Associations between leaf structure, orientation, and sunlight exposure in five Western Australian communities

Five plant communities in Western Australia, as well as selected desert and Rocky Mountain species of the western USA, were surveyed to evaluate associations among leaf structure, orientational properties, and the sunlight exposure and precipitation characteristic of each community. Selected leaf structural features have been associated previously with photosynthetic function and included shape, thickness, the ratio of thickness to width, stomatal distribution, leaf surface coloration, and the number and distribution of palisade cell layers. Decreases in annual precipitation (<4 to over 15 cm/yr) and increases in total daily sunlight (4.2 to 29.2 mol photons/m1) corresponded strongly to an increase in the percentage of species in a given community with more inclined (more inclined than +/- 45 degrees from horizontal) or thicker leaf mesophyll (>0.4 mm) leaves. Also, the percentage of species with a leaf thickness to width ratio >0.1, which were amphistomatous, or which had palisade cell layers beneath both leaf surfaces, increased from >20% in the highest rainfall and lowest sunlight community to >80% in the community with least rainfall but greatest sunlight exposure. Over 70% of the species in the most mesic, shaded community had lighter abaxial than adaxial leaf surfaces (leaf bicoloration). All of the above structural features were positively associated with a more inclined leaf orientation (r1 = 0.79), except for leaf bicoloration, which was negatively associated (r1 = 0.75). The ratio of adaxial to abaxial light was more strongly associated with leaf bicoloration (r1 = 0.83) and the presence of multiple adaxial and isobilateral palisade cell layers(r1 = 0.80) than with total incident sunlight on just the adaxial leaf surface (r1 = 0.69 and 0.73, respectively). These results provide field evidence that leaf orientation and structure may have evolved in concert to produce a photosynthetic symmetry in leaf structure in response to the amount of sunlight and other limiting factors of the community. This structural symmetry may serve fundamentally to regulate the distribution of both light and CO2 levels inside the leaf and, thus, increase photosynthetic CO2 uptake per unit leaf biomass.     

Attenuation of incident light in Galax urceolata (Diapensiaceae): concerted influence of adaxial and abaxial anthocyanic layers on photoprotection

Although anthocyanin coloration in lower (abaxial) leaf cells has been documented for numerous species, the functional significance of this character has not been comprehensively investigated according to habitat or leaf orientation. Here, we demonstrate that abaxial anthocyanin may function as a photoprotectant, similarly to its purported role in upper (adaxial) cells, in leaves vulnerable to high irradiance incident on abaxial surfaces. Spectral scans were derived for Galax urceolata leaves with the following phenotypes: abaxial or adaxial anthocyanin only, abaxial and adaxial anthocyanin, and no anthocyanin. To determine whether anthocyanins conferred protection from photoinhibition, maximum photosystem II efficiencies of red (anthocyanic) and green (acyanic) surfaces were compared during and after exposure to photoinhibitory conditions. Leaves were either positioned with their adaxial surfaces facing the light source or inverted to expose abaxial surfaces. Spectral scans showed increased absorptance of 500-600 nm wavelengths by red surfaces (consistent with the absorbance spectrum of anthocyanin), regardless of whether that surface was abaxial or adaxial. Leaves with anthocyanin in either illuminated surface were also photoinhibited less than leaves lacking anthocyanin in that surface. These results suggest that anthocyanic layers reduce absorbed sunlight in the mesophyll not only for adaxial surfaces, but also for the abaxial. Adaxial/abaxial anthocyanin plasticity may therefore be adaptive in high-light environments or during light-sensitive developmental stages where leaf orientation and/or substrate albedo are variable.     

