Bundle Sheath Extensions Research Articles

Leaf anatomy affects optical properties and enhances photosynthetic performance under oblique light.

Photosynthesis under oblique illumination has not been studied extensively despite being the prevailing light regime under natural conditions. We studied how photosynthetic rate (An) is affected by the geometrical arrangement between leaf lamina and light rays, in conjunction with key anatomical features; studied plant species selected based on the absence (homobaric) or the occurrence of bundle sheath extensions (BSEs; heterobaric) and the arrangement of these structures, that is, parallel (monocots) or reticulated (dicots). The direction of light ray affected leaf absorptance (Abs) and An; both were maximal when the angle of incidence of light on leaf surface (polar angle, θ) was 90°. For any lower θ, both Abs and An were higher when the angle between the leaf axis and the light rays (azimuthal angle, φ) was zero. The dependence of Abs and An from φ was only evident in monocots and, especially, in heterobaric compared to homobaric leaves. In some species, An was substantially higher than predicted from calculated photon flux density of oblique light. The occurrence of BSEs, especially in monocots, significantly alters leaf optical properties, resulting in more efficient photosynthesis under oblique illumination conditions.

Leaf anatomy of seven endemic species of Glicophyllum (Malpighiaceae) in Brazil.

Glicophyllum is well supported, presenting four homoplasies, easily differentiated from the other genera of the clade due to characteristics related to the reproductive organs, which can make the identification of their species difficult when they are not in the reproductive phase. Therefore, there are provided the leaf anatomical and morphological description of the leaf glands of seven species of Glicophyllum to assist in the identification of their species. The samples for the study were obtained from several national and international herbaria, sectioned freehand, stained with basic fuchsin - astra blue and compared through a binary matrix using the Sorensen's coefficient in the MVSP software. Among the leaf anatomical characters found, the following stand out: presence/absence of trichomes; petiole contour; contour of the main vein; organization of the mesophyll, presence/absence of bundle sheath extension and the surface of the glands. In this study, an identification key with leaf anatomical data is presented for the first time, demonstrating the applicability of leaf anatomy for the taxonomy of Glicophyllum. In the multivariate analysis, it is observed that the characteristics of leaf venation and topology of the glands are more representative to differentiate the taxa. Therefore, the data obtained can support future taxonomic and phylogenetic studies of the genus.

Taxonomic significance and evolution of homobaric and heterobaric leaves in Adesmia clade species (Leguminosae – Papilionoideae)

The presence/absence of bundle sheath extensions (BSE) in a leaf blade allows the classification of the leaves as heterobaric or homobaric, respectively. The compartmentalization of the mesophyll by BSE is associated with differences in functional aspects of heterobaric and homobaric leaves, such as conduction of water and luminosity, traits related to the successful colonization of different environments by plants. Therefore, studies on the distribution of different leaf types in a taxon can provide information on ecological, systematic, physiological, and evolutionary aspects. The Adesmia clade belongs to the legume tribe Dalbergieae (Papilionoideae, Leguminosae) and comprises five genera with a neotropical or pantropical distribution. We mapped the distribution of Adesmia clade species with either homobaric or heterobaric leaves, searching for associations with their habit and anatomical evolution. Leaves from 75 subshrubby, shrubby, and climbing species from different environments were obtained from worldwide herbaria. Leaflet blade samples were processed according to standard techniques in plant anatomy. We found that 53.4% of the species studied presented heterobaric leaves and 46.6% presented homobaric leaves. All species of the genus Adesmia DC. had homobaric leaves, while all species of Amicia Kunth and Nissolia Jacq. had heterobaric leaves. In contrast, Zornia J.F. Gmel. and Poiretia Vent. presented some species with homobaric leaves and others with heterobaric leaves. All climbers studied had heterobaric leaves. For the shrubby and subshrubby species analysed, 64,8% had heterobaric leaves and 35,2% had homobaric leaves. Our results show that within the Adesmia clade the BSE in the leaf blade possibly evolved from heterobaric leaves with the extensions in the second and third order (occasionally fourth order) veins to heterobaric leaves with BSE only in second order veins, and for homobaric leaves in earlier lineages (Zornia and Adesmia).

