Horticultural Responses to Light

1. Horticultural Responses to Light By C. Kohn, Waterford, WI

2. Light & Plant Processes • 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

3. The Nature of Light • 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

4. Light as Energy • 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

5. Light Quantity = Intensity • 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

6. Light Intensity • 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.

7. Light Intensity & Seasons • 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

9. Light Frequency = Quality • 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

10. Light Intensity & Frequency • 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

11. Detection of Light • 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

12. Plant Pigments • 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 carotenoidsare 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.

13. Absorption of light • 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.

14. How do they know? • 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 Pr Pfr Phytochromes

15. For example… • 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.

16. Germination • 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.

17. Cryptochromes • 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

18. Differences in Responses 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

19. Impact of Colors of Light Under Red Light Only Under Blue Light Only • Tall & Spindly • Short & Stocky

20. More Than Meets the Eye:New Findings on How Mulch Color Can Affect Food Plants • Plant physiologist Michael J. Kasperbauer made a career of "seeing" light the way plants do: in wavelengths, some of which cannot be detected by the human eye. • In research done with Clemson University, he and ARS soil scientist Patrick G. Hunt found that tomato plants grown over red mulch yielded about 20 percent more fruit than those grown over standard black mulch. • He later found that strawberries grown over red mulch smelled better, tasted sweeter, and yielded more than those grown over black mulch. • http://www.ghorganics.com/New%20Findings%20on%20How%20Mulch%20Color%20Can%20Affect%20Food%20Plants.htm

21. 3 Factors of Light • 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.

22. Photoperiodism • 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!