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In this article we will discuss about the process of healing of wounds in plants, explained with the help of suitable diagrams.
Plant organs often are wounded by adverse calamities of nature, grazing animals and human interference etc. As a result the living cells are exposed and they are subjected to desiccation and attack by bacteria, fungi and other pathogenic microorganisms etc. Higher plants have their own arrangement of healing of wounds.
There are different methods of healing of wounds. When the wound is superficial the exposed cells dry up and eventually die. Suberin deposits in the cells situated below the wound and thus a protective layer is formed. Some plants have laticifers and secrete latex. Latex covers the superficial damaged surface where it coagulates and thus forms a protective cover over the wound.
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In plants having deep wounds callus or wound tissue is formed. Callus is composed of masses of soft parenchyma cells. It develops over and below the damaged surface of stem and root. It is formed, by the division of parenchyma cells that occur either in the phloem, in the cortex or in the vascular rays.
Most frequently cambium donates callus. During the formation of callus the cambium cells proliferate and produce masses of parenchyma. The peripheral cells of callus become suberized thus sealing the exposed surface of damaged tissue. Periderm (also called wound periderm) develops in the callus.
Wound periderm forms a protective bark that inhibits the entry of pathogen. Below the periderm a recognized cambium occurs and it forms new vascular tissues normally. Suberinization seems to create an internal condition that is favourable for periderm formation.
Experiments with potato tuber and sweet potato root reveal that suberinization is preceded by an accumulation of phenolic substances, particularly of chlorogenic acid. It also reveals that lignification is associated with wound healing. In potato tuber wounding stimulates mitosis and the wound healing is accompanied by suberinization and wound cork formation.
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Increased temperature and high humidity are favourable for wound healing. Type of soil determines the number of layers formed in the wound cork of potato. It is observed in potato tuber that during wound healing suberin or cutin deposits over the damaged surface. Thus the surface is sealed and the formation of phellogen follows.
Experiment with potato tuber reveals that covering of cut surface is necessary for phellogen formation. In an experiment the cut surface was covered with paraffin wax. It was observed that phellogen formed without prior suberinization. Later researches reveal that in wounded potato tubers rapid increase in endogenous gibberellins like substances occur.
In trees where wound is deep, the uninjured living cells below the wound are stimulated to divide. As a result callus is formed. It overgrows the wound. Phellogen appears on the peripheral side of callus and divides to form wound cork. Callus with wound cork appears as swelling over the stem surface. Such structure is usually referred to as knot.
In trees where the wound is due to pruning of branches callus is formed on the circumference of wounds (Fig. 28.1 A, B). This happens in the early growing season. Production of secondary xylem continues in the uninjured surrounding tissues by the cambium. Annual ring is also formed in undamaged area around the scar.
The base of branch dies, shrivels up and loses the capacity of growth. As more and more growth rings are formed the base of branch gets buried more and more deeply. The cambium layer becomes continuous over the scar and thus the wound is completely covered by the secondary xylem (Fig. 28.1C). In a timber the base of branch appears as knot.
In case of deep wound where the cambium is damaged, normal callus is formed in the damaged area provided the uninjured tissues are protected from drying out immediately. In the callus a new cambium develops and it unites with the uninjured cambium. Thus the cambium becomes continuous and the production of secondary tissues continues.
The differentiation of cambium in callus and its subsequent fusion with neighboring cambium form the basis of grafting. In grafting the junction of stock and scion is filled up with callus. Within callus cambium differentiates and it fuses with the cambia of stock and scion. Thus a continuous cambium is formed and it donates normal vascular tissues.
