chapter 16 pt 5

Question 1

Growing in the dark:
pt 1

Answer 1

• If a plant is in the dark the biological imperative is to grow upwards rapidly to reach the light to be able to photosynthesise.
• The seedlings that break through the soil first will not have to compete with other seedlings for light.
• Evidence suggests that it is gibberellins that are responsible for the extreme elongation of the internodes when a plant is grown in the dark.

Question 2

Growing in the dark:
pt 2

Answer 2

• Once a plant is exposed to the light, a slowing of upwards growth is valuable.
• Resources can be used for synthesising leaves, strengthening stems, and overall growth.
• Scientists have demonstrated that levels of gibberellin fall once the stem is exposed to light.
• Gardeners sometimes use this response to ‘force’ growth in plants - early rhubarb is famously grown in dark sheds in Yorkshire.
• The rapid upward growth which takes place in a plant grown in the dark is known as etiolation.
• Etiolated plants are thin and pale - because the plant is deprived of light little chlorophyll develops in the leaves.

Question 3

Geotropisms:

Answer 3

• Plants are also sensitive to gravity, and the different responses of the roots and shoots are very important in the control of plant growth.
• In normal conditions, plants always receive a unilateral gravitational stimulus - gravity always acts downwards.
• The response of plants to gravity can be seen in the laboratory using seedlings placed on their sides either in all-round light or in the dark.
• Shoots are usually negatively geotropic (grow away from gravitational pull) and roots are positively geotropic (grow towards gravitational pull).
• This adaptation ensures that the roots grow down into the soil and the shoots grow up to the light.
• Geotropisms are also known as gravitropisms.

Question 4

Practical investigations into geotropisms:

Answer 4

• The geotropic response can be investigated in shoots and roots using a rotating drum known as a clinostat.
• The plants can be grown on a slowly rotating clinostat (about four revolutions per hour) so the gravitational stimulus is applied evenly to all sides of the plant - and the root and (in the dark) shoot grow straight.
• Alternatively, the seeds can be placed in petri dishes stuck to the wall of the lab, and the dishes rotated 90° at intervals as the seedlings grow.
• A geotropic response in the roots can be seen within about two hours.

Question 5

diagram Practical investigations into geotropisms:

Answer 5

Question 6

commercial use of plant hormones

Answer 6

Control of ripening
Hormone rooting powders and micropropagation:

Hormonal weedkillers:

Question 7

Control of ripening
pt 1

Answer 7

• The gaseous plant hormone ethene is involved in the ripening of climacteric fruits.
• These are fruits that continue to ripen after they have been harvested.
• Their ripening is linked to a peak of ethene production triggering a series of chemical reactions including a greatly increased respiration rate.
• Climacteric fruits include bananas, tomatoes, mangoes, and avocados.
• Non-climacteric fruit (such as oranges, strawberries, and watermelon) do not produce large amounts of ethene and do not ripen much after picking.
• The effect of ethene on climacteric fruit can easily be seen if part of a bunch of green bananas is put in a bag with a single ripe banana.
• The bunch with the ripe banana will ripen faster than the rest of the bunch, even if the temperature is exactly the same in both cases.
• Ethene from the ripe banana stimulates the rapid ripening of the green ones.

Question 8

Control of ripening
pt 2

Answer 8

• Ethene is widely used commercially in the production of perfectly ripe climacteric fruit for greengrocers and supermarkets.
• These fruit are harvested when they are fully formed but long before they are ripe. and then cooled, stored, and transported.
• The unripe fruit is hard and much less easily damaged during transport around the world than the ripe versions.
• When the fruit are needed for sale, they are exposed to ethene gas under controlled conditions.
• This ensures that each batch of fruit ripens at the same rate and are all at the same stage to be put on the shelves for sale to the public.
• This careful control of ripening prevents a lot of wastage of fruit during transport, and increases the time available for them to be sold.

Question 9

graph to show effect of ethene on r. rate of fruit

Answer 9

Question 10

Hormone rooting powders and micropropagation:
pt 1

Answer 10

• Auxin affects the growth of both shoots and roots.
• Scientists have discovered that the application of auxin to cut shoots stimulates the production of roots.
• This makes it much easier to propagate new plants from plant cuttings.
• A cutting is a small piece of the stem of a plant, usually with some leaves on.
• If this is placed in compost or soil - or even water - roots may eventually appear and a new plant forms
• Dipping the cut stem into hormone rooting powder increases the chances of roots forming, and of successful propagation taking place.

Question 11

Hormone rooting powders and micropropagation:
pt 2

Answer 11

• This has made it much easier for horticulturists to develop cuttings to sell and for individuals taking their own cuttings.
• In both horticulture and agriculture, many plants are now propagated on a large scale by micropropagation, when thousands of new plants are grown from a few cells of the original plant.
• Plant hormones are essential in this process - they control the production of the mass of new cells and then the differentiation of the clones into tiny new plants.

Question 12

Hormonal weedkillers:
pt 1

Answer 12

• the interactions between the different plant hormones are finely balanced to enable the plant to grow.
• If this balance is lost it can interrupt the metabolism of the whole plant and may lead to plant death.
• Sometimes, this is exactly what we want to achieve, and plant hormones can help us.
• Weeds are plants that grow where they are not wanted.
• Commercial food crops are vital globally for producing the food people need to eat.
• Weeds interfere with crop plants, competing for light, space, water, and minerals.

Question 13

Hormonal weedkillers:
pt 2

Answer 13

• Scientists have developed synthetic auxins which act as very effective weedkillers.
• Many of the main staple foods around the world are narrow-leaved monocot plants such as rice, maize, and wheat.
• Most of the weeds are broad-leaved dicots.
• If synthetic dicot auxins are applied as weedkiller, they are absorbed by the broad-leaved plants and affect their metabolism.
• The growth rate increases and becomes unsustainable, so they die.
• The narrow-leaved crop plants are not affected and continue to grow normally, freed from competition.
• The synthetic auxins used by farmers and gardeners are simple and cheap to produce, have a very low toxicity to mammals, and are selective.

Question 14

ways in which plant hormones are used commercially:

Answer 14

• Auxins can be used in the production of seedless fruit.
• Ethene is used to promote fruit dropping in plants such as cotton, walnuts, and cherries.
• Cytokinins are used to prevent ageing of ripened fruit and products such as lettuces, and in micropropagation to control tissue development.
• Gibberellins can be used to delay ripening and ageing in fruit, to improve the size and shape of fruits, and in beer brewing to speed up the malting process.