How Plants Grow (a Level)

Growth is a permanent increase in the size of an organism or of some part of it.

It occurs due to

A. cell division

B. assimilation of new material within the cells that resulted from the division

C. cell expansion

The main areas of cell division in plants are known as the meristems and they occur just behind the tip of a root or shoot. The regions of cell division and cell elongation are particularly sensitive to plant growth substances. These chemical messages act in a number of ways.

Some affect cell division by increasing the number of divisions that occur.

Others make it easier for the cellulose walls to be stretched, and this makes it easier for the cells to expand and grow.

AUXINS

Auxins, for example indoleacetic acid (IAA), are powerful growth stimulants that are effective in very low concentrations.

Auxins are produced in young shoots and always move down the plant from the shoots to the roots.

This movement involves some active transport and calcium ions.

Auxins are involved in apical dominance, by suppressing the growth of lateral shoots so that the main stem grows fastest.

In low concentrations, they promote root growth.

The more auxin that is transported down the stem, the more roots grow If the tips of the stems are removed, removing the source of auxins, the stimulation of root growth is removed and root growth slows and stops.

Auxins are also involved in the tropic responses of plant shoots to unilateral light (light from one side).

The response of a plant to auxins often depends on both the concentration of the hormone and the region of the plant.

THE EFFECT OF IAA- HOW AUXINS WORK

Auxins affect the ability of the plant cell walls to stretch.

1. IAA is made in the tip of the shoot and diffuses back towards the zone of elongation.

2. The molecules of IAA bind to specific receptor sites on the cell surface membranes, activating the active pumping of hydrogen ions into the cell wall spaces.

3. This changes the hydrogen ion concentration, providing the optimum pH of around 5 for enzymes that break bonds between neighbouring cellulose microfibrils

4. This allows the microfibrils to slide past each other very easily, so the walls stay very plastic and flexible.

5. The cells absorb water by osmosis and, as a result of turgor pressure, the very flexible cell walls stretch allowing the cells to elongate and expand.

6. Eventually, as the cells mature, the IAA is destroyed by enzymes, the pH of the cell walls rises, the enzymes are inhibited and bonds form between the cellulose microfibrils.

7. Consequently, the cell wall becomes more rigid again and the cell can no longer expand