Measurement of leaf epidermal transmittance of UV radiation by chlorophyll fluorescence

In higher plants one of the important functions of the leaf epidermis is the effective screening of ultraviolet-B (280–320 nm, UV-B) radiation, due mostly to phenolic compounds. The assessment of the contribution of this function is necessary for an evaluation of the impact of increasing UV-B radiation. A method is proposed to estimate epidermal transmittance on the basis of chlorophyll fluorescence measurements. Fluorescence of chlorophyll induced by UV-A (320–400 nm, measuring beam centered at 366 nm, half band width 32 nm) or UV-B (measuring beam centered at 314 nm, half band width 18 nm) is compared to that induced by a blue-green measuring light (475 nm, half band width 140 nm). It is shown that the ratios of UV-and blue-green (BG)-induced fluorescence, F(UV-A)/F(BG) and F(UV-B)/F(BG), are relatively constant among leaf samples of various species ( Vicia faba, Spinacia oleracea, Rumex scutatus ) from which the epidermis was removed. In epidermis-free leaves no significant differences were found between adaxial and abaxial leaf sides, suggesting that leaf structure has negligible influence on the F(UV)/F(BG) ratios. On the other hand, fluorescence excitation ratios varied over a vast range when intact leaves from different species and habitats were investigated. Ratios were low in sun leaves and relatively high in shade- and greenhouse-grown leaves. By relating these results to those obtained with epidermis-free leaves, epidermal transmittances for UV-B radiation could be estimated, with values ranging between 1 and 45%. The data demonstrate a large adaptability of epidermal UV-A and UV-B transmittance in higher plants. The proposed method may prove a versatile and relatively simple tool for investigating epidermal UV transmittance complementing established methods.     

Radiative properties of hardwood leaves to ultraviolet irradiation

Spectral reflectance and transmittance of leaves to ultraviolet irradiation were determined under laboratory conditions for seven species of hardwood trees, namely red oak (Quercus rubra, L), black oak (Q. velutina, Lamarch), white oak (Q. alba, L.), sugar maple (Acer saccharum), Norway maple (A. plantanoides), hickory (Carya tomemtosa), sweetgum (Liquidambar styraciflua), and black oak litter. The experimental system consisted of a solar simulator, an integrating sphere, and a spectroradiometer. Experiments were repeated three to five times for both adaxial and abaxial surfaces of fresh leaves chosen at randomly. The spectral distributions and simple averages of the radiative properties in the wavelength ranges of ultraviolet-B (UV-B, 280–320 nm) and ultraviolet-A (UV-A, 320–400 nm) were determined. The spectral distributions of reflectance were similar between adaxial and abaxial surfaces, although the magnitude varied among tree species. Leaf reflectance was very low for the ultraviolet spectrum in general and varied among species and between adaxial and abaxial surfaces. It was generally higher over the UV-A waveband compared to UV-B, and higher on the abaxial than adaxial surface. The broadband reflectance in the UV-A range (over all species) was 5.0 and 3.9% for abaxial and adaxial surface, respectively, compared to 3.5 and 2.8% in UV-B. The transmittance through leaves was extremely small in the UV-B (<0.1%) and nearly zero in the UV-A spectral range. Consequently, the absorptance of ultraviolet radiation by leaves, as determined from the measured reflectance and transmittance, was quite high, being more than 90% for all the combinations of species and wavebands examined. The reported results are useful for studies requiring spectral radiative properties of the examined leaves with respect to ultraviolet irradiation.     

