1. Light in Horticulture

2. Objectives Learn about light as a form of radiation
Review the plant growth processes that relate to light
Study the characterisitics of light that are relevant to horticulturists
Consider ways horticulturists manipulate light to meet horticultural goals

3. What is “light”? Visible Light
But, it is only a small (“visible”) part of a huge electromagnetic spectrum
Visible Light
Microwaves
short waves
long waves
Gamma rays
X-rays
Ultra-violet
Infrared
Radio
high energy
low energy
Electromagnetic Spectrum

4. What is “light”? Visible Spectrum 380 430 470 500 560 600 650 760
Nanometers of wavelength
Nanometer = meters

5. Radiation that Strikes an Object
Incoming
Radiation
(from sun or
lamp)
Absorption
Reflection
off & back
Transmission
through

6. Radiation that Strikes an Object
Incoming
Radiation
(from sun or
lamp)
Glass pane
(greenhouse)
Absorption
Reflection
off & back
Transmission
through

7. Explains “Greenhouse Effect”
1. Transmission of radiation through glass
Shorter wavelengths only
2. Absorption by objects in greenhouse
3. Objects in greenhouse heat up
4. Objects re-radiate as heat
long wavelengths only
5. Long waves cannot get back out through
glass - heat trapped in greenhouse

8. Greenhouse Effect Transmission Re-radiation (heat) Heat (long waves)
can’t pass through
glass - heat is trapped
and builds up
Absorption

9. Greenhouse Effect - Global
Heat (long waves)
can’t pass through
atmosphere (CO2 &
water vapor) - heat is
trapped and builds up
Transmission
through
atmosphere
Re-radiation
(heat)
Absorption
by Earth

10. Light’s Characteristics
Spectral quality (color or wavelength)
Intensity (no. of photons, amt. of energy)
Duration (length of lighted period, i.e. “day”)
Direction (angle of incidence)

11. SPECTRAL QUALITY (Kind)
Visible Spectrum
430
470
500
560
600
650
760
Nanometers of wavelength

12. Photosynthesis Photosynthetically Active Radiation
In photosynthesis, all light’s colors, or parts of the visible spectrum are not equal!
Only some light is absorbed by chlorophyll and other pigments in photosynthesis
The absorbed part of the spectrum is called PAR
Photosynthetically Active Radiation

13. Absorption Incoming Radiation (from sun or lamp) Reflection off & back
Transmission
through

14. P A R 380 430 470 500 560 600 650 760 Nanometers of wavelength
These wavelengths are either
reflected off or transmitted
through the leaf
Absorption of chorophyll a & b
380
430
470
500
560
600
650
760
Nanometers of wavelength

15. LIGHT INTENSITY (Quantity)
Intensity is measured as micro moles / square meter / second (mmols/m2/sec)
Sunlight ranges between mmols/m2/sec
Interiors as low as 50 mmols/m2/sec

16. Plants & Light Intensity
photosynthesis
CO2 Uptake (photosynthesis)
CO2 Release (respiration)
respiration
dark
bright light
Increasing Light Intensity

17. Plants & Light Intensity
Compensation
Point
photosynthesis
CO2 Uptake (photosynthesis)
CO2 Release (respiration)
respiration
dark
bright light
Increasing Light Intensity

18. Photosynthesis = Respiration
Compensation Point
At this light intensity, a plant captures as much energy through photosynthesis as it needs to survive
At the compensation point, a plant will not grow, but it will not die (maintenance level)
Below the C.P., a plant will die
Above the C.P., a plant may grow
Photosynthesis = Respiration

19. Plants & Light Intensity
Maintenance
Compensation
Point
photosynthesis
CO2 Uptake (photosynthesis)
Plant Dies
Plant May Grow
CO2 Release (respiration)
respiration
dark
bright light
Increasing Light Intensity

20. Plants & Light Intensity
photosynthesis
CO2 Uptake (photosynthesis)
Saturation Point
CO2 Release (respiration)
respiration
dark
bright light
Increasing Light Intensity

21. Saturation Point
At this light intensity, a plant captures the maximum amount of energy through photosynthesis that it is able to do
At lower light intensities, a plant will respond (grow faster) to brighter light
Above the S. P., brighter light has no effect on growth

22. Plants & Light Intensity
photosynthesis
CO2 Uptake (photosynthesis)
Saturation Point
Grows Faster
No Increase
CO2 Release (respiration)
respiration
dark
bright light
Increasing Light Intensity

23. C. P. & S. P. Compensation Point & Saturation Point vary with species
“Sun-loving” species tend to have a higher saturation point (make more efficient use of brightly lighted conditions)
“Shade-loving” species tend to have a lower compensation point (survive at lower light intensities)

24. “Sun” or “Shade” Plant? “Sun-loving” species “Shade-loving” species
CO2 Uptake (photosynthesis)
“Shade-loving” species
CO2 Release (respiration)
dark
bright light
Increasing Light Intensity

25. DURATION (How Long?)
Plant growth rate and productivity is dependent upon:
how much time the plant receives PAR above the compensation point, and
how far above the C. P. it is

26. Horticultural Practices
Plant spacing - light exposure
Pruning - allow light penetration into tree or shrub canopy
Eliminate weeds that compete for light
Reduce weeds by dense crop canopy (“shade out the weeds”)
Supplemental lighting (interiors/greenhouse)

27. Summary Light is a critical aspect of the environment for plant growth
The characteristics of light impact many plant developmental processes

28. (Constitution of the American Society for Photobiology)
"Photobiology is broadly defined to include all biological phenomena involving non-ionizing radiation. It is recognized that photobiological responses are the result of chemical and/or physical changes induced in biological systems by non-ionizing radiation.”
(Constitution of the American Society for Photobiology)

29. Photobiology - Areas
Photosensitization - Chlorophylls UV Radiation effects – DNA mutations Environmental photobiology – Plant Productivity Non-visual photoperception – Circadian, photoperiodism Photomorphogenesis – Development of organism Phototropism – Plant movement towards light Photosynthesis – conversion of light energy into chemical energy
Environmental photobiology – Plant Productivity
Photomorphogenesis – Development of organism
Phototropism – Plant movement towards light
Photosynthesis – conversion of light energy into chemical energy

30. Properties of light
Particle - photons
Wave

31. Properties of light Wave Particle - photons
Photomorphogenesis – Development of organism
Phototropism – Plant movement towards light
Photoreceptors – Phytochrome, Cryptochrome, Phototropins etc.
Calcium
Hormones
Several – bitter pit, corking etc.
Particle - photons
Photosynthesis – conversion of light energy into chemical energy
Carbon capture – Starch production
Field physiological disorders
Sunburn, Scald

32. Light absorption and emission by chlorophyll
PP5e-Fig jpg

33. Photosynthesis conversion of light energy into chemical energy
CO2 + H2O = (CH2O) + O2
An absorption spectrum provides information about the amount of light energy taken up or absorbed by a molecule or substance as a function of the wavelength of the light.
An action spectrum is the rate of a physiological activity plotted against wavelength of light.
What is the significance of the overlapping absorption and action spectra?

34. Action spectrum compared with an absorption spectrum
PP5e-Fig jpg

35. Photosynthesis
T. W. Engelmann Experiment

36. Photosynthesis - initiation

39. Background: LEF NADPH FNR FD NADP+ H+ PSII PSI e-
Chloroplast Stroma
NADP+ H+ PQ PQ PQ
PSII
PSI
LHC
PQH2
PQH2
LHC
b6f e- PC
Thylakoid lumen
LEF= Linear Electron Flow
Aaron Livingston, Ph.D

40. Background: LEF NADPH NADPH NADPH H+ FNR NADP+ ATP NADP+ H+ H+ FD
Chloroplast Stroma
b6f
PSI PQ
PSII
LHC
LHC
Thylakoid lumen
Aaron Livingston, Ph.D

41. Beyond light reaction

42. Importance of LEF to crop production
Paraquat
DCMU