and linolenic acid extracted from waxes of rye Secale cereale L. Swingle exhibited fungistatic activity against the pathogen that causes withertip disease Martin Similarly, an acidic substance extracted from apple leaf wax using organic solvents was found to be toxic' to apple mildew Martin et al. There is also indirect evidence that fatty acids within epicuticular waxes may influence sorghum resistance to two fungal pathogens, E.

Jenks, P. Peters, and ]. Axtell, unpublished. Whether differential susceptibility was due to differences in these compounds is not clear, but using such mutants provides an excellent model system for assessing the role of wax fatty acids in plant fungal-susceptibility. Such studies are particularly suited for sorghum, since it is the only plant known to have fatty acids as the major leaf wax constituents Jenks et al.

While fatty acids appear to be antifungal, other long-chain wax constituents appear to stimulate fungal development. For example, 1,16hexadecanedial and 1,hexadecanediol induced appressorium development in the rice blast fungus Magnaporthe grisea Gilbert et al. By comparison, the CZ4 and longer carbon length primary alcohols from avocado Persea americana Mill.

Interestingly, non-host wax extracts with even greater amounts of these long-chain primary alcohols actually inhibited fungal development. These authors speculated that a balance between appressorium-inducing primary alcohols and the absence of inhibitors may serve as a trigger for germination. Many aromatic epicuticular wax constituents have also been implicated in host plant resistance to fungal pathogens. For instance, isomeric diols, ex and f3 isomers of 4,8,duvatriene-1,3-diol found in the chloroform-soluble leaf surface extracts from tobacco Nicotiana tabacum L. Moreover, N.

However, when these surface compounds were reapplied to these same leaves, rates of P. Since epicuticular waxes often do not recrystallize in the same patterns as seen in situ without special procedures Jeffree et al. Jenks, unpublished , wax chemistry and not wax crystallization pattern was likely playing the dominant role in determining tobacco plant susceptibility to these pathogens. It is not known whether pathogenic fungi secrete epicuticular-waxspecific degrading enzymes to facilitate host penetration.

However, it is known that fungal cutinase genes and their gene products can be induced by contact with constituents of plant cuticles Woloshuk and Kolattukudy ; Podila et al. Cutin, which exists as a polyester composed of primarily C16 and C18 fatty acids having hydroxyl groups in ro- and midchain positions, is a major constituent of the cuticle that underlies the epicuticular wax layer. The cuticle also contains significant amounts of waxes. Many cutinases have specificity for primary alcohol esters of the cutin polyester, however, chain-length specificity for ester constituents appears to vary widely with fungal genera Kolattukudy Further studies are needed to assay fungal secretions for their ability to degrade epicuticular wax constituents.

The chemical composition of epicuticular waxes is diverse, and it is difficult to determine what role particular wax constituents play in plant-pathogen interactions, and whether changing wax chemistry would be an effective way to enhance plant resistance. Such investigations would be facilitated by using mutation induction, backcross breeding, or recombinant DNA technology to develop near-isogenic lines with different epicuticular wax chemical profiles. The disease susceptibility of these lines could be subsequently compared. Another means by which epicuticular waxes may influence plant fungalpathogen susceptibility is by altering moisture levels on plant surfaces.

In order to germinate, most fungal spores except conidia like those of powdery mildews require free water, or relative humidities above 95 percent, for a finite period of time Blakeman The water-shedding properties of epicuticular waxes could therefore indirectly impede fun- 1. Raspberry Rubus spp. Potentially, plant surface waxes may serve as a sort of "raincoat" that sheds irrigation, precipitation, and condensed moisture, thereby improving plant resistance to fungal pathogens.

Wettability of a leaf surface is a function of water droplet contact angles, and contact angles are affected by the structure and chemical composition of leaf epicuticular waxes. For instance, large wax structures tend to hold water droplets above the leaf surface, creating large contact angles Holloway Likewise, hydrophobic wax constituents, such as alkanes, secondary alcohols, ketones, and esters, create high surface water contact angles and would shed water more efficiently than leaf surfaces covered with less hydrophobic compounds, and thereby indirectly decrease susceptibility to foliar fungal pathogens.

Whether differences in surface wettability in turn affected spore germination and pathogen development were not investigated. A variety of chemical constituents have been observed to diffuse from internal tissues to the plant surface. For example, simple sugars, amino acids, organic acids, growth regulators, vitamins, alkaloids, and phenols have all been found within water droplets on plant surfaces Blakeman In fact, competition for nutrients on leaf surfaces may limit spore germination of certain plant pathogens Blakeman As a barrier to water movement, epicuticular waxes also have the potential to influence the diffusion of nutrients, growth factors, and antifungal compounds phytoalexins to leaf surfaces, but this has not been examined in detail.

Mechanical removal of plant epicuticular waxes has been shown to influence fungal development. For example, Cruikshank demonstrated that formation of the penetration peg and hyphae development of Colletotrichum gloeosporioides were enhanced by removal of surface waxes from tomato Lycopersicon esculentum Mill fruit. This effect was thought due to increased diffusion of nutrients and plant substances that stimulated fungal metabolism Cruikshank Studies to measure the amount of fungi-active compounds that diffuse through plant 16 M.

ASHWORTH wax and cuticle layers of various thickness, especially those of genetically similar lines, may provide an effective means of elucidating what role these surface lipids play in determining the amount of plant leachates that actually diffuse onto plant surfaces. Presumably, the diffusion of pathogen-derived compounds into plant organs would also be affected by properties of epicuticular waxes.

Epicuticular Waxes and Phytophagous Insects As the interface between insect pests and potential hosts, epicuticular waxes play an important role in plant-insect interactions. Chapman and Bernays proposed that all phytophagous insects make some sensory examination of plant surfaces prior to feeding, and several thorough reviews on the effects of plant epicuticular waxes on insect feeding and behavior are available Woodhead and Chapman ; Juniper ; Eigenbrode and Espelie ; Eigenbrode The observations that chemical and physical properties of surface waxes can alter the interactions between insect pests and crop plants has important agricultural implications and has been an active area of research.

