9.3 Growth in Plants

Auxin

a plant hormone that causes the elongation of cells in shoots and is involved in regulating plant growth.

Meristem

a region of plant tissue, found chiefly at the growing tips of roots and shoots and in the cambium, consisting of actively dividing cells forming new tissue.

• Meristems are undifferentiated cells in plants capable of intermediate growth (analogous to stem cells). Meristems tissues have specific regions of growth in plants (allows regrowth or vegetative propagation)

Micropropagation

the propagation of plants by growing plantlets in tissue culture and then planting them out.

Shoot apex

the tip of a shoot, the apical or lateral shoot meristematic dome together with the leaf primordial, from which emerge the leaves and sub-adjacent stem tissue.

Stem

the main body or stalk of a plant or shrub, typically rising above ground, but occasionally subterranean.

Tropisms

the turning or bending movement of an organism or a part of an organism toward or away from an external stimulus, such as light, heat, or gravity.

• Tropism is the turning of an organism in response to a directional external stimulus

  1. PHOTOTROPISM: growth in response to light

  2. GEOTROPISM: growth in response to gravity

Comparison of growth: Apical & Lateral meristems

APICAL:

• occurs in the tips of roots & shoots

• responsible for primary growth (adds length)

• develops into primary xylem & phloem

• produces new leaves & flowers

LATERAL:

• occurs at the cambium

• responsible for secondary growth (adds width)

• produces secondary xylem & phloem

• produces the bark on trees

Plant hormones & their functions

Auxin: involved in primary growth & tropic responses

Gibberellin: involved in seed germination

Cytokinin: involved in secondary growth (eg. branching)

Abscisic Acid: responsible for abscission & regulating transpiration

Ethylene: A gas which stimulates ripening

role of auxin in apical dominance

Auxin released by the apical meristem in shoots promotes apical growth (verticality). It additionally inhibits growth in lateral buds (a condition called apical dominance). As shoots grow further from lateral buds, inhibition is diminished, allowing spread.

Auxin concentration gradients within plant tissue

Auxin efflux pumps set up concentration gradients of auxin in response to stimuli. These pumps control growth direction by determining areas with high auxin levels.

role of auxin in phototropism in shoots & roots

Plant tropisms are caused by differential elongation of cells in response to stimulus. Auxin controls growth rates by chnaging patterns of gene expression in plant cells. In shoots, auxin promotes cell elongation, whereas in roots, auxin inhibits cell elongation. Plant turns away from side with cell elongation, so the shoots grow towards light (positive tropism), & the roots grow away from light (negative tropism)

Process of micropropagation & examples

The process involves:

1. Tissue sample (explant) grown in nutrient agar

2. Development promoted by growth hormones

3. Growing shoots divided (multiplication phase)

4. Cloned plants transferred to new soil

• Rapid bulking = cloning desirable stock plants

• Virus-free strains = cloning non-infected tissue

• Propagating rate species = prevents extinction