Asymmetric responses of adaxial and abaxial stomata to elevated CO2: impacts on the control of gas exchange by leaves

The response of adaxial and abaxial stomatal conductance in Rumex obtusifolius to growth at elevated atmospheric concentrations of CO₂ (250 μmol mol^?1 above ambient) was investigated over two growing seasons. The conductance of both the adaxial and abaxial leaf surfaces was found to be reduced by elevated concentrations of CO₂. Elevated CO₂ caused a much greater reduction in conductance for the adaxial surface than for the abaxial surface. The absence of effects upon stomatal density indicated that the reductions were probably the result of changes in stomatal aperture. Partitioning of gas exchange between the leaf surfaces revealed that increased concentrations of CO₂ caused increased rates of photosynthesis only via the abaxial surface. Additionally, leaf thickness was found to increase during growth at elevated concentrations of CO₂. The tendency for these amphistomatous leaves to develop a distribution of conductance approaching that of hypostomatous leaves clearly reduced their maximum photosynthetic potential. This conclusion was supported by measurements of stomatal limitation, which showed greater values for the adaxial surfaces, and greater values at elevated CO₂. This reduction in photosynthesis may in part be caused by higher diffusive limitations imposed because of increased leaf thickness. In an uncoupled canopy, asymmetrical stomatal responses of the kind identified here may appreciably reduce transpiration. Species which show symmetrical responses are less likely to show reduced transpirational rates, and a redistribution of water loss between species may occur. The implications of asymmetrical stomatal responses for photosynthesis and canopy transpiration are discussed.     

Temperature response of carbon isotope discrimination and mesophyll conductance in tobacco

The partial pressure of CO₂ at the sites of carboxylation within chloroplasts depends on the conductance to CO₂ diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (g_m). We investigated the temperature response of g_m in tobacco (Nicotiana tabacum) by combining gas exchange in high light, ambient CO₂ in either 2 or 21% O₂ with carbon isotope measurements using tuneable diode laser spectroscopy. The g_m increased linearly with temperature in 2 or 21% O₂. In 21% O₂, isotope discrimination associated with g_m decreased from 5.0 ± 0.2 to 1.8 ± 0.2‰ as temperature increased from 15 to 40 °C, but the photorespiratory contribution to the isotopic signal is significant. While the fractionation factor for photorespiration (f = 16.2 ± 0.7‰) was independent of temperature between 20 and 35 °C, discrimination associated with photorespiration increased from 1.1 ± 0.01 to 2.7 ± 0.02‰ from 15 to 40 °C. Other mitochondrial respiration contributed around 0.2 ± 0.03‰. The drawdown in CO₂ partial pressure from ambient air to intercellular airspaces was nearly independent of leaf temperature. By contrast, the increase in g_m with increasing leaf temperature resulted in the drawdown in CO₂ partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.     

Inclination of sun and shade leaves influences chloroplast light harvesting and utilization

Light harvesting and utilization by chloroplasts located near the adaxial vs the abaxial surface of sun and shade leaves were examined by fluorometry in two herbaceous perennials that differed in their anatomy and leaf inclination. Leaves of Thermopsis montana had well-developed palisade and spongy mesophyll whereas the photosynthetic tissue of Smilacina stellata consisted of spongy mesophyll only. Leaf orientation depended upon the irradiance during leaf development. When grown under low-light levels, leaves of S. stellata and T. montana were nearly horizontal, whereas under high-light levels, S. stellata leaves and T. montana leaves were inclined 60⁰ and 30⁰, respectively. Leaf inclination increased the amount of light that was intercepted by the lower leaf surfaces and affected the photosynthetic properties of the chloroplasts located near the abaxial leaf surface. The slowest rates of quinone pool reduction and reoxidation were found in chloroplasts located near the adaxial leaf surface of T. montana plants grown under high light, indicating large quinone pools in these chloroplasts. Chloroplasts near the abaxial surface of low-light leaves had lower light utilization capacities as shown by photochemical quenching measurements. The amount of photosystem II (PSII) down regulation, measured from each leaf surface, was also found to be influenced by irradiance and leaf inclination. The greatest difference between down regulation monitored from the adaxial vs abaxial surfaces was found in plants with horizontal leaves. Different energy dissipation mechanisms may be employed by the two species. Values for down regulation in S. stellata were 2–3 times higher than those in T. montana, while the portion of the PSII population which was found to be Q_B nonreducing was 4–6 times lower in high light S. stellata leaves than in T. montana. All values of Stern-Volmer type nonphotochemical quenching (NPQ) from S. stellata leaves were similar when quenching analysis was performed at actinic irradiances that were higher than the irradiance to which the leaf surface was exposed during growth. In contrast, with T. montana, NPQ values from the abaxial leaf surface were up to 45% higher than those from the adaxial leaf surface regardless of growth conditions. The observed differences in chloroplast properties between species and between the adaxial and abaxial leaf surfaces may depend upon a complex interaction among light, leaf anatomy and leaf inclination.     

