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Acta Agronomica Sinica ›› 2020, Vol. 46 ›› Issue (6): 914-923.doi: 10.3724/SP.J.1006.2020.94141

• TILLAGE & CULTIVATION·PHYSIOLOGY & BIOCHEMISTRY • Previous Articles     Next Articles

Anatomy and microscopic observation of Ricinus communis seed structure

GUO Xue-Min(),ZHAO Xiao-Man,XU Ke,WANG Xin-Rui,ZHANG Chen-Yu,DONG-FANG Yang   

  1. College of Agronomy and Biotechnology, Heibei Normal University of Science & Technology, Changli 066600, Hebei, China
  • Received:2019-09-23 Accepted:2020-01-15 Online:2020-06-12 Published:2020-02-17
  • Contact: Xue-Min GUO E-mail:xueminguo@126.com
  • Supported by:
    Doctor Foundation of Hebei Normal University of Science & Technology(2013YB021)

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Abstract:

We systematically observed the structure and distribution of vascular bundles of castor (Ricinus communis) testa, endosperm and embryo with germination, separation, paraffin sectioning technique, and microscopic observation techniques, stained with safranine and fast green, along with schiff’s reagent, respectively. (1) Micropyle at the dorsal base of the seed was not covered by the caruncle. (2) The testa consisted of three parts: episperm, endopleura and the caruncle, in which the episperm was composed of the long columnar epidermal layer, sponge parenchma and palisade parenchma from outside to inside, while the endopleura was composed of Malpighian layer, sponge parenchma and the inner integument layer, with an air chamber surrounded by the inner layer and outer layer of endopleura at the end of the caruncle, similar to that of eggs. (3) Both the episperm and endopleura were distributed with vascular bundles, and the large ones of the episperm only distributed in the spongy parenchma of the ventral raphe of the seed, which extended from the hilum to the opposite end of the caruncle; while those of endopleura were also distributed in the spongy parenchtma, and they branched step by step through the bundle band, ending at the edge of the air chamber at the end of the caruncle; those of both the episperm and endopleura were continuous through the junction point of the chalazal end to form the vascular system of the testa. (4) The caruncle was composed of epidermis and parenchyma, in which parenchyma consisted of small parenchyma cells on the outside and large parenchyma cells on the inside. In the middle and ventral part of the caruncle, there were only two vascular bundles, which constituted the vascular system of the caruncle independently. In the caruncle, caruncle channel was also found, with the inner end opening in the air chamber of the endopleura and the outer end being blind. (5) No vascular bundles were found in the endosperm, which consisted of parenchymal cells containing a large number of aleurone grains. In the embryo, the vascular bundles were collateral, which differentiated in the hypocotyle and entered the main vein of two cotyledons from both sides of the top of the hypocotyle respectively, gradually reduced from 4 to 1 in the main veins, and the branches became thinner in the cotyledon, forming the vascular system of embryo. The cotyledon epidermis and mesophyll cells contained aleurone grains, but no aleurone grains were found in the plumule, hypocotyle and radicle. Two cotyledons separated from each other on the adaxial surface, while their abaxial surfaces were connected with the endosperm through the mucilaginous layers. The hypocotyle and radicle were spatially separated from the endosperm. These results provide an anatomical basis for comprehensively understanding the structure of the seeds and studying the seed development and germination and their material transportation.

Key words: Ricinus communis, seed, vascular bundle, anatomical structure

Fig. 1

Shape of R. communis seed A: ventral view of a seed, showing the ornamentation and raphe (arrow) of a seed; B: a seed without episperm, showing the distribution of vascular bundles (arrow) in endopleura; C: a seed without testa, showing the milky white endosperm; D: median longitudinal section perpendicular to the plane of cotyledon, showing the position of an embryo in a seed; E: a free embryo, showing cotyledon, hypocotyle and radicle; F: a germinating seed with partial testa and radicle breaking through the micropyle, showing the micropyle on the dorsal side of the caruncle and not covered by the caruncle. HI: Hilum; CA: Caruncle; EN: Endosperm; TE: Testa; EM: Embryo; CO: Cotyledon; HY: Hypocotyle; RA: Radicle. Bar = 2 mm. "

Fig. 2

Anatomical structure of R. communis testa A: transection of episperm on the back of a seed perpendicular to the long axis, showing columnar epidermis, spongy parenchyma and palisade parenchyma; B: transection of epidermal cells from the episperm, showing epidermal cells in oval shape; C: transection of lateral episperm of a seed perpendicular to the long axis, showing columnar epidermis, spongy parenchyma and palisade parenchyma; D: transection of episperm at the ventral surface of a seed along the direction of raphe extension, showing columnar epidermis, spongy parenchyma, palisade parenchyma, and the large bundle in spongy parenchyma; E: transection of episperm at the ventral surface of a seed perpendicular to the direction of ridge extension, showing large vascular bundle; F: a schematic diagram of the distribution of endopleura, showing an air chamber surrounded by inner and outer layers of endopleura at caruncle end; G: stereomicroscopic image from the inner side of the ventral episperm, showing the nonseparable pattern of episperm and endopleura, and the junction point (arrow) between the large bundle of raphe and the bundle band of endopleura at the opposite end of the caruncle; H: transection of endopleura from the vein-islet, showing malpighian layer, flat spongy parenchyma and inner layer of inner integument; I: transection of endopleura along the direction of bundle extension, showing the vascular bundle located in spongy parenchyma. EP: epidermis; SP: spongy parenchyma; PP: palisade parenchyma; LB: large bundle; IE: inner layer of endopleura; OE: outer layer of endopleura; ML: malpighian layer; IL: inner layer of inner integument; EB: endopleura vascular bundle. "

