The structure of the endodermis during the development of wheat (Triticum aestivum L.) roots

Four stages of development in the process of differentiation of the root endodermis of wheat (<em>Triticum aestivum</em> L. var. Grana) are described. The proendodermis cells have a meristematic nature. Their vacuoles accumulated an osmophilic material. In the next stage, Casparian strips arose in the walls of the endodermis. Dictyosomes and ER cisterns were numerous in the protoplasts of these cells. In the following stage, a suberin lamella was deposited over the entire internal surface of the primary cell wall. In the final stage, a secondary cell wall, thickened in the form of a letter U, was formed. In secondary wall simple pits arose. The endodermis of the wheat root developed asynchronously, more quickly over the phloem bundles.

Development of cell wall modifications in the endodermis and exodermis of Allium cepa roots

The cytological events of wall modification in the endodermis and exodermis of Allium cepa L. roots were examined with fluorescence and transmission electron microscopy. In the endodermis, Casparian bands, suberin lamellae, and tertiary walls developed in succession. At the site of the future Casparian band, the plasma membrane was bound to the wall before deposition of detectable hydrophobic components in the radial wall. Suberin lamellae were deposited on the inner faces of the primary walls, first along the outer tangential walls and then the inner tangential walls. On both walls, segments of the lamellae were formed earlier in primary pit fields than at nonprimary pit field regions. Suberin lamellae then extended to the radial walls. When they reached the Casparian bands, the lamellae intruded between the bound plasma membranes and the walls, so that the cells' plasma membranes remained intact. In this way, suberin lamellae that were continuous around the cells were laid down. Later, tertiary walls were deposited internal to the suberin lamellae. None of the wall modifications interrupted the symplastic connections of the endodermis. During suberin lamella and tertiary wall formation, more dictyosomes and ER profiles appeared than during Casparian band development. In the exodermis, although Casparian bands were readily detected with fluorescence microscopy, they were rarely detected with electron microscopy. Suberin lamellae were formed in long cells severing their plasmodesmata. As in the endodermis, dictyosomes and ER were prominent during suberin lamella formation. Tertiary walls were not formed in the exodermis.Key words: Allium cepa, Casparian band, endodermis, exodermis, suberin lamella, ultrastructure.

Suberin deposition and band plasmolysis in the corn (Zea mays L.) root exodermis

The exodermal Casparian band in corn (Zea mays L.) was first seen 10 mm distal to the kernel 4 days after planting. From its inception, the band usually occupied most of the radial wall (as seen in a cross section of the root). Subsequent maturation of the band around the root was asynchronous into the region of emerging lateral roots. Thus, a continuous apoplastic barrier would have been absent over much of the young root surface. Suberin lamellae development was also asynchronous, as these structures formed in those cells which had Casparian bands. Frequently, a lamella was initially deposited in patches, progressing centripetally until a continuous lipid layer was formed around the cell protoplast. Many instances of band plasmolysis (typical of the endodermis) were observed in the developing uniform exodermis. It could occur in cells with no detectable Casparian bands, suggesting that the tight connection between the plasmalemma and the wall that causes this phenomenon is not due to hydrophobic attractions. The results are consistent with the idea that there are strong attractions between proteins of the membrane and wall in the region of the Casparian band. The tight connection between the plasmalemma and the wall was broken during the later stages of suberin lamella development. Key words: Zea mays L., Poaceae, band plasmolysis, exodermis, Casparian band, suberin lamella.