Simulation of photon transport in a three-dimensional leaf: implications for photosynthesis

A model to evaluate photon transport within leaves and the implications for photosynthesis are investigated. A ray tracing model, Raytran, was used to produce absorption profiles within a virtual dorsiventral plant leaf oriented in two positions (horizontal/vertical) and illuminated on one of its two faces (adaxial/abaxial). Together with chlorophyll profiles, these absorption profiles feed a simple photosynthesis model that calculates the gross photosynthetic rate as a function of the incident irradiance. The differences observed between the four conditions are consistent with the literature: horizontal‐adaxial leaves, which are commonly found in natural conditions, have the greatest light use efficiency. The absorption profile obtained with horizontal‐abaxial leaves lies below this, but above those obtained for vertical leaves. The latter present similar gross photosynthetic rates when irradiated on either the adaxial or abaxial surfaces. Vertical profiles of photosynthetic rates across the leaf confirm that carbon fixation occurs mainly in the palisade parenchyma, that the leaf anatomy is integral to its function and that leaves cannot be considered as a single homogeneous unit. Finally, the relationships between leaf structure, orientation and photosynthesis are discussed.     

Ontogenetic differences in mesophyll structure and chlorophyll distribution in Eucalyptus globulus ssp. globulus

Mesophyll structure has been associated with the photosynthetic performance of leaves via the regulation of internal light and CO(2) profiles. Differences in mesophyll structure and chlorophyll distribution within three ontogenetically different leaf types of Eucalyptus globulus ssp. globulus were investigated. Juvenile leaves are blue-grey in color, dorsiventral (adaxial palisade layer only), hypostomatous, and approximately horizontal in orientation. In contrast, adult leaves are dark green in color, isobilateral (adaxial and abaxial palisade), amphistomatous, and nearly vertical in orientation. The transitional leaf type has structural features that appear intermediate between the juvenile and adult leaves. The ratio of mesophyll cell surface area per unit leaf surface area (A(mes)/A) of juvenile leaves was maximum at the base of a single, adaxial palisade layer and declined through the spongy mesophyll. Chlorophyll a + b content showed a coincident pattern, while the chlorophyll a:b ratio declined linearly from the adaxial to abaxial epidermis. In comparison, the mesophyll of adult leaves had a bimodal distribution of A(mes)/A, with maxima occurring beneath both the adaxial and abaxial surfaces within the first layer of multiple palisade layers. The distribution of chlorophyll a + b content had a similar pattern, although the maximum ratio of chlorophyll a:b occurred immediately beneath the adaxial and abaxial epidermis. The matching distributions of A(mes)/A and chlorophyll provide further evidence that mesophyll structure may act to influence photosynthetic performance. These changes in internal leaf structure at different life stages of E. globulus may be an adaptation for increased xeromorphy under increasing light exposure experienced from the seedling to adult tree, similar to the characteristics reported for different species according to sunlight exposure and water availability within their native habitats.     

Temperature and water relation patterns in subalpine understory plants

Summary The daily temperature and water relations of 7 perennial subalpine, understory species (1 shrub, 1 subshrub, 5 herbs) were compared in the Rocky Mountains of southeastern Wyoming with an emphasis on the effects of natural sun and shade exposure. Field measurements of rainfall; leaf, air, and soil temperatures; stomatal conductance to water vapor diffusion; and plant and soil water potentials were supplemented with leaf and root morphological measurements to evaluate potential adaptive patterns in understory species.Morphologically, all 7 species had relatively broad leaves that were hypostomous and bicolored with the abaxial leaf surface lighter than the abaxial surface. Root systems tended to be shallow (<20 cm), especially for the herbaceous species. Although soil water potentials from 4 to 40 cm depths remained relatively high throughout the summer (>-1.0 MPa), plant xylem water potentials for sunlit plants decreased to below-2.0 MPa during midday. During these sunlit periods, leaf temperatures and conductances increased substantially, leading to severe wilting for 4 of the 5 herbaceous species. Stomatal conductance and density for 6 of the 7 species monitored were much greater on abaxial compared to adaxial leaf sides and substantial stomatal closure occurred when either leaf side was oriented to receive direct sunlight. Moreover, stomatal opening on abaxial leaf sides corresponded to the amount of sunlight incident upon the adaxial rather than abaxial leaf surfaces. The 2 shrubby species did not wilt during these periods and were characterized by the highest leaf temperatures (>30°C). These 2 species also had consistantly lower xylem water potentials throughout the summer growth period. These results are discussed in terms of the possible adaptive significance of midday wilting, leaf hypostomy and bicoloration and stomatal behavior to the water and photosynthetic relations of understory species.     

