Effect of nitrogen-deficiency on midday photoinhibition in flag leaves of different rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars

Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H₂O₂) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H₂O₂ accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H₂O₂. Therefore, H₂O₂-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition.     

Dorsoventral variations in dark chilling effects on photosynthesis and stomatal function in Paspalum dilatatum leaves

The effects of dark chilling on the leaf-side-specific regulation of photosynthesis were characterized in the C(4) grass Paspalum dilatatum. CO(2)- and light-response curves for photosynthesis and associated parameters were measured on whole leaves and on each leaf side independently under adaxial and abaxial illumination before and after plants were exposed to dark chilling for one or two consecutive nights. The stomata closed on the adaxial sides of the leaves under abaxial illumination and no CO(2) uptake could be detected on this surface. However, high rates of whole leaf photosynthesis were still observed because CO(2) assimilation rates were increased on the abaxial sides of the leaves under abaxial illumination. Under adaxial illumination both leaf surfaces contributed to the inhibition of whole leaf photosynthesis observed after one night of chilling. After two nights of chilling photosynthesis remained inhibited on the abaxial side of the leaf but the adaxial side had recovered, an effect related to increased maximal ribulose-1,5-bisphosphate carboxylation rates (V(cmax)) and enhanced maximal electron transport rates (J(max)). Under abaxial illumination, whole leaf photosynthesis was decreased only after the second night of chilling. The chilling-dependent inhibition of photosynthesis was located largely on the abaxial side of the leaf and was related to decreased V(cmax) and J(max), but not to the maximal phosphoenolpyruvate carboxylase carboxylation rate (V(pmax)). Each side of the leaf therefore exhibits a unique sensitivity to stress and recovery. Side-specific responses to stress are related to differences in the control of enzyme and photosynthetic electron transport activities.     

Expression of a mutant maize gene in the ventral leaf epidermis is sufficient to signal a switch of the leaf's dorsoventral axis

Maize leaves are initiated from the shoot apex with an inherent leaf dorsoventral polarity; the leaf surface closest to the meristem is the adaxial (upper, dorsal) surface whereas the opposite leaf surface is the abaxial (lower, ventral) surface. The Rolled leaf1 (Rld1) semi-dominant maize mutations affect dorsoventral patterning by causing adaxialization of abaxial leaf regions. This adaxialization is sometimes associated with abaxialization of the adaxial leaf regions, which constitutes a "switch". Dosage analysis indicates Rld1 mutants are antimorphs. We mapped Rld1's action to a single cell layer using a mosaic analysis and show Rld1 acts non cell-autonomously along the dorsoventral axis. The presence of Rld1 mutant product in the abaxial epidermis is necessary and sufficient to induce the Rolled leaf1 phenotype within the lower epidermis as well as in other leaf layers along the dorsoventral axis. These results support a model for the involvement of wild-type RLD1 in the maintenance of dorsoventral features of the leaf. In addition, they demonstrate the abaxial epidermis sends/receives a cell fate determining signal to/from the adaxial epidermis and controls the dorsoventral patterning of the maize leaf.     

Leaf dimorphism in Thuja plicata and Platycladus orientalis (thujoid Cupressaceae s. str., Coniferales): the changes in morphology and anatomy from juvenile needle leaves to mature scale leaves

Thuja plicata and Platycladus orientalis initially produce only bifacial needle leaves. When the first lateral shoots develop, the leaf morphology and anatomy changes dramatically. Subsequently, only greatly reduced, bifacial scale leaves are developed. A new kind of “superimposed bifaciality” occurs with the change from juvenile needle leaves to mature scale leaves. Anatomical dorsiventrality affects not only the individual leaf, but also the complete plagiotropic lateral shoots of Thuja, which have a sun- and shade-exposed side. The upper light-exposed median leaves show adaxial leaf anatomy, contrary to the lower shaded median leaves showing abaxial leaf anatomy. Due to their mixed exposure, the lateral leaves show a lateral differentiation. At vertical lateral shoots of Platycladus, a predominant light-exposed side is absent. Thus, the anatomical dorsiventrality does not affect the complete shoot. Here the morphological abaxial side of a scale leaf becomes functionally and physiologically adaxial by reorientation of the palisade parenchyma and stomata. In juvenile needle leaves, the palisade parenchyma is located adaxial, with the majority of stomata being located abaxial. Conversely, in mature scale leaves, the palisade parenchyma is abaxial and the majority of stomata are adaxial.     