Leaf toughness increases with tree height and is associated with internal leaf structure and photosynthetic traits in a tropical rain forest

Although leaf toughness is an essential plant adaptation to herbivore pressure and environmental stress, the relationships of leaf toughness with leaf anatomy and photosynthetic traits, and its spatial variations within tropical rainforests, remain poorly understood. We measured these traits in 103 tree species belonging to 27 families from the canopy to understory using a canopy crane system in a tropical rainforest in Sarawak, Malaysia. We focused on the leaf anatomical trait of bundle-sheath extensions (BSEs) around the vascular bundle due to their diverse ecophysiological functions. We divided the trees into heterobaric species with BSEs and homobaric species lacking BSEs, to investigate the relationships of leaf toughness with tree height, leaf functional traits such as carbon (C) and nitrogen (N) content, thickness, leaf mass per area (LMA) and the maximum photosynthetic rate (Pmax). Leaf toughness, LMA, thickness and C and N contents increased with height regardless of BSE presence. Heterobaric leaves had greater toughness than homobaric leaves, whereas leaf thickness, LMA and C were similar between the two leaf types throughout the height gradient. We found that standardized toughness per thickness or C was greater in heterobaric species, as BSEs consist mainly of fibrous tissue. Pmax was higher for heterobaric than homobaric leaves in the upper canopy presumably due to the functions of BSEs, including water conductivity, but did not differ with plant type in the lower layers. In other words, heterobaric species efficiently exploit the advantages of tougher leaves and higher Pmax by having BSEs. The increased proportion of heterobaric species, with their tougher leaves and higher Pmax, in the upper canopy is consistent with adaptation to physically stressful conditions in the tropical rainforest canopy, including high herbivore pressure and strong light.

Auxin-driven ecophysiological diversification of leaves in domesticated tomato.

Heterobaric leaves have bundle sheath extensions (BSEs) that compartmentalize the parenchyma, whereas homobaric leaves do not. The presence of BSEs affects leaf hydraulics and photosynthetic rate. The tomato (Solanum lycopersicum) obscuravenosa (obv) mutant lacks BSEs. Here, we identify the obv gene and the causative mutation, a nonsynonymous amino acid change that disrupts a C2H2 zinc finger motif in a putative transcription factor. This mutation exists as a polymorphism in the natural range of wild tomatoes but has increased in frequency in domesticated tomatoes, suggesting that the latter diversified into heterobaric and homobaric leaf types. The obv mutant displays reduced vein density, leaf hydraulic conductance and photosynthetic assimilation rate. We show that these and other pleiotropic effects on plant development, including changes in leaf insertion angle, leaf margin serration, minor vein density, and fruit shape, are controlled by OBV via changes in auxin signaling. Loss of function of the transcriptional regulator AUXIN RESPONSE FACTOR 4 (ARF4) also results in defective BSE development, revealing an additional component of a genetic module controlling aspects of leaf development important for ecological adaptation and subject to breeding selection.

Desiccation of the leaf mesophyll and its implications for CO2 diffusion and light processing.

Leaves balance CO2 and radiative absorption while maintaining water transport to maximise photosynthesis. Related species with contrasting leaf anatomy can provide insights into inherent and stress‐induced links between structure and function for commonly measured leaf traits for important crops. We used two walnut species with contrasting mesophyll anatomy to evaluate these integrated exchange processes under non‐stressed and drought conditions using a combination of light microscopy, X‐ray microCT, gas exchange, hydraulic conductance, and chlorophyll distribution profiles through leaves. Juglans regia had thicker palisade mesophyll, higher fluorescence in the palisade, and greater low‐mesophyll porosity that were associated with greater gas‐phase diffusion (g IAS), stomatal and mesophyll (g m) conductances and carboxylation capacity. More and highly‐packed mesophyll cells and bundle sheath extensions (BSEs) in Juglans microcarpa led to higher fluorescence in the spongy and in proximity to the BSEs. Both species exhibited drought‐induced reductions in mesophyll cell volume, yet the associated increases in porosity and g IAS were obscured by declines in biochemical activity that decreased g m. Inherent differences in leaf anatomy between the species were linked to differences in gas exchange, light absorption and photosynthetic capacity, and drought‐induced changes in leaf structure impacted performance via imposing species‐specific limitations to light absorption, gas exchange and hydraulics.

