By C. Kohn, Waterford, WI

The most obvious and important plant process affected by light is photosynthesis, the creation of sugar from water and carbon dioxide by using the energy of light.

Many plant processes, particularly transpiration, change during the course of the day due to changing levels of light

Plant growth can also be affected by light; the same plants grown in different types of light will have different characteristics

Visible white light is composed of all the colors in the visible spectrum

Light is simply a form of energy Light energy travels in waves Three aspects of light affect plants –

1. Quantity – brightness of light (or height of each wave in a wavelength)

2. Quality – color of light (or width of each wavelength, i.e. frequency)

3. Duration – amount of time light is present

Visible light is only one portion of the electromagnetic spectrum.

Visible light is comprised of energy with a wavelength between 400 and 700 nanometers (nm = 0.000000001 m. wide from peak to peak)

Longer wavelengths create radio waves, etc.

Shorter wavelengths are much more intense; e.g. X-rays,UV rays

Light intensity is determined by the size of the waves.

The ‘taller’ the waves, the more intense the light

The shorter the waves, the less intense the light

In general, the more light a plant receives, the higher the rate of photosynthesis This in turn should translate into more plant

growth and production However, if a plant is not acclimated to

bright light (e.g. if it was started indoors and moved outside too quickly), the pigments of the plant can be ‘bleached’ by intense sunlight Plants started inside must be ‘hardened off’ or

they may suffer in bright light.

Light intensity increases in summer because the rays of the sun are directly overhead This causes them to be

‘concentrated’ on the area just below

In winter months, the sunlight is spread out over a larger area; this causes the intensity to decrease

Light frequency is the number of times the peak of a wavelength passes a point.

The smaller the wavelength, the greater the frequency, causing more energy to be carried by the light

The longer the wavelength, the smaller the frequency, and less energy is carried by light

Light frequency can be thought of as the color The longer the wavelengths, the redder the color The shorter the wavelengths, the bluer the color

Light intensity can be thought of as brightness The taller the waves, the brighter the light The shorter the waves, the dimmer the light

These are independent of each other You can have dark (short)

red (long) wavelengths

You can have bright (tall) blue (short) wavelengths

Pigments are any chemical substance that absorb light

The color of the pigment is determined by the light that is not used by the pigment, or by the light that is reflected back into your eye

Plants have several kinds of pigments, each that absorb different kinds of light

Chlorophyll a is found in all photosynthetic plants Chlorophyll a absorbs light mostly in the violet-blue

range and reddish orange range Chlorophyll b and carotenoids are secondary

pigments; not all plants have them These pigments absorb light in the orange and green

ranges Because these types of light are less effective,

chlorophyll b and carotenoids tend to have less of an impact on plants

The color of these pigments is usually evident only in fall as the chlorophyll a pigments shut down.

As seen to the right, most of the light used by a plant is in the blue-range and the red range.

Far-red and green are the least utilized by a plant

The blue and red ranges of light are called the Action Spectra for plants because they stimulate most plant activity.

Receptors in plants called phytochromes enable the plant to not only detect light, but also detect the quality of light. Phytochromes detect red light

Under changing intensity and wavelength of light, the phytochrome’s physical structure will change

This can cause a chain reaction insidethe cell, creating a physical response Pfr

PrPhytochrome

s

For example, if a plant is blocked by the leaves of another tree, the plant will receive more far red light than red light

This will cause the phytochrome to change its shape, creating a cascade of changes that will cause a longer stem and more branching.

Because smaller seeds have little in the way of energy reserves, their germination tends to be stimulated by light

Light will strike the phytochromes in small seeds, causing them to change shape

The change phytochrome stimulates or inhibits genes in the DNA of the plant that create proteins related to germination.

Cryptochromes detect blue light These structures work similarly to

phytochromes. Cryptochromes are responsible for…

Inhibiting stem elongation Moving the plant towards sunlight Opening the stomata Creating a circadian rhythm for a plant

Without the cryptochromes and phytochromes, plants would have a circadian rhythm that varied between 21 and 27 hours

RED LIGHT – HEIGHT LIGHT

BLUE LIGHT – LEAF LIGHT

Causes stem elongation

Germination Branching Promotion of

flowering (only with blue light)

Detection of day length

Inhibits stem elongation

Phototropism (moving a plant towards light)

Opening Stomata Circadian Rhythm Leaf Growth

UNDER RED LIGHT ONLY UNDER BLUE LIGHT ONLY

Tall & Spindly Short & Stocky

So far, we’ve covered 2 of the three factors – 1. Light Intensity – 2. Light Quality – color of light

The third is light duration How long the light lasts each day (or more

accurately, how long the dark lasts each night) affects crucial plant processes, particularly flowering.

The flowering period of many plants is controlled by the photoperiod, or length of uninterrupted darkness

There are three kinds of photoperiod: 1. Short-day plants – need long nights and short

days to flower; e.g. poinsettias 2. Long-day plants – need short nights and long

days to flower; e.g. most vegetable crops 3. Day-neutral – unaffected by length of day; e.g.

dandelions It is not the length of the day, but the length

of the night that determine this aspect of plants E.g. a greenhouse of poinsettias should not be lit

up at night!