The functional significance of leaf orientation in the sand dune herb Pennywort

Heather M. JoestingWake Forest University

Light + CO2 + H2O Sucrose (C6H12O6) + O2

www.eoearth.org

Sunlight and Photosynthesis

Energy dissipated via photochemical processes –Photosynthesis

Sunlight and Photosynthesis

Incident sunlight

Maximum

Ph

otos

ynth

esis

rat

e

~700 µmol m-2 s-1

~1000 µmol m-2 s-1

Photoinhition

Photodamage

Sunlight and Leaf Temperature

Energy dissipated via photochemical processes –Photosynthesis

Energy dissipated via non photochemicalprocesses

Heat = increase in leaf temperature

Leaf temperature

Phot

osyn

thes

is ra

te

40°C

Sunlight and Leaf Temperature

Optimum

Photo from georgeweigel.net

Leaf death

Leaf Temperature and Water Loss

Energy dissipated via photochemical processes –Photosynthesis

Energy dissipated via non photochemicalprocesses

Heat = increase in leaf temperature

Leaf cools by evaporative heat loss (transpiration)

Transpiration

Increased sunlight exposure

Increased leaf temperature

Increased transpiration rate

Increased water loss

Wilting

Light Processing in LeavesFunctions for gas exchange, primarily takes up CO2 and loses water vapor

Light Processing in Leaves

Controls opening of stomata, opens by filling with water

High light environment

Solar time (hh:mm)

05:00 09:00 13:00 17:00 21:00

PAR

( μm

ol m

-2 s-1

)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Mt. Mitchell, NCTopsail Beach, NC

Lig

ht

Adaptations to High Light Stress

• Anthocyanin production

• Small leaf size

• Hairs and wax to make surface more reflective

• Leaf succulence

• CAM photosynthesis

• Leaf orientation

Leaf Orientation

Leaf orientation reduces midday light exposure and decreases leaf temperature and transpiration rate Werk and Ehleringer, 1984; Smith and Ullberg, 1989; James and Bell, 2000

nps.gov

Rumex densiflorus in alpine tree-line, WyomingGeller and Smith, 1982

Perezia nana in Sonoran DesertSylvertsen and Cunningham, 1979

Swibiodiversity.org

Leaf Orientation – Angle

PM

Midday

AM

Top (adaxial)

Bottom (abaxial)

Top/bottom = high

Increased leaf temperature, potential water loss, and risk of photoinhibition

Top/bottom ≈ 1

Lower leaf temperature, potential water loss, and risk of photoinhibtion

PM

Midday

AM

Top PM (adaxial)

Bottom AM (abaxial)

Leaf Orientation – Azimuth

EveningWest

MorningEast

Pennywort – Hydrocotyle bonariensis

Pennywort – Hydrocotyle bonariensis

What is the function of observed leaf

orientation in Pennywort (Hydrocotyle bonariensis)?

1. Is there daily and/or seasonal leaf orientation, and what is the effect on sunlight exposure?

2. What is the effect of leaf orientation on leaf temperature, photosynthetic gas exchange, and water loss?

Questions

N

S

EW

Topsail Island

North Carolina

Study Site

What is the function of observed leaf

orientation in Pennywort (Hydrocotyle bonariensis)?1. Is there daily and/or seasonal leaf orientation, and

what is the effect on sunlight exposure?

2. What is the effect of leaf orientation on leaf temperature, photosynthetic gas exchange, and water loss?

Questions

Q1 – Methods

– Measured leaf angle every two hours from 08:00 h to 18:00 h on single mature leaves

– Early season (May), mid-season (June and August), and late season (September)

Leaf angle (+)

Leaf angle (-)Adaxial surface

North Leaf azimuth

Q1 – Methods– Light measured as

Photosynthetically active radiation (PAR; describes amount of incident light seen by leaf)

– Measured for top of leaf and bottom of leaf every 2 hours from 08:00 h to 18:00 h in May, June, August, and September

Solar time (hh:mm)

07:00 11:00 15:00 19:00

Lea

f ang

le (o

)

40

50

60

70

80

90 May JuneJulySeptember

Daily Leaf Orientaiton Same general trend for leaf azimuth

May June August September

Lea

f ang

le (ο

)

40

50

60

70

80

90

May June August September

Lea

f azi

mut

h (ο

)

60

80

100

120

140

160

180

200

220

240

Seasonal Leaf Orientation

South

East

Increase in mean leaf angle over the growing season, from 55° to 82°

Increase in mean leaf azimuth over growing season, from 94° to 205°

90°

Seasonal but Not Daily Orientation

• Daily orientation:

– There was little to no variation daily in mean leaf angle and mean leaf azimuth

• Seasonal orientation:

– Increase in mean leaf angle and mean leaf azimuth over growing season

• First study to show seasonal but not diurnal orientation

55°

Early season (May)82°

East (94°)

Southwest (205°)

Late season (August - September)