Photosynthetic and structural acclimation to light direction in vertical leaves of Silphium terebinthinaceum

The azimuth of vertical leaves of Silphium terebinthinaceum profoundly influenced total daily irradiance as well as the proportion of direct versus diffuse light incident on the adaxial and abaxial leaf surface. These differences caused structural and physiological adjustments in leaves that affected photosynthetic performance. Leaves with the adaxial surface facing East received equal daily integrated irradiance on each surface, and these leaves had similar photosynthetic rates when irradiated on either the adaxial or abaxial surface. The adaxial surface of East-facing leaves was also the only surface to receive more direct than diffuse irradiance and this was the only leaf side which had a clearly defined columnar palisade layer. A potential cost of constructing East-facing leaves with symmetrical photosynthetic capcity was a 25% higher specific leaf mass and increased leaf thickness in comparison to asymmetrical South-facing leaves. The adaxial surface of South-facing leaves received approximately three times more daily integrated irradiance than the abaxial surface. When measured at saturating CO₂ and irradiance, these leaves had 42% higher photosynthetic rates when irradiated on the adaxial surface than when irradiated on the abaxial surface. However, there was no difference in photosynthesis for these leaves when irradiated on either surface when measurements were made at ambient CO₂. Stomatal distribution (mean adaxial/abaxial stomatal density = 0.61) was unaffected by leaf orientation. Thus, the potential for high photosynthetic rates of adaxial palisade cells in South-facing leaves at ambient CO₂ concentrations may have been constrained by stomatal limitations to gas exchange. The distribution of soluble protein and chlorophyll within leaves suggests that palisade and spongy mesophyll cells acclimated to their local light environment. The protein/chlorophyll ratio was high in the palisade layers and decreased in the spongy mesophyll cells, presumably corresponding to the attentuation of light as it penetrates leaves. Unlike some species, the chlorophyll a/b ratio and the degree of thylakoid stacking was uniform throughout the thickness of the leaf. It appears that sun-shade acclimation among cell layers of Silphium terebinthinaceum leaves is accomplished without adjustment to the chlorophyll a/b ratio or to thylakoid membrane structure.     

Variations in the dorso-ventral organization of leaf structure and Kranz anatomy coordinate the control of photosynthesis and associated signalling at the whole leaf level in monocotyledonous species

Photosynthesis and associated signalling are influenced by the dorso-ventral properties of leaves. The degree of adaxial/abaxial symmetry in stomatal numbers, photosynthetic regulation with respect to light orientation and the total section areas of the bundle sheath (BS) cells and the surrounding mesophyll (M) cells on the adaxial and abaxial sides of the vascular bundles were compared in two C₄[ Zea mays (maize) and Paspalum dilatatum ] and one C₃[ Triticum turgidum (Durum wheat)] monocotyledonous species. The C₃ leaves had a higher degree of dorso-ventral symmetry than the C₄ leaves. Photosynthetic regulation was the same on each side of the wheat leaves, as were stomatal numbers and the section area of the BS relative to that of the M cells (BS/M section area ratio). In contrast, photosynthetic regulation in maize and P. dilatatum leaves showed a marked surface-specific response to light orientation. Compared to the adaxial sides of the C₄ monocotyledonous leaves, the abaxial surfaces had more stomata and the BS/M section area ratio was significantly higher. Differences in dorso-ventral structure, particularly in Kranz anatomy, serve not only to maximize photosynthetic capacity with respect light orientation in C₄ monocotyledonous leaves but also allow adaxial and abaxial-specific signalling from the respective M cells.     

