Periderm

Compared with control plants, no visible differences were observed on neither the plants nor the tubers. However, the periderm formed after wounding the surface was thicker on transgenic tubers synthesizing PL3. 

In contrast to the exterior localization of cutin, suberin can be deposited in both external and internal tissues. External deposition occurs in the periderm of secondary roots and stems and on cotton fibers, whereas internal deposition occurs in the root endodermis and the bundle sheath of monocots. The Casparian strip of the root en-dodermis contains suberin, which produces a barrier isolating the apoplast of the root cortex from the central vascular cylinder. Suberin also produces a gas-impermeable barrier between the bundle sheath and mesophyll cells in C4 plants. The bark of trees contains periderm-derived cork cells that have a high suberin content. 

Lenticels Loosely arranged group of cells in the periderm Cells of the vascular system Involved in gas exchange inward and outward... 

Peridermal tissue of stems Cork cells and cork cambium Mechanical protection... 

Peterson JK, Harrison HE (1991) Isolation of Substance from Sweet Potato Ipomoea batatas) Periderm Tissue that Inhibits Seed Germination. J Chem Ecol 17 943... 

Figure 1. The location of plant polyesters cutin attached to the epidermal wall (top) and suberin within the cell wall of a plant periderm (bottom). Reproduced by permission of the National Research Council of Canada, from Ref. 1.

In certain instances, however, factors other than the cell wall polymers of the phellem may be important in the protection provided by the secondary surface. Rosellinia desmazieresii inoculated in a food base onto the underground stems of a resistant Salix repens hybrid (5. x Friesiana) exhibited greatly reduced epiphytic growth and cord formation compared with inoculations onto susceptible S. repens itself. Attempted penetration was not observed on the resistant hybrid (30). This behaviour suggests that diffusible chemical inhibitors at the stem surface may be important in resistance to this pathogen, which has a demonstrated ability to degrade suberin and penetrate the surface periderm (30). 

Although in an unwounded, healthy, plant periderms are normally superficial, interxylary cork has been reported from at least 40 species of di-... 

Table I. Histochemical responses of cell walls associated with the necrophylactic periderm response in Picea sitchensis challenged with Armillaria obscura...

Figure 1. Structural responses of the bark of Picea sitchensis to wounding and inoculation with Phaeolus schweinUzii. IW, inoculated wound SP, surface periderm NT, necrotic tissue TC, thickened cells SIT, relic of suberized impervious tissue NP, necrophylactic periderm P, phloem VC, vascular cambium.

Figure 3. Hyphae (arrowed) of Armillaria obscura in thick walled cells (TC) overlying the necrophylactic periderm (NP) in Picea siichensis. Bark sectioned and stained with toluidine blue as previously described (15). Scale bar = 25 /im.

Figure 4. Penetration of necrophylactic periderms (NP) in the root of Ar-moracia rusiicana by rhizomorph-like aggregations of hyphae (R). Scale bar = 100 pm.

Because irradiation interferes with the natural wound-healing process, it is an important prerequisite of successful application of irradiation that mature tubers with fully developed periderm are suitable for radiation processing and tubers must be properly cured from harvesting and handling injuries before irradiation. 

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