The geometry of light interception by shoots of Heteromeles arbutifolia: morphological and physiological consequences for individual leaves

The influence of leaf orientation and position within shoots on individual leaf light environments, carbon gain, and susceptibility to photoinhibition was studied in the California chaparral shrub Heteromeles arbutifolia with measurements of gas exchange and chlorophyll fluorescence, and by application of a three-dimensional canopy architecture model. Simulations of light absorption and photosynthesis revealed a complex pattern of leaf light environments and resulting leaf carbon gain within the shoots. Upper, south-facing leaves were potentially the most productive because they intercepted greater daily photon flux density (PFD) than leaves of any other orientation. North-facing leaves intercepted less PFD but of this, more was received on the abaxial surface because of the steep leaf angles. Leaves differed in their response to abaxial versus adaxial illumination depending on their orientation. While most had lower photosynthetic rates when illuminated on their abaxial as compared to adaxial surface, the photosynthetic rates of north-facing leaves were independent of the surface of illumination. Because of the increasing self-shading, there were strong decreases in absorbed PFD and daily carbon gain in the basipetal direction. Leaf nitrogen per unit mass also decreased in the basipetal direction but on a per unit area basis was nearly constant along the shoot. The decrease in leaf N per unit mass was accounted for by an increase in leaf mass per unit area (LMA) rather than by movement of N from older to younger leaves during shoot growth. The increased LMA of older lower leaves may have contributed directly to their lower photosynthetic capacities by increasing the limitations to diffusion of CO₂ within the leaf to the sites of carboxylation. There was no evidence for sun/shade acclimation along the shoot. Upper leaves and especially south-facing upper leaves had a potential risk for photoinhibition as demonstrated by the high PFDs received and the diurnal decreases in the fluorescence ratio F _v/F _m. Predawn F _v/F _m ratios remained high (>0.8) indicating that when in their normal orientations leaves sustained no photoinhibition. Reorientation of the leaves to horizontal induced a strong sustained decrease in F _v/F _m and CO₂ exchange that slowly recovered over the next 10–15?days. If leaves were also inverted so that the abaxial surface received the increased PFDs, then the reduction in F _v/F _m and CO₂ assimilation was much greater with no evidence for recovery. The heterogeneity of responses was due to a combination of differences between leaves of different orientation, differences between responses on their abaxial versus adaxial surfaces, and differences along the shoot due to leaf age and self-shading effects.     

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.     

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.     

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.     

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.     

Effects of nitrogen supply restriction on gas exchange and photosystem 2 function in flag leaves of a traditional low-yield cultivar and a recently improved high-yield cultivar of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The effects of nitrogen (N) supply restriction on the CO₂ assimilation and photosystem 2 (PS2) function of flag leaves were compared between two contrastive Japanese rice cultivars, a low-yield cultivar released one century ago, cv. Shirobeniya (SRB), and a recently improved high-yield cultivar, cv. Akenohoshi (AKN). Both cultivars were solution-cultured at four N supply levels from N4 (control) to N1 (the lowest). With a reduction in N-supply, contents of N (LNC), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and chlorophyll (Chl) in flag leaves decreased in both cultivars. In parallel with this, the net photosynthetic rate (P _N), mesophyll conductance (g _m), and stomatal conductance (g _s) decreased. P _N was more dominantly restricted by g _m than g _s. The values of P _N, g _m, and RuBPCO content were larger in AKN than SRB at the four N supply levels. The content of Chl greatly decreased with N deficiency, but the reduction in the maximum quantum yield of PS2 was relatively small. Quantum yield of PS2 (Φ_PS2) decreased with N deficiency, and its significant cultivar difference was observed between the two cultivars at N1: a high value was found in AKN. The content ratio of Chl/RuBPCO was also significantly low in AKN. The low Chl/RuBPCO is one of the reasons why AKN maintained a comparatively high P _N and Φ_PS2 at N deficiency. The adequate ratio of N distribution between Chl and RuBPCO is the important prerequisite for the efficient and sustainable photosynthesis in a flag leaf of rice plant under low N-input.     