Effects of low concentrations of O3, leaf age and water stress on leaf diffusive conductance and water use efficiency in soybean

Ozone, leaf age and water stress each affected leaf conductance in soybean [ Glycine max (L.) Merr. Hodgson], but there were no interactions among these factors. Exposure to increased concentrations of O₃ (0.01, 0.05, 0.09. and 0.13 μl l⁻¹) resulted in linear declines in abaxial and adaxial conductances in leaves of all ages. There were no differences in relative response to O₃ between the two leaf surfaces. For well-watered plants, water use efficiency also decreased with exposure to increased O₃ concentrations (water-stressed plants were not tested). Abaxial conductance increased as leaves aged from 4 to 10 days and then declined with further aging. Adaxial conductance decreased with all increases in leaf age beyond 4 days, and the ratio of abaxial/adaxial conductance increased continuously throughout the leaf lifespan. During water-stress cycles (water withheld for 2–3 days) leaves of water-stressed plants had lower conductances than those from well-watered plants, and there was no difference in relative response between abaxial and adaxial stomata.     

Interactions of colonisation density and leaf environments on survival of Trioza eugeniae nymphs

1. Female eugenia psyllids Trioza eugeniae oviposit on the margins of expanding young Syzygium paniculatum leaves. The developing nymphs, feeding within pit‐shaped galls on the leaves, cause the leaves to become curled and deformed. The degree of leaf curling was correlated positively with densities of T. eugeniae nymphs. 2. High relative humidity increased persistence of nymphs on leaves at low insect densities, but persistence did not differ between high or low relative humidity conditions when nymphal densities were high and leaves were greatly curled. 3. Direct insolation increased nymphal mortality. Nymphs on the abaxial leaf surface in the direct sun had lower mortality than similarly exposed nymphs on the adaxial leaf surface. 4. Field populations showed high preference for abaxial leaf surfaces and a stronger preference for shaded adaxial surfaces than for exposed adaxial surfaces. 5. Adverse environmental conditions of direct insolation and low relative humidity may be mitigated by leaf curl associated with moderate populations, however competition at high nymphal density supersedes any potential benefit arising from leaf curling and has a negative effect on nymphal survival.     

An apparent anomaly in peanut leaf conductance

Conductance to gaseous transfer is normally considered to be greater from the abaxial than from the adaxial side of a leaf. Measurements of the conductance to water vapor of peanut leaves (Arachis hypogaea L.) under well watered and stress conditions in a controlled environment, however, indicated a 2-fold higher conductance from the adaxial side of the leaf than from the abaxial. Studies of conductance as light level was varied showed an increase in conductance from either surface with increasing light level, but conductance was always greater from the adaxial surface at any given light level. In contrast, measurements of soybean (Glycine max [L.] Merr.) and snapbean (Phaseolus vulgaris L.) leaf conductance showed an approximate 2-fold greater conductance from the abaxial surface than from the adaxial. Approximately the same number of stomata were present on both peanut leaf surfaces and stomatal size was similar. Electron microscopic examination of peanut leaves did not reveal any major structural differences between stomata on the two surfaces that would account for the differences in conductance. Light microscope studies of leaf sections revealed an extensive network of bundle sheaths with achloraplastic bundle sheath extensions; the lower epidermis was lined with a single layer of large achloraplastic parenchyma cells. Measurements of net photosynthesis made on upper and lower leaf surfaces collectively and individually indicated that two-thirds of the peanut leaf's total net photosynthesis can be attributed to diffusion of CO₂ through the adaxial leaf surface. Possibly the high photosynthetic efficiency of peanut cultivars as compared with certain other C₃ species is associated with the greater conductance of CO₂ through their upper leaf surfaces.     

