Light in Horticulture

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

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

What is “light”? Visible Spectrum 380 430 470 500 560 600 650 760 Nanometers of wavelength Nanometer = 0.000000001 meters

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

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

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

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

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

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)

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

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

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

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

LIGHT INTENSITY (Quantity) Intensity is measured as micro moles / square meter / second (mmols/m2/sec) Sunlight ranges between 1000-2000 mmols/m2/sec Interiors as low as 50 mmols/m2/sec

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

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

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

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

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

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

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

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)

“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

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

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)

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

(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)

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

Properties of light Particle - photons Wave

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

Light absorption and emission by chlorophyll PP5e-Fig-07-05-0.jpg

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?

Action spectrum compared with an absorption spectrum PP5e-Fig-07-08-0.jpg

Photosynthesis T. W. Engelmann Experiment

Photosynthesis - initiation

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

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

Beyond light reaction

Importance of LEF to crop production Paraquat DCMU