The effect of suberin lamellae on the vitality and symplasmic permeability of the onion root exodermis

The onion exodermis is made up of two cell types, i.e., long and short cells. Both form Casparian bands, but suberin lamella development is absent or delayed in the short cells. Long cells did not accumulate fluorescein, a common test for cell vitality, because of reduced wall permeability due to suberin lamella development. Immature, long cells without lamellae stained in 15 min, whereas mature cells with lamellae required a 3.5- to 4-h treatment before staining was visible. Long exposure to fluorescein was needed to show that mature long cells were alive. Their vitality appeared to decline slowly with age but was not affected by drought stress. Fluorescein staining was apparent in the long cells only after treatment of paradermal sections; when dye was applied only externally to root segments, it did not enter the long cells from the epidermis or from the neighbouring short cells. This result indicates that the long cells were connected symplasmically to the cells of the cortex but were either unconnected, or connected by plasmodesmata of small functional diameter, to the epidermal and short cells. If they were unconnected, they would not form part of the symplasmic path of ion transfer into the root. Keywords: Allium cepa L., drought, exodermis, suberin lamella, vitality.

Development of Casparian bands and suberin lamellae in the endodermis of onion roots

The endodermal Casparian band in onion (Allium cepa L.) roots is first evident 10 mm from the apex. It occupies a small region (20–30%) in the middle of the radial wall for a distance of 70 mm from the apex. However, with the development of suberin lamellae in most cells at greater distances from the root tip, the band expands symmetrically through the wall until, at 80 mm from the root tip, it fills the entire radial wall. Passage cells overlying some xylem poles have a delayed development of suberin lamellae, and in these cells the Casparian bands remain narrow. The sensitive stain, Fluorol yellow 088, detects precursors of the suberin lamellae in the outer tangential walls of the endodermal cells. The presence of complete lamellae is indicated by Sudan red 7B staining and resistance to acid digestion. Growth of the Casparian band during suberin lamella development may play a role in maintaining the apoplastic barrier in this layer by enlarging the contact area between the band and the lamellae. Key words: Casparian band, endodermis, suberin lamella, Allium cepa.

Deposition of Casparian bands and suberin lamellae in the exodermis and endodermis of young corn and onion roots

The Casparian band of the exodermis of corn and onion roots matures further from the root tip than its counterpart in the endodermis. A complete suberin lamella in the exodermal cells usually develops about 10 mm proximal to the exodermal Casparian band. The distance between the exodermal Casparian band and suberin lamella in the endodermis was usually much greater than in the exodermis. Both the exodermal Casparian band and suberin lamella matured closer to the tip in onion than in corn roots. The distance from the root tip at which the exodermal Casparian band matured increased with root age during the 5-day period studied in corn grown in hydroponics and vermiculite, and onion grown in hydroponics. This difference was most pronounced in corn, in which the Casparian band matured 20 mm from the root tip when the root was 20 mm long but matured 120 mm from the tip 4 days later when the root was 170 mm long. When the growth rate of corn roots was drastically inhibited by adding polyethylene glycol to the hydroponic medium, the exodermal Casparian band and suberin lamellae were present within 10 mm of the root tip. The position in the root at which the exodermis matures is thus highly variable and can depend on the plant species, and the age and growth rate of the individual root.

IV.—The Occurrence of Cell Division in the Endodermis

Summary1. Endodermal cells frequently show considerable increase in tangential dimension at a late stage in their existence, as a result of which the continuity of the endodermis may be maintained, despite the rapid expansion of the intra-endodermal cylinder following secondary growth.2. This tangential increase in size of the endodermal cells is usually accompanied by their division. The divisions may actually cause the increase in size of the cells, that is, there is active growth of the cells. Or they may follow a passive stretching. In either case the new walls fulfil a mechanical function, lending added strength to the stretched walls of the original cell.3. Two types of division are distinguishable:(a) Those occurring in primary endodermal cells. The new walls soon develop Caspary strips, linking up with those of the parent cells.(b) Divisions in tertiary endodermal cells. The new walls here show neither Caspary strip nor suberin lamella.4. It is pointed out that in neither case does the development of new walls interfere with the impermeability of the endodermis.5. The occurrence of the divisions is considered in relation to the supposed impermeability of the suberin lamella of the secondary endodermal cell, and to the conditions resulting in the deposition of the Caspary strip.