Production of Betacyanins in Transgenic Nicotiana tabacum Increases Tolerance to Salinity.

Although red betalain pigments (betacyanins) have been associated with salinity tolerance in some halophytes like Disphyma australe, efforts to determine whether they have a causal role and the underlying mechanisms have been hampered by a lack of a model system. To address this, we engineered betalain-producing Nicotiana tabacum, by the introduction of three betalain biosynthetic genes. The plants were violet-red due to the accumulation of three betacyanins: betanin, isobetanin, and betanidin. Under salt stress, betacyanic seedlings had increased survivability and leaves of mature plants had higher photochemical quantum yields of photosystem II (F v /F m ) and faster photosynthetic recovery after saturating light treatment. Under salt stress, compared to controls betacyanic leaf disks had no loss of carotenoids, a slower rate of chlorophyll degradation, and higher F v /F m values. Furthermore, simulation of betacyanin pigmentation by using a red filter cover improved F v /F m value of green tissue under salt stress. Our results confirm a direct causal role of betacyanins in plant salinity tolerance and indicate a key mechanism is photoprotection. A role in delaying leaf senescence was also indicated, and the enhanced antioxidant capability of the betacyanic leaves suggested a potential contribution to scavenging reactive oxygen species. The study can inform the development of novel biotechnological approaches to improving agricultural productivity in saline-affected areas.

Differential ROS Generation in Response to Stress in Symbiodinium spp.

Oxidative stress inside cells occurs when the production of reactive oxygen species (ROS) is no longer efficiently counterbalanced by the generation of antioxidants. In this study, we measured the intracellular production of ROS, including hydrogen peroxide (H2O2), superoxide (O2-), and singlet oxygen (1O2), in cultured dinoflagellates of the genus Symbiodinium under thermal and oxidative stress. ROS tagged with fluorescent probes were measured by flow cytometry. Dissimilar Symbiodinium internal transcribed spacer 2 (ITS2) clades or phylotypes (A1, B2, E, F1) produced ROS in different quantities in response to stress. For example, when comparing the control (26 °C) to the high-temperature treatment (35 °C), Symbiodinium E showed no change in the intracellular concentrations of any of the ROS; but phylotype A1 displayed a 10-fold increase in the overall ROS concentration and a 4-fold increase in O2-. Under oxidative stress, when 8 mmol l-1 H2O2 was added to the cells, these same two Symbiodinium phylotypes increased their overall concentrations of ROS, but only Symbiodinium E showed an increase in the concentrations of O2- (2×) and 1O2 (3×). Therefore, not only were the stress responses of the various Symbiodinium phylotypes different but also the responses of individual phylotypes to thermal and oxidative stress were different in terms of ROS production. Variation in the quality and quantity of ROS generation and its implications for subsequent antioxidant production suggest that different stress mechanisms are at play. While our experiments were done under laboratory conditions that did not necessarily mirror ecological ones, these results provide new insight into processes inside Symbiodinium cells during stress events and add new explanations for a phylotype's susceptibility to stress.

High concentrations of aromatic acylated anthocyanins found in cauline hairs in Plectranthus ciliatus.

