Mention the characteristics and types of plant growth regulators.

(N/A) The plant growth regulators $(PGRs)$ are small,simple molecules of diverse chemical composition.
They can be indole compounds (indole-$3$-acetic acid,$IAA$); adenine derivatives ($N^{6}$-furfurylamino purine,kinetin); derivatives of carotenoids (abscisic acid,$ABA$); terpenes (gibberellic acid,$GA_{3}$); or gases (ethylene,$C_{2}H_{4}$).
Plant growth regulators are variously described as plant growth substances,plant hormones,or phytohormones in literature.
Types of $PGRs$:
The $PGRs$ can be broadly divided into two groups based on their functions in a living plant body:
$1$. Plant Growth Promoters: These are involved in growth-promoting activities,such as cell division,cell enlargement,pattern formation,tropic growth,flowering,fruiting,and seed formation. Examples include auxins,gibberellins,and cytokinins.
$2$. Plant Growth Inhibitors: The $PGRs$ of this group play an important role in plant responses to wounds and stresses of biotic and abiotic origin. They are also involved in various growth-inhibiting activities such as dormancy and abscission. Abscisic acid $(ABA)$ belongs to this group.
Note: The gaseous $PGR$,ethylene,could fit into either of the groups,but it is largely an inhibitor of growth activities.