Notes on leaf anatomy: Additional information for Indonesian Pandanus spp. (Pandanaceae)

In Indonesia, Pandanus spp. were distributed from Sumatra to Papua. These Pandanus species has broader variation of anatomical leaf characters whether epidermis or transversal section traits. This study was conducted to obtain an information of anatomical characters of Pandanus leaf and supporting character on Pandanus identification. Pandanus collections of Herbarium Bogoriense were used for this study, and some of Pandanus species from Papua New Guinea and Malay Peninsula were added as well. Paraffin method and simple peeling of the leaf surface were used to get leaf transversal sections and epidermis characters. In general, anatomical characters of examined species in this study relatively similar to other Pandanus species which were studied by former researchers. Pandanus has broad range of stomata types on the lower leaf surface, from simple stomata to the most specialized stomata. The upper leaf surface of Pandanus spp. has similar stomata type, viz. class 1 (simple). Zonation always presents on all species, except on P. atropurpureus, P. conoideus and P. nitidus. Mostly, crystals CaCO3 were found in three forms, namely rectangular and needle-like shape in idioblast cells, while prism-shape is scattered in the mesophyll area. These crystal shapes absent in P. odorifer and there is one additional crystal shape, drusse which present in P. conoideus. Silica bodies located in the upper and lower hypodermis of leaf. Bundle sheath extension was seen in all species except P. beguinii, P. conoideus, and P. fusinus.

OBV (obscure vein), a C2H2 zinc finger transcription factor, positively regulates chloroplast development and bundle sheath extension formation in tomato (Solanum lycopersicum) leaf veins

Leaf veins play an important role in plant growth and development, and the bundle sheath (BS) is believed to greatly improve the photosynthetic efficiency of C4 plants. The OBV mutation in tomato (Solanum lycopersicum) results in dark veins and has been used widely in processing tomato varieties. However, physiological performance has difficulty explaining fitness in production. In this study, we confirmed that this mutation was caused by both the increased chlorophyll content and the absence of bundle sheath extension (BSE) in the veins. Using genome-wide association analysis and map-based cloning, we revealed that OBV encoded a C2H2L domain class transcription factor. It was localized in the nucleus and presented cell type-specific gene expression in the leaf veins. Furthermore, we verified the gene function by generating CRISPR/Cas9 knockout and overexpression mutants of the tomato gene. RNA sequencing analysis revealed that OBV was involved in regulating chloroplast development and photosynthesis, which greatly supported the change in chlorophyll content by mutation. Taken together, these findings demonstrated that OBV affected the growth and development of tomato by regulating chloroplast development in leaf veins. This study also provides a solid foundation to further decipher the mechanism of BSEs and to understand the evolution of photosynthesis in land plants.

The Optical Properties of Leaf Structural Elements and Their Contribution to Photosynthetic Performance and Photoprotection.