Feeding-related behaviors of many insects are inhibited on plant surfaces that have thick coatings of epicuticular wax crystals. For example, mature leaves of two Eucalyptus species E. These differences were apparently due to developmental differences in epicuticular wax coatings. The juvenile Eucalyptus leaves had a glaucous coating created by dense epicuticular wax crystals, whereas the adult leaves had a nonglaucous surface with reduced amounts of wax crystals. Experimental observations showed that beetles clung less effectively onto the glaucous juvenile leaves compared to the nonglaucous adult leaves.

Reduced clinging efficiency meant less time spent by the beetles feeding and ovipositing on juvenile Eucalyptus leaves. Setae on mustard beetle tarsi appear adapted for clinging to smooth plant surfaces, since wax particles tended to accumulate on the beetle's setae and inhibit insect movement. Likewise, studies by Mulroy suggested that the glaucous ecotypes of Dudleya britonii Johans.

Whether, in fact, properties of surface waxes were the basis for pyralid 1. Additional evidence for the role of epicuticular waxes in inhibiting insect feeding behaviors came from studies on cabbage and rape. In the same manner, mechanical removal of the waxy bloom from cabbage and rape increased feeding by flea beetles, P. Similarly, polishing or rinsing glaucous surfaces with solvents to remove wax deposits increased ovipositing on cabbage leaves by both diamondback moth Plutella xyJostella L.

Uematsu and Sakanoshita and the cabbage root fly Delia radicum L. Prokopy et al. In many plant-insect associations, it is a reduction in epicuticular wax crystals that impedes the selection of host plants by phytophagous insects. Likewise, the populations of cabbageworm larvae Pieris rapae L. When ovipositional nonpreference was removed as a factor by artificially infesting leaves with both cabbageworm larvae and eggs, the glossy lines were still more resistant than the normal glaucous lines to both insect species.

Similarly, glossy cabbage lines exhibited reduced survival of diamondback moth larvae when compared to the wildtype glaucous lines in the field Eigenbrode et al. Recent findings by Eigenbrode et al. Specifically, three predator species, Chrysopa bicarnea Banks , Hippodamia convergens Guerrin-Menneville , and Orius insidiosus Say , were more mobile and more effective predators of diamondback moth larvae on glossy cabbage wax mutants than on wild-type varieties Eigenbrode et al. Glossy and glaucous lines were equally susceptible to diamondback moth in greenhouse studies where entomophagous insects were not present.

This observation contributed support to the idea that surface waxes affected these tritrophic interactions. In like manner, glossy lines of pea Pisum sativum L. Similar aphid numbers on glossy and 18 M.

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ASHWORTH glaucous lines in controlled cage studies contribute to evidence that lack of surface wax crystals may cause increased predation by certain generalist predators. Investigations in sorghum noted that bloomless and sparse bloom mutants were more resistant in the field to the greenbug aphid Schizaphis graminum Rondani than wildtype.

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Horticultural Reviews presents state-of-the-art reviews on topics in horticultural science and technology covering both basic and applied research. Topics. View Table of Contents for Horticultural Reviews. Horticultural Reviews, Volume Editor(s). Jules Janick permissions · xml. CHAPTER 2.

As in previous studies, this difference did not exist in field cages Starks and Weibel ; Weibel and Starks Thus, a combination of observations indicate that epicuticular waxes may influence insect pest resistance in the field by affecting the mobility and adherence of phytophagous and entomophagous insects on plant surfaces. In addition to crystallization patterns, insects often use the chemical compositions of epicuticular waxes as cues for host-plant selection.

For example, many insects prefer to feed on artificial mediums impregnated with epicuticular waxes from their host plant rather than comparable artificial mediums in which the host wax extracts were omitted. In addition, insects presumably avoid feeding on mediums containing plant waxes from a non-host Woodhead and Padgham ; Braker and Chazdon Thus, a common research objective has been to identify individual wax constituents that, in situ, serve as primary cues for host-plant selection.

Long-chain alipathic epicuticular wax constituents have been shown to influence plant-insect interactions. For example, primary alcohols and free fatty acids will stimulate feeding or ovipositing behaviors of aphids Greenway et al. In whole plant studies, leafcutter bees Megachile sp. In addition to lacking wax crystals on the adaxial leaf surface, the glossy ecotype had a 6-fold reduction in the relative content of triacontanol C30 primary alcohol in its surface waxes. There are other reports of long-chain alipathic epicuticular waxes affecting insect feeding preferences.

For example, third instar locust Locusta migratoria L. It was proposed that gradual increases in the relative hydrocarbon abundance and hydrocarbon chain-length distribution with increasing age in sorghum leaf waxes explain, at least in part, the increased deterrence exhibited by young plants. By comparison, wax alkanes and esters on sorghum apparently deterred feeding by L. Moreover, greater proportions of wax alkanes induced more intense searching behavior on leaves of maize by fall armyworm Spodoptera jrugiperda J.

Smith 1. Bergman et al. Interestingly, other evidence has arisen to implicate short-chain alcohols in plant insect-resistance. Greater resistance in tobacco to the tobacco budworm Heliothis viresens Fabricius was associated with higher levels of docosanol C 22 primary alcohol Johnson and Severson Thus, the long-chain hydrocarbons clearly play an important role in host-plant selection by phytophagous insects, but the mechanism is unclear.

In addition to the hydrocarbon fractions, there are reports that aromatic constituents also affect insect selection of plant hosts. Increased amounts of a- and p-amyrins in the surface waxes of Rhododendron species were correlated with resistance to the azalea lace bug Stephanitis pyrioides Scott Balsdon et al. Likewise, amyrins in grasses were shown to be deterrents to Locusta migratoria Bernays and Chapman By comparison, free and esterified triterpenols increased aphid resistance in sorghum when present at high levels Heupel Prophenylbenzenes, coumarins, and a polyacetylene in leaf epicuticular waxes of carrot Daucus carota L.

