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Environment -Light

What (three) environmental factors influence the direction of plant growth and what do we call these responses?

light

  • phototropism - shoot positive, root negative

gravity

  • gravitropism - shoot negative, root positive

touch

  • thigmotropism - tendrils positive, but most plant parts respond in some way (usually negative) to touch or mechanical stimulation)

What parts of the plant respond to light , gravity and touch?

shoots and roots

What is the basis of the response to the environmental factors of phototropism, gravitropism and thigmotropism?

Perception of the signal (photo-receptors for light, starch grains for gravity) leads to localized build-up of hormones (auxin for light and gravity, ethylene for touch). Hormone affects growth positively or negatively.

What do we mean by a "biological clock" and what evidence is there that it exists?

A cellular time keeping mechanism which controls such things such as nictinastic movements, continue even when environmental stimulus (light/dark cycles in this case) are removed.

What are some examples of plants which require long days or short days for flowering?

long day:

  • henbane, spinach

short day

  • cocklebur, primroses, chrysanthemums

Is it the day or the night length which is important?

night length (effect of "night breaks")

What part of the plant "perceives" the light/dark cycle?

leaves

What is the effect of interrupting the night on the two types of plants?

Night breaks in short days will stop short day plant, but promote flowering in a long day plant.

What are two other aspects of plant growth in which this pigment is involved?

  • seed germination - chlorophyll development and elongation in seedlings and shoots
  • etiolation - effects of darkness and light filtering through leaf canopy

When plants grow towards the light they often respond most strongly to:

  1. red light
  2. green light
  3. blue light

  1. The red/far-red system may be involved in some phototropic responses; if so it is far-red light that causes stretching towards the light.

     

  2. Green light is about the least effective part of the spectrum in plant growth responses.

     

  3. A blue-light absorbing pigment (cryptochrome) seems to be involved in phototropic responses.

The Pfr form of phytochrome is:

  1. physiologically inactive
  2. formed on exposure to red light
  3. formed on exposure to far-red light

  1. No, Pfr is generally thought to be the form that causes flowering , seed germination and other responses (although the response may be inhibition)

     

  2. It sounds like Pfr should be formed by far-red light; but this is the form in which the phytochrome molecule absorbs far-red light ,and is itself formed by red light.

     

  3. It sounds like Pfr should be formed in far-red light, but it is formed in red light.

Broad, thin leaves and elongated stems develop when plants are growing in the shade of other plants because the light is enriched with:

  1. red light
  2. far-red light
  3. green light

  1. The plants causing the shade would be expected to absorb red light for their own photosynthesis.

     

  2. The light filtering through the leaves will be enriched with far-red which decreases the proportion of Pfr in relation to Pr and this affects growth.

     

  3. The light in the shade would certainly be green but these wavelengths are physiologically inactive.

Long day plants can be encouraged to flower by interrupting:

  1. the days with periods of darkness
  2. nights with far-red light
  3. nights with red light

  1. Although we talk of "long-day" and "short-day" plants the length of night is more critical for flowering than the length of day.

     

  2. In long-day plants flowering is promoted by Pfr (which would be low under far-red light.)

     

  3. Interrupting the night with red or white light would restore levels of Pfr that promotes flowering in long-day plants.

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