The effect of exposure to enhanced UV-B radiation on the penetration of monochromatic and polychromatic UV-B radiation in leaves of Brassica napus

Using quartz optical fibres, penetration of both monochromatic (310 nm) and polychromatic UV-B (280–320 nm) radiation in leaves of Brassica napus L. (cv. Ceres) was measured. Plants were grown under either visible light (750 μmol m⁻² s⁻¹ photosynthetically active radiation) or with the addition of 8. 9 KJ m⁻² day⁻¹ biologically effective UV-B (UV-B_BE) radiation. Results showed that of the 310 nm radiation that penetreated the leaf, 90% was within the intial one third of the leaf with high attenuation in the leaf epidermis, especially in UV-treated plants. Polychromatic UV-B radiation, relative to incident radiation, showed a relatively uniform spectral distribution within the leaf, except for collimated radiation. Over 30% of the UV-screening pigments in the leaf, including flavonoids, were found in the adaxial epidermal layer, making this layer less transparent to UV-B radiation than the abaxial epidermis, which contained less than 12% of the UV-screening pigments. UV-screening pigments increased by 20% in UV-treated leaves relative to control leaves. Densely arranged epicuticular wax on the adaxial leaf surface of UV-treated plants may have further decreased penetration of UV-B radiation by reflectance. An increased leaf thickness, and decreases in leaf area and leaf dry weight were also found for UV-treated plants.     

Penetration of UV-A, UV-B and blue light through the leaf trichome layers of two xeromorphic plants, olive and oak, measured by optical fibre microprobes

Quartz fibre-optic microprobes were used to monitor the light microenvironment beneath trichome layers of the xeromorphic leaves of two Mediterranean evergreen sclerophylls, Olea europaea and Quercus ilex . Young developing leaves of both plants were densely pubescent on both surfaces of the lamina, whereas the mature leaves were pubescent only on the abaxial side. Trichome layers of young as well as of mature leaves of both plants attenuated almost all incident ultraviolet (UV)-B (310 nm) and UV-A (360 nm) radiation and a considerable portion of blue light (430 nm). Abaxial trichome layers of young leaves were more effective in screening out the incident radiation compared to the adaxial ones of the same leaves and also compared to the abaxial layer of the mature leaves. The abaxial epidermis of dehaired mature leaves of O. europaea was ineffective in absorbing most of the incident UV-B and UV-A radiation. UV and visible spectra beneath trichome layers of O. europaea in mature leaves confirmed that the light microenvironment on the epidermis was deprived in the UV-B, UV-A and partly in the blue spectral regions. It is proposed that the occurrence of a dense trichome layer, especially in young leaves, may play a protective role against not only UV-B radiation damage, but also against high visible irradiance. This function is performed irrespective of the differing anatomy of individual hairs of both plants. The protection provided by the trichomes could afford advantages under stress conditions, especially during leaf development.     

Epidermal transmittance and phenolic composition in leaves of atrazine-tolerant and atrazine-sensitive cultivars of Brassica napus grown under enhanced UV-B radiation

Experiments were conducted on the atrazine-tolerant mutant Stallion and the atrazine-sensitive cv. Paroll of Brassica napus L., which were grown under either visible light or with the addition of UV-B radiation (280–320 nm) for 15 days. The mutant has been shown to be sensitive to high levels of visible light as compared to the atrazine-sensitive cultivar and therefore we wished to determine plant response to UV-B radiation with respect to potential pigment changes, certain anatomical features, radiation penetration and partial photosynthesis. With regard to pigment changes, we were particularly interested in whether the compositional shift in flavonol pigments under enhanced UV-B radiation, previously suggested to favour increased antioxidant activity, is confined to the adaxial epidermis, which generally receives most UV-B radiation or whether the pigment shift is also inducible in the abaxial epidermis.As was to be expected, the penetration of UV-B radiation (310 nm) was lower in the UV-B-exposed plants, which was correlated with an increased amount of UV-screening pigments in the adaxial and abaxial epidermal layers. The main flavonoid glycosides showed the largest shift from kaempferol to quercetin as aglycone moiety in the adaxial epidermal layer. However, in the abaxial epidermal layer the hydroxycinnamic acid (HCA) derivatives and kaempferol glycosides were predominant. Penetration of 430 nm light was higher after UV-B exposure, and probably contributed to the fact that photosynthetic efficiency of photosystem II was unchanged or higher after UV-B exposure. UV-B radiation decreased leaf area in the atrazine-tolerant mutant only. Both cultivars showed an increased leaf thickness after UV-B exposure due to cell elongation mainly of the palisade tissue. This was especially evident in the mutant.     