Mixtures of wax constituents have been shown to act synergistically in affecting insect behavior SHidler and Buser ; Spencer Aromatic components of epicuticular wax vary widely, and can be extremely diverse on plant species, being found in only trace amounts on some plants and as dominant constituents on others, and it is likely that many of these compounds will affect plantinsect interactions. Epicuticular Waxes and Drought Epicuticular waxes play an important role in plant-water relations.

The chenlical constituents of epicuticular waxes are thought to create a continuous hydrophobic water barrier, which impedes water loss from plant organs. The presence of wax structures can also create a still-air 20 M. As with wax hydrophobicity, an enhanced boundary layer would likely reduce the rate of transpiration from plant tissues.

In addition, epicuticular waxes on some plants reflect light, decreasing the radiation heat load, and thus reducing the transpiration rate. Therefore, the mechanisms by which surface waxes affect water loss can be varied, involving both chemical and physical methods. Effect of Waxes on Plant Water Loss. Reduced amounts of epicuticular wax on plant surfaces have been shown to be associated with increased rates of transpiration. Brushing waxes off the excised leaves significantly increased the rate of water loss Hall and Jones Similarly, leaves of rice O.

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Taxon identifiers. Rice Production Worldwide. Also, the C3l homologues are the major alkanes on arabidopsis leaves, whereas the C29 homologues are the predominant stem wax alkanes. The white waxy coating on this succulent plant reflects about 60 percent of the incident light, and thus reduces the amount of light that reaches the interior of the leaf. Kandiannan, N. And in Vietnam , the blossoming peach flower is the signal of spring.

Excised wheat leaves from nonglaucous lines had 28 percent less epicuticular wax and 33 percent higher water loss rates than genetically similar glaucous lines Clarke and Richards ; Johnson et a1. The amount of epicuticular wax on various sorghum cultivars was negatively correlated with excised-leaf water loss rates when wax loads were between 0. However, these transpiration rates did not decrease significantly as wax loads increased above 0.

Drought-stressed plants generally have greater amounts of epicuticular wax per unit leaf area than non-stressed plants.

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However, the increased amount of wax is not always associated with greater plant resistance to water loss. For example, crested wheatgrass Agropyron desertorum Willd. Jefferson et a1. As might be expected, the drought-stressed sorghum plants had reduced stomatal conductance to water vapor, compared to respective irrigated plants Blum ; Johnson et a1.

Somewhat surprisingly, however, the relationship between stomatal conductance and wax content was not observed in crested wheatgrass, even though the amount of wax was significantly higher in the stressed plants Jefferson et a1. Similarly, other studies showed no correlation 1.

For example, the amounts of leaf wax were not related to either epidermal Araus et a1. Bengston et a1. The absence of a correlation between the amounts of epicuticular wax and water loss in these studies may have. While such an explanation is plausible, it does not explain why drought-stressed plants often accumulate additional epicuticular wax.

Water loss through plant cuticles may be more complicated than simple diffusion through a waxy layer. For instance, water flow through the cuticle layers may be directed through the preferred polar pathways described by Schonherr a. Theoretically, plant cuticles could differ greatly in the number of these polar pathways. As water is a polar molecule, more and larger polar pathways in a cuticle would presumably lead to greater amounts and rates of water loss.

Studies using isolated cuticular membranes suggested that subcuticular waxes, and not epicuticular waxes, played the major role in reducing cuticular water permeability Schonherr a, b. Individual chemical constituents and mixtures of constituents that compose the plant epicuticular wax layer likely differentially affect epidermal conductance to water vapor. For example, laboratory studies using plastic membranes coated with either grape epicuticular waxes or selected classes of wax components at Jlg cm-2 found that the hydrocarbon, alcohol, and aldehyde fractions effectively limited water transport through the artificial membranes.

By contrast, fatty acids restricted water transport only slightly, whereas wax triterpenoid conjugates had no effect Grncarevic and Radler Such results are not surprising since fatty acids and terpenoids are much less hydrophobic than those other wax constituents. Thus, increasing amounts of hydrophobic components within the surface waxes should lower epidermal conductance to water vapor.

In addition to producing greater amounts of wax, plants also have been found to alter the chemical compositions of their epicuticular waxes in response to water deficits. In cotton, the leaves, bracts, and bolls produced longer-chain-length epicuticular wax alkanes in drought-stressed plants compared to irrigated plants Bondada et a1.

Longer-chain alkanes are more hydrophobic and their induction by drought suggests their importance in reducing water loss during periods of water shortage. In addition to their assumed role as a hydrophobic barrier, evidence suggests that some epicuticular wax structures may reflect significant amounts of solar radiation and thereby reduce the radiation heat load and water use via transpiration. For example, Clarke and Richards determined that among seven genetically similar populations of wheat, which had similar levels of wax but differed in surface glaucousness based on visual ratings , the glaucous lines had, on average, 10 percent lower rates of excised-leaf water loss compared with the nonglaucous lines.

Thus, it is suggested that increased reflectance apparently reduced leaf water loss. However, this result is not definitive, since potential differences in wax fine structure and chemistry among these populations may have reduced water loss through other means. Moreover, Johnson et a1. Other evidence for a role of wax reflectance in reducing water loss comes from observations of desert plants. Plant species adapted to arid environments, in general, have a whitish surface and reflect more radiation than do mesophytic plants Gates et a1.

Epicuticular wax crystals that protrude above the plant surface may increase the thickness of the still-air boundary layer. This can reduce transpiration by increasing surface resistance to diffusion of water vapor. For example, the lower excised-leaf water loss rates in glaucous wheat may be due to the greater surface boundary layer created by the protruding wax crystals. Potentially, these wheat lines could provide a model system for dissecting the role of wax "induced" boundary layers in plant water loss.

