Plant Hormones

Auxins

Auxin is a plant hormone that causes the tips of roots and shoots to grow. They are produced at the tips and work backwards, elongating the cells just behind the tip. 

Uneven distribution of auxins causes the tips of the roots and shoots to grow in different directions. Plants use auxins to their advantage, making the plant shoots grow towards light (phototropism) and roots grow in the direction of gravity (gravitropism or geotropism). This allows the shoot to absorb light for photosynthesis and the roots to absorb water and nutrients from the soil and anchor the plant to the ground. 

 

Required Practical

Investigating the affect of light on the growth of seedlings.

You can carry out simple experiments to investigate the effects of light or gravity on plant seedings. In this example we will vary the direction and intensity of light.

Doing the experiment

1. Line 3 petri dishes with moist cotton wool or filter paper.
2. Put 10 cress or mustard seeds into each petri dish and leave them in a warm place to germinate.
3. Once germinated, measure the height of the seedlings using a ruler.
4. Put one petri dish in a dark cupboard or room with no light source, one under an overhead light source and one under a light source at an angle and leave for a week.
5. Measure the height of the seedings again and observe how they have grown under each light condition.

You must make sure to control other variables:

• Use the same number of seeds in each dish.
• Use the same type of seeds from the same packet.
• Keep the temperature the same for all the petri dishes.
• Give each petri dish the same amount of water.

Analysing the results

Observations from the petri dishes:

• Those grown in the dark will be the longest as they will keep trying to grow upwards in search of a light source. However their leaves will be small and yellow. 
• Those with the overhead light source will grow directly upwards and will be healthy.
• Those with the light source at an angle will have grown in the direction of the light (phototropism) and will be healthy. 

You can then calculate and compare the mean height of the seedlings grown under the overhead light and in the dark.

Conclusions

• The seedlings grow towards the light (phototropism).
• The seedlings grew taller in the dark than in the light.

Uses of Plant Hormones

Plant hormones can be extracted or artificially copied by humans and used in many ways.

Auxin– plant growth hormone in roots and shoots.

• Weed killers– Most weeds have broad leaves compared to the grasses and cereals that they grow next to. Selective weed killers use auxins to make the broad leaf plants grow too much and die. However, removing these unwanted plants with weed killers reduces biodiversity.
• Rooting powders– Auxins are used in rooting powders to grow full new plants from plant cuttings. They provide a very quick method to produce clones of the same plant.
• Tissue culture– To grow a full plant from just a few cells, scientists add auxins and nutrients to help the cells divide.

 Gibberellin– plant growth hormone that stimulates germination, flowering and growth of stems.

• Dormancy– seeds may not germinate until they have gone through periods of cold and drought (the seeds are dormant). Adding gibberellins can cause these seeds to germinate at a time it wouldn’t usually and all at the same time. 
• Flowering– adding gibberellins can cause plants to flower regardless of the weather and can make flowers grow bigger.
• Large fruit– gibberellins can make fruits grow larger. It is often used to make seedless fruits grow to the same size as regular fruits. 

Ethene– speeds up ripening of fruits and controls cell division.

• Ripening– ethene is added to ripen fruits so they are ready to be sold in shops. 
• Delaying ripening– ripening can be temporarily stopped by adding chemicals that stop the fruit producing ethene or remove ethene from the plant all together. This means that fruits can be transported when unripe and less easily damaged, then ripened when closer to the destination with artificially produced ethene.