Hormone control in plants

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

This page will answer many questions e.g.

How do hormones control plant functions?

What are auxins?

What is phototropism?

What is gravitropism or geotropism?

What controls the ripening of plants?

Hormone Control in plants

  • Know that plants also produce hormones and respond to external stimuli.

  • a) Know and understand that plants are sensitive to, and respond to, light, moisture and gravity:

    • their shoots grow towards light and against the force of gravity,

    • their roots grow towards moisture and in the direction of the force of gravity.

  • b) Know that plants produce hormones to coordinate and control growth.

    • Auxin is a plant hormone that controls the growth of the tips of shoots and roots

    • Auxin acts by enabling the plant to respond to, and ...

      • ... the tips of shoots to grow towards light, the effect is called phototropism,

      • ... the tips of shoots to grow upwards (against gravity) and the tips of roots downwards (with gravity), the effects are called gravitropism,

      • ... the tips of roots to seek moisture in the soil.

    • Know that auxins controls phototropism and gravitropism (geotropism).

      • Auxin is produced in the tips and moves back by diffusion to stimulate cell growth - cell enlargement-elongation.

      • If the tip of shoot is cut off, the shoot may stop growing because the auxin hormone is no longer available.

      • Auxin can promote growth in shoots but a high concentration of auxin can inhibit growth in the root to ensure it grows in the right direction.

    • You should understand the role of auxins in phototropism and gravitropism.

  • c) The responses of plant roots and shoots to light, gravity and moisture are the result of unequal distribution of hormones like auxin, causing unequal growth rates and changes in growth direction.

    • How the plant growth hormone auxin works!

      • Shoot tips growing towards light - positive phototropism (positively phototropic)

        • When light shines on a shoot, more auxin concentrates on the side that is in the shade (less light side).

        • This stimulates growth to elongate the cells on the shaded side so the shoot bends towards the light.

      • Shoots growing up against gravity - negative gravitropism/geotropism (negatively gravitropic/geotropic)

        • If a shoot starts to grow sideways-horizontally, gravity causes more auxin to concentrate on the lower side.

        • Therefore the lower side cells are stimulated to grow faster causing the shoot to grow upwards.

      • Roots growing down with gravity - positive gravitropism/geotropism (positively gravitropic/geotropic)

        • If a root is tending to grow horizontally, then, due to gravity it tends to have more auxin on its lower side, BUT excess auxin can inhibit growth and so the upper cells tend to elongate faster the lower side cells, causing the root to bend round downwards and become more firmly embedded in the soil.

      • Roots growing towards moisture - positive hydrotropism (positively hydrotropic)

        • If a root is exposed to an uneven distribution of moisture ie one side of the root is more moist than the other, more auxin concentrates on the side with the most moisture.

        • Consequently the increased auxin level inhibits growth on the moist side and causes the greater growth rate on the least moist side to make the root bend towards the moisture.

  • d) Plant growth hormones are used in agriculture and horticulture as weed killers and as rooting hormones and is a very important use of plant hormones like auxin.

    • Some plant growth hormones can be used as selective weed killers to disrupt the growth of weeds but leave the crops unaffected.

      • Desired crops of grasses and cereals are narrow leafed plants but many weeds have broad leaves.

      • Selective plant growth hormone based weed-killers have been developed that affect the growth development of broad-leafed weeds and eventually kill them, BUT, do not affect the grasses and cereals with narrow leaves.

    • Some plant cuttings won't always readily grow in soil or compost, but by adding a rooting powder to the compost containing a plant hormone like auxin, the growth of roots and subsequent shoots are greatly encouraged.

      • It then enables a flower grower or market gardener to rapidly produce lots of clones of a particular plant - ideally, of the best quality plants.

    • Plant hormones can be used to control the ripening of fruit or produce seedless fruit.

      • The ripening can be controlled while the fruit are still on the tree/bush or during transport to the warehouses/shops.

        • You can therefore pick fruit before it is ripe and still quite firm - which means its less easily damaged in transport.

        • You can then choose the time when the ripening hormone is added so the fruit is as fresh as it can be for you the consumer via the wholesaler, market vendor, small shop or giant supermarket!

      • Most fruit plants with seeds in the core, require pollination by insects, otherwise the fruits and seeds will not grow.

        • By applying growth hormones to the unpollinated flowers of some fruit plants, the fruits grow BUT not the seeds! Sometimes the plant hormones are applied after pollination, but still prevent the seeds developing.

        • A very handy way of producing common varieties of seedless fruits like watermelons, grapes, bananas and many seedless citrus fruits, such as oranges, lemons and limes.

    • Gibberellin is another plant growth hormone used to stimulate seed germination, stem growth (taller) and flowering.

      • If a variety of a dwarf plant is treated with Gibberellin, it can grow as high as a tall variety!

    • Sometimes combinations of hormones eg auxins plus gibberellins can be used in conjunction with each other to have greatly enhanced effect eg producing very tall plants.

  • You are expected to use appropriate skills, knowledge and understanding to:

  • Any practical work and investigations you did should also be revised  (which should also be revised, helps in understanding 'how science works' and context examination questions):

    • evaluate the use of plant hormones in horticulture as weedkillers and to encourage the rooting of plant cuttings.

    • plant growth investigation eg ...

      • ... the effect of light on the growth of seedlings,

      • ... the effect of gravity on growth in germinating seedlings,

      • ... the effect of water on the growth of seedlings,

      • ... using a motion sensor to measure the growth of plants and seedlings,

      • ... the effect of rooting compounds and weed killers on the growth of plants,


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