Another excellent model plant system for studying the effect of leaf wax on boundary layers may be the near-isogenic wax mutants of sorghum. The epicuticular wax layer over the wildtype abaxial sheath surfaces of sorghum can reach 2 mm in thickness, while the bJoomless and sparse-bloom mutants have wax layers whose thicknesses are reduced in a continuum down to a glossy surface. Leaf waxes may also directly affect stomatal conductance via effects on boundary layers. Jeffree et a1.

Presumably, waxes impeded the diffusion of water vapor within the stomatal antechamber. A similar situation may also occur in sorghum. Scanning electron micrographs show that waxes can occlude the stomatal pore McWhorter et a1. The observation by Chatterton et a1. Whether, in fact, these differences were due primarily to wax effects on stomatal conductance, or epidermal conductance through the cuticle, still needs to be established. Epicuticular Waxes and Freezing Temperatures There are two ways in which epicuticular waxes may facilitate plant survival at sub-zero temperatures.

One way is by reducing winter desiccation in evergreen species. The leaves of evergreen species are subjected to desiccation stress during the winter Sakai ; Tranquillini During cold periods, water within the soil, stems, and branches is often frozen and thus the uptake of water to replenish water lost via transpiration is prevented. Since the path of water loss in both cases is primarily through the cuticle, it would seem likely that leaves having well-developed layers of epicuticular wax may have an adaptive advantage in these conditions.

Herrick and Friedland observed that red spruce Picea ruben Sarg. Although the reason for the lowered cuticular resistance was not determined in this study, a well-developed epicuticular wax layer could directly impede water loss, or indirectly reduce cuticular transpiration by reducing the absorption of incident solar radiation and lowering leaf temperature. The presence of epicuticular wax has also been postulated to affect frost damage. Thomas and Barber a observed that glaucousness of leaf surfaces was more prevalent in Eucalyptus urnigera Hook. They observed that glaucous leaves shed water more effectively than comparable non-glaucous leaves, and that dry leaves would supercool to much lower temperatures prior to freezing.

Other 24 M. ASHWORTH studies have also noted that leaves with surface moisture freeze at warmer temperatures than comparable leaves having a dry exterior surface Ashworth The most likely explanation is that water first freezes on the leaf surface, and subsequently inoculates freezing internally. Therefore, a well-developed layer of epicuticular wax would both shed surface water and impede ice propagation into subtending tissues.

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However, while this appears to be an attractive hypothesis, it is neither clear how surface moisture affects leaf freezing, nor how external ice crystals trigger freezing within the leaf. There have been no other reports linking leaf glaucousness and frost susceptibility, despite the diversity of wax levels present in some species. Epicuticular Waxes and Solar Radiation It is a common observation that plants growing in environments with high levels of solar radiation, as occurs in desert and alpine regions, often have leaf surface features that reflect light Billings and Morris ; Gates et at One such feature is the presence of a thick layer of epicuticular wax.

Plants growing in high solar radiation environments are often glaucous, and the presence of epicuticular wax crystals on leaf surfaces attenuates light exposure to the subtending tissues. For example, several investigators have demonstrated that plants having thick layers of epicuticular wax can reflect between 20 and 80 percent of the incoming radiation, whereas non-glaucous plants typically reflect less than 10 percent Thomas and Barber b; Clark and Lister ; Reicosky and Hanover ; Mulroy ; Vogelmann The reflective character of glaucous leaves was reduced to the level of the non-glaucous leaves by treatments that removed the epicuticular waxes Thomas and Barber b; Clark and Lister ; Reicosky and Hanover ; Mulroy The size, distribution, and orientation of wax crystals, and other surface features, determines the extent to which light is scattered at the tissue surface Barnes and Cardoso-Vilhena Generally, radiation is scattered across the spectrum, and increased reflectance of ultraviolet UV , visible, and infrared radiation has been observed Thomas and Barber b; Clark and Lister ; Reicosky and Hanover ; Mulroy ; Vogelmann ; Barnes and Cardoso-Vilhena ; and others.

However, in some plant species there is preferential scattering of shorter wavelength radiation. For example, leaves of blue spruce Picea pungens Engelm. The bluish appearance of these leaves, and the enhanced reflection of UV radiation was shown 1. Dense deposits of epicuticular wax apparently provide an adaptive advantage to plants growing in high light environments. Much of the research to this point has focused on whether the presence of epicuticular wax deposits would reduce exposure levels of damaging UV radiation.

UV irradiance increases at higher elevations, and plants that grow at higher elevations often have thick layers of epicuticular wax Billings and Morris ; Gates et a1. Measurements of UV penetration into tissues have shown that species vary in their ability to screen out UV-B radiation, and that most of the UV radiation is attenuated in the cuticle and epidermal cell layer Bornman and Vogelmann ; Day et a1. Most of the attenuating effect of the cuticle and epidermal layer is due to the presence of flavonoids and related phenolic compounds that absorb UV-B Vogelmann ; Barnes and Cardoso-Vilhena ; Krauss et a1.

Epicuticular waxes from most species do not absorb significant amounts of radiation in this portion of the spectrum. However, epicuticular waxes on some species may provide protection by scattering and reflecting incoming UV radiation, and thus reducing exposure levels in underlying tissues Clark and Lister ; Mulroy ; Vogelmann ; Grant et a1. Epicuticular wax deposits can also act as a photoprotectant in the visible portion of the spectrum. Robinson and co-workers , found that removal of reflective surface waxes from Cotyledon orbiculata L.