Stomatal distribution patterns change according to leaf development and leaf water status in <Emphasis Type="Italic">Salix miyabeana</Emphasis>

Salix is a pioneer woody plant genus characterized by a strong plasticity in leaf morphology. The aims of this paper were to determine stomatal distribution (1) in mature leaves in response to environmental conditions, and (2) during leaf development. Stomata of abaxial and adaxial faces of mature leaves of Salix miyabeana SX67 (cultivated in short rotation coppice) were analyzed at the end of summer 2012 and 2013 at six locations in Quebec, Canada. Within each site and across the two growing seasons, stomatal density of abaxial faces was diluted by an increase in area of mature leaves due to higher rainfall. For shrubs with more than one growing season, stomatal density of abaxial faces was affected by annual rainfall, independently of site, whereas leaf area was predominantly influenced by site but was also modulated in part by rainfall. The number of stomata per leaf was site-specific, independently of rainfall. These leaves were mainly hypostomatic, although those collected on shrubs during their first growing season after coppicing (i.e. with a high root:shoot ratio) were amphistomatic. Similarly, at early development stages (surface area <2.8 cm²), leaves were amphistomatic, whereas stomata on adaxial faces of larger leaves were occluded. Nevertheless, stomatal conductance of abaxial faces increased with leaf area, whereas stomatal density was best described by a quadratic relationship. This strategy allows for a maximum uptake of carbon while limiting water loss during leaf development and to adapt the morphology of mature leaves depending on moisture and site conditions.     

Ontogenetic changes in stomatal size and conductance of sunflowers

The ontogenetic changes in stomatal size, frequency and conductance (g_s) on abaxial and adaxial leaf surfaces of sunflower plants (Helianthus annuus L. Russian Mammoth) were examined under controlled environmental conditions. The stomatal frequency on the adaxial and abaxial leaf surfaces decreased with leaf ontogeny and insertion level. The ratio of adaxial to abaxial stomatal frequency did not change with leaf ontogeny and insertion level, and 42–44% of total stomata was apportioned to the adaxial surface. Ontogenetic changes in stomatal pore length were detected and increased with ontogenesis. The stomatal length of both leaf surfaces had linear relationships with leaf area. Ontogenetic changes in g_s were similar between the two surfaces. However the adaxial g_s was lower than abaxial g_s in leaves of higher insertion levels. Conductance had a linear relationship with width x frequency but not with pore area.     

Physical features of grass leaves influence the placement of eggs within the plant by the Hessian fly

Hessian fly eggs are more likely to be found on adaxial rather than abaxial surfaces of wheat leaves. These leaf surfaces differ in their physical features: the adaxial side of the leaf has parallel grooves and ridges while the abaxial side is relatively smooth. We used leaf models to investigate the relationship between Hessian fly egglaying and these physical features. When both sides of a green paper leaf model were treated with a wheat leaf extract, but only one side of the model was scored with parallel grooves, the grooved side received more eggs than the smooth side. As the number of grooves per surface increased from 0 to 10, eggs per model increased. When grooves and the wheat leaf extract were tested together and separately, the grooves significantly increased egg numbers in the presence, but not the absence, of wheat extract. In contrast, wheat extract increased egg numbers both in the absence and presence of grooves. Molding techniques were used to recreate the physical features of the adaxial and abaxial leaf surfaces of five grasses. For four of the grass genotypes (a triticale, two common wheats, and a durum wheat), patterns of egglaying on real leaves and molded models were similar, i.e., adaxial leaf surfaces and adaxial molded models were preferred over abaxial leaf surfaces and abaxial models. On the fifth grass, oat, preferences for the adaxial side of real leaves and for adaxial models were not as obvious. We conclude that the adult female Hessian fly obtains information about the leaf surface through her tactile and/or kinesthetic senses and uses this information when making egglaying decisions.     