Can dual chlorophyll fluorescence excitation be used to assess the variation in the content of UV-absorbing phenolic compounds in leaves of temperate tree species along a light gradient?

The present study assesses light-induced variations in phenolic compounds in leaves of saplings of two co-occurring temperate species (Acer platanoides L., and Fraxinus excelsior L.) along a light gradient using a new non-invasive optical method (Dualex). The Dualex-derived UV absorbance of leaf epidermis (the sum of the adaxial and abaxial faces, AUV) increased significantly with increasing light in both species. AUV values were correlated with absorbance of the leaf extract at 305 nm and 375 nm (A305 and A375) in both species with similar slopes for both species. However, a large difference in intercept was observed between the two species when A305 was regressed against AUV. Similarly, AUV values were well correlated with the amount of phenolics in the leaf extracts assessed by the Folin-Ciocalteu method, but slopes were significantly different for the two species. Thus, the UV-A epidermal transmittance, despite being a reliable indicator of the UV-screening capacity of the leaf epidermis, cannot be used for any quantitative estimate of UV-B screening capacity or of energetic requirement for leaf construction without a species-specific calibration.     

Protective mechanisms and acclimation to solar ultraviolet-B radiation in Oenothera stricta

Abstract Mechanisms of plant protection and acclimation to potentially damaging solar ultraviolet-B (UV-B, 280–320 nm) radiation incident on the Earth's surface were examined in Oenothera stricta. Attenuation of this radiation in the upper leaf epidermis reduces the penetration of UV-B radiation to the mesophyll where damage to physiologically sensitive targets can occur. The epidermis is a highly selective radiation filter that can attenuate up to 95% of the incident UV-B radiation and yet transmit between 70% and 80% of the visible radiation. Exposure to UV-B radiation significantly reduced the degree of epidermal UV-B transmittance by as much as 33%. No significant reduction in epidermal transmittance of visible radiation was observed as a result of UV-B exposure. The plasticity in epidermal UV-B transmittance results from production of flavonoid and related phenolic compounds in the tissue. Absorbance of UV-B radiation in llavonoid extract solutions from epidermal and mesophyll tissues significantly increased by as much as 100% and 35%, respectively, after exposure to UV-B radiation. Photosynthetic rates of leaves exposed to UV-B radiation were not significantly reduced at dose rates representative of the radiation flux found in the habitat of this species, but significant photosynthetic depression was observed at dose rates that exceed the field UV-B flux. The phenotypic plasticity in epidermal UV-B transmittance resulting in decreased penetration of damaging UV-B radiation to the mesophyll may reduce the rate of damage to a level where repair mechanisms can keep pace with reduced injury.     

Temporal variation in epidermal flavonoids due to altered solar UV radiation is moderated by the leaf position in Betula pendula

The physiological mechanisms controlling plant responses to dynamic changes in ambient solar ultraviolet (UV) radiation are not fully understood: this information is important to further comprehend plant adaptation to their natural habitats. We used the fluorimeter Dualex to estimate in vivo the epidermal flavonoid contents by measuring epidermal UV absorbance (A(375) ) in Betula pendula Roth (silver birch) leaves of different ages under altered UV. Seedlings were grown in a greenhouse for 15 days without UV and transferred outdoors under three UV treatments (UV-0, UV-A and UV-A+B) created by three types of plastic film. After 7 and 13 days, Dualex measurements were taken at adaxial and abaxial epidermis of the first three leaves (L1, L2 and L3) of the seedlings. After 14 days, some of the seedlings were reciprocally swapped amongst the treatments to study the accumulation of epidermal flavonoids in the youngest unfolded leaves (L3) during leaf expansion under changing solar UV environments. A(375) of the leaves responded differently to the UV treatment depending on their position. UV-B increased the A(375) in the leaves independently of leaf position. L3 quickly adjusted A(375) in their epidermis according to the UV they received and these adjustments were affected by previous UV exposure. The initial absence of UV-A+B or UV-A, followed by exposure to UV-A+B, particularly enhanced leaf A(375) . Silver birch leaves modulate their protective pigments in response to changes in the UV environment during their expansion, and their previous UV exposure history affects the epidermal-absorbance achieved during later UV exposure.     

