Germination and effect of gibberellin

What is germination?

The biochemical and physiological processes through which a seed becomes a photosynthesising plant.

What are the 3 main requirements for successful germination?

1. A suitable temperature

2. Water

3. Oxygen

Why is a suitable temperature needed for successful germination to occur?

The optimum temperature for germination is the optimum for the enzymes involved in the process.

Why is water needed for successful germination to occur?

• To mobilise enzymes for transport in the xylem and phloem.

• To vacuolate cells, making them turgid.

Why is oxygen needed for successful germination to occur?

For aerobic respiration which releases ATP; ATP fuels metabolism and growth.

Describe the process of germination in a non-endospermic / dicotyledonous seed e.g. broad bean.

• Water enters through the micropyle.

• The cotyledons swell, and the testa ruptures, allowing entry of more water and oxygen for aerobic respiration.

• Starch and protein reserves in the cotyledons are hydrolysed.

Amylase hydrolyses starch into maltose.

Proteases hydrolyse proteins into amino acids.

• Some of the sugars are converted into cellulose for cell wall synthesis. Aerobic respiration releases energy from sugars (glucose) and amino acids are used to synthesis new proteins.

What happens to the dry masses of the embryo and cotyledons as a seed germinates?

• The dry mass of the embryo developing into a seedling increases.

• The dry mass of the cotyledons decreases.

What plant hormone is involved in the germination of endospermic / monocotyledonous seeds such as Maize?

Gibberellin / Gibberellic acid (GA).

Describe how gibberellin is involved in the process of germination.

• The embryo secretes gibberellic acid, which diffuses through the endosperm to the aleurone layer.

The aleurone layer is the outer layer of the endosperm.

• The gibberellic acid switches on genes in the cell of the aleurone layer, resulting in transcription and translation, producing enzymes including protease and amylase.

• The proteases hydrolyse proteins in the aleurone layer to amino acids, which are used to make amylase enzymes.

• The amylase diffuses out of the aleurone layer and hydrolyses the starch stored in the endosperm.

• The maltose and glucose produced diffuse back through the endosperm to the plumule and radicle of the embryo and is used in growth.