tropisms in plants Flashcards
tropisms as a response to environmental cues
To be able to make the maximum use of the environmental conditions, plants must grow and respond to variations in those conditions. For example, once a seed begins to germinate in the soil, the shoot and root must keep growing in the right direction if the developing plant is to survive. The shoot must grow up towards the light source for photosynthesis to take place. The roots must grow downwards into the soil which will provide support, minerals, and water for the plant. The movements of the root and shoot take place in direct response to environmental stimuli. The direction of the response is related to the direction from which the stimulus comes.
These responses are examples of tropisms.
Much of the research on tropisms uses germinating seeds and very young seedlings. They are easy to work with and manipulate and as they are growing and responding rapidly, any changes show up quickly. Changes also tend to affect the whole organism rather than a small part (as with a mature plant) and this makes any tropisms much easier to observe and measure. The seedlings of monocotyledonous plants - usually cereals such as oats and wheat - are most commonly used as the shoot that emerges is a single spike with no apparent leaves known as a coleoptile. It is easier to manipulate and observe than a dicotyledonous shoot. However, coleoptiles are relatively simple plant systems, so it is important to remember that the control of the responses to light in an intact adult plant may be more complex.
phototropism
The basic model of the way plants respond to light as they grow was based on experiments where shoots were kept entirely in the dark or in full illumination. However, this is rarely the case in real life.
Phototropisms are the result of the movement of auxins across the shoot or root if it is exposed to light that is stronger on one side than the other.
If plants are grown in bright, all-round light in normal conditions of gravity they grow more or less straight upwards. In even but low light they will also grow straight upwards - in fact in these conditions they will grow faster and taller than in bright light. If plants, however, are exposed to light which is brighter on one side than another, or to unilateral light that only shines from one side, then the shoots of the plant will grow towards that light and the roots, if exposed, will grow away. Shoots are said to be positively phototropic and roots are negatively phototropic. This response has an obvious survival value for a plant. It helps to ensure that the shoots receive as much all-round light as possible, allowing the maximum amount of photosynthesis to take place. Also, if the roots should emerge from the soil - as they might do after particularly heavy rain, for example - they will rapidly turn back to the soil.
the effect of unilateral light
Examples of the response of plants to unilateral light can be seen in any garden or woodland. Where plants are partially shaded the shoots grow towards the light and then grow on straight towards it. This response appears to be the result of the way auxin moves within the plant under the influence of light.
Figure 2 shows that the side of a shoot exposed to light contains less auxin than the side which is not illuminated. It appears that light causes the auxin to move laterally across the shoot, so there is a greater concentration on the unilluminated side. This in turn stimulates cell elongation and growth on the dark side, resulting in observed growth towards the light.
Once the shoot is growing directly towards the light, the unilateral stimulus is removed. The transport of auxin stops and the shoot then grows straight towards the light. The original theory was that light destroyed the auxin, but this has been disproved by experiments showing that the levels of auxin in shoots are much the same regardless of whether they have been kept
in the dark or under unilateral illumination.
growing in the dark
The fact that plants grow more rapidly in the dark than when they are illuminated can at first seem illogical. If a plant, however, 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. 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.
geotropisms
Light is not the only thing to which plants are sensitive. 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.