On the relationship between leaf anatomy and CO2 diffusion through the mesophyll of hypostomatous leaves

Internal conductances to CO₂ transfer from the stomatal cavity to sites of carboxylation (g_i) in hypostomatous sun-and shade-grown leaves of citrus, peach and Macadamia trees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m^?2) for all leaves, including sclerophyllous Macadamia, whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high g_i, the chloro-plast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a), the lowest tissue density (T_d) and the highest chloro-plast surface area (S_c) exposed to ias. There were negative correlations between g_i and leaf thickness or D/a, but positive correlations between g_i and S_c or S_c/T_d. We developed a one-dimensional diffusion model which partitioned g_i into a gaseous diffusion conductance through the ias (g_ias) plus a liquid-phase conductance through mesophyll cell walls (g_cw). The model accounted for a significant amount of variation (r₂=0.80) in measured g_i by incorporating both components. The g_ias component was related to the one-dimensional path-length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The g_cw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between g_i and leaf thickness or D/a related to g_ias whereas positive correlations between g_i and S_c or S_c/T_d, related to g_cw. The g_cw was consistently lower than g_ias, and thus represented a greater constraint on CO₂ diffusion in the mesophylls of these hypostomatous species.     

Infection Process of Botrytis cinerea on Eucalypt Leaves

This study aimed to elucidate the infection process of Botrytis cinerea on eucalypt leaves. Tests were conducted to evaluate the influence of leaf side (adaxial or abaxial), leaf age and luminosity on conidial germination, appressorium formation and grey mould (GM) severity. The adaxial and abaxial surfaces of detached eucalypt leaves were inoculated with a conidial suspension of B. cinerea and kept under constant light or dark. Subsequently, the adaxial surface of young and old leaves was inoculated and kept in the dark. To evaluate the percentage of conidia germination and appressorium formation, leaf samples were collected 6 hours after inoculation (hai), clarified (alcohol and chloral hydrate) and evaluated under a light microscope. The severity of GM was assessed 10 days after inoculation. For scanning electron microscopy analysis, samples were collected from 2 to 168 hai. A higher percentage of conidia germination (92%) and GM severity (21%) occurred on the adaxial surfaces of leaves kept in the dark. There was no statistical difference between the surfaces of young and old leaves for conidia germination. No appressorium was formed by B. cinerea. The GM severity on young leaves (17.3%) was 34 times higher than on old leaves (0.5%). The micrographs showed germinating conidia emitting 1–4 germ tubes in samples at 4 hai. The fungus penetration occurred through intact leaf surfaces, and both extra‐ and intracellular colonization of the mesophyll cells by the hyphae of the pathogen were observed at 120 hai. Sporulation occurred on the adaxial and abaxial surfaces (macronematous conidiophores) and below the epidermis (micronematous conidiophores).     

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.     

Leaf bisection for the enzymatic isolation of mesophyll protoplasts fromSaintpaulia ionantha

We describe a rapid and efficient procedure for isolating leaf mesophyll protoplasts fromSaintpaulia ionantha (African violet). Because of the unusual surface properties of the leaf, efficient digestion required bisection of the lamina into abaxial and adaxial halves to expose the mesophyll. Enzymatic digestion occurred during a 3 h incubation with 2% (m/v) driselase in 0.50 M mannitol, yielding 2–3 × 10⁵ protoplasts g⁻¹ (f.m.), with 70% viability. Protoplasts ranged in size from 50 to 100μm, and were derived from mostly non-photosynthetic mesophyll parenchyma.     

