Gibberellins: Plant Hormones for Stem Elongation

Gibberellins are plant hormones that strongly promote stem elongation, seed germination, and flowering in some plants. They were first discovered from a fungus that caused rice seedlings to grow abnormally tall and weak. The most studied gibberellin is gibberellic acid, GA3.

These hormones matter in agriculture for increasing fruit size, breaking seed dormancy, and speeding up germination in brewing. Understanding gibberellins also explains dwarf varieties of wheat and rice that helped increase grain yield during the Green Revolution.

Key Concepts

Gibberellin: A diterpenoid hormone that stimulates cell elongation and division in stems.

GA3: Gibberellic acid, a commonly used gibberellin.

Bolting: Rapid elongation of stem in rosette plants before flowering, induced by gibberellins.

Dormancy: A period when seeds do not germinate even under favourable conditions. Gibberellins help break it.

Dwarfism: Short stature due to reduced gibberellin synthesis or response.

Aleurone layer: Protein rich layer in cereal grains where gibberellin induces enzyme production.

How It Works

1. Gibberellins are made in young leaves, root tips, and developing seeds.
2. They travel to target tissues and bind to receptors in cells.
3. In stems, they increase cell elongation and also stimulate division in the subapical meristem.
4. In germinating barley seeds, gibberellin from the embryo moves to the aleurone layer.
5. Aleurone cells then produce alpha-amylase enzyme.
6. Amylase breaks starch in endosperm into sugars that feed the growing embryo.

Worked Example

A dwarf pea plant is 15 centimetres tall. After treatment with GA3, it grows to 45 centimetres in 10 days. Increase in height is 45 minus 15 equals 30 centimetres. Average growth rate after treatment is 30 divided by 10 equals 3 centimetres per day. An untreated dwarf grows only 5 centimetres in the same period, which is 0.5 centimetres per day. The treated plant grows six times faster, showing gibberellin can replace the missing natural hormone.

Common Misconceptions

• Gibberellins only make plants tall. They also affect germination, flowering, fruit size, and enzyme production.
• Tall plants always yield more. Excess elongation can cause lodging, where stems fall over. Dwarf varieties often yield more grain.
• All seeds need gibberellin spray to germinate. Many seeds make enough of their own gibberellin after absorbing water.

Real-World Applications

GA3 is sprayed on grapes to increase berry size and bunch elongation. In brewing, it speeds uniform germination of barley for malt production. It is used to break dormancy in potato tubers and seeds. Dwarf wheat and rice varieties with reduced gibberellin response produce shorter, stronger stems and higher harvest index.

Gibberellins act as powerful growth signals that lengthen stems, awaken seeds, and mobilise stored food. Used wisely, they improve fruit quality and germination, but balance is essential for healthy yield.