Leaves have evolved to effectively harvest light, and, in parallel, to balance photosynthetic CO2 assimilation with water losses. At times, leaves must operate under light limiting conditions while at other instances (temporally distant or even within seconds), the same leaves must modulate light capture to avoid photoinhibition and achieve a uniform internal light gradient. The light-harvesting capacity and the photosynthetic performance of a given leaf are both determined by the organization and the properties of its structural elements, with some of these having evolved as adaptations to stressful environments. In this respect, the present review focuses on the optical roles of particular leaf structural elements (the light capture module) while integrating their involvement in other important functional modules. Superficial leaf tissues (epidermis including cuticle) and structures (epidermal appendages such as trichomes) play a crucial role against light interception. The epidermis, together with the cuticle, behaves as a reflector, as a selective UV filter and, in some cases, each epidermal cell acts as a lens focusing light to the interior. Non glandular trichomes reflect a considerable part of the solar radiation and absorb mainly in the UV spectral band. Mesophyll photosynthetic tissues and biominerals are involved in the efficient propagation of light within the mesophyll. Bundle sheath extensions and sclereids transfer light to internal layers of the mesophyll, particularly important in thick and compact leaves or in leaves with a flutter habit. All of the aforementioned structural elements have been typically optimized during evolution for multiple functions, thus offering adaptive advantages in challenging environments. Hence, each particular leaf design incorporates suitable optical traits advantageously and cost-effectively with the other fundamental functions of the leaf.

Can hydraulic design explain patterns of leaf water isotopic enrichment in C3 plants?

H2 18 O enrichment develops when leaves transpire, but an accurate generalized mechanistic model has proven elusive. We hypothesized that leaf hydraulic architecture may affect the degree to which gradients in H2 18 O develop within leaves, influencing bulk leaf stable oxygen isotope enrichment (ΔL ) and the degree to which the Péclet effect is relevant in leaves. Leaf hydraulic design predicted the relevance of a Péclet effect to ΔL in 19 of the 21 species tested. Leaves with well-developed hydraulic connections between the vascular tissue and the epidermal cells through bundle sheath extensions and clear distinctions between palisade and spongy mesophyll layers (while the mesophyll is hydraulically disconnected) may have velocities of the transpiration stream such that gradients in H2 18 O develop and are expressed in the mesophyll. In contrast, in leaves where the vascular tissue is hydraulically disconnected from the epidermal layers, or where all mesophyll cells are well connected to the transpiration stream, velocities within the liquid transport pathways may be low enough that gradients in H2 18 O are very small. Prior knowledge of leaf hydraulic design allows informed selection of the appropriate ΔL modelling framework.

Bioimaging Techniques Reveal Foliar Phosphate Uptake Pathways and Leaf Phosphorus Status.

Global demand for phosphorus (P) requires new agronomic practices to address sustainability challenges while increasing food production. Foliar P fertilization could increase P use efficiency; however, leaf entry pathways for inorganic phosphate ion (Pi) uptake remain unknown, and it is unclear whether foliar P applications can meet plant nutrient demands. We developed two techniques to trace foliar P uptake in P-deficient spring barley (Hordeum vulgare) and to monitor the effectiveness of the treatment on restoring P functionality. First, a whole-leaf P status assay was developed using an IMAGING PAM system; nonphotochemical quenching was a proxy for P status, as P-deficient barley developed nonphotochemical quenching at a faster rate than P-sufficient barley. The assay showed restoration of P functionality in P-deficient plants 24 h after foliar P application. Treated leaves reverted to P deficiency after 7 d, while newly emerging leaves exhibited partial restoration compared with untreated P-deficient plants, indicating Pi remobilization. Second, vanadate was tested as a possible foliar Pi tracer using high-resolution laser ablation-inductively coupled plasma-mass spectrometry elemental mapping. The strong colocalization of vanadium and P signal intensities demonstrated that vanadate was a sensitive and useful Pi tracer. Vanadate and Pi uptake predominantly occurred via fiber cells located above leaf veins, with pathways to the vascular tissue possibly facilitated by the bundle sheath extension. Minor indications of stomatal and cuticular Pi uptake were also observed. These techniques provided an approach to understand how Pi crosses the leaf surface and assimilates to meet plant nutrient demands.