Vegetative shoots of a naturalized population of purple-leaved plectranthus (Plectranthus ciliatus, Lamiaceae) were found to contain four main anthocyanins: peonidin 3-(6″-caffeoyl-ß-glucopyranoside)-5-ß-glucopyranoside, peonidin 3-(6″-caffeoyl-ß-glucopyranoside)-5-(6‴-malonyl-ß-glucopyranoside), peonidin 3-(6″-E-p-coumaroyl-ß-glucopyranoside)-5-(6‴-malonyl-ß-glucopyranoside), and peonidin 3-(6″-E-p-coumaroyl-ß-glucopyranoside)-5-ß-glucopyranoside. The first three of these pigments have not been reported previously from any plant. They all follow the typical anthocyanin pattern of Lamiaceae, with universal occurrence of anthocyanidin 3,5-diglucosides and aromatic acylation with p-coumaric and sometimes caffeic acids; however, they differ by being based on peonidin. The four anthocyanins were present in the leaves (22.2 mg g(-1) DW), and in the xylem and interfascicular parenchyma of the stem. They were exceptionally abundant, among the highest reported for any plant organ, in epidermal hairs on some of the stem internodes (101 mg g(-1) DW). Anthocyanin content in these hairs increased more than three-fold from the youngest to the fourth-youngest internodes. In situ absorbances (λmax ≈ 545 nm) were bathochromic in comparison to absorbances of the isolated anthocyanins in their flavylium form in acidified aqueous solutions (λmax = 525 nm), suggesting that the anthocyanins occur both in quinoidal and flavylium forms in constant proportions in the anthocyanic hair cells. The most distinctive observation with respect to relative proportions of individual anthocyanins was found in de-haired internodes, for which anthocyanin caffeoyl-derivatives decreased, and anthocyanin coumaroyl-derivatives increased, from the youngest to the fourth-youngest internode.

Effects of boiling and in vitro gastrointestinal digestion on the antioxidant activity of Sonchus oleraceus leaves.

Leaves of Sonchus oleraceus L. are especially rich in phenolic compounds and have potent extractable antioxidants. However, it is not known how their antioxidant activity changes after cooking and gastrointestinal digestion. We recorded the profile of phenolics and their associated antioxidant activity in both raw and boiled S. oleraceus leaf extracts after in vitro gastric and intestinal digestion, and quantified their antioxidant potentials using Caco-2 and HepG2 cells. Boiling significantly diminished the oxygen radical absorbance capacity (ORAC) and concentrations of ascorbate and chicoric acid in the soluble fractions. In contrast, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and concentrations of caftaric and chlorogenic acids were unaffected. Phenolics in the soluble fraction were absorbed into cultured human cells and exerted antioxidant activity. Only chlorogenic acid content remained stable during gastrointestinal digestion. S. oleraceus appears to be an excellent dietary source of phenolic antioxidants.

Characterisation of betalain biosynthesis in Parakeelya flowers identifies the key biosynthetic gene DOD as belonging to an expanded LigB gene family that is conserved in betalain-producing species.

Plant betalain pigments are intriguing because they are restricted to the Caryophyllales and are mutually exclusive with the more common anthocyanins. However, betalain biosynthesis is poorly understood compared to that of anthocyanins. In this study, betalain production and betalain-related genes were characterized in Parakeelya mirabilis (Montiaceae). RT-PCR and transcriptomics identified three sequences related to the key biosynthetic enzyme Dopa 4,5-dioxgenase (DOD). In addition to a LigB gene similar to that of non-Caryophyllales species (Class I genes), two other P. mirabilis LigB genes were found (DOD and DOD-like, termed Class II). PmDOD and PmDOD-like had 70% amino acid identity. Only PmDOD was implicated in betalain synthesis based on transient assays of enzyme activity and correlation of transcript abundance to spatio-temporal betalain accumulation. The role of PmDOD-like remains unknown. The striking pigment patterning of the flowers was due to distinct zones of red betacyanin and yellow betaxanthin production. The major betacyanin was the unglycosylated betanidin rather than the commonly found glycosides, an occurrence for which there are a few previous reports. The white petal zones lacked pigment but had DOD activity suggesting alternate regulation of the pathway in this tissue. DOD and DOD-like sequences were also identified in other betalain-producing species but not in examples of anthocyanin-producing Caryophyllales or non-Caryophyllales species. A Class I LigB sequence from the anthocyanin-producing Caryophyllaceae species Dianthus superbus and two DOD-like sequences from the Amaranthaceae species Beta vulgaris and Ptilotus spp. did not show DOD activity in the transient assay. The additional sequences suggests that DOD is part of a larger LigB gene family in betalain-producing Caryophyllales taxa, and the tandem genomic arrangement of two of the three B. vulgaris LigB genes suggests the involvement of duplication in the gene family evolution.

