A computational model of the cereal leaves hydraulics

Environmental factors and plant architectonics significantly determine its water regime, namely, the water content and its movement through the tissues forced by the difference in water potentials, turgor pressure in cells, etc. In turn, the cumulative water regime affects the functioning and growth of cells, photosynthesis, and, as a result, the plant’s growth. The leaf contributes to the formation of the water regime and characterizes the contour of the plant’s adaptive system. Nevertheless, the data on the contribution of leaves to the total resistance to water transport in the plant and the structure of the hydraulic resistance of the leaf itself is still contradictory. This paper presents the formulation and justification of the monocots leaf hydraulics model based on Darcy’s law. The model was tested in computational experiments in the Comsol 4.3b package on idealized geometric models of leaf blades. Simulations showed the dependence of water potential distribution in xylem vessels and leaf mesophyll on the permeability of these tissues and on microclimatic parameters around the leaf. The adequacy of the model parameters selected as a result of testing is discussed.

Leaf Anatomical Study of Solanum Species (Solanaceae) in Iran

Solanum (Solanaceae) comprises cultivated and wild plants with 1400 species in the world and 14 species in Iran. Solanum is a taxonomically complex genus due to morphological similarities, phenotypic plasticity and hybridisation. Limited studies were done on anatomical features of this important genus. In this project, 10 native and exotic species of Solanum in Iran belonging to two subgenera were examined anatomically. Leaf mesophyll and midrib and indumentum were analysed using light microscope. Hand-made cross section method and Toluidine blue as colouring agent were used. Characters as length and width of main vascular bundle, thickness of collenchyma, trichome density, thickness of parenchyma strand, thickness of lamina and length and shape of midrib were diagnostic features among species studied. In UPGMA tree and PCA ordination, species of two subgenera were separated from each other. Results of this study confirmed the taxonomic importance of anatomical characters in Solanum species studied.

Comparative Transcriptome Profiling Indicated that Leaf Mesophyll and Leaf Vasculature have Different Drought Response Mechanisms in Cassava

Drought stress is one of the major environmental factors that limited crop’s growth and production. Cassava known as a tropical crop that is widely distributed in Sub-Saharan Africa. It has a strong drought tolerance and can grow well under tough environmental conditions. Therefore, understanding how cassava responds to drought stress and coordinates survival and accumulation has great theoretical significance for improving crop drought resistance breeding. Many studies on cassava drought responses mainly focused on the leaf and whole seedling. Nevertheless, how the vasculature plays an important role in plant response to water deficiency remains to be fully elucidated. Here, comparative transcriptome analysis was performed on isolated mesophyll tissue and leaf vein vascular tissue of cassava variety KU50 after mild drought treatment to determine the molecular mechanism behind drought resistance in cassava vasculature. Our results showed that KU50 leaves had increased leaf temperature, with characters of rapidly decreased net photosynthetic rate, stomatal conductance, and transpiration rate in leaves, and the intercellular CO2 concentration accumulated under drought stress. Comparative transcriptome profiling revealed that under drought stress, leaf mesophyll tissue mainly stimulated the biosynthesis of amino acids, glutamic acid metabolism, and starch and sucrose metabolism. In particular, the arginine biosynthesis pathway was significantly enhanced to adapt to the water deficiency in leaf mesophyll tissue. However, in vascular tissue, the response to drought mainly involved ion transmembrane transport, hormone signal transduction, and depolymerization of proteasome. Concretely, ABA signaling and proteasome metabolism, which are involved in ubiquitin regulation, were changed under drought stress in KU50 leaf vascular tissue. Our work highlights that the leaf vasculature and mesophyll in cassava have completely different drought response mechanisms.

ABA homeostasis and long-distance translocation is redundantly regulated by ABCG ABA importers

The effects of abscisic acid (ABA) on plant growth, development and response to the environment depend on local ABA concentrations. Here, we exploited a genome-scale amiRNA screen, targeting the Arabidopsis transportome, to show that ABA homeostasis is regulated by two previously unknown ABA transporters. ABCG17 and ABCG18 are localized to the plasma membranes of leaf mesophyll and stem cortex cells to redundantly promote ABA import, leading to conjugated inactive ABA sinks, thus restricting stomatal closure. ABCG17 and ABCG18 double knockdown revealed that the transporters encoded by these genes not only limit stomatal aperture size, conductance and transpiration while increasing water-use efficiency but also control ABA translocation from the shoot to the root to regulate lateral root emergence. Under abiotic stress conditions, ABCG17 and ABCG18 are transcriptionally repressed, promoting active ABA movement and response. The transport mechanism mediated by ABCG17 and ABCG18 allows plants to maintain ABA homeostasis under normal growth conditions.