The white waxy coating on this succulent plant reflects about 60 percent of the incident light, and thus reduces the amount of light that reaches the interior of the leaf. A third way in which epicuticular waxes may provide an adaptive advantage in high radiation environments is by reflecting a portion of the incident solar radiation and reducing the absorbance of visible and infrared radiation. This, in turn, should lead to cooler leaf temperatures, and thus reduced transpiration rates Reicosky and Hanover ; Mulroy ; Barnes and Cardoso-Vilhena Consistent with this hypothesis are reports indicating that plants with well-developed layers of epicuticular wax have lower leaf and canopy temperatures, reduced rates of transpiration, and improved water status relative to comparable controls Johnson et a1.

While the presence of epicuticular wax apparently affects the exposure of underlying tissues to solar radiation, it is interesting to note that the 26 M. Such indirect evidence also supports the hypothesis that epicuticular waxes may provide an adaptive advantage in high solar radiation environments. Epicuticular Waxes and Agricultural Sprays A diverse array of agricultural chemicals are sprayed onto horticultural and agronomic plants for a variety of reasons, including insect, pathogen, and weed control, foliar fertilization, and growth regulation.

Epicuticular waxes are a significant barrier to both spray retention and subsequent penetration into plant organs. Nearly all plant surface waxes are hydrophobic and thus tend to repel water-based liquid sprays. For that reason, most agricultural sprays are formulated as either oil-based solutions, or adjuvants like wetting agents, spreaders, or stickers are added to facilitate spray droplet retention, distribution, and penetration into the plant surface.

While permeability of the cuticular membrane to agricultural chemicals has been examined in numerous studies, fewer studies have examined the specific role of epicuticular waxes on surface retention, permeability, and sorption of these sprays. Epicuticular Waxes and Surface Retention. It has been demonstrated that the more polar the surface wax components, the lower the contact angle of water droplets on the plant surface, and that this generally correlates with better spray retention Holloway Similarly, in peach Prunus persica L.

Batsch 'Red Haven' , increasing amounts of alkanes, esters, and total surface waxes during leaf development correlated with a decrease in wettability as measured by increased droplet contact angles Bukovac et a!. Formulating sprays and their adjuvants to improve their interaction with hydrophobic surface waxes has been used commercially to enhance spray retention.

Future approaches might involve crop improvement strategies to alter surface waxes and enhance the retention of pesticides. Alternatively, reducing herbicide retention on the surface of crop plants might also be useful. Epicuticular waxes could be made thicker or more hydrophobic using genetic approaches such that herbicides could be applied at greater rates in field cropping systems without damaging the crop. Epicuticular Waxes and Surface Penetration. As an external barrier, epicuticular waxes often impede the penetration of agricultural chemicals into the interior of leaves and other organs.

For example, several 1. A negative correlation was also reported between foliar penetration of the 14C-radiolabeled growth regulator, napthaleneacetic acid NAA , and total surface wax levels per leaf area during leaf expansion Bukovac et a1. In other studies, Santier and Chamel demonstrated that glyphosate herbicide penetration through tomato cuticles was greater in organs that had less cuticular wax. Greater amounts of epicuticular waxes reduced leaf phytotoxicity to surfactants, likely due to reduced penetration of the surfactant through the leaf cuticle Knoche et a1.

The more wax crystals on the surface of plant leaves, the more tortuous the pathway that agricultural chemicals must traverse before entering epidermal cells Riederer and Schreiber In situations where the amount of surface wax controls penetration of spray materials, partial removal, disruption, or increased diffusion through plant surface wax layers would likely increase permeability of agricultural chemicals. Interestingly, the thinnest areas of the plant surface waxy barrier may be found associated with the stomatal pore and substomatal chamber; however, whether this general situation is true for most plants has not been verified.

If so, spray adjuvants targeted to the stomatal cuticular boundaries might be effective. While many studies have demonstrated a strong negative correlation between the amount of epicuticular wax and permeability of agricultural chemicals, some studies have reported little or no such correlation. For example, Baker and Hunt could not demonstrate a clear correlation between foliar penetration of the 14C-radiolabeled growth regulator NAA and the total amounts of wax per leaf surface area. Norris similarly found no correlation between the amount of cuticular waxes and the permeability of 2,4-D through the cuticles often different plant species.

Leaf surfaces of tomato had higher permeabilities than leaf surfaces of pepper, even though the tomato leaves had much higher proportions of wax associated with their cuticles Chame While it has been assumed that the amount of epicuticular and subcuticular waxes plays a major role in limiting the conductance of agricultural chemicals through plant cuticular layers, instances where no correlation between the amount of cuticular waxes and cuticular permeability suggest that wax thickness alone does not fully explain the physicochemical basis for permeability.

ASHWORTH Possible explanations for the lack of correlation between wax amounts and spray penetration may be that differences in the specific chemical constituents of both the plant cuticular waxes and agricultural sprays influence the permeability of plant surfaces. For example, increasing amounts of alkanes and esters during leaf development in peach correlated with a decrease in permeability of 14C-radiolabeled NAA Bukovac et a1. These differences may be due to reduced wettability of the surface or to reduced penetration through the cuticular wax layers. By comparison, the rate of chemical penetration into leaves was affected by the lipophilicity of spray formulations; that is, cuticular permeability tended to increase in the order of increasing lipophilicity of constituents in the spray Schreiber and Schonherr b; Schonherr and Baur One fate of agricultural chemicals applied to plants is their sorption Le.

Charnel et a1. The epicuticular waxes themselves have low sorption capacity for most agricultural chemicals; nevertheless, it is unclear why removal of waxes from the cuticle membrane leads to increased sorption of these chemicals to the cuticle matrix Bukovac et a1. Charnel et al. Still, there is no direct evidence supporting this hypothesis.

Epicuticular Waxes and Air Pollutants Since epicuticular waxes are present at the interface between plants and the atmosphere, it is not surprising that both the effects of atmospheric pollutants on epicuticular waxes, and the role of these waxes in tolerance to such pollutants have been investigated. The effects of gaseous pollutants, acid precipitation, and atmospheric deposition on epicuticular waxes has been reviewed by Turunen and Huttunen and Percy et al. A common symptom observed on plants growing in polluted environments is the degradation of surface wax crystals and their accelerated aging.