Dynamics of flavonol accumulation in leaf tissues under different UV-B regimes in Centella asiatica (Apiaceae).

MAIN CONCLUSION: A cumulative effect of UV-B doses on epidermal flavonol accumulation was observed during the first week of a time course study in Centella asiatica (Apiaceae). However, once flavonol levels had peaked, additional accumulation was possible only if higher daily UV-B irradiances were applied. We aimed to understand the dynamics of flavonol accumulation in leaf tissues using non-destructive spectroscopy and HPLC-mass spectrometry. When leaves that had grown without UV-B were given brief daily exposures to low-irradiance UV-B, they accumulated flavonols, predominantly kaempferol-3-O-ß-D-glucuronopyranoside and quercetin-3-O-ß-D-glucuronopyranoside, in their exposed epidermis, reaching a plateau after 7 days. More prolonged UV-B exposures or higher doses eventually augmented flavonol concentrations even in non-exposed tissues. If UV-B irradiance was subsequently reduced, leaves appeared to lose their ability to accumulate further flavonols in their epidermis even if the duration of daily exposure was increased. A higher irradiance level was then necessary to further increase flavonol accumulation. When subsequently acclimated to higher UV-B irradiances, mature leaves accumulated less flavonols than did developing ones. Our study suggests that levels of epidermal flavonols in leaves are governed primarily by UV-B irradiance rather than by duration of exposure.

Examining the photoprotection hypothesis for adaxial foliar anthocyanin accumulation by revisiting comparisons of green- and red-leafed varieties of coleus (Solenostemon scutellarioides).

Although plants rely on light to drive energy production via photosynthesis, excess light can be harmful. Plants have evolved photoprotective mechanisms to mitigate this threat, including thermal energy dissipation, the most common form of which involves de-epoxidized constituents of the xanthophyll cycle facilitating the conversion of excess excitation energy to heat. A role in photoprotection has also been proposed for red anthocyanins when they accumulate near the adaxial leaf surface. Here, we compared the response to experimental light stress of a red-leafed (anthocyanin rich) and a green-leafed variety of coleus [Solenostemon scutellarioides (L.) Codd], examining chlorophyll fluorescence emission and pigment composition. After experimentally imposed intense white light, red- and green-leafed coleus exhibited manifestations of light stress (decreased photosystem II quantum efficiency) of a similar magnitude. This, considered alone, could be interpreted as evidence that anthocyanins do not serve a photoprotective role. However, during excess light exposure, the green-leafed variety employed a greater level of thermal energy dissipation and possessed correspondingly higher xanthophyll cycle pool sizes and de-epoxidation states. During exposure to red light, which anthocyanins absorb very poorly, levels of thermal energy dissipation did not differ between coleus varieties. Taken together, our findings suggest that adaxial anthocyanins minimize stress associated with excess light absorption and that the green-leafed variety of coleus compensated for its much lower levels of adaxial anthocyanins by invoking higher levels of energy dissipation. Thus, anthocyanin accumulation should be considered alongside the suite of photoprotective mechanisms employed by photosynthetic tissues.

Betalain induction by l-DOPA application confers photoprotection to saline-exposed leaves of Disphyma australe.