Separating the contribution of the upper and lower mesophyll to photosynthesis in Zea mays L. leaves

The appearance of transverse sections of maize leaves indicates the existence of two airspace systems serving the mesophyll, one connected to the stomata of the upper epidermis and the other to the stomata of the lower surface, with few or no connections between the two. This study tests the hypothesis that the air-space systems of the upper and lower mesophyll are separated by a defined barrier of measurable conductance. A mathematical procedure, based on this hypothesis, is developed for the quantitative separation of the contributions made by the upper and lower halves of the mesophyll to carbon assimilation using gasexchange data. Serial paradermal sections and three-dimensional scanning-electron-microscope images confirmed the hypothesis that there were few connections between the two air-systems. Simultaneous measurements of nitrous-oxide diffusion across the leaf and of transpiration from the two surfaces showed that the internal conductance was about 15% of the maximum observed stomatal conductance. This demonstrates that the poor air-space connections, indicated by microscopy, represent a substantial barrier to gas diffusion. By measuring the CO₂ and water-vapour fluxes from each surface independently, the intercellular CO₂ concentration (c _i) of each internal air-space system was determined and the flux between them calculated. This allowed correction of the apparent CO₂ uptake at each surface to derive the true CO₂ uptake by the mesophyll cells of the upper and lower halves of the leaf. This approach was used to analyse the contribution of the upper and lower mesophyll to CO₂ uptake by the leaf as a whole in response to varying light levels incident on the upper leaf surface. This showed that the upper mesophyll was light-saturated by a photon flux of approx. 1000 mol·m^-2·s^-1 (i.e. about one-half of full sunlight). The lower mesophyll was not fully saturated by photon fluxes of nearly double full sunlight. At low photon fluxes the c _i of the upper mesophyll was significantly less than that of the lower mesophyll, generating a significant upward flux of CO₂. At light levels equivalent to full sunlight, and above, c _i did not differ significantly between the two air space systems. The physiological importance of the separation of the air-space systems of the upper and lower mesophyll to gas exchange is discussed.Abbreviations and symbols A net leaf CO₂ uptake rate - A _upper ^app. and A _lower ^app. net rates of CO₂ uptake across the upper and lower surfaces - A _upper and A _lower derived net rates of CO₂ uptake by the upper and lower mesophyll - A _upward net flux of CO₂ from the lower to upper mesophyll - c _a, c _{a, upper} and c _{a, lower} the CO₂ concentrations in the air around the leaf above the upper surface and below the lower surface - c _N2O the concentration of N₂O in the air around the leaf - c _i, c _{i, upper} and c _{i, lower} the mesophyll intercellular CO₂ concentration of the whole leaf, the upper mesophyll and the lower mesophyll - g _i leaf internal conductance to CO₂ - g _s, g _{s, lower} and g _{s, upper} the stomatal conductance of the whole leaf, the lower surface and the upper surface - g the total conductance across the leaf - Q the photosynthetically active photon flux density     

葛(豆科)叶的解剖学研究

利用光学显微镜和扫描电镜观察了葛(Pueraria lobata)叶的解剖学特征。结果表明,葛叶片的上、下表皮都只有一层表皮细胞,上表皮比下表皮厚。上、下表皮都有腺毛和非腺毛。气孔主要分布在下表皮,下表皮的气孔密度为(261±17)mm-2,上表皮只有(6±3)mm-2。叶肉由两层栅栏组织细胞和一层海绵组织细胞构成。叶肉细胞中有丰富的叶绿体。在栅栏组织和海绵组织之间有一层平行于叶脉的薄壁细胞。叶脉中含有大量的草酸钙晶体。葛叶的这些形态特征与其喜阳、耐旱的特点相适应。     

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.     

Leaf traits variation during leaf expansion in <Emphasis Type="Italic">Quercus ilex</Emphasis> L.

