The growth of
plants depend on nutrition and environmental factors. The absence or short
supply of any of the factors in the environment may exert stress on plants
affecting their health and survival. If plants are grown without light, they
become extremely long and fail to form chlorophyll. They are said to be
etiolated. Due to short supply of mineral nutrient in the soil, the plant fail
to form sufficient chlorophyll, and becomes yellowish in color. This condition
is known as chlorosis.
PLANT’S DEFENSE MECHANISM AGAINST PATHOGENS
Plants can be
under attack by several types of pathogens. These include fungi, bacteria,
viruses, nematodes. In the absence of induced antibiotics to protect
themselves, plants must rely on various structural and anatomical attributes
for, either avoid infection entirely or prevent infection initially. There are
certain defense structures that are already present in the plants. For example
(a) Surface waxes (b) Structure of epidermal cell walls (c) Position of stomata
and lenticels (d) Thick cell walls (e) Leaf hairs.
Some defense
structures are formed in response to infection by the pathogen. Once infection
occurs, the plant may respond by producing structural barriers to further entrance
of the parasite. The mechanisms are either at the tissue level, termed
histological defense or sub-cellular level known as cellular defense reaction.
Histological Defense Structures
(a) Formation of cork layers.
(b) Formation of abscission layer.
(c) Formation of tyloses (a chemical
substance).
(d) Deposition of gums,
(e) Cellular Defense Structures
(f) Callus thickening of the cell wall i.e. a
group of cells called callus which quickly covers the wound.
(g) Necrotic defense reaction or hypersensitivity
(death of cells surrounding infection site).
In addition to
that some plants exude onto their surface compounds which inhibit the
germination of fungi e.g. tomato. Other plants e.g. onion produce phenolic
compounds that protect them from infection.
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