Stomatal response of hybrid poplar to incident light, sudden darkening and leaf excision

Responses of abaxial and adaxial stomata of Populus trichocarpa Torr. & Gray. × P. deltoides Bartr. (ex Marsh.) cv. Unal to incident light, sudden darkening and leaf excision in the light and in the dark were studied on 5-year-old trees in the field using diffusion porometry. Stomatal closure in the dark was found to be incomplete in most cases studies. Stomata closed after leaf excision in the dark within 90 min. Stomatal closure after darkening of an entire tree or an entire branch (white the rest of the tree was in the light) was slower, and complete stomatal closure was noticed only for adaxial stomata after 3 h. Adaxial stomata were more reactive and sensitive than abaxial stomata to sudden darkening and leaf excision in the light and the dark. In all treatments, stomatal response was more responsive in mature leaves than in young, still expanding leaves.     

Kinetic characterization of reduced pyridine nucleotide dehydrogenases (duroquinone-dependent) in cucurbita microsomes

Stomatal conductances of normally oriented and inverted leaves were measured as light levels (photosynthetic photon flux densities) were increased to determine whether abaxial stomata of Vicia faba leaves were more sensitive to light than adaxial stomata. Light levels were increased over uniform populations of leaves of plants grown in an environmental chamber. Adaxial stomata of inverted leaves reached maximum water vapor conductances at a light level of 60 micromoles per square meter per second, the same light level at which abaxial stomata of normally oriented leaves reached maximum conductances. Abaxial stomata of inverted leaves reached maximum conductances at a light level of 500 micromoles per square meter per second, the same light level at which adaxial stomata of normally oriented leaves reached maximum conductances. Maximum conductances in both normally oriented and inverted leaves were about 200 millimoles per square meter per second for adaxial stomata and 330 millimoles per square meter per second for abaxial stomata. Regardless of whether leaves were normally oriented or inverted, when light levels were increased to values high enough that upper leaf surfaces reached maximum conductances (about 500 micromoles per square meter per second), light levels incident on lower, shaded leaf surfaces were just sufficient (about 60 micromoles per square meter per second) for stomata of those surfaces to reach maximum conductances. This `coordinated' stomatal opening on the separate epidermes resulted in total leaf conductances for normally oriented and inverted leaves that were the same at any given light level. We conclude that stomata in abaxial epidermes of intact Vicia leaves are not more sensitive to light than those in adaxial epidermes, and that stomata in leaves of this plant do not respond to light alone. Additional factors in bulk leaf tissue probably produce coordinated stomatal opening on upper and lower leaf epidermes to optimally meet photosynthetic requirements of the whole leaf for CO₂.     

Difference in light-induced increase in ploidy level and cell size between adaxial and abaxial epidermal pavement cells of Phaseolus vulgaris primary leaves

Changes in nuclear DNA content and cell size of adaxial andabaxial epidermal pavement cells were investigated using brightlight-induced leaf expansion of Phaseolus vulgaris plants. Inprimary leaves of bean plants grown under high (sunlight) ormoderate (ML; photon flux density, 163 µmol m^–2s^–1) light, most adaxial epidermal pavement cells hada nucleus with the 4C amount of DNA, whereas most abaxial pavementcells had a 2C nucleus. In contrast, plants grown under lowintensity white light (LL; 15 µmol m^–2 s^–1)for 13 d, when cell proliferation of epidermal pavement cellshad already finished, had a 2C nuclear DNA content in most adaxialpavement cells. When these LL-grown plants were transferredto ML, the increase in irradiance raised the frequency of 4Cnuclei in adaxial but not in abaxial pavement cells within 4d. On the other hand, the size of abaxial pavement cells increasedby 53% within 4 d of transfer to ML and remained unchanged thereafter,whereas adaxial pavement cells continuously enlarged for 12d. This suggests that the increase in adaxial cell size after4 d is supported by the nuclear DNA doubling. The differentresponses between adaxial and abaxial epidermal cells were notinduced by the different light intensity at both surfaces. Itwas shown that adaxial epidermal cells have a different propertythan abaxial ones. Key words: Cell enlargement, endopolyploidization, epidermal pavement cells, incident light intensity, leaf expansion, nuclear DNA content, Phaseolus vulgaris     