Maximum CO 2 diffusion inside leaves is limited by the scaling of cell size and genome size

Maintaining high rates of photosynthesis in leaves requires efficient movement of CO 2 from the atmosphere to the mesophyll cells inside the leaf where CO 2 is converted into sugar. CO 2 diffusion inside the leaf depends directly on the structure of the mesophyll cells and their surrounding airspace, which have been difficult to characterize because of their inherently three-dimensional organization. Yet faster CO 2 diffusion inside the leaf was probably critical in elevating rates of photosynthesis that occurred among angiosperm lineages. Here we characterize the three-dimensional surface area of the leaf mesophyll across vascular plants. We show that genome size determines the sizes and packing densities of cells in all leaf tissues and that smaller cells enable more mesophyll surface area to be packed into the leaf volume, facilitating higher CO 2 diffusion. Measurements and modelling revealed that the spongy mesophyll layer better facilitates gaseous phase diffusion while the palisade mesophyll layer better facilitates liquid-phase diffusion. Our results demonstrate that genome downsizing among the angiosperms was critical to restructuring the entire pathway of CO 2 diffusion into and through the leaf, maintaining high rates of CO 2 supply to the leaf mesophyll despite declining atmospheric CO 2 levels during the Cretaceous.

Distribution of cystoliths in the leaves of Acanthaceae and its effect on leaf surface anatomy

Cystoliths are large outgrowths of cell wall material and calcium carbonate with a silicon-containing stalk found in the leaves, stems and roots of only a handful of plant families. Each cystolith is contained within a cell called a lithocyst. In leaves, lithocysts may be found in the mesophyll or the epidermis. A study by Koch et al. (2009) reported unique, indented features on the surface of superamphiphilic Ruellia devosiana (Acanthaceae) leaves which the authors named ‘channel cells’. We report herein that such ‘channel cells’ in the Acanthaceae are actually lithocysts containing fully formed cystoliths in which only a portion of the lithocyst is exposed at the epidermis, forming a leaf epidermal impression. Intact leaves and isolated cystoliths from 28 Acanthaceae species (five in the non-cystolith clade and 23 in the cystolith clade) were examined using light and scanning electron microscopy and X-ray microanalysis. All 23 members of the cystolith clade examined contained cystoliths within lithocysts, but not all showed leaf epidermal impressions. In four species, the lithocysts were in the leaf mesophyll, did not contact the leaf surface, and did not participate in leaf epidermal impression formation. The remaining 19 species had lithocysts in the epidermis and possessed leaf epidermal impressions of differing sizes, depths and morphologies.

Chloroplast Distribution in the Stems of 23 Eucalypt Species

Small diameter branchlets and smooth barked stems and branches of most woody plants have chloroplasts. While the stems of several eucalypt species have been shown to photosynthesise, the distribution of chloroplasts has not been investigated in detail. The distribution of chloroplasts in branchlets (23 species) and larger diameter stems and branches with smooth bark (14 species) was investigated in a wide range of eucalypts (species of Angophora, Corymbia and Eucalyptus) using fresh hand sections and a combination of bright field and fluorescence microscopy. All species had abundant stem chloroplasts. In both small and large diameter stems, the greatest concentration of chloroplasts was in a narrow band (usually 100–300 μm thick) immediately beneath the epidermis or phellem. Deeper chloroplasts were present but at a lower density due to abundant fibres and sclereids. In general, chloroplasts were found at greater depths in small diameter stems, often being present in the secondary xylem rays and the pith. The cells of the chlorenchyma band were small, rounded and densely packed, and unlike leaf mesophyll. A high density of chloroplasts was found just beneath the phellem of large diameter stems. These trees gave no external indication that green tissues were present just below the phellem. In these species, a thick phellem was not present to protect the inner living bark. Along with the chlorenchyma, the outer bark also had a high density of fibres and sclereids. These sclerenchyma cells probably disrupted a greater abundance and a more organised arrangement of the cells containing chloroplasts. This shows a possible trade-off between photosynthesis and the typical bark functions of protection and mechanical strength.