The morphological, anatomical and physiological variations of leaf traits were analysed during Quercus ilex L. leaf expansion. The leaf water content (LWC), leaf area relative growth rate (RGR_l) and leaf dry mass relative growth rate (RGR_m) were the highest (76±2 %, 0.413 cm² cm⁻² d⁻¹, 0.709 mg mg⁻¹ d⁻¹, respectively) at the beginning of the leaf expansion process (7 days after bud break). Leaf expansion lasted 84±2 days when air temperature ranged from 13.3±0.8 to 27.6±0.9 °C. The net photosynthetic rate (P _N), stomatal conductance (g _s), and chlorophyll content per fresh mass (Chl) increased during leaf expansion, having the highest values [12.62±1.64 μmol (CO₂) m⁻² s⁻¹, 0.090 mol (H₂O) m⁻² s⁻¹, and 1.03±0.08 mg g⁻¹, respectively] 56 days after bud break. Chl was directly correlated with leaf dry mass (DM) and P _N. The thickness of palisade parenchyma contributed to the total leaf thickness (263.1±1.5 μm) by 47 %, spongy layer thickness 38 %, adaxial epidermis and cuticle thickness 9 %, and abaxial epidermis and cuticle thickness 6 %. Variation in leaf size during leaf expansion might be attributed to a combination of cells density and length, and it is confirmed by the significant (p<0.001) correlations among these traits. Q. ilex leaves reached 90 % of their definitive structure before the most severe drought period (beginning of June — end of August). The high leaf mass area (LMA, 15.1±0.6 mg cm⁻²) at full leaf expansion was indicative of compact leaves (2028±100 cells mm⁻²). Air temperature increasing might shorten the favourable period for leaf expansion, thus changing the final amount of biomass per unit leaf area of Q. ilex.     

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.     

Reaction of nitric oxide with the oxidized di-heme and heme–copper oxygen-reducing centers of terminal oxidases: Different reaction pathways and end-products

Inhibition of terminal oxidases by nitric oxide (NO) has been extensively investigated as it plays a role in regulation of cellular respiration and pathophysiology. Cytochrome bd is a tri-heme (b₅₅₈, b₅₉₅, d) bacterial oxidase containing no copper that couples electron transfer from quinol to O₂ (to produce H₂O) with generation of a transmembrane protonmotive force. In this work, we investigated by stopped-flow absorption spectroscopy the reaction of NO with Escherichia coli cytochrome bd in the fully oxidized (O) state. We show that under anaerobic conditions, the O state of the enzyme binds NO at heme d with second-order rate constant k_on = 1.5 ± 0.2 × 10² M⁻¹ s⁻¹, yielding a nitrosyl adduct (d³⁺–NO or d²⁺–NO⁺) with characteristic optical features (an absorption increase at 639 nm and a red shift of the Soret band). The reaction mechanism is remarkably different from that of O cytochrome c oxidase in which the heme–copper binuclear center reacts with NO approximately three orders of magnitude faster, forming nitrite. The data allow us to conclude that in the reaction of NO with terminal oxidases in the O state, Cu_B is indispensable for rapid oxidation of NO into nitrite.     

Influence of growth regulators on direct embryo formation from leaf explants of Phalaenopsis orchids

Leaf explants of two Phalaenopsis, P. amabilis and P. Nebula, were used to test the effects of auxins (2,4-D, IAA, IBA, NAA), cytokinins (2iP, BA, kinetin, TDZ, zeatin), GA₃, ancymidol, polyamines (putrescine, spermine, spermidine), ACC, AgNO₃ and CoCl₂ on the amount of direct embryo formation on different leaf locations (the cut end, the adaxial side, the abaxial side and the leaf tip). The results showed that there was a genotypic effect on direct embryo formation induced by cytokinins that 13.32 μM BA and 4.92 μM 2iP was the most effective in P. amabilis and P. Nebula, respectively. Besides, explant position highly affected embryogenic competence of leaf cells in both species that the cut end showed highest embryogenic response, the adaxial side was the second, and then the abaxial side and the leaf tip. Altogether, cytokinins tested were all effective in both species, and ACC at 20 μM had 35% of embryogenic response in P. amabilis. However, auxins, GA₃, ancymidol and polyamines were inhibitory in both species.     

