Phototropism

It is the shoot tip that is sensitive to light, as when it is covered a response does not occur. Plant stems produce a hormone called auxin in the tip. When a stem is illuminated from one side this hormone tends to accumalate more on the non-illuminated side. The result is that the plant stem bends towards the light. So growth on this side speeds up. This means the shoots and leaves are turned towards the light for photosynthesis.

Auxin is produced in the tip of growing shoots. If the tips are removed, they cannot produce auxin, so phototropism cannot occur. If the tips are covered, light cannot break down the auxin, so phototropism cannot then occur either.

Auxin distribution controls phototropism.

1.Sunlight strikes the plant from directly above. Auxin (pink dots) encourages growth straight up.

2, 3, 4.Sunlight strikes the plant at an angle. Auxin is concentrated on one side, encouraging growth at an angle from the stem.

As the effect of the hormone is increased growth this leads to the non-illuminated side growing more rapidly than the side that is recieving most light. The differential growth that occurs when one side of the stem grows more than the other side leads to the steam bending in the direction of the light.

The auxin is produced at the tip of the shoot and accumulates on the shaded side as it diffuses down the shoot. The bending is caused by the auxin promoting greater cell elongation in regions where it is inb high concentration.

• When light is from above there is an equal distribution of auxin  on both sides of the shoot grows straight up. Auxin causes cell elongation (growth).
• When a shoot tip is removed, no auxin is produced so there is no growth (no elongation).
• Auxin is produced by a shoot tip. Whwn light is coming from one direction (unilateral light) this cvauses an uneven distribution of auxin. Most auxin gathers on the dark side causing more growth/cell elongation on the dark side causing shoot to bend towards the light.

.Plants grow in the direction of a light source. Plants left on window-sill or against the wall of a house usually do not grow up straight up, but bend towards the light source. This response ensures that the plant stem and leaves recieve more light than they would otherwise would do if there was no such response. This means that more photosynthesis takes place and there will be more growth.

Phototropism is the growth of an organism which responds to a light stimulus(plant's growth response to light). When the stem grows towards the light, the plant can photosynthesise more. More food is produced, so the plant can grow faster. This increases the plant's chances of survival.

Although it is easy to observe the effet of phototropism, what causes it to occur?

The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest side from the light.

Plants are sensitive to:-light -temperature -water -touch.