Leaf and scape anatomy of Leiothrix Ruhland (Eriocaulaceae) from a taxonomic and ecological perspective

Leiothrix is endemic to South America with 47 species, only two of which do not occur in Brazil. Its distinguishing characters are habit, dialypetalous or gamopetalous corolla of the male flowers and presence or not of pseudovivipary. According to Ruhland's monograph (1903) there are five subgenera. Recent molecular and morphological analyses of the family show the genus to be monophyletic, but infrageneric relationships are not well established. The study objective was to investigate leaf and scape anatomy of representatives of the four subgenera that occur in Brazil in order to contribute to the taxonomy and better understand the plants' environmental adaptations. Leiothrix anatomy is similar to that of other Eriocaulaceae genera: leaves and scapes have a single-layered epidermis, collateral vascular bundles are surrounded by a double sheath, leaves have stomata only on the abaxial side, and scapes have alternating vascular bundles of larger and smaller size. Leiothrix subg. Rheocaulon is monospecific, leaves have a single vascular bundle, scapes lack ribs, the epidermal cells have slightly thickened walls, and the cortex contains collenchyma and chlorophyllous parenchyma. In L. subg. Eleutherandra the leaf epidermis cell walls have homogeneous thickening at the margins, specialized substomatal chambers, a hypodermis, and sclerenchymatous extensions of the vascular bundle sheaths. Most taxa of L. subg. Calycocephalus have smaller vascular bundle sheath extensions facing both surfaces and ribbed scapes. Species from Minas Gerais have a hypodermis and up to seven leaf vascular bundles; those from Bahia lack a hypodermis and have more than seven leaf vascular bundles. L. subg. Stephanophyllum has scapes with a cylindric multilayered pericycle with thickened and lignified cell walls. These anatomical structures partly corroborate the subgenera proposed by Ruhland, suggest the existence of other infrageneric groups and reflect the environmental adaptations of the studied species.

Leaflet anatomy of Poiretia Vent. (Leguminosae, Papilionoideae, Dalbergieae) with emphasis on internal secretory structures in support of taxonomy

Poiretia Vent. is a Leguminosae (Papilionoideae, Dalbergieae) genus comprising 12 species which are distinguished mainly by external morphology of the leaves and inflorescences; however, overlapping in variation of several morphological traits has led to taxonomic problems, including erroneous identifications of species and varieties. Furthermore, attributes of recognized importance among legumes, such as characteristics of secretory structures, have been neglected in the distinction of Poiretia species. Here we described the leaflet blade anatomy of all 12 Poiretia species emphasizing the internal secretory structures in order to identify characters with potential taxonomic value. Leaflets were obtained from herbarium and from plants occurring in natural environments. Samples were processed according to standard techniques for plant anatomy. The leaflets of P. angustifolia and P. punctata were hypostomatic, whereas those of the other species were amphistomatic. The adaxial face of the main vein was flat in P. bahiana, P. coriifolia, P. latifolia and P. unifoliolata, slightly convex in P. elegans and concave in the other species. Bundle sheath extensions were observed in eight of the 12 species. A fiber cap with lignified walls was present externally to the phloem of the midrib, except P. bahiana, P. latifolia, P. marginata and P. punctata. Secretory cavities were present in the mesophyll of all species; in P. angustifolia, P. latifolia, P. longipes, P. marginata and P. tetraphylla secretory cavities occurred in the region of the mesophyll near the midrib, while in P. latifolia the secretory cavities occurred in the cortex of the midrib. The number of epithelial layers varied among species, with secretory cavities having two to six cell layers. Idioblasts in the epidermis of P. crenata, P. latifolia and P. mattogrossensis were quite voluminous. The distinctive characteristics observed among the species were used to develop an identification key for the species of Poiretia.

