Download ANSWERS TO REVIEW QUESTIONS – CHAPTER 18

ANSWERS TO REVIEW QUESTIONS – CHAPTER 18
1. Define the terms ‘hormone’ and ‘tropism’. List the major types of plant hormones. (p. 401)
Plant hormones are molecules normally produced within the plant that affect growth and development
at very low levels. There are five main groups of plant hormones: auxins, gibberellins, cytokinins,
abscisic acid and ethylene.
Tropism is the directional response by a plant to a stimulus. It may be positive or negative (i.e. towards
or away from the source of stimulus). There are a number of ‘tropisms’ related to particular stimuli,
e.g. phototropism is a response to light, geotropism is a response to gravity and chemotropism is a
response to a chemical substance.
2. (a) What is the best-known naturally occurring auxin in plants? (pp. 402–403)
The main auxin occurring naturally in plants is indole-3-acetic acid (IAA) (see Figure 24.4). When
applied to intact plants, IAA does not elicit large responses but is very effective in promoting growth of
portions of stems or coleoptile segments by promoting elongation of cells.
(b) Explain the acid growth hypothesis for the action of auxin on cell elongation. (p. 403)
The acid growth hypothesis is that auxin stimulates the release of hydrogen ions (H +) from within a cell
(see Table 17.1), which results in a loosening of bonds within the cell wall, making it more flexible. As
a result of turgor pressure, cells elongate.
3. Explain how roots probably detect gravity. (pp. 404–405)
In roots, the root cap detects the stimulus of gravity. Detection of gravity appears to involve the
sedimentation of plastids (statoliths), usually amyloplasts, which contain starch granules. When a root
is moved to a horizontal position, amyloplasts settle on the side of the cells that were originally
vertical. The movement of amyloplasts may result in a gradient of growth hormones (auxins) within
cells, causing growth and bending of the root in the direction of the force of gravity. There is also
evidence that Ca2+ is an important messenger in gravitropism responses—such that a downward
movement of Ca2+ towards the lower side of the cells may be responsible for subsequent bending of the
root.
4.
Synthetic hormones are used in agriculture. What, for example, are 2,4-D and 2,4,5-T and
what have been their uses? (pp. 402–403)
Both 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) are well
known herbicides. Treatment of cereals with 2,4-D has been a standard practice for many years. It is
used for selective killing of dicotyledon weeds in cereal crops, since it has relatively little effect on
grasses (monocotyledons) that have inherently higher levels of resistance. 2,4,5-T works in a similar
manner and, as a component of Agent Orange, was used as a defoliant in jungle warfare during the
Vietnam War. This herbicide is now banned due to trace contaminants of the toxic substance dioxin.
5.
A plant is described as a long-day plant with an absolute requirement for vernalisation.
Explain in simple terms what this means. (pp. 415–417)
This plant requires a combination of two environmental cues before flowering will be induced. It
requires a photoperiod that exceeds a critical day length (long-day plant) and also a period of low
temperature (vernalisation) in order to flower. This plant is therefore likely to flower in spring or early
summer when both of these requirements will have been met.
6. Compare the ethylene and brassinosteroid signal transduction pathways. (pp. 412–414)
Refer to Figures 18.17 and 18.21.
7.
Explain how stratification influences seed dormancy and what the expected effect on abscisic
acid levels might be. What is its biological function? (p. 411)
Stratification is a period of cold that breaks seed dormancy and causes germination. The stratification
period of most plants is greater than two weeks, at temperatures often less than 5ºC. By the time the
stratification requirement in the plant is met, winter will have passed and then germination will proceed
in the more favourable conditions of spring. Abscisic acid (ABA) is a growth inhibitor; therefore, ABA
levels would decline after stratification.
8.
Explain the role of cytokinins and auxin in the induction of organogenesis in tissue culture.
(pp. 401–402)
Refer to Figure 18.14.
9. How do gibberellins cause the mobilisation of starch in the grain of cereal crops? (p. 406)
GA1 produced in the embryo moves to the aleurone layer of the cereal grain where it activates genes
that control the synthesis of the enzyme that converts starch to sugar, which is a much smaller molecule
and is easier to transport.
10. How do photoperiodic plants measure the length of the dark period? (pp. 415–417)
Photochrome pigment in leaves enables plants to detect light and darkness. This pigment interacts with
an internal clock mechanism to measure the length of the dark period.