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 PHOTOPERIOD 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 Read page 224 in Dynamic Agriculture Read and answer questions in Workbook pg 13

Answers to questions on page 13 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. The shade plant curve is steeper in dim light indicating that the shade plant uses a greater proportion of the light 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. 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 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. Time of day

The earth’s axis is tilted at an angle of 23. 5o to the sun 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. Seasons of the year

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 51.5 (psu) The increase in light intensity increased the rate of photosynthesis until it slowed at 51 lux and then reached a limit of 55 psu. Temperature, amount of carbon dioxide and water in the air. 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.

visible light Infra red Red Orange Yellow Green Blue Indigo Violet wavelength A0 800 700 600 500 400 300 Infra red Red Orange Yellow Green Blue Indigo Violet Ultraviolet visible light Plant responds to Plants, however do not use all wavelengths equally.

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 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. Chlorophyll Violet, indigo, blue and red Wavelengths between 400 and 450nm get absorbed most by chlorophyll. Wavelengths between 450 and 650nm do not get absorbed by chlorophyll. 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 Photoperiodism Phototropism