Leaf structural traits of three species of <i>Qualea</i> Mart. (Vochysiaceae) in a cerradão area in the Cerrado-Amazonian Forest transition

Qualea genus is commonly found in different phytophysiognomies of the Brazilian Cerrado, growing in open ecosystems to closed forest areas, under high irradiances, fires, low availability of nutrients and seasonal water deficit. We compared the leaf structural traits of Qualea grandiflora Mart., Qualea multiflora Mart., and Qualea parviflora Mart., in a cerradão area in the Cerrado-Amazonian forest transition. Leaf samples were collected at Mário Viana Municipal Biological Reserve, in Nova Xavantina, Mato Grosso state, Brasil, and processed according to the usual techniques for optical microscopy. Thick cuticle, stomatal ridges, stomata distributed between the stomatal crypts, density and distribution of tector trichomes, hypodermis, isobilateral mesophyll with few intercellular spaces, abundance of sclerenchyma associated to vascular bundles and the presence of vascular bundle sheath extensions are typical anatomic traits of xerophytic environments. We found qualitative structural differences among the species, with Qualea grandiflora having stomata with ridges distributed among stomatal crypts, and the presence of a hypodermis in Qualea multiflora, and Qualea parviflora. The results suggest that the species are xeromorphic in the region of Cerrado-Amazonian transition having leaf adaptive traits to deal with the environmental variations of their habitat.

A new species and putative hybrids of Manihot (Euphorbiaceae) from Brazil

This work describes and illustrates the new species Manihot reflexifolia, sp. nov. and compares its leaf anatomy to Manihot reniformis and probable hybrids between the two species. Manihot reflexifolia is also compared to Manihot jacobinensis from which it can be distinguished by its cylindrical petioles, elliptical to short lanceolate, reflexed leaf lobes and staminate flowers restricted to the terminal portion of the inflorescence (upper half), while M. jacobinensis has dorsiventrally flattened petioles, suborbicular to obovate leaf lobes, and staminate flowers along the entire inflorescence axis. The new species is restricted to rocky outcrops of the Chapada Diamantina in central Bahia State, Brazil. An identification key to Manihot species from rocky outcrops of Chapada Diamantina is provided. The leaf anatomy of M. reflexifolia, M. reniformis and the putative hybrids was analyzed using optical and scanning electron microscopy. Leaf anatomy was sufficient to characterize M. reflexifolia, M. reniformis and the putative hybrids, as well as to identify the first two wild taxa as potential progenitors of the hybrids (M. reflexifolia × M. reniformis). The most useful anatomical characteristics were the morphology of the petiole and midrib, bundle sheath extension, organization and quantity of vascular bundles, organization and composition of the mesophyll, and the presence of epidermal papillae.

Bundle sheath extensions affect leaf structural and physiological plasticity in response to irradiance

Coordination between structural and physiological traits is key to plants' responses to environmental fluctuations. In heterobaric leaves, bundle sheath extensions (BSEs) increase photosynthetic performance (light-saturated rates of photosynthesis, Amax ) and water transport capacity (leaf hydraulic conductance, Kleaf ). However, it is not clear how BSEs affect these and other leaf developmental and physiological parameters in response to environmental conditions. The obscuravenosa (obv) mutation, found in many commercial tomato varieties, leads to absence of BSEs. We examined structural and physiological traits of tomato heterobaric and homobaric (obv) near-isogenic lines grown at two different irradiance levels. Kleaf , minor vein density, and stomatal pore area index decreased with shading in heterobaric but not in homobaric leaves, which show similarly lower values in both conditions. Homobaric plants, on the other hand, showed increased Amax , leaf intercellular air spaces, and mesophyll surface area exposed to intercellular airspace (Smes ) in comparison with heterobaric plants when both were grown in the shade. BSEs further affected carbon isotope discrimination, a proxy for long-term water-use efficiency. BSEs confer plasticity in traits related to leaf structure and function in response to irradiance levels and might act as a hub integrating leaf structure, photosynthetic function, and water supply and demand.

Competition for epidermal space in the evolution of leaves with high physiological rates.