In addition, accompanying changes in chemical 1. Although the rates at which epicuticular waxes degrade in polluted atmospheres is well correlated with the level of air pollution, it is not clear as to which gaseous pollutant s are responsible for symptom development, and the mechanisms involved Turunen and Huttunen Exposure to elevated levels of ozone altered epicuticular waxes on conifer needles as measured by altered contact angles increased wettability in Norway spruce Picea abies L. Barnes and Brown , and changes in needle wettabilty, wax chemical composition, and reduced rates of wax synthesis in red spruce Percy et a1.

Such responses are likely dose dependent, as fumigation of Norway spruce with ozone at levels below those used in the previously mentioned investigations, but well above ambient concentrations, did not alter wax structure, surface wettability, or total wax levels Dixon et a1. Interestingly, fumigation of in vitro recrystallized epicuticular waxes from Norway spruce, which are composed of nonacosanol, had no effect on wax crystal structure or chemical composition Jetter et a1.

Epicuticular waxes are also affected by S02 and N0 2 exposure. Symptoms associated with exposure to elevated levels of these gaseous pollutants either singularly or in combination include altered wax crystal morphology Karhu and Huttunen , enhanced erosion of epicuticular waxes Huttunen and Laine ; Riding and Percy ; Crossley and Fowler ; Sauter and Voss ; Tuomisto , reduced wax deposition Riding and Percy , and decreased wettability of needles Cape How exposure to the atmospheric pollutants causes such changes is unknown.

Direct exposure of recrystallized epicuticular waxes to S02 changed neither the chemical composition nor the morphology of wax crystals, indicating that direct interaction between the pollutant and epicuticular waxes is not the cause for the accelerated erosion observed on plant surfaces Jetter et a1. However, exposure to lower concentration of N0 2 0. Since the concentration of N02 used in these in vitro fumigation studies were well above ambient concentrations reported at polluted sites, Jetter and co-workers concluded that, as with S02' the accelerated erosion of epicuticular waxes was not due 30 M.

ASHWORTH to a direct chemical interaction between the gaseous pollutant and epicuticular waxes, and suggest that the degradation of epicuticular waxes must be a secondary effect of tissue interaction with these pollutants. Acidic rain and fog can also affect epicuticular waxes. This has been well documented in conifer species, where a reduction in the hydrophobicity of needle surfaces and an erosion of crystalline wax structures have been reported at both polluted sites, and in response to simulated acid rain treatments Cape ; Huttunen and Laine ; Percy and Baker ; Barnes and Brown ; Turunen and Huttunen , ; Percy et al.

Simulated acid rain and acidic fog treatments increased the wettability of leaf surfaces, as measured by decreased water droplet contact angle in several conifer species, including Norway spruce Barnes and Brown , red spruce Percy et al. Cape ; Turunen et al. A similar effect has also been noted in several crop species, including rape, bean Phaseolus vulgaris L. Percy and Baker , Although the reason for the decreased hydrophobicity of leaf surfaces has yet to be resolved, it could occur in response to either changes in surface structural features, altered wax chemical composition, or a combination of both factors.

All of these possibilities seem likely, as there is evidence that both the crystalline structure of epicuticular waxes and their chemical composition change in response to simulated acid rain. Epicuticular wax crystals typically weather and degrade to form a more amorphous layer of wax as tissues age, and numerous investigators have reported that this erosion of wax crystalline structures is accelerated in response to acidic precipitation treatments Cape ; Huttunen and Laine ; Percy and Baker ; Turunen and Huttunen , ; Percy et al.

In addition, changes in wax chemical composition in response to simulated acid rain have been reported to occur in several species Percy and Baker , How acid rain exposure causes a change in wax crystalline structure, and whether the structural change is linked to changes in wax chemical composition is unclear.

Changes in wax structure as a result of chemical interactions between the acidic precipitation and epicuticular wax crystals seems unlikely Riederer ; Percy et al. In addition, the immersion of recrystallized waxes into sulfuric acid and nitric acid 'mixtures pH 3 to simulate acid deposition had no effect on wax crystalline structure Percy et al.

As an alternative, Percy et al. They note that enzymes in the wax biosyn- 1. These authors also note that exposure to simulated acid precipitation can affect both the rates of wax biosynthesis and the chemical composition Percy and Baker ,; Percy et al. Although this is an attractive hypothesis, it has not yet been demonstrated that cellular pH levels change to that extent in response to acid rain, or that such changes would lead to altered wax biosynthesis.

The well-documented effects of air pollutants and acid rain on epicuticular waxes have been linked to reduced plant growth and forest decline Turunen and Huttunen The erosion of epicuticular waxes and the increase in leaf wettabilty would increase the time that water remains on leaf surfaces and facilitate the leaching of mineral nutrients.

Erosion of the surface waxes and those associated with stomata may also lead to increased rates of transpiration and predispose tissues to pathogen infection. Therefore, while changes in epicuticular waxes may be one of the first symptoms of air pollution damage, they may also indicate a mechanism by which gaseous pollutants damage plant tissues. In the following discussion, epicuticular wax production is divided into two general categories including epicuticular wax biosynthesis and epicuticular wax secretion. Epicuticular wax biosynthesis refers primarily to the enzymatic steps and biochemical regulation of wax production.

Secretion refers primarily to the physical pathway and transport processes used to move molecules from within epidermal cells to the plant surface, where they are deposited as epicuticular wax. Fatty acids, aldehydes, primary alcohols, and esters are the primary products of the acyl elongation reduction pathway, whereas fatty acids, aldehydes, M.