PMMidday

AM PMMidday

AM

Daily Light on Top and Bottom Leaf Surface

Reduction in midday light on top leaf surface with more inclined leaf in late season

Shift in peak (09:00 – 10:00 h early season, 12:00 – 14:00 h late season) on both top and bottom leaf

Solar time (hh:mm)

05:00 09:00 13:00 17:00A

baxi

al P

AR

( μm

ol m

-2 s-1

)0

50

100

150

200

250

300

350

400

Leaf azimuth EastLeaf azimuth Southwest

Solar time (hh:mm)

05:00 09:00 13:00 17:00

Ada

xial

PA

R ( μ

mol

m-2

s-1)

0

200

400

600

800

1000

1200

1400MayJuneAugustSeptember

Leaf angle 50 – 70° Leaf angle 80 - 85°

Top Bottom

Seasonal Ratio of Top: Bottom Light Exposure

May June August September

Top PPFD 674 ± 67.8 406 ± 29.2 528 ± 20.6 620 ± 57.7

Bottom PPFD 172 ± 16.5 245 ± 16.1 162 ± 18.1 200 ± 22.1

Top/Bot 3.92 1.66 3.26 2.07

Month

May June August September

PAR

(µm

ol m

-2 s-1

)

0

100

200

300

400

500

600

700

800AdaxialAbaxialTop

Bottom

Q1 – Conclusion

• Seasonal increase in leaf angle (more vertical) and leaf azimuth (seasonally tracks the sun)

• Daily light regulation:

– ↑ a.m., peak midday, ↓ p.m.

• Seasonal light regulation:

– ↓ in top light exposure, shift in peak in bottom light exposure

May: ~4x more light on top leaf surface (mean leaf angle = 54°)June: ~1.5 x more light on top leaf surface (75°)August: ~3x more light on top leaf surface (82°)September: ~2x more light on top leaf surface (82°)

Q2 – Expectations

↑ leaf temperature and transpiration (E)

↓ photosynthesis (A)

↓ leaf temperature and transpiration (E)

↑ photosynthesis (A)

PM

Midday

AM PM

Midday

AM

What is the function of observed leaf

orientation in Pennywort (Hydrocotyle bonariensis)?1. Is there daily and/or seasonal leaf orientation, and

what is the effect on sunlight exposure?

2. What is the effect of leaf orientation on leaf temperature, photosynthetic gas exchange, and water loss?

Questions

Q2 – Methods

Measured monthly from 06:00 – 21:00 h with leaf thermocouple (N = 2 – 4 pairs)

Measured monthly from 09:00 to 17:00 h with infrared gun (N = 10 pairs)

Q2 – MethodsPhotosynthetic gas exchange measured on experimental and control leaves in July and August at middayLICOR LI-6400 portable photosynthesis system

Measures gas exchange in plant leaves

Known amount of CO2 and water vapor to leaf –Amount of CO2 and water vapor back to system = Amount of CO2 taken up by leaf and water vapor released

Daily Leaf TemperatureT

leaf

(o C)

20

22

24

26

28

30

32

34

36

38ControlForced

May June

Solar time (hh:mm)

05:00 09:00 13:00 17:00 21:00

Tle

af (o C

)

24

26

28

30

32

34

36

38

40July

Solar time (hh:mm)

05:00 09:00 13:00 17:00 21:00

August

Gas Exchange

A ( μ

mol

m-2

s-1)

0

2

4

6

8

10

12ControlForced

g ( μ

mol

m-2

s-1)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

July August

E (m

mol

m-2

s-1)

0

2

4

6

8

10

A

B

C

Control

Experimental (forced)

Inclined leaves have greater:

• Photosynthesis

- Leaf converting more light and CO2to sucrose

• Leaf conductance

- Stomata are open and exchanging more CO2 and water vapor

• Transpiration

- Leaf is losing more water by evaporative heat loss

Photosynthesis

Transpiration

Leaf conductance

July August

• What is the influence of transpiration in reduced leaf temperature in inclined leaves?

Role of Transpiration

Three groups of six similar sized leaves with similar leaf inclination

3 leaves control, 3 leaves covered in Vaseline (experimental)

Leaf temperature measured every hour from 11:00 – 14:00 h with infrared gun

Solar time (hh:mm)

10:00 11:00 12:00 13:00 14:00 15:00

Tle

af (o C

)

31

32

33

34

35

36

37

38VaselineControl

Midday Leaf Temperatures

Vaseline covered leaves

95 – 99.5°F

Control leaves

89 - 93°F

Q2 – Conclusions

• Inclined leaves have lower leaf temperatures and greater photosynthetic gas exchange

• There is also an important role of evaporative heat loss via transpiration maintaining leaf temperatures

– Leaves covered in vaseline could not lose water vapor and had higher leaf temperatures as result

• Function of leaf orientation in Pennywort

– Leaf angle increases over season to reduce midday incident sunlight

– Increase in leaf azimuth seasonally tracks sun to maximize