BOTANY Higher Secondary Second Year - Textbooks Online

Plant growth substances
The growth of a plant is regulated through gene action and
environmental conditions. There are substances, which are produced by
plants themselves, which regulate their growth and many physiological
and biochemical activities. These are called plant growth substances.
Regulation of plant growth through chemical mechanisms frequently involves
certain molecules known as hormones.
Based on the origin and biological activities plant growth substances
are grouped into three - growth regulators, phytohormones and growth
inhibitors.
Growth regulator
It is a hormone like synthetic organic compound. In small amounts, it
modifies the growth and development either by promoting or inhibiting
the growth. eg. Naphthalene acetic acid (NAA).
Phytohormones
These are organic substances produced by the plant. They are active
in very minute quantities. They are synthesised in one of the parts of the
plant and translocated to another part where they influence specific
physiological, biochemical and morphological changes. The phytohormones
are broadly grouped under five major classes namely auxins, gibberellins,
cytokinins, ethylene and abscisic acid.
Auxins
Auxin was the first plant hormone to be discovered. They were
isolated initially from human urine. The term auxin is given to generally
IAA and other natural and synthetic compounds having similar structure
and growth regulating properties. Generally, auxins are produced in the
growing apices of stem and root where from they migrate to the other
parts of the plant. Auxins such as IAA and phenyl acetic acid (PAA) are
natural auxins. Synthetic auxins are chemicals synthesised in the laboratory.
They are considered as plant growth regulators. eg. Naphthalene acetic
acid, 2,4 – Dichlorophenoxy acetic acid.
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