δ13C of CO2 respired in the dark in relation to δ13C of leaf metabolites: comparison between Nicotiana sylvestris and Helianthus annuus under drought

The variations of δ¹³C in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO₂ respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well‐watered conditions, CO₂ respired in the dark was ¹³C‐enriched compared to sucrose by about 4‰ in N. sylvestris and by about 3‰ and 6‰ in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris, we observed a constant ¹³C‐enrichment by about 6‰ in respired CO₂ compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the ¹³C‐enrichment in respired CO₂ increased in dehydrated N. sylvestris and decreased in dehydrated H. annuus in comparison with control plants. We conclude that (i) carbon isotope fractionation during dark respiration is a widespread phenomenon occurring in C₃ plants, but that (ii) this fractionation is not constant and varies among species and (iii) it also varies with environmental conditions (water deficit in the present work) but differently among species. We also conclude that (iv) a discrimination during dark respiration processes occurred, releasing CO₂ enriched in ¹³C compared to several major leaf reserves (carbohydrates, lipids and organic acids) and whole leaf organic matter.     

Studies on octylphenoxy surfactants: IX. Effect of oxyethylene chain length on GA3 absorption by sour cherry leaves

The effect of a homologous series of octylphenoxy surfactants, -[4-(1,1,3,3-tetramethylbutyl)phenyl]--hydroxypoly-(oxy-1,2-ethanediyl), condensed with 5, 7–8, 9–10, 16, and 30 oxyethylene (EO) units on enhancement of gibberellic acid (GA₃) absorption by leaves ofPrunus cerasus cv. Montmorency was studied. Increasing EO chain length (5–30 EO) increased surface tension (27.5–35.3 mN m^–1) and contact angles on adaxial (21–36°) and abaxial (28–49°) leaf surfaces. With increasing EO content, the form of GA₃ deposits from droplets on the leaf surface changed from an annulus shape (5 and 7–8 EO) to globular forms covering increasingly smaller interface areas (9–10 to 30 EO). The surfactants increased GA₃ uptake, the magnitude decreased with an increase in oxyethylene chain length. Similar trends were found for both the adaxial and abaxial surfaces. Penetration through the abaxial surface was linearly related to the logarithm of the oxyethylene content of the surfactant molecule (r ²=0.934^**) and to the hydrophilic: lipophilic balance (r ²=0.926^**). Absorption by the abaxial surface was approximately one order of magnitude greater than by the adaxial surface.     

Measurement of gradients of absorbed light in spinach leaves from chlorophyll fluorescence profiles

Profiles of chlorophyll fluorescence were measured in spinach leaves irradiated with monochromatic light. The characteristics of the profiles within the mesophyll were determined by the optical properties of the leaf tissue and the spectral quality of the actinic light. When leaves were infiltrated with 10^?4M DCMU [3‐(3,4‐dichlorophenyl)‐1, 1‐dimethyl‐urea] or water, treatments that minimized light scattering, irradiation with 2000 μmol m^?2 s^?1 green light produced broad Gaussian‐shaped fluorescence profiles that spanned most of the mesophyll. Profiles for chlorophyll fluorescence in the red (680 ± 16 nm) and far red (λ > 710 nm) were similar except that there was elevated red fluorescence near the adaxial leaf surface relative to far red fluorescence. Fluorescence profiles were narrower in non‐infiltrated leaf samples where light scattering increased the light gradient. The fluorescence profile was broader when the leaf was irradiated on its adaxial versus abaxial surface due to the contrasting optical properties of the palisade and spongy mesophyll. Irradiation with blue, red and green monochromatic light produced profiles that peaked 50, 100 and 150 μm, respectively, beneath the irradiated surface. These results are consistent with previous measurements of the light gradient in spinach and they agree qualitatively with measurements of carbon fixation under monochromatic blue, red and green light. These results suggest that chlorophyll fluorescence profiles may be used to estimate the distribution of quanta that are absorbed within the leaf for photosynthesis.     