In many plants, the acyl elongation-decarbonylation pathway is extended to synthesize secondary alcohols and ketones via enzymatic hydroxylation and oxidation reactions, respectively. The other important group of plant Activated Fatty Acids Cuticle Synthesis bm2 bm6, bm7,g, waxl C37 cerl Fig. Model pathway describing biochemical reactions in plant epicuticular wax production by leaves and putative sites for genetic lesions. Designations of bm represent the bloomless mutants of sorghum, whereas h represents the sparse-bloom mutants of sorghum Rich , the cer-j59 mutant is an eceriferum mutant of barley Avato et al.

The superscript S indicates a stem-specific arabidopsis mutation Jenks et a1. Jenks, unpublished. The waxt and beft are also leaf epicuticular wax mutants in arabidopsis Jenks et al. The glt, g17, gI8, and gl18 mutations in maize are not shown but were thought to inhibit the production of Ct6 and Ct8 fatty acid wax precursors Bianchi et a1. The following section will focus on the more prevalent acyl elongation-reduction and the acyl elongation-decarbonylation pathways and their associated reactions.

Plant epicuticular wax hydrocarbons arise from a pool of Ct6 and Ct8 free fatty acids synthesized within plastids Post-Beittenmiller Other studies suggest that cytoplasmic membranes are also sites for synthesis of the Ct6 and Ct8 acid precursors of epicuticular wax Lessire et a1. The Ct6 and Ct8 fatty acids arise by activity of the enzyme complex, fatty acid synthetase, which coordinates malonyl-acyl carrier protein's ACP sequential donation of seven C2 acyl units to an initial primer, acetyl-Coenzyme A CoA , to produce the Ct6 fatty acid palmitoyl-ACP. Once synthesized, these precursors can be used as substrates in the synthesis of a variety of important plant compounds, including phospholipids, storage lipids, cutin, suberin, and epicuticular waxes.

The Ct8 precursors destined for modification into epicuticular waxes undergo further elongation reactions that create chain lengths up to 36 carbons long Fig. In the past, it was presumed that Ct6 and Ct8 fatty acid precursors of epicuticular waxes were being further elongated by several chain-length-specific acyl-CoA elongases, since certain mutations and chemical inhibitors appeared to suppress production of wax homologues longer than a certain length. For example, the cer-j59 mutant of barley apparently inhibited the putative C24 acyl-CoA elongase, since wax homologues longer than 24 carbons were greatly reduced Avato et a1.

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The bm4 mutation in sorghum Rich , the cer2 and cer6 mutations in arabidopsis Jenks et a1. Elongation of the C28 constituents appeared to be inhibited in the cer19 mutant in arabidopsis M. Jenks, unpublished , the g13 mutant in maize Bianchi et a1. Moreover, the g12, g14 and gliB mutations in maize Bianchi et a1. Although research by Bessoule et a1.

Thus, whether separate elongases exist for each two-carbon addition to growing acyl chains longer than 20 carbonases has not been proven. Furthermore, thioesterases associated with fatty acid synthase were shown to govern the chain length distribution for synthesis of the 8 to 20 carbon length fatty acids Voelker et a1. Thus, a single elongase complex could be responsible for synthesis of all acyl-CoA chains longer than 20 carbons through the mediating activity of thioesterases and other elongase-associated enzymes.

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In the mutation and inhibitor studies mentioned above, evidence for suppression of single elongation steps could be explained by direct effects on enzymes mediating the activity of a single elongase, rather than inhibition of one of several elongases directly. For these reasons, it is still unclear whether elongation of the very long chain acyl-CoAs in epicuticular wax biosynthesis is performed by one or more elongases.

Previous research suggested that fatty acyl chains were released from elongase complex es by either fatty acyl-CoA thioester hydrolysis to fatty acids and CoA, or by fatty acyl-CoA reduction to aldehydes. Acyl-CoA thioesterase activities have been reported in plants Ohlrogge et a1. Specifically, Liu and Post-Beittenmiller isolated an epidermally-expressed acyl carrier protein thioesterase with high specificity for stearoyl-CoA C 18 substrates. Moreover, Pollard et a1. Although only acyl-ACP thioesterases have been isolated to date, it is possible that additional thioentrases may cleave acyl-CoA and thereby play an important role in determining chain lengths of products released from this pathway.

Alterations in epicuticular wax chemical profiles among certain mutant lines may be explained by mutations affecting thioesterases or associated reactions. It has been proposed that low amounts of fatty acids and aldehydes and longer wax chainlengths on cer3, cer7, ceri0, and cer13 mutants in arabidopsis may be due to inhibited release of acyl chains from elongation compartments Fig. The h9 mutant of sorghum Rich and the g14 mutant of cabbage Macey and Barber b had similarly increased chain lengths among epicuticular wax constituents.

Potentially, the longer chain length constituents on these mutants could have resulted from suppressed thioesterase activity. Reductases and Decarbonylases.

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The conversion of fatty acyl-CoA to aldehydes is thought to be catalyzed by a putative microsomal fatty acyl-CoA reductase that lacks chain-length specificity Kolattukudy Recently, two separate acyl-CoA elongases were solubilized from pea Vioque and Kolattukudy ; Kolattukudy , one that apparently generates primary alcohols and another that generates aldehydes from the acyl-CoA precursors. These results suggest that the model in Fig. In the next metabolic step, aldehydes can be converted to either primary alcohols or alkanes. A microsomal aldehyde reductase that lacks chain length specificity may produce primary alcohols from aldehydes Kolattukudy or alternately via a two-step reduction from the acyl-CoA, Aldehydes may also be converted to alkanes by an aldehyde decarbonylase that, like the reductases, appears to lack chain-length specificity Cheesbrough and Kolattukudy Heavy sucrose gradient fractions containing cell wall and cuticle fragments were capable of enzymatic decarbonylation of aldehydes with chain lengths of Ct6 to C32 , while fractions lacking wall and cuticle fragments lacked decarbonylation activity Cheesbrough and Kolattukudy These findings suggest that aldehydes produced in intra-cytoplasmic membranes of epidermal cells were likely converted to alkanes by a decarbonylase enzyme located in the cell wall or cuticle region.

