Development and Structure of Internal Glands and External Glandular Trichomes in <i>Pogostemon cablin</i>

<div><p><i>Pogostemon cablin</i> possesses two morphologically and ontogenetically different types of glandular trichomes, one type of bristle hair on the surfaces of leaves and stems and one type of internal gland inside the leaves and stems. The internal gland originates from elementary meristem and is associated with the biosynthesis of oils present inside the leaves and stems. However, there is little information on mechanism for the oil biosynthesis and secretion inside the leaves and stems. In this study, we identified three kinds of glandular trichome types and two kinds of internal gland in the <i>Pogostemon cablin</i>. The oil secretions from internal glands of stems and leaves contained lipids, flavones and terpenes. Our results indicated that endoplasmic reticulum and plastids and vacuoles are likely involved in the biosynthesis of oils in the internal glands and the synthesized oils are transported from endoplasmic reticulum to the cell wall via connecting endoplasmic reticulum membranes to the plasma membrane. And the comparative analysis of the development, distribution, histochemistry and ultrastructures of the internal and external glands in <i>Pogostemon cablin</i> leads us to propose that the internal gland may be a novel secretory structure which is different from external glands.</p></div

Histochemistry of the mature secretory glands of <i>Pogostemon cablin</i>.

<p>Histochemistry of the mature secretory glands of <i>Pogostemon cablin</i>.</p

Ultrastructural aspects of internal glands in stems of <i>Pogostemon cablin</i>.

<p>(A) Longitudinal section of internal glands showing that the internal gland has a long head cell (HC), a stalk cell (SC) and a basal cell (BC). (B) The sub-cuticular space (SCS) of internal glands has thin cuticle (arrow) and is filled with oil droplets (_*). (C) The higher magnification of (B) showing one big oil droplet (_*) between cell wall (CW) and plasma membrane. (D) Oil droplets in SCS contain lipid droplets (L) and membrane-like structure (arrow) outside the oil droplet. (E) Portion of head cell showing numerous mitochondria (M), small vacuoles (V) and plastids (P). (F) Vacuoles with electron-opaque material (arrows) are in close to plastid (P) and mitochondria (M). (G) The smooth endoplasmic reticulum (arrows) is observed to be in close to plasma membrane (PM). (H) Portion of mature internal glands showing electron-opaque material (arrows) in close contact with vacuoles. (I) The smooth endoplasmic reticulum (arrows) in close to plastids (P). (J) The stalk cell with thickened lateral wall (arrow) contains big vacuoles (V), the nucleus (N) and numerous plastids (P). (K) The details of cell wall showing plasmodesmata (arrows) that connect the narrow stalk cell and head cell. (L) The details of head cell showing that vesicles (V) near plasma membrane (PM) are in close connect with electron-opaque material (arrows).</p

The development and histochemistry of internal glands in stems.

<p>Transverse (A) or longitudinal (B) section of stems showing the internal glands (arrows) among the cortical cells. (C–G) Light micrographs of internal glands in different developmental phases showing the process of development: (C) the initial cell with nucleus and few vacuoles; (D) two-celled stage with two cytoplasmically dense cells after cell divisions; (E) three-celled stage with one vacuolate basal cell, one narrow stalk cell, and the apical initial cell; (F) pre-secretory stage with one big apical cell; (G) mature internal glands with a long secretory cell. (H–M) Bright field and fluorescence micrographs of internal glands in stems showing histochemical characterization of secretory products. The staining for total lipids with Neutral red (H), Sudan III (I) and Sudan Black B (J) suggests the accumulation of total lipids in the internal glands of stems. In addition, the presence of unsaturated lipids is demonstrated by the reaction with OsO₄ (K). NADI staining for terpenes is positive in mature internal glands (L). Secreted material in the sub-cuticular space of internal glands reacts positively with Naturstoffreagent A for the detection of flavones (M).</p

Ultrastructural aspects of three glandular trichome types of <i>Pogostemon cablin</i>.