Penetration of UV-B radiation in foliage: evidence that the epidermis behaves as a non-uniform filter

In some plants, particularly herbaceous species, a considerable proportion of incident ultraviolet-B radiation (UV-B, 280-320 nm) penetrates into the leaf mesophyll where it is potentially damaging to nucleic acids and the photosyn-thetic machinery. We used optical techniques to look at the spatial variation in UV-B penetration through the epidermis of foliage of two herbaceous species (Chenopodium album and Smilacina stellata)and a conifer (Picea pun-gens). Measurements of UV-B penetration in intact foliage with a fibre-optic microprobe revealed that 300 nm radiation reached 161±36μm (mean±SD) into leaves of C. album, 154±40μm in S. stellata and 17±2μm in P. pungens, with epidermal transmittance being 39±14%, 55±19% and 0%, respectively. A thin polymer film was developed which fluoresced blue when irradiated by UV-B. Fresh epidermal leaf peels were placed over the film and irradiated with UV-B, and microscopic examination of the film from below allowed us to determine the spatial pattern of UV-B penetration through the epidermis. In herbaceous species, film fluorescence below cell walls, but not epidermal and guard cell protoplasts indicated that UV-B transmittance was much greater through anticlinal cell wall regions than protoplasts. Ultraviolet-B transmittance through large areas of epidermal cells could be induced by plasmolysis. Epidermal transmittance was also relatively high through stomal pores (and what appear to be nuclei in Smilacina), but relatively low through stomatal guard cells. Results from the fluorescing film technique were substantiated by direct measurements of UV-B transmittance through epidermal peels with a fibre-optic microprobe run paradermally along the bottom or inner side of irradiated peels. In Smilacina, we estimate that UV-B epidermal transmittance was up to 90% through anticlinal cell wall regions, but <10% through protoplast areas. In contrast to herbaceous species, we did not detect any UV-B transmittance through the epidermis of P. pungens with either the fluorescing film or the fibre-optic microprobe technique. The epidermis appears to be a much more spatially uniform UV-B filter in conifers than in these herbaceous species.     

Epidermal UV-screening in vascular plants from Svalbard (Norwegian Arctic)

Stratospheric ozone depletion is most pronounced at high latitudes, and the concurring increased UV-B radiation might adversely affect plants from polar areas. However, vascular plants may protect themselves against UV-B radiation by UV-absorbing compounds located in the epidermis. In this 3-year study, epidermal UV-B (_max 314 nm) and UV-A (_max 366 nm) screening was assessed using a fluorescence method in 12 vascular species growing in their natural environment at Svalbard. The potential for acclimation to increased radiation was studied with artificially increased UV-B, simulating 11% ozone depletion. Open-top chambers simulated an increase in temperature of 2–3°C in addition to the UV-B manipulation. Adaxial epidermal UV-B transmittance varied between 1.6 and 11.4%. Artificially increased UV-B radiation and temperature did not consistently influence the epidermal UV-B transmittance in any of the measured species, suggesting that they may not have the potential to increase their epidermal screening, or that the screening is already high enough at the applied UV-B level. We propose that environmental factors other than UV-B radiation may influence epidermal UV-B screening.     

Are some plant life forms more effective than others in screening out ultraviolet-B radiation?

