Transcript plants and light

PLANTS AND LIGHT
LIGHT QUALITY
• The quality, intensity, and duration of light directly
impact plant growth.
• Light quality refers to the colour or wavelength reaching
the plant's surface.
• A prism (or raindrops) can divide sunlight into
respective colours of red, orange, yellow, green, blue,
indigo and violet.
• Red and blue have the greatest impact on plant growth.
• Green light is least effective (the reflection of green light
gives the green colour to plants).
• Blue light is primarily responsible for vegetative leaf
growth.
• Red light, when combined with blue light, encourages
flowering
Figure 1.
Relative efficiency of various light colours
in photosynthesis.
LIGHT INTENSITY
• The more sunlight a plant receives, to a degree, the higher
the photosynthetic rate will be.
• Leaves of plants growing in low light readily sun scorch
when moved to a bright location.
• Over time, as the wax content on a leaf increases, it
will become more sun tolerant.
• Light is measured in Lux.
LIGHT DURATION
Light duration refers to the amount of time that a plant is
exposed to sunlight.
PHOTOPERIOD
The flowering response of many plants is controlled by the
photoperiod (the length of uninterrupted darkness).
Photoperiod response can be divided into three types.
Short day plants flower in response to long periods of night
darkness. Examples include poinsettias, Christmas cactus,
chrysanthemums, and single-crop strawberries.
Long day plants flower in response to short periods of night
darkness. Examples include onions and spinach.
Day neutral plants flower without regard to the length of the
night, but typically flower earlier and more profusely under
long daylight regimes. Day neutral strawberries provide
summer long harvesting (except during heat extremes).
STUDENT ACTIVITIES
1. Read page 224 in Dynamic Agriculture
2. Read and answer questions in Workbook pg 13
Answers to questions on page 13
1. The greater the amount of sunlight and therefore light
intensity a plant receives and is absorbed by the plant, the
greater the rate of photosynthesis that takes place.
2. The shade plant curve is steeper in dim light indicating that
the shade plant uses a greater proportion of the light
3. The rate of respiration is indicated by the point at which the
curve intercepts the vertical axis. In this case, shade plants
intersects the axis at a lower point than the sun plants.
4. Sun plants use bright light more effectively than shade
plants, as shade plants become light saturated at a much
lower level of light intensity. This means that their
photosynthetic rate is also much lower than sun plants.
Student Practical activities
Work through the exercises on pg 14 in Workbook.
Homework
Work on New Zealand’s Light intensity exercise
New Zealand’s Light Intensity Answers
Time of day
The angle at which the sunlight hits the
earth changes the size of the area
receiving the light.
At midday or a 90o angle, the light is
constricted to a very narrow area, so
the light is intensified.
At greater angles such as in the
morning or afternoon the size of the
area over which the light is spread is
greater, so the light is less intense.
Seasons of
the year
The earth’s axis is tilted at an angle of 23.5o to
the sun. It is this tilt that is responsible for the
seasons. When the southern hemisphere is
tilted towards the sun it is experiencing
summer and higher intensity of light, while the
northern hemisphere is turned away, where it
is winter with lower intensity.
The oval pathway of the Earth’s orbit is
responsible for the changing length of the day.
Latitude
Latitude is measured north or south in degrees from
the equator. The sun’s intensity is the highest at the
equator and lowest at the poles due to the Earth’s tilt
and orbit around the sun.
Topography
The slope of the land also determines how much
light is received. A north facing slope receives more
light intensity than a south facing slope, as the light
is restricted to a narrow area. Whereas on the
south facing slope, the light id spread over a large
area.
LIGHT INTENSITY AND PHOTOSYNTHESIS
Page 16
What factors affect rate of photosynthesis?
Temperature, water availability, oxygen levels, light: type,
intensity and colour, carbon dioxide levels.
Graph the results from the experiment in your workbooks.
Answers to questions page 16
1. 51.5 (psu)
2. The increase in light intensity increased the rate of
photosynthesis until it slowed at 51 lux and then reached a
limit of 55 psu.
3. Temperature, amount of carbon dioxide and water in the
air.
4. Species C because it thrive best in lower light conditions.
LIGHT WAVELENGTHS AND COLOUR
The nature of light
Light is electromagnetic energy usually derived from the
sun.
Visible light is a small part of this energy.
Within the visible light there are different wave lengths.
Different wavelengths of visible light appear to us as
colours.
Plants respond to the visible light and small sections of the
invisible light just out side the visible wavelengths.
Infra red
Red
visible
light
Orange
Yellow
Green
Blue
wavelength A0
800
700
600
500
Indigo
Violet
400
Ultraviolet
300
Plants, however do not use all wavelengths equally.
Plant
responds to
HOW DOES COLOUR OF LIGHT AFFECT PLANT GROWTH?
Blue Light
Sunlight gives off all colors of light, but the sunlight of the
lengthening days of spring and the long days of summer is
predominately blue.
Plants that were dormant in the winter begin growing in the
spring.
Seeds begin to germinate.
Therefore plants need blue light for the growth of stems, stalks
and leaves.
Germinating seeds, seedlings and transplants need blue light.
Red Light
As the days shorten with the ending of summer, the blue
light that dominated the spectrum of sunlight is slowly
overtaken by light in the red band.
Ripening fruits and vegetables are ordinarily harvested in
late summer or early autumn.
Red light is needed for flowering and the production of
fruit.
Read page 18 in workbook and answer 1-6
Page 18 answers
1. Light is captured by the pigment chlorophyll in the leaves
of the plant (allowing the process of photosynthesis to
occur) which absorbs all the different wavelengths
(colours) except green which is reflected hence giving
them a green colour.
2. Chlorophyll
3. Violet, indigo, blue and red
4. Wavelengths between 400 and 450nm get absorbed most
by chlorophyll.
5. Wavelengths between 450 and 650nm do not get
absorbed by chlorophyll.
6. Leaves are green as they reflect the green colour and
absorb all the other colours.
Read page 19 Plant Responses to Light
Highlight the important points.
Then answer 1-10
Answers
1. Photoperiodism
2. Phototropism
3. Photoperiodism
4. Photoperiodism
5. Photoperiodism
6. Phototropism
7. Phototropism
8. Photoperiodism
9. Phototropism
10. Phototropism