The capacity to synthesize betalains has arisen in diverse phylogenetic lineages across the Caryophyllales, and because betalainic plants often grow in deserts, sand dunes, or salt marshes, it is likely that these pigments confer adaptive advantages. However, possible functional roles of foliar betalains remain largely unexplored and are difficult to test experimentally. We adopted a novel approach to examine putative photoprotective roles of betalains in leaves for which chloroplast function has been compromised by salinity. Responses of l-DOPA-treated red shoots of Disphyma australe to high light and salinity were compared with those of naturally red- and green-leafed morphs. Betalain content and tyrosinase activity were measured, and Chl fluorescence profiles and H2 O2 production were compared under white, red or green light. Green leaves lacked tyrosinase activity, but when supplied with exogenous l-DOPA they produced five betacyanins. Both the naturally red and l-DOPA-induced red leaves generated less H2 O2 and showed smaller declines in photosystem II quantum efficiency than did green leaves when exposed to white or green light, although not when exposed to red light. Light screening by epidermal betalains effectively reduces the propensity for photoinhibition and photo-oxidative stress in subjacent chlorenchyma. This may assist plant survival in exposed and saline environments.

Photoprotection by foliar anthocyanins mitigates effects of boron toxicity in sweet basil (Ocimum basilicum).

Boron (B) toxicity is an important agricultural problem in arid environments. Excess edaphic B compromises photosynthetic efficiency, limits growth and reduces crop yield. However, some purple-leafed cultivars of sweet basil (Ocimum basilicum) exhibit greater tolerance to high B concentrations than do green-leafed cultivars. We hypothesised that foliar anthocyanins protect basil leaf mesophyll from photo-oxidative stress when chloroplast function is compromised by B toxicity. Purple-leafed 'Red Rubin' and green-leafed 'Tigullio' cultivars, grown with high or negligible edaphic B, were given a photoinhibitory light treatment. Possible effects of photoabatement by anthocyanins were simulated by superimposing a purple polycarbonate filter on the green leaves. An ameliorative effect of light filtering on photosynthetic quantum yield and on photo-oxidative load was observed in B-stressed plants. In addition, when green protoplasts from both cultivars were treated with B and illuminated through a screen of anthocyanic protoplasts or a polycarbonate film which approximated cyanidin-3-O-glucoside optical properties, the degree of photoinhibition, hydrogen peroxide production, and malondialdehyde content were reduced. The data provide evidence that anthocyanins exert a photoprotective role in purple-leafed basil mesophyll cells, thereby contributing to improved tolerance to high B concentrations.

Epidermal coumaroyl anthocyanins protect sweet basil against excess light stress: multiple consequences of light attenuation.

The putative photoprotective role of foliar anthocyanins continues to attract heated debate. Strikingly different experimental set-ups coupled with a poor knowledge of anthocyanin identity have likely contributed to such disparate opinions. Here, the photosynthetic responses to 30 or 100% solar irradiance were compared in two cultivars of basil, the green-leafed Tigullio (TG) and the purple-leafed Red Rubin (RR). Coumaroyl anthocyanins in RR leaf epidermis significantly mitigated the effects of high light stress. In full sunlight, RR leaves displayed several shade-plant traits; they transferred less energy than did TG to photosystem II (PSII), and non-photochemical quenching was lower. The higher xanthophyll cycle activity in TG was insufficient to prevent inactivation of PSII in full sunlight. However, TG was the more efficient in the shade; RR was far less able to accommodate a large change in irradiance. Investment of carbon to phenylpropanoid biosynthesis was more in RR than in TG in the shade, and was either greatly enhanced in TG or varied little in RR because of high sunlight. The metabolic cost of photoprotection was lower whereas light-induced increase in biomass production was higher in RR than in TG, thus making purple basil the more light tolerant. Purple basil appears indeed to display the conservative resource-use strategy usually observed in highly stress tolerant species. We conclude that the presence of epidermal coumaroyl anthocyanins confers protective benefits under high light, but it is associated with a reduced plasticity to accommodate changing light fluxes as compared with green leaves.

The functional significance of black-pigmented leaves: photosynthesis, photoprotection and productivity in Ophiopogon planiscapus 'Nigrescens'.