Summary The unprecedented rate of depletion of the stratospheric ozone layer will likely lead to appreciable increases in the amount of ultraviolet-B radiation (UV-B, 280–320 nm) reaching the earth's surface. In plants, photosynthetic reactions and nucleic acids in the mesophyll of leaves are deleteriously affected by UV-B. We used a fiber-optic microprobe to make direct measurements of the amount of UV-B reaching these potential targets in the mesophyll of intact foliage. A comparison of foliage from a diverse group of Rocky Mountain plants enabled us to assess whether the foliage of some plant life forms appeared more effective at screening UV-B radiation. The leaf epidermis of herbaceous dicots was particularly ineffective at attenuating UV-B; epidermal transmittance ranged from 18–41% and UV-B reached 40–145 m into the mesophyll or photosynthetic tissue. In contrast to herbaceous dicots, the epidermis of 1-year old conifer needles attenuated essentially all incident UV-B and virtually none of this radiation reached the mesophyll. Although the epidermal layer was appreciably thinner in older needles (7 y) at high elevations (Krumholtz), essentially all incident UV-B was attenuated by the epidermis in these needles. The same epidermal screening effectiveness was observed after removal of epicuticular waxes with chloroform. Leaves of woody dicots and grasses appeared intermediate between herbaceous dicots and conifers in their UV-B screening abilities with 3–12% of the incident UV-B reaching the mesophyll. These large differences in UV-B screening effectiveness suggest that certain plant life forms may be more predisposed than others to meet the challenge of higher UV-B levels resulting from stratospheric ozone depletion.     

叶子花表皮特征的研究及意义

对叶子花（Bougainvillea spectabilis）正常叶和变态叶上气孔密度、气孔指数和保卫细胞大小进行了研究。结果表明：正常叶上表皮的表皮细胞为多边形,垂周壁平直;下表皮的表皮细胞为不规则型,垂周壁浅波状;气孔类型为不规则型。变态叶上表皮没有发现气孔,变态叶下表皮的表皮细胞垂周壁则由浅波形逐渐变为深波形,气孔类型为不规则型和轮列型。随着变态叶的发育,变态叶下表皮的气孔密度降低,气孔指数升高;变态叶保卫细胞的长增大,宽减小。变态叶的平均气孔密度和平均气孔指数明显低于正常叶。正常叶和变态叶的保卫细胞均呈肾形。     

Spectral balance and UV-B sensitivity of soybean: a field experiment

Soybean [Glycine max (L.) Merr.] cultivar Essex was grown and tested for sensitivity to UV-B radiation (280–320 nm) under different combinations of UV-A (320–400 nm) and PFD (400–700 nm) radiation in four simultaneous field experiments. The radiation conditions were effected with combinations of filtered solar radiation and UV-B and UV-A lamps electronically modulated to track ambient radiation. Significant UV-B-caused decreases in total aboveground production and growth were seen only when PFD and UV-A were reduced to less than half their flux in sunlight. When PFD was low, UV-A appeared to be particularly effective in mitigating UV-B damage. However, when PFD was high, substantial UV-A did not appear to be required for UV-B damage mitigation. Leaf chlorophyll fluorescence did not indicate photosynthetic damage under any radiation combination. With UV-B, leaves in all experiments exhibited increased UV-absorbing pigments and decreased whole-leaf UV transmittance. Results of these field experiments indicate difficulties in extrapolating from UV-B experiments conducted in glasshouse or growth cabinet conditions to plant UV-B sensitivity in the field. Implications for UV radiation weighting functions in evaluating atmospheric ozone reduction are also raised.     

Visible and ultraviolet reflectance characteristics of arctic homeotherms

Reflectance characteristics of the epidermal coverings (hair, feathers) of several white birds and mammals were examined in the visible and ultraviolet regions of the solar spectrum. Non-white phases of the same species, and other non-white animals were examined for comparison. As expected, non-white animals exhibited lower reflectance values than white animals in the visible spectrum. Most species examined demonstrated reduced reflectance in the ultraviolet, reaching minimum values between 290 and 310 nm. In white animals, significant differences were found in the reflectance of UV-A (320–400 nm) and UV-B (280–320 nm) radiation. This accounts for the apparent differences in ultraviolet reflectance among various arctic mammals detected previously with ultraviolet photography. Reflectance patterns in the visible and ultraviolet were not obviously correlated with phylogenetic relationship, nor with the gross structure of hair or feathers.     