Inverted vascular bundles in the leaf of Cladium (Cyperaceae)

Illustrations of seedling development and the adult plant of Cladium jamaicense Crantz are given. The course of vascular bundles in the leaf is described in detail. The structure of seedling and adult leaves are compared. Development of leaf primordia and initiation of vascular bundles are followed. Changes in the anatomy within one leaf and differences between foliage leaves and scales are related to their development. The adaxial vascular bundles that are inversely orientated are initiated within a procambial complex in close association with the largest abaxial bundles. Normally orientated adaxial bundles have an origin independent of other bundles. A hypothesis is presented which accounts for the differentiation of inverted bundles in morphogenetic terms. No developmental evidence was found to support the phylogenetic derivation of the leaf of Cladium by adaxial folding and fusion of laminar halves with their own adaxial surfaces.     

Untangling metabolic and spatial interactions of stress tolerance in plants. 1. Patterns of carbon metabolism within leaves

The localization of the key photoreductive and oxidative processes and some stress-protective reactions within leaves of mesophytic C₃ plants were investigated. The role of light in determining the profile of Rubisco, glutamate oxaloacetate transaminase, catalase, fumarase, and cytochrome-c-oxidase across spinach leaves was examined by exposing leaves to illumination on either the adaxial or abaxial leaf surfaces. Oxygen evolution in fresh paradermal leaf sections and CO₂ gas exchange in whole leaves under adaxial or abaxial illumination was also examined. The results showed that the palisade mesophyll is responsible for the midday depression of photosynthesis in spinach leaves. The photosynthetic apparatus was more sensitive to the light environment than the respiratory apparatus. Additionally, examination of the paradermal leaf sections by optical microscopy allowed us to describe two new types of parenchyma in spinach—pirum mesophyll and pillow spongy mesophyll. A hypothesis that oxaloacetate may protect the upper leaf tissue from the destructive influence of active oxygen is presented. The application of mathematical modeling shows that the pattern of enzymatic distribution across leaves abides by the principle of maximal ecological utility. Light regulation of carbon metabolism across leaves is discussed.     

Specification of adaxial and abaxial stomata, epidermal structure and photosynthesis to CO2 enrichment in maize leaves

Acclimation to CO2 enrichment was studied in maize plants grown to maturity in either 350 or 700 microl l-1 CO2. Plants grown with CO2 enrichment were significantly taller than those grown at 350 microl l-1 CO2 but they had the same number of leaves. High CO2 concentration led to a marked decrease in whole leaf chlorophyll and protein. The ratio of stomata on the adaxial and abaxial leaf surfaces was similar in all growth conditions, but the stomatal index was considerably increased in plants grown at 700 microl l-1 CO2. Doubling the atmospheric CO2 content altered epidermal cell size leading to fewer, much larger cells on both leaf surfaces. The photosynthesis and transpiration rates were always higher on the abaxial surface than the adaxial surface. CO2 uptake rates increased as atmospheric CO2 was increased up to the growth concentrations on both leaf surfaces. Above these values, CO2 uptake on the abaxial surface was either stable or increased as CO2 concentration increased. In marked contrast, CO2 uptake rates on the adaxial surface were progressively inhibited at concentrations above the growth CO2 value, whether light was supplied directly to this or the abaxial surface. These results show that maize leaves adjust their stomatal densities through changes in epidermal cell numbers rather than stomatal numbers. Moreover, the CO2-response curve of photosynthesis on the adaxial surface is specifically determined by growth CO2 abundance and tracks transpiration. Conversely, photosynthesis on the abaxial surface is largely independent of CO2 concentration and rather independent of stomatal function.     

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.