Oxidases and Transacylases. In many plant species, secondary alco- hols and ketones constitute a significant portion of epicuticular waxes. For instance, the stems of arabidopsis have a C29 ketone and C29 secondary alcohol as the second and third most abundant constituents, with the C29 alkane being the first Hannoufa et a1. Kolattukudy et al. These alcohols are then converted to the corresponding ketone by an oxidase. However, genes or the enzymes responsible for these oxidative reactions have yet to be isolated. Esters likely arise from esterification of primary alcohols and fatty acylCoA by a acyl-CoA-fatty alcohol transacylase, whose activity was detected in microsomal fractions Kolattukudy This transacylase may have 36 M.

Nevertheless, although an acyl-CoA:fatty alcohol acyltransferase involved in the synthesis of liquid seed storage wax esters has been isolated from jojoba Simmondsia chinensis Link seeds Shockey et al, , neither genes nor enzymes responsible for esterification reactions in the synthesis of crystalline epicuticular wax esters have yet been isolated. An important question concerning epicuticular wax biosynthetic pathways in plants is whether multiple biosynthetic pathways exist within the same plant. Potentially, more than one pathway might exist in the same tissue. Whether multiple acyl elongation pathways exist in single organs has also not been ruled out.

Other studies suggest that multiple pathways could exist in different organs of the same plant. For example, many plants have distinct epicuticular wax composition on various tissues Table 1. Also, studies using glossy mutants in maize suggest that two independent epicuticular wax enzymatic systems, designated EDI and EDII elongation-decarboxylation systems I and II , are predominantly involved in seedling and adult leaf epicuticular wax biosynthesis, respectively Bianchi et al.

Furthermore, many wax mutations in sorghum were specific to individual tissue types Jenks et al. Interestingly, even different epidermal cell types may have separate epicuticular wax biosynthetic pathways. The bm3 and bmll mutations in sorghum inhibited wax production from epidermal cork cells but not epidermal long cells, whereas most other wax mutations affected both cork cell and long cells. Wildtype maize has wax crystals over the entire seedling leaf epidermal surface, except that wax crystals are not present over guard cells.

It was surprising then to discover that the gIl mutation in maize seedlings inhibited wax production on all epidermal long cells except stomatal accessory cells, which exhibited near-normal wax crystals Lorenzoni and Salamini Thus, the GIl gene product may not be as highly expressed in accessory cells as long cells. While such ideas are intriguing, it is still yet to be determined whether multiple wax pathways could exist in single organs, whether wax biosynthetic pathways could be organ- or cell-specific, or whether the findings discussed above are better explained by complicated regulation of a single wax pathway.

By comparison, gi15 in maize affected the developmental transition from juvenile to adult leaf epicuticular wax biosynthesis and the recently sequenced GL15 gene has sequence homology to the arabidopsis floral-development regulatory APETALA2 gene family Moose and Sisco Arabidopsis cer2 inhibits stem specific C26 elongation but does not affect leaf epicuticular waxes Jenks et a1.

Likewise, the cer7 and cer13 mutations in arabidopsis may affect wax chain length in a stem-specific manner Jenks et a1. Genes that dramatically reduce the total amount of epicuticular waxes when mutated are thought to inhibit major regulatory or metabolic functions of early substrate conversion in the wax pathway.

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For example, cerl and cer16 in arabidopsis Jenks et a1. The bm2 mutation also reduced the deposition, and altered the ultrastructure of the cuticle Jenks et a1. These mutants likely affect genes that playa role in important early steps in wax biosynthesis. Recently, a new class of epicuticular wax mutants was isolated from aT-DNA mutagenized population of arabidopsis and designated wax, bcf bicentifoJia , and knb knobhead Jenks et a1.

These mutants exhibited plieotropic effects on surface wax chemistry, wax crystallization pattern, and leaf cell morphology.

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Interestingly, tissues on the waxl mutant fused together very early in organ development similar to the fdhl fiddleheadl mutant in arabidopsis Lolle et a1. The knb mutants are similar to a class of sorghum wrinkled-leaf epicuticular wax mutants identified in an chemically mutagenized population M. Conversely, altered cell wall morphology could simply alter secretion of epicuticular wax precursors from the epidermal cytoplasm to the plant surface.

Regardless, this class of mutants should provide opportunities for investigating wax biosynthesis and secretion. Epicuticular Wax Secretion Previous studies have shown that the basic mechanisms controlling cell secretion processes are similar in cells as divergent as yeasts, plants, and mammalian brain neurons Moore et al. Thus, secretion of epicuticular waxes likely involves many of these same highly conserved mechanisms. Presumably, surface wax precursors are first synthesized in the cytoplasm of epidermal cells and then secreted in sequence through the cytoplasm, apical plasmalemma, secondary cell wall, primary cell wall, pectic layer, and cuticle Fig.

Early Studies of Epicuticular Wax Secretion. In , Malpighi reported using a compound light microscope to identify the outermost plant epidermal layer see Hallam The first microscopic description of epicuticular wax deposits may have been Brongniarts's description of a granular morphology on certain plant surfaces. Von Mohl used the light microscope to describe two cuticle layers, the primary cuticle and a fibrilla-filled secondary cuticle directly below the Epicuticular Wax Primary Cuticle Secondary Cuticle Epidermal Cytoplasm Fig.

Diagram showing the arrangement of cuticle and cell wall in the apical portion of the leaf epidermis. The secondary cuticle contains mostly cutin and carbohydrates, the lamellate primary cuticle consist mostly of cutin and subcuticular waxes, whereas the epicuticular wax layer is dominated by aliphatic wax constituents although aromatic constituents can also occur at high levels in many plant species.