Leaves with high photosynthetic capacity require high transpiration capacity. Consequently, hydraulic conductance, stomatal conductance, and assimilation capacities should be positively correlated. These traits make independent demands on anatomical space, particularly due to the propensity for veins to have bundle sheath extensions that exclude stomata from the local epidermis. We measured density and area occupation of bundle sheath extensions, density and size of stomata and subsidiary cells, and venation density for a sample of extant angiosperms and fossil and living nonangiosperm tracheophytes. For most nonangiosperms, even modest increases in vein density and stomatal conductance would require substantial reconfigurations of anatomy. One characteristic of the angiosperm syndrome (e.g. small cell sizes, etc.) is hierarchical vein networks that allow expression of bundle sheath extensions in some, but not all veins, contrasting with all-or-nothing alternatives available with the single-order vein networks in most nonangiosperms. Bundle sheath modulation is associated with higher vein densities in three independent groups with hierarchical venation: angiosperms, Gnetum (gymnosperm) and Dipteris (fern). Anatomical and developmental constraints likely contribute to the stability in leaf characteristics - and ecophysiology - seen through time in different lineages and contribute to the uniqueness of angiosperms in achieving the highest vein densities, stomatal densities, and physiological rates.

Absorption of foliar-applied Zn in sunflower (Helianthus annuus): importance of the cuticle, stomata and trichomes.

The pathways whereby foliar-applied nutrients move across the leaf surface remain unclear. The aim of the present study was to examine the pathways by which foliar-applied Zn moves across the sunflower (Helianthus annuus) leaf surface, considering the potential importance of the cuticle, stomata and trichomes. Using synchrotron-based X-ray florescence microscopy and nanoscale secondary ion mass spectrometry (NanoSIMS), the absorption of foliar-applied ZnSO4 and nano-ZnO were studied in sunflower. The speciation of Zn was also examined using synchrotron-based X-ray absorption spectroscopy. Non-glandular trichomes (NGTs) were particularly important for foliar Zn absorption, with Zn preferentially accumulating within trichomes in ≤15 min. The cuticle was also found to have a role, with Zn appearing to move across the cuticle before accumulating in the walls of the epidermal cells. After 6 h, the total Zn that accumulated in the NGTs was approx. 1.9 times higher than in the cuticular tissues. No marked accumulation of Zn was found within the stomatal cavity, probably indicating a limited contribution of the stomatal pathway. Once absorbed, the Zn accumulated in the walls of the epidermal and the vascular cells, and trichome bases of both leaf sides, with the bundle sheath extensions that connected to the trichomes seemingly facilitating this translocation. Finally, the absorption of nano-ZnO was substantially lower than for ZnSO4, with Zn probably moving across the leaf surface as soluble Zn rather than nanoparticles. In sunflower, both the trichomes and cuticle appear to be important for foliar Zn absorption.

Four distinct leaf types in the Brazilian Cerrado, based on bundle sheath extension morphology

Plant species have been classified as being either heterobaric or homobaric based on the presence or absence (respectively) of bundle sheath extensions (BSE) in leaf veins. These leaf types differ in their physiological performance, and their occurrences tend to be associated with environment. This study investigated the presence/absence of BSE and their structural peculiarities in 49 woody species (28 angiosperm families) in the Brazilian Cerrado using anatomical methods. We measured the gas exchange indices and stomatal density. In addition to typical homobaric (61.2%) and heterobaric (22.4%) species, we also found species with intermediate BSE features. These we call “larger heterobaric” (12.2%) and “semi-heterobaric” (4.1%) species. In the former, BSE occurs around only the secondary veins, whereas in the latter the BSE projects only towards the adaxial leaf surface. The net CO2 assimilation rates, the stomatal conductance, the transpiration rates, and the carboxylation efficiencies were statistically higher in heterobaric than in homobaric species. In the larger heterobaric species, the photosynthetic indices were similar to those in the heterobaric species. Meanwhile, semi-heterobaric species showed photosynthetic indices similar to the homobaric species. We demonstrated that structural variation in the BSE is linked to physiological leaf performance, suggesting some correlation between leaf morphology and plant adaptations.

Bundle sheath extensions are linked to water relations but not to mechanical and structural properties of leaves

Key message Bundle sheath extensions (BSEs) are linked to leaf gas exchange and drought tolerance but not to leaf mechanical and structural properties across species from a cool temperate forest.