Effects of Natural Intensities of Visible and Ultraviolet Radiation on Epidermal Ultraviolet Screening and Photosynthesis in Grape Leaves

Grape (Vitis vinifera cv Silvaner) vine plants were cultivated under shaded conditions in the absence of ultraviolet (UV) radiation in a greenhouse, and subsequently placed outdoors under three different light regimes for 7 d. Different light regimes were produced by filters transmitting natural radiation, or screening out the UV-B (280-315 nm), or screening out the UV-A (315-400 nm) and the UV-B spectral range. During exposure, synthesis of UV-screening phenolics in leaves was quantified using HPLC: All treatments increased concentrations of hydroxycinnamic acids but the rise was highest, reaching 230% of the initial value, when UV radiation was absent. In contrast, UV-B radiation specifically increased flavonoid concentrations resulting in more than a 10-fold increase. Transmittance in the UV of all extracted phenolics was lower than epidermal UV transmittance determined fluorimetrically, and the two parameters were curvilinearly related. It is suggested that curvilinearity results from different absorption properties of the homogeneously dissolved phenolics in extracts and of the non-homogeneous distribution of phenolics in the epidermis. UV-B-dependent inhibition of maximum photochemical yield of photosystem II (PSII), measured as variable fluorescence of dark-adapted leaves, recovered in parallel to the buildup of epidermal screening for UV-B radiation, suggesting that PSII is protected against UV-B damage by epidermal screening. However, UV-B inhibition of CO(2) assimilation rates was not diminished by efficient UV-B screening. We propose that protection of UV-B inactivation of PSII is observed because preceding damage is efficiently repaired while those factors determining UV-B inhibition of CO(2) assimilation recover more slowly.     

Secondary phenolic products in isolated guard cell,epidermal cell and mesophyll cell protoplasts from pea (Pisum sativum L.) leaves: Distribution and determination

Summary Guard cells and epidermal cells of the abaxial (lower) and adaxial (upper) epidermis ofPisum sativum L., mutant Argenteum, are the predominant sites of flavonoid accumulation within the leaf. This was demonstrated by the use of a new method of simultaneous isolation and separation of intact, highly-purified guard cell and epidermal cell protoplasts from both epidermal layers and of protoplasts from the mesophyll. Isolated guard and epidermal protoplasts retained flavonoid patterns of the parent epidermal tissue; quercetin 3-triglucoside and its p-coumaric acid ester as major constituents, kaempferol 3-triglucoside and its p-coumaric acid ester as minor compounds. Total flavonoid content in the lower epidermis was estimated to be ca. 80 fmol per guard cell protoplast and 500 fmol per epidermal cell protoplast. Protoplasts isolated from the upper epidermis had about 20–30% as much of these flavonoids. Mesophyll protoplasts retained only about 25 fmol total flavonoid per protoplast.By fluorescence microscopy, using the alkaline-induced yellow-green fluorescence characteristics of flavonols, we suggest that these flavonol glycosides are present in cell vacuoles. There was no indication for the presence of flavine-like compounds.Abbreviations uE adaxial (upper) epidermis - IE abaxial (lower) epidermis - GCP guard cell protoplasts - ECP epidermal cell protoplasts - MCP mesophyll cell protoplasts - PP protoplasts - HPLC high performance liquid chromatography - TLC thin layer chromatography - CC column chromatography - HOAc acetic acid     

Growth and morphological responses to different UV wavebands in cucumber (Cucumis sativum) and other dicotyledonous seedlings