Two sides of a leaf blade: <Emphasis Type="Italic">Blumeria graminis</Emphasis> needs chemical cues in cuticular waxes of <Emphasis Type="Italic">Lolium perenne</Emphasis> for germination and differentiation

Plant surface characteristics were repeatedly shown to play a pivotal role in plant–pathogen interactions. The abaxial leaf surface of perennial ryegrass (Lolium perenne) is extremely glossy and wettable compared to the glaucous and more hydrophobic adaxial surface. Earlier investigations have demonstrated that the abaxial leaf surface was rarely infected by powdery mildew (Blumeria graminis), even when the adaxial surface was densely colonized. This led to the assumption that components of the abaxial epicuticular leaf wax might contribute to the observed impairment of growth and development of B. graminis conidia on abaxial surfaces of L. perenne. To re-assess this hypothesis, we analyzed abundance and chemical composition of L. perenne ab- and adaxial epicuticular wax fractions. While the adaxial epicuticular waxes were dominated by primary alcohols and esters, the abaxial fraction was mainly composed of n-alkanes and aldehydes. However, the major germination and differentiation inducing compound, the C₂₆-aldehyde n-hexacosanal, was not present in the abaxial epicuticular waxes. Spiking of isolated abaxial epicuticular Lolium waxes with synthetically produced n-hexacosanal allowed reconstituting germination and differentiation rates of B. graminis in an in vitro germination assay using wax-coated glass slides. Hence, the absence of the C₂₆-aldehyde from the abaxial surface in combination with a distinctly reduced surface hydrophobicity appears to be primarily responsible for the failure of normal germling development of B. graminis on the abaxial leaf surfaces of L. perenne.     

Ozone-induced ethylene release from leaf surfaces

Ozone-induced stress ethylene emissions from the adaxial and abaxial leaf surfaces of four plant species (Glycine max [L.] Merr. cv. Dare, Lycopersicon esculentum Mill cv. Roma VF, Eucalyptus globulus Labill. and Hedera helix L.) were studied to determine if the stress ethylene diffused through the stomata or cuticle. In plants not exposed to ozone, basal ethylene was detected above both the adaxial and abaxial leaf surfaces of all the plant species examined, indicating that some ethylene can diffuse across the leaf cuticle. Ozone induced stress ethylene production in all species examined. Significant ozone-induced ethylene concentrations were detected above both surfaces of amphistomatous soybean (Glycine) and tomato (Lycopersicon) leaves. In contrast, ozone-induced ethylene production was associated only with the leaf surface (abaxial) that contained stomata for hypostomatous blue gum eucalyptus and English ivy (Hedera) leaves; the leaf surface (adaxial) of the eucalyptus and ivy leaves which did not contain stomata did not release significant amounts of stress ethylene. These data indicate that ozone-induced stress ethylene primarily diffuses from the leaf via the stomata.     

Association genetics,geography and ecophysiology link stomatal patterning in Populus trichocarpa with carbon gain and disease resistance trade‐offs

Stomata are essential for diffusive entry of gases to support photosynthesis, but may also expose internal leaf tissues to pathogens. To uncover trade‐offs in range‐wide adaptation relating to stomata, we investigated the underlying genetics of stomatal traits and linked variability in these traits with geoclimate, ecophysiology, condensed foliar tannins and pathogen susceptibility in black cottonwood (Populus trichocarpa). Upper (adaxial) and lower (abaxial) leaf stomatal traits were measured from 454 accessions collected throughout much of the species range. We calculated broad‐sense heritability (H²) of stomatal traits and, using SNP data from a 34K Populus SNP array, performed a genome‐wide association studies (GWAS) to uncover genes underlying stomatal trait variation. H² values for stomatal traits were moderate (average H² = 0.33). GWAS identified genes associated primarily with adaxial stomata, including polarity genes (PHABULOSA), stomatal development genes (BRASSINOSTEROID‐INSENSITIVE 2) and disease/wound‐response genes (GLUTAMATE‐CYSTEINE LIGASE). Stomatal traits correlated with latitude, gas exchange, condensed tannins and leaf rust (Melampsora) infection. Latitudinal trends of greater adaxial stomata numbers and guard cell pore size corresponded with higher stomatal conductance (g_s) and photosynthesis (A_max), faster shoot elongation, lower foliar tannins and greater Melampsora susceptibility. This suggests an evolutionary trade‐off related to differing selection pressures across the species range. In northern environments, more adaxial stomata and larger pore sizes reflect selection for rapid carbon gain and growth. By contrast, southern genotypes have fewer adaxial stomata, smaller pore sizes and higher levels of condensed tannins, possibly linked to greater pressure from natural leaf pathogens, which are less significant in northern ecosystems.