Black pigmented leaves are common among horticultural cultivars, yet are extremely rare across natural plant populations. We hypothesised that black pigmentation would disadvantage a plant by reducing photosynthesis and therefore shoot productivity, but that this trait might also confer protective benefits by shielding chloroplasts against photo-oxidative stress. CO2 assimilation, chlorophyll a fluorescence, shoot biomass, and pigment concentrations were compared for near isogenic green- and black-leafed Ophiopogonplaniscapus 'Nigrescens'. The black leaves had lower maximum CO2 assimilation rates, higher light saturation points and higher quantum efficiencies of photosystem II (PSII) than green leaves. Under saturating light, PSII photochemistry was inactivated less and recovered more completely in the black leaves. In full sunlight, green plants branched more abundantly and accumulated shoot biomass quicker than the black plants; in the shade, productivities of the two morphs were comparable. The data indicate a light-screening, photoprotective role of foliar anthocyanins. However, limitations to photosynthetic carbon assimilation are relatively small, insufficient to explain the natural scarcity of black-leafed plants.

Betalain production is possible in anthocyanin-producing plant species given the presence of DOPA-dioxygenase and L-DOPA.

BACKGROUND: Carotenoids and anthocyanins are the predominant non-chlorophyll pigments in plants. However, certain families within the order Caryophyllales produce another class of pigments, the betalains, instead of anthocyanins. The occurrence of betalains and anthocyanins is mutually exclusive. Betalains are divided into two classes, the betaxanthins and betacyanins, which produce yellow to orange or violet colours, respectively. In this article we show betalain production in species that normally produce anthocyanins, through a combination of genetic modification and substrate feeding. RESULTS: The biolistic introduction of DNA constructs for transient overexpression of two different dihydroxyphenylalanine (DOPA) dioxygenases (DODs), and feeding of DOD substrate (L-DOPA), was sufficient to induce betalain production in cell cultures of Solanum tuberosum (potato) and petals of Antirrhinum majus. HPLC analysis showed both betaxanthins and betacyanins were produced. Multi-cell foci with yellow, orange and/or red colours occurred, with either a fungal DOD (from Amanita muscaria) or a plant DOD (from Portulaca grandiflora), and the yellow/orange foci showed green autofluorescence characteristic of betaxanthins. Stably transformed Arabidopsis thaliana (arabidopsis) lines containing 35S: AmDOD produced yellow colouration in flowers and orange-red colouration in seedlings when fed L-DOPA. These tissues also showed green autofluorescence. HPLC analysis of the transgenic seedlings fed L-DOPA confirmed betaxanthin production. CONCLUSIONS: The fact that the introduction of DOD along with a supply of its substrate (L-DOPA) was sufficient to induce betacyanin production reveals the presence of a background enzyme, possibly a tyrosinase, that can convert L-DOPA to cyclo-DOPA (or dopaxanthin to betacyanin) in at least some anthocyanin-producing plants. The plants also demonstrate that betalains can accumulate in anthocyanin-producing species. Thus, introduction of a DOD and an enzyme capable of converting tyrosine to L-DOPA should be sufficient to confer both betaxanthin and betacyanin production to anthocyanin-producing species. The requirement for few novel biosynthetic steps may have assisted in the evolution of the betalain biosynthetic pathway in the Caryophyllales, and facilitated multiple origins of the pathway in this order and in fungi. The stably transformed 35S: AmDOD arabidopsis plants provide material to study, for the first time, the physiological effects of having both betalains and anthocyanins in the same plant tissues.

Red leaf margins indicate increased polygodial content and function as visual signals to reduce herbivory in Pseudowintera colorata.