Fig. 3

Distribution of endopleura vascular bundles of R. communis A: partial free endopleura, showing the reddish xylems representing vascular bundles and branching pattern of vascular bundles; B: detailed view of box a in figure A, showing that the vascular bundle band, connecting with large vascular bundle in the raphe from episperm at the ventral surface of a seed, branched into three larger vascular bundles; C: detailed view of box b in figure A, showing terminal side wall junction (arrow) of the tracheids; D: detailed view of box c in figure A, showing the pattern of branching of vascular bundles; E: detailed view of box d in figure A, showing the helical tracheid (arrow) at the end of vascular bundle of endopleura. BB: bundle band. "

Fig. 4

Shape and structure of the caruncle of R. communis seed A-C: stereomicrograph, showing the shape of the caruncle; D-J: transection; K and L: longitudinal section; D and H-J: images stained with Schiff’s reagent, showing cell walls composed of carbohydrates; E-G, K, L: images with double staining of saffron and fixed green, showing cell walls without lignin and cuticle. A: view on the back of the caruncle, showing the shallow ditch in the middle part of the caruncle, which divided its upper part into two spherical protrusions; B: ventral view of the caruncle, showing the tight combination of caruncle and episperm; C: view on the bottom of the caruncle, showing an opening (arrow) of caruncle channel between the inner and outer layers of the endopleura and near the dorsal surface of the seed, and the outer layer of the white endopleura; D: transection of the upper part of the caruncle, showing two spherical protrusions; E: transection of shallow ditch base of the caruncle, showing no caruncle channel; F: transection of the middle lateral side of the caruncle, showing three layers of small and cuticle-free cells on the outside and large parenchyma cells inside; G: detailed view of transection of the middle part of the caruncle, showing developed pits (arrow); H: transection of middle part of the caruncle, showing the caruncle channel; I: detailed view of box a in figure H, showing Y-shaped caruncle channel; J: detailed view of box b in figure H, showing vascular bundle in the caruncle; K: longitudinal section of the caruncle a short distance from the central axis of the caruncle channel, showing the upper end of the caruncle channel (arrow) which did not connected with the base of the shallow ditch; L: longitudinal section along the central axis of the caruncle channel, showing the opening (arrow) at the base of the caruncle channel. EM: episperm; CC: caruncle channel. "

Fig. 5

Anatomical structure of R. communis embryos stained with periodic acid Schiff (PAS) A: transection of middle cotyledon, showing the pattern of two cotyledons surrounded by endosperm; B: local enlargement of figure A, showing plenty of aleurone grains containing crystalloids and phosphate spheres in endosperm cells; C: local enlargement of endosperm margin in figure A, showing the outer 9-11 layers of small endosperm cells and the inner large ones containing aleurone grains; D: transection of the middle cotyledon in Figure A, showing small cotyledon cells and large endosperm ones, which contained aleurone grains, and the mucilaginous layer between the cotyledon and endosperm (arrow). E: transection of the radicle, showing ground meristem and procambia; F: local enlargement of figure E, showing ground meristem and procambia cells lacking aleurone grains; G: transection of the hypocotyle, showing cortex, procambia and vascular cylomnder; H: local enlargement of figure G, showing cortex, procambia and vascular cylomnder cells lacking aleurone grains; I: transection of plumule, showing two plumule protuberances surrounded by two main veins of cotyledons; J: local enlargement of figure I, showing plumule cells lacking aleurone grains; K: transection a short distance above the level in figure I in cotyledon base, showing anatomical structure of main veins of two cotyledons; L: cells in main veins lacking aleurone grains. The circles in the figure represented collateral vascular bundles. GM: ground meristem; PR: procambia; CX: cortex; PL: plumule; VC: vascular cylinder. "

Fig. 6

Anatomical structure of R. communis embryos stained with saffron and green fixation A and B: the transverse section of the middle part of the seed; C-F: longitudinal section perpendicular to the plane of the cotyledon. A: transection of the middle cotyledon, showing the patterns of cotyledon separation, cotyledon-endosperm interconnection through the mucilaginous layer (arrows), large endosperm cells and small cotyledon ones with aleurone grains; B: transection of the endosperm, showing endosperm cells containing large amounts of aleurone grains; C: longitudinal section of embryos with most cotyledons removed, showing partial cotyledon, plumule, hypocotyle and radicle; D: longitudinal section of the radicle in figure C, showing differentiation pattern of primary epidermis, ground meristem and procambium; E: longitudinal section of the hypocotyle in figure C, showing differentiation pattern of epidermis, cortex and vascular cylomnder; F: longitudinal section of plumule and partial cotyledon, showing the plumule and vascular bundles (arrows) connecting the hypocotyle and cotyledon. "

Fig. 7

Diagram showing the distribution of vascular bundles in R. communis seed A: median longitudinal section perpendicular to the plane of cotyledon, showing the relationship between vascular bundles of endosperm and episperm and main vascular bundles of cotyledons originating from hypocotyle; B: median longitudinal section parallel to the plane of cotyledon, showing spatial separation of vascular bundles in endopleura and cotyledon; C: median transverse section perpendicular to the long axis of seed, showing spatial relationship of vascular bundles in episperm, endopleura and cotyledon; D: local transverse section of middle part of caruncle, showing bundles in caruncle. VC: the vascular bundles in the main vein of cotyledons; HB: hypocotyle bundles; AC: air chamber; BV: branched vascular bundles in cotyledons; BC: bundles in the caruncle. "

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