We examined the influence of short-term exposure of different UV wavebands on the fine-scale kinetics of hypocotyl growth of dim red light-grown cucumbers (Cucumis sativus L.) and other selected dicotyledonous seedlings to evaluate: (1) whether responses induced by UV-B radiation (280-320 nm) are qualitatively different from those induced by UV-A (320-400 nm) radiation, and (2) whether different wavebands within the UV-B elicit different responses. Responses to brief (30 min) irradiations with 3 different UV wavebands all included transient inhibition of elongation during irradiation followed by wavelength specific responses. Irradiations with proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm) induced inhibition of hypocotyl elongation within 20 min of onset of irradiation, while UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation with a lag of 1-2 h. The response to short wavelength UV-B was persistent for at least 24 h, while the response to long wavelength UV-B lasted only 2-3 h. The UV-A treatment induced reductions in elongation rates of approximately 6-9 h following exposure followed by a continued decline in rates for the following 15-18 h. Short wavelength UV-B also induced positive phototropic curvature in both cucumber and Arabidopsis seedlings, and this response was present in nph-1 mutant Arabidopsis seedlings defective in normal blue light phototropism. Reciprocity was not found for the response to short wavelength UV-B. The short wavelength and long wavelength UV-B responses differed in dose-response relationships and both short wavelength responses (phototropic curvature and elongation inhibition) increased sharply at wavelengths below 300 nm. These results indicate that different photosensory processes are involved in mediating growth and morphological responses to short wavelength UV-B (280-300 nm), long wavelength UV-B (essentially 300-320 nm) and UV-A. The existence of two separate types of hypocotyl inhibition responses to UV-B, with one that depends on the intensity of the light source, provides alternate interpretations to findings in other studies of UV-B induced photomorphogenesis and may explain inconsistencies between action spectra for inhibition of stem growth.     

Protection by the epidermis of photosynthesis against UV-C radiation estimated by chorophyll a fluorescence

The epidermis of Argenteum mutant of Pisum sativum L. and Vicia faba L. was shown to be effective in protecting mesophyll photosynthesis from UV-C irradiation (peak 254 nm; 1.5 W m⁻²). These plants were chosen because it is easy to peel the epidermis from both sides of the leaf in Argenteum and the abaxial side in Vicia . Chlorophyll a fluorescence induction was decreased to the same extent by UV-C radiation in both leaf sides when the epidermis was removed. The fluorescence from the leaf with the epidermis inact was not affected. With irradiation of leaves with higher intensity (3.4 W m⁻²) of UV-C, the variable fluorescence was decreased by UV-C impinging on the abaxial side, but not the adaxial side in either of the plant species. A methanol extract from the mutant Pisum epidermis had a high absorbance in the UV region, and the absorbance was more than two-fold larger in adaxial epidermis than in abaxial epidermis. These results indicate that the epidermal layer, which contains substances that have high UV absorbance, protects mesophyll photosynthesis against UV radiation.     

Epidermis effects on spectral properties of leaves of four herbaceous species

The spectral properties of the leaves of the herbaceous species Brassica oleracea L. var. botrytis L., Cerastum tomentosum L., Petunia hybrida Vilm., and Talinum paniculatum (Jacq.) Gaertn. were examined to see what effect the epidermis had on leaf absorptance, reflectance and transmittance. Removal of the epidermis from the side of the leaf surface being illuminated resulted in increases in leaf absorptance and transmittance, and a decrease in reflectance in the 400–800 nm waveband. Removal of the epidermis from the opposite side of an illuminated leaf (effect was similar in both abaxial and adaxial surfaces) resulted in small decreases in both absorptance and reflectance, and corresponding increases in transmittance. Removal of both the upper and lower epidermis resulted in a marked increase in transmittance, while both leaf reflectance and absorptance were decreased. The results suggest that the presence of the epidermis significantly increases leaf absorptance in the photosynthetic wavebands.     

山东广义苦荬菜属(菊科)叶表皮微形态的研究

对山东广义苦荬菜属植物叶表皮微形态进行了光镜下的观察研究。结果表明:山东广义苦荬菜属9种植物叶表皮微形态可分为四种类型:(1)小苦荬型:上、下表皮均有气孔器,气孔器为不规则型,偶有非典型不等型,上、下表皮细胞为不规则形,垂周壁波状;(2)黄瓜菜型:上表皮无气孔器,下表皮气孔器为不规则型,偶有非典型不等型,上、下表皮细胞为不规则形,垂周壁深波状;(3)沙苦荬型:上、下表皮均有气孔器,气孔器为不规则型,偶有非典型不等型,上、下表皮细胞为多边形,垂周壁平直、弓形;(4)苦荬菜型:上、下表皮均有气孔器,气孔器为不规则型,偶有非典型不等型,上表皮细胞为不规则形,垂周壁波状,下表皮细胞为多边形,垂周壁弓形;与中国植物志把广义苦荬菜属划分为4个属的意见相一致。