<p>(A–C) Short-stalked capitate trichomes in secretory stage: (A) the glandular trichomes with two head cells (HC), a narrow stalk cell (SC) and a basal cell (BC) and the sub-cuticular space (SCS); (B) portion of head cell showing Golgi (G) and plastids (P) in close contact with rough endoplasmic reticulum (RER) near cell wall (CW); (C) numerous Golgi (G) with many vesicles near the plasma membrane (PM). (D–F) Peltate glandular trichomes in secretory stage: (D) longitudinal section through a secretory peltate glandular trichome with plentiful electron-light lipid deposits (_*) in the sub-cuticular space; (E) the higher magnification of (D) showing Golgi (G) and plastids (P) in close contact with the short segments of smooth endoplasmic reticulum (SER); (F) vesicles (arrows) are found between cell wall (CW) and plasma membrane (PM). (G–K) Ultrastructural aspects of the long-stalked capitate trichomes: (G) the mature glandular trichomes with a cytoplasmically dense apical cell, a narrow stalk cell and an elongated vacuolated stalk cell; (H) the higher magnification of (G) showing the secretory cell with the sub-cuticular space (SCS), many small vacuoles (V), abandent mitochondria (M) and numerous small plastids (P); (I) the details of the cytoplasm of the secretory cell showing prevalence of mitochondria (M) and plastids (P) with plastoglobuli in the apical region; (J) the sparse smooth endoplasmic reticulum (SER) close to a lipid-filled plastid (P) (arrow); (K) a bigger vesicle (arrow) between cell wall (CW) and plasma membrane (PM).</p

The development and histochemistry of internal glands in leaves.

<p>(A)The images (SEM) of <i>Pogostemon cablin</i> leaves showing the internal gland (arrow) among palisade cells. (B) The semithin section of leaves showing the morphology of internal gland. (C–F) Semithin sections of internal glands in different developmental phases showing the developmental process: (C) the initial cell of internal glands with the nucleus (arrow); (D) the initial cell with a vacuolate basal cell and a apical cell after apericlinal cell division (arrows); (E) internal glands at three-celled stage with twocytoplasmically dense cells after the apical cell divisions (arrows); (F) mature internal glands with one big secretory cell, one narrow stalk cell and one vacuolate basal cell. (G–L) Bright field and fluorescence micrographs of internal glands in leaves showing histochemical characterization of secretory products. Secretory material reacts positively for total lipids with Neutral Red (G), Sudan III (H) and Sudan Black B (I). The reaction for unsaturated lipids using OsO₄ (J) is positive. The essential oil within the sub-cuticular space has reacted positively with the Nadi reagent for terpenes (K). And the staining for flavones with Naturstoffreagent A suggests the presence of flavones in the internal glands of leaves (L).</p

Reduced Graphene Oxide-Incorporated SnSb@CNF Composites as Anodes for High-Performance Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries because of the low cost and natural abundance of sodium resources. Nevertheless, low energy density and poor cycling stability of current SIBs unfavorably hinder their practical implementation for the smart power grid and stationary storage applications. Antimony tin (SnSb) is one of the most promising anode materials for next-generation SIBs attributing to its high capacity, high abundance, and low toxicity. However, the practical application of SnSb anodes in SIBs is currently restricted because of their large volume changes during cycling, which result in serious pulverization and loss of electrical contact between the active material and the carbon conductor. Herein, we apply reduced graphene oxide (rGO)-incorporated SnSb@carbon nanofiber (SnSb@rGO@CNF) composite anodes in SIBs that can sustain their structural stability during prolonged charge-discharge cycles. Electrochemical performance results shed light on that the combination of rGO, CNF, and SnSb alloy led to a high-capacity anode (capacity of 490 mAh g⁻¹  at the 10th cycle) with a high capacity retention of 87.2% and a large Coulombic efficiency of 97.9% at the 200th cycle. This work demonstrates that the SnSb@rGO@CNF composite is a potential and attractive anode material for next-generation, high-energy SIBs