Red-pigmented leaf margins are common, but their functional significance is unknown. We hypothesized that red leaf margins reduce leaf herbivory by signalling to herbivorous insects the presence of increased chemical defences. Leaves were collected from a natural population of Pseudowintera colorata. Margin size, herbivory damage, anthocyanin content and concentrations of polygodial, a sesquiterpene dialdehyde with antifeedant properties, were quantified. Feeding trials involving larvae of Ctenopseustis obliquana, a generalist herbivore, were conducted on red- and green-margined P. colorata leaves in darkness, or under white, green or red light. Leaves with wider red margins contained higher concentrations of polygodial and anthocyanins, and incurred less natural herbivory. In trials under white light, C. obliquana consumed disproportionately more green- than red-margined leaf laminae. Larvae exhibited no feeding preference when light was manipulated such that leaf colour discrimination was impaired. Red leaf margins provide a reliable and effective visual signal of chemical defence in P. colorata. Ctenopseustis obliquana larvae perceive and respond to the colour of the leaf margins, rather than to olfactory signals. Our study provides direct experimental evidence for aposematic coloration in red leaves.

Antioxidant activity of puha (Sonchus oleraceus L.) as assessed by the cellular antioxidant activity (CAA) assay.

There is considerable interest in antioxidant dietary components that can be protective against degenerative diseases in humans. Puha (Sonchus oleraceus L.) is a rich source of polyphenols, and exhibits strong antioxidant activity as measured by the 2,2-diphenylpicrylhydrazyl (DPPH) assay. However, the potential of puha to protect against degenerative diseases requires that low molecular weight antioxidants (LMWA) are absorbed by, and active in, human cells. The cellular antioxidant activity (CAA) assay was used to investigate the antioxidant activity of puha leaf extracts. Preparation methods of freezing and freeze-drying reduced the total polyphenolic content compared with fresh puha, but did not affect the LMWA potential as determined by the DPPH assay. The IC(50) values were 0.012 ± 0.003 mg/mL and 0.010 ± 0.005 mg/mL for freeze-dried and fresh puha leaves, respectively. Using the CAA assay, it was shown that LMWAs from foliar extracts of puha were effectively absorbed into HepG2 cells, and exerted antioxidant activity at levels comparable to those of extracts from blueberry fruits, the much-touted antioxidant superfood. Methylene blue staining of HepG2 cells indicated that puha extracts were not cytotoxic at concentrations below 100 mg DW/mL. The data indicate the potential of puha as a nutraceutical supplement for human health.

Hydraulic responses of whole vines and individual roots of kiwifruit (Actinidia chinensis) following root severance.

Whole vine (K(plant)) and individual root (K(root)) hydraulic conductances were measured in kiwifruit (Actinidia chinensis Planch. var. chinensis 'Hort16A') vines to observe hydraulic responses following partial root system excision. Heat dissipation and compensation heat pulse techniques were used to measure sap flow in trunks and individual roots, respectively. Sap flux and measurements of xylem pressure potential (Ψ) were used to calculate K(plant) and K(root) in vines with zero and ∼80% of roots severed. Whole vine transpiration (E), Ψ and K(plant) were significantly reduced within 24 h of root pruning, and did not recover within 6 weeks. Sap flux in intact roots increased within 24 h of root pruning, driven by an increase in the pressure gradient between the soil and canopy and without any change in root hydraulic conductance. Photosynthesis (A) and stomatal conductance (g(s)) were reduced, without significant effects on leaf internal CO(2) concentration (c(i)). Shoot growth rates were maintained; fruit growth and dry matter content were increased following pruning. The woody roots of kiwifruit did not demonstrate a rapid dynamic response to root system damage as has been observed previously in monocot seedlings. Increased sap flux in intact roots with no change in K(root) and only a moderate decline in shoot A suggests that under normal growing conditions root hydraulic conductance greatly exceeds requirements for adequate shoot hydration.

Why some stems are red: cauline anthocyanins shield photosystem II against high light stress.

