Q.Which of the following is a stress hormone?

Plant Growth Regulators: The Plant's Chemical Messengers

Plants can't run away from trouble. They can't chase food or find a mate. So how do they decide when to grow, when to flower, when to drop leaves, or when to defend themselves? They use chemical signals — tiny molecules that act like a postal system inside the plant, carrying instructions from one cell to another.

These chemical signals are called Plant Growth Regulators (PGRs). Think of them as the plant's hormones, but with a twist: unlike animal hormones that mostly travel through blood, plant regulators can work right where they're made, move to nearby cells, or travel through the plant's vascular system. They work in tiny amounts — parts per million or even parts per billion — but their effects are dramatic.

The Two Big Families

All PGRs fall into two broad groups based on what they do:

Growth Promoters — these encourage cell division, cell enlargement, flowering, fruit formation. They make the plant grow bigger and develop faster.

Growth Inhibitors — these slow down growth, promote dormancy, help the plant survive stress, and cause aging and ripening.

The Five Classical Regulators

Indian exam syllabi (CBSE, ICSE, state boards) focus on five major PGRs. Here they are, grouped by function:

How They Work Together: A Real Example

Take a seed buried in soil. It's full of abscisic acid — the inhibitor that keeps it dormant. The seed waits. When enough water soaks in and temperature rises, the balance shifts: gibberellins are produced, which break down the stored food and push the shoot upward. Auxin then takes over at the tip, making cells elongate and guiding the shoot toward light. Meanwhile, cytokinins from the growing root tip encourage cell division in the shoot. Later, when fruit forms, ethylene triggers ripening and eventually leaf fall.

No single regulator works alone. They interact, oppose each other, and amplify each other's effects.

Why This Matters for Exams

You will be asked to: …