Photosynthetic costs and benefits of abaxial versus adaxial anthocyanins in Colocasia esculenta ‘Mojito’

Main conclusion

Anthocyanins in upper (adaxial) leaf tissues provide greater photoprotection than in lower (abaxial) tissues, but also predispose tissues to increased shade acclimation and, consequently, reduced photosynthetic capacity. Abaxial anthocyanins may be a compromise between these costs/benefits. Plants adapted to shaded understory environments often exhibit red/purple anthocyanin pigmentation in lower (abaxial) leaf surfaces, but rarely in upper (adaxial) surfaces. The functional significance of this color pattern in leaves is poorly understood. Here, we test the hypothesis that abaxial anthocyanins protect leaves of understory plants from photo-oxidative stress via light attenuation during periodic exposure to high incident sunlight in the forest understory, without interfering with sunlight capture and photosynthesis during shade conditions. We utilize a cultivar of Colocasia esculenta exhibiting adaxial and abaxial anthocyanin variegation within individual leaves to compare tissues with the following color patterns: green adaxial, green abaxial (GG), green adaxial, red abaxial (GR), red adaxial, green abaxial (RG), and red adaxial, red abaxial (RR). Consistent with a photoprotective function of anthocyanins, tissues exhibited symptoms of increasing photoinhibition in the order (from least to greatest): RR, RG, GR, GG. Anthocyanic tissues also showed symptoms of shade acclimation (higher total chl, lower chl a/b) in the same relative order. Inconsistent with our hypothesis, we did not observe any differences in photosynthetic CO₂ uptake under shade conditions between the tissue types. However, GG and GR had significantly (39 %) higher photosynthesis at saturating irradiance (A _sat) than RG and RR. Because tissue types did not differ in nitrogen content, these patterns likely reflect differences in resource allocation at the tissue level, with greater nitrogen allocated toward energy processing in GG and GR, and energy capture in RG and RR (consistent with relative sun/shade acclimation). We conclude that abaxial anthocyanins are likely advantageous in understory environments because they provide some photoprotection during high-light exposure, but without the cost of decreased A _sat associated with adaxial anthocyanin-induced shade syndrome.     

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.     

Auxin polar transport flanking incipient primordium initiates leaf adaxial‐abaxial polarity patterning

The leaves of most higher plants are polar along their adaxial‐abaxial axis, and the development of the adaxial domain (upper side) and the abaxial domain (lower side) makes the leaf a highly efficient photosynthetic organ. It has been proposed that a hypothetical signal transported from the shoot apical meristem (SAM) to the incipient leaf primordium, or conversely, the plant hormone auxin transported from the leaf primordium to the SAM, initiates leaf adaxial‐abaxial patterning. This hypothetical signal has been referred to as the Sussex signal, because the research of Ian Sussex published in 1951 was the first to imply its existence. Recent results, however, have shown that auxin polar transport flanking the incipient leaf primordium, but not the Sussex signal, is the key to initiate leaf polarity. Here, we review the new findings and integrate them with other recently published results in the field of leaf development, mainly focusing on the early steps of leaf polarity establishment.     

Photosystem II efficiency of the palisade and spongy mesophyll in <Emphasis Type="Italic">Quercus coccifera</Emphasis> using adaxial/abaxial illumination and excitation light sources with wavelengths varying in penetration into the leaf tissue

The existence of major vertical gradients within the leaf is often overlooked in studies of photosynthesis. These gradients, which involve light heterogeneity, cell composition, and CO₂ concentration across the mesophyll, can generate differences in the maximum potential PSII efficiency (F _V/F _M or F _V/F _P) of the different cell layers. Evidence is presented for a step gradient of F _V/F _P ratios across the mesophyll, from the adaxial (palisade parenchyma, optimal efficiencies) to the abaxial (spongy parenchyma, sub-optimal efficiencies) side of Quercus coccifera leaves. For this purpose, light sources with different wavelengths that penetrate more or less deep within the leaf were employed, and measurements from the adaxial and abaxial sides were performed. To our knowledge, this is the first report where a low photosynthetic performance in the abaxial side of leaves is accompanied by impaired F _V/F _P ratios. This low photosynthetic efficiency of the abaxial side could be related to the occurrence of bundle sheath extensions, which facilitates the penetration of high light intensities deep within the mesophyll. Also, leaf morphology (twisted in shape) and orientation (with a marked angle from the horizontal plane) imply direct sunlight illumination of the abaxial side. The existence of cell layers within leaves with different photosynthetic efficiencies makes appropriate the evaluation of how light penetrates within the mesophyll when using Chl fluorescence or gas exchange techniques that use different wavelengths for excitation and/or for driving photosynthesis.     

Characterization and density of trichomes on three common bean cultivars

Trichomes have been implicated as a mechanism which can confer resistance to both plant pests and drought. A study was conducted to provide information regarding genetic variability for trichome distribution and density among three diverse dry bean (Phaseolus vulgaris L.) cultivars, and to characterize the types of trichomes present among the cultivars. Trichomes on the leaf surfaces were micrographed with a scanning electron microscope (SEM) and counted using a stereomicroscope on both the abaxial and adaxial leaf surfaces of the cultivars ‘Bill Z’, ‘Pompadour Checa’ and ‘Diacol Calima’. Straight, hooked, and glandular trichomes were observed on the leaf surfaces of each cultivar. SEM micrographs are presented for the leaf surfaces of each cultivar and trichome type. The abaxial leaf surface had more straight trichomes than the adaxial leaf surface for ‘Pompadour Checa’ and ‘Diacol Calima’, however ‘Bill Z’ had more on the adaxial surface. The opposite relationship existed among the cultivars and leaf surfaces for the hooked trichomes.     

Distinct light responses of the adaxial and abaxial stomata in intact leaves of Helianthus annuus L

Using a laboratory-constructed system that can measure the gas exchange rates of two leaf surfaces separately, the light responses of the adaxial and abaxial stomata in intact leaves of sunflower ( Helianthus annuus L.) were investigated, keeping the intercellular CO₂ concentration ( C _i) at 300  µ L L⁻¹. When evenly illuminating both sides of the leaf, the stomatal conductance ( g _s) of the abaxial surface was higher than that of the adaxial surface at any light intensity. When each surface of the leaf was illuminated separately, both the adaxial and abaxial stomata were more sensitive to the light transmitted through the leaf (self-transmitted light) than to direct illumination. Relationships between the whole leaf photosynthetic rate ( A _n) and the g _s for each side highlighted a strong dependence of stomatal opening on mesophyll photosynthesis. Light transmitted through another leaf was more effective than the direct white light for the abaxial stomata, but not for the adaxial stomata. Moreover, green monochromatic light induced an opening of the abaxial stomata, but not of the adaxial stomata. As the proportion of blue light in the transmitted light is less than that in the white light, there may be some uncharacterized light responses, which are responsible for the opening of the abaxial stomata by the transmitted, green light.     

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.     

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.     

The Effects of Drought Stress and Leaf Ageing on Leaf Photosynthesis and Electron Transport in Photosystem 2 in Sweet Potato (Ipomoea batatas Lam.) Cultivars

Of the four tested sweet potato cultivars having different features in growth and yield, cv. Koganesengan (KOG) was sustainable in photosynthetic activity through young to aged leaves under drought. One of the causes for this phenomenon may be stomatal conductance (g _s) of this cultivar that was relatively high in both aged and drought-imposed leaves. In these leaves the non-photochemical quenching (NPQ) was low and the quantum yield of photosystem 2 (Φ_e) was high, compared to those of the other cultivars. This helps to prevent excessive accumulation of chemical energy in leaves and a decrease in photoinhibition damage to the photosynthetic function, by which KOG sustains a relatively high photosynthetic activity under the drought and alleviates functional deterioration caused by leaf age. This revised version was published online in August 2006 with corrections to the Cover Date.