Red-stemmed plants are extremely common, yet the functions of cauline anthocyanins are largely unknown. The possibility that photoabatement by anthocyanins in the periderm reduces the propensity for photoinhibition in cortical chlorenchyma was tested for Cornus stolonifera. Anthocyanins were induced in green stems exposed to full sunlight. PSII quantum yields (capital EF, Cyrillic(PSII)) and photochemical quenching coefficients were depressed less in red than in green stems, both under a light ramp and after prolonged exposures to saturating white light. These differences were primarily attributable to the attenuation of PAR, especially green/yellow light, by anthocyanins. However, the red internodes also had less chlorophyll and higher carotenoid:chlorophyll ratios than the green, and when the anthocyanic periderm was removed, small differences in the capital EF, Cyrillic(PSII) of the underlying chlorenchyma were retained. Thus, light screening by cauline anthocyanins is important, but is only part of a set of protective acclimations to high irradiance. Hourly measurements of capital EF, Cyrillic(PSII) on established trees under natural daylight indicated a possible advantage of red versus green stems under sub-saturating diffuse, but not direct sunlight. To judge the wider applicability of the hypothesis, responses to high light were compared for red and green stems across five further unrelated species. There was a strong, linear, interspecific correlation between photoprotective advantage and anthocyanin concentration differences among red and green internodes. The photoprotective effect appears to be a widespread phenomenon.

Red (anthocyanic) leaf margins do not correspond to increased phenolic content in New Zealand Veronica spp.

BACKGROUND AND AIMS: Red or purple coloration of leaf margins is common in angiosperms, and is found in approx. 25 % of New Zealand Veronica species. However, the functional significance of margin coloration is unknown. We hypothesized that anthocyanins in leaf margins correspond with increased phenolic content in leaf margins and/or the leaf entire, signalling low palatability or leaf quality to edge-feeding insects. METHODS: Five species of Veronica with red leaf margins, and six species without, were examined in a common garden. Phenolic content in leaf margins and interior lamina regions of juvenile and fully expanded leaves was quantified using the Folin-Ciocalteu assay. Proportions of leaf margins eaten and average lengths of continuous bites were used as a proxy for palatability. KEY RESULTS: Phenolic content was consistently higher in leaf margins compared with leaf interiors in all species; however, neither leaf margins nor more interior tissues differed significantly in phenolic content with respects to margin colour. Mean phenolic content was inversely correlated with the mean length of continuous bites, suggesting effective deterrence of grazing. However, there was no difference in herbivore consumption of red and green margins, and the plant species with the longest continuous grazing patterns were both red-margined. CONCLUSIONS: Red margin coloration was not an accurate indicator of total phenolic content in leaf margins or interior lamina tissue in New Zealand Veronica. Red coloration was also ineffective in deterring herbivory on the leaf margin, though studies controlling for variations in leaf structure and biochemistry (e.g. intra-specific studies) are needed before more precise conclusions can be drawn. It is also recommended that future studies focus on the relationship between anthocyanin and specific defence compounds (rather than general phenolic pools), and evaluate possible alternative functions of red margins in leaves (e.g. antioxidants, osmotic adjustment).

Anthocyanidin-containing compounds occur in the periderm cell walls of the storage roots of sweet potato (Ipomoea batatas).

Anthocyanins, which are O-glycosylated derivatives of anthocyanidins, are responsible for the red, blue and purple coloration of many organs of angiosperms where they have previously been reported to occur in vacuoles and cytoplasm. However, bright-field microscopy of sections of the storage roots of the sweet potato (Ipomoea batatas) clone 99N1/222, which contains high concentrations of anthocyanins, showed that the walls of the periderm cork cells (skin), but not those of the adjacent parenchyma cells, were coloured bright red/purple. In situ absorption spectra of the periderm cell walls were similar to those of methanolic solutions of anthocyanins. Dry cell-wall preparations obtained from the periderm and parenchyma tissues were coloured deep reddish-brown and white, respectively. Pigment was extracted from the periderm cell-wall preparation by treating with 1 M NaOH. Acid hydrolysis of this extract followed by HPLC indicated the presence of the anthocyanidins cyanidin and peonidin. It is concluded that the periderm cell walls contain covalently bound anthocyanidin-containing compounds, possibly anthocyanins.