Learning to Read Light

Martha Carlson, Sept. 2011

Pioneers in A New Language

What did the sugar maple tell the alien?

After a long discussion of its internal biochemistry, the positive and negative aspects of its site, the changeability of weather, and recent changes in climate and atmospheric chemistry, the tree tipped its leaves and reflected a few photons of near infrared light towards something on the ground. The alien turned to notice a small animated creature that was vibrating with excitement. The alien smiled and bent down to look at the creature. The thin tissues that projected from the alien’s skin flickered with blue, green and red light.

This lecture was developed for Monitoring Forest Health, an upper level college course in the Dept. of Natural Resources, College of Life Sciences, UNH. Carlson taught this in Fall 2011 with Dr. Barrett N. Rock. This power point explores pioneer work in understanding the role of light in photosynthesis. This lecture is presented on the Forest Watch website for teachers and, with help from their teachers, for upper level high school students.

“What is it? It doesn’t reply,” the alien asked with surprise.

“That’s a human. They don’t have very good receptors. They don’t light. They’re

ilphotonic.,” the maple reflected.(By the end of this lecture, you will be able to define “ilphotonic”.)

Gates et al.1965

What is the relationship between light and plants? What makes plants grow? Heat? Light?

“…the interaction of radiant energy with the plant leaf…”

Shull, 1929, McNicolas, 1931, Rabideau, 1946—Step by step, scientists began unraveling the mystery. Dr. David M. Gates, a biologist at the National Bureau of Standards at the University of Colorado, experimented with spectroscopy and microscopy to find the answers.

Gates measured the size of cells in leaves(15 microns x 15 microns x 60 microns), counting the chloroplasts—he counted 50 inside one palisade cell.

Gates’ New Concepts• The Energy Environment!

“The energy absorbed selectively at certain wavelengths by chlorophyll will be converted into heat or fluorescence, and converted photochemically into stored energy in the form of organic compounds through photosynthesis.”

Anatomical Ideas• The cells of the leaf are designed to receive

maximum light.

• The anatomical structures respond to light as in:

Stomatal openings seemed to be connected to light. And leaves absorbed light along a light

“action spectra”.

Gates Noticed Differences Among Plants

Cactus—Reflect moreLichens—Wet and dry

Conifers—Design of needles, holding heat?

In Visible Light

Different reflectance and absorption of light with different:• Angles• Wetness or Dryness• Age of Leaf• Pigments• Species

Different Wavelengths

• Green is Different• Red is Different, changing as leaf grows• Discovers the Red Edge Inflection Point.• The REIP changes with growing leaf as

more chlorophyll is developed.

Near Infrared Light

• Differences in NIR reflectance might be a function of cell shape, size and amount

of intercellular space.• Thicker leaves have greater reflectance.

• Why is NIR so reflectant? (Gates doesn’t know about porphyrin ring size and

photon lengths).

Other Unanswered Questions

Heat energy and light energy. Measuring both. Not sure about how

the photosynthesis systems dump heat.

Fluorescence at a slightly longer wavelength—how does it work?

Mini-Lesson before we proceed: Photons Are Fast and Very Small

There are more femtoseconds in 1 second than there are seconds in 30 million years!

1 year= 31,536,000 sec. so 30,000,000 x 31,536,000 =9.4608 x 1014 or 946,080,000,000,000 seconds in 30 million years!

1/1,000,000,000,000,000th, 1/10-15 part of one second in which a photon is absorbed! Femtosecond.

1 billion = 1,000,000,000 1 million = 1,000,000

Time:Femtosecond 10-15 billion millionthsPico second 10-12 million millionthsNano second 10-9 billionthsMicrosecond 10-6 millionthsMillisecond 10-3 thousandsOne second 11

Reading Spectral Analyses

Sample VIRIS, September 15, 2008

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400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Wavelength (nm)

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cen

t R

efle

cta

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Tree 835

Tree 812

LeafPigments

Near InfraredLight

Blue Green RedVisible Light

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lue

Ligh

t, TM

1

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en L

ight

, TM

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d Li

ght,

TM3

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REIP NIR 1 NIR3

Water

NIR

, TM

5

Sample VIRIS, September 15, 2008

0

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400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Wavelength (nm)

Per

cen

t R

efle

cta

nce

Tree 835

Tree 812

LeafPigments

Near InfraredLight

Blue Green RedVisible Light

B

lue

Ligh

t, TM

1

Gre

en L

ight

, TM

2

Re

d Li

ght,

TM3

N

IR, T

M4

REIP NIR 1 NIR3

Water

NIR

, TM

5

What are Nanometers?

Micrometer one-millionth of a meter, 106

Nanometer one-billionth of a meter, 109

Gates calls it a milli micrometer.

So to change 1.65 micrometers to 1650 nanometers, MOVE DECIMAL 3 spaces.

Rock et al., Remote Detection of Forest Damage, 1986

Purpose of Research: “This…may allow scientists to discriminate and identify unique spectral signatures associated with the response of vegetation to various stress agents…spectral ‘fingerprints’ associated with specific causal agents of forest damage and decline.”

Dr. Barrett Rock is the founder of Forest Watch. He is a professor of Natural Resources at the University of New Hampshire.

New Concepts

• Remote sensing can “see” the forest.• What Thematic Mapper, aboard Landsat

satellite, 500 miles above Earth, sees can be the same as leaf reflectances in the lab.

• Stress changes the absorption and reflectances of light.

• Those changes in light disclose changes in anatomy and pigments

Visible Light

Confident about what blue, green and red light show.The red edge “is directly correlated with leaf

chlorophyll concentrations”.The REIP changes with health and with normal

senescence.Concentrations of pigments will vary according to

stress.

Infrared Light

• NIR tells about leaf health, refractions along cell-wall-water-air interfaces.

• Stress leads to less reflectance in NIR plateau.• Water content may be seen in SWIR area.

Sensing Tools

Some spectrometers cannot detect enough detail to see the blue shift in the REIP.

Satellite imagery can be displayed in “false color composites.”

What Do We Know NowWhy does a leaf reflect NIR—porphyrins, photons are too long and

slow waves.Fluorescence—a non-photochemical quenching of light energy—

photoinhibition --not energy dependent (as carotenoids dump heat) and not a state transition (as antenna detach). Indication of

activity in chlorophyll.Still mulling over the differences between temperature and light

energy. See Morton (my body produces 3x its weight in ATP on an average day!)

Today we are inside those chloroplasts and know that there are 600 million chlorophyll molecules in each chloroplast. In one square cm

of grass, you would have more than 30 million chloroplasts!

A single palisade cell with chloroplasts (Photosynthesis.com).

What Do We Know Now• More information about structure of PSII

and PSI and how they respond to stresses and how they are damaged by stress.

• Rock’s “water stress index” recognized as TM5/4.

• New satellites will measure fluorescence.• Still working on REIP.

What Can These Two Papers Tell Us about Stress in Trees Today?

Today scientists know about 80 “indices” or messages in light waves which they can interpret to understand tree health. We are trying to learn more of the language of

light, to become as “photonic” as we are “literate.”Copyright 2011 University of New Hampshire.