Use of Plant Hormones by Humans (OCR Gateway GCSE Biology)

Higher Tier Only

• Plant hormones can be extracted or artificially made and used by gardeners and farmers in horticulture and agriculture to usefully control plant growth to obtain larger yields
  • Auxins, ethene and gibberellins are commercially used plant hormones in the food and decorative plant industry
• However, the everyday use of hormones as weed killers can have a negative effect on biodiversity; as the growth of unwanted but natural plants such as weeds is inhibited
• Many different species of plants are classed as weeds commercially, but to other organisms they are a food source and potential habitat, so destroying them can have negative effects on other organisms in the ecosystem

Auxins

• Auxins can be used as selective weed killers; negatively affecting the growth of broad-leaved plants which are weeds in comparison to the narrow-leaved grasses and cereals grown as crops for food production (which are desired)
  • The growth of weeds is controlled by farmers who don’t want their yields to be smaller as a result of competition between crops and weeds for space and nutrients from the soil
  • Selective weed killers disrupt the growth of weeds only, causing them to die
  • However once applied to a crop their spread cannot be controlled, and they could affect other plant species negatively
• If a gardener or farmer wants to easily and cheaply produce lots of clones of a desirable plant, then they can take cuttings of the plant and dip the tips in auxins which are sold as ‘rooting powders’ as they encourage the rapid development of roots
• The same principle as above can be used to clone plants in the lab; auxins in this way are used to promote growth in tissue culture where scrapings of cells can be taken from a desirable plant and used to produce clones in a petri dish that are then planted and allowed to grow in soil

Ethene (also known as ethylene)

• Ethene is used in the food industry to control ripening of fruit during storage and transport
  • It is far more effective to transport unripe fruit, as ripe fruit is softer and therefore more easily damaged and spoiled
  • The production of ethene can be inhibited to delay ripening of fruits in storage; this can either be achieved directly by adding chemicals that prevent ethene from being produced, or by reacting ethene in the air around fruit with substances that can remove it
  • When ripening needs to be encouraged (e.g. when fruit is in the supermarket), artificially produced ethene gas can be released to speed up the process

Gibberellins

• Gibberellins can be used to:
  • End seed dormancy - a high concentration of gibberellin, promotes seed germination.
    • Gibberellin levels naturally rise after a period of dormancy (exposure to cold and dry conditions) - usually, dormancy ends with an intake of water into the seed and warmer weather
  • Promote flowering - regardless of the weather conditions the plant is in
  • Increase fruit size - higher levels of gibberellin promote the development and growth of fruit
  • Produce seedless fruit (parthenocarpic fruit development) - normally, fruit with seeds in the middle only grows on flowering plants that have been pollinated.
    • If gibberellins are applied to the unpollinated flowers of some plants, the fruit grows but no seeds develop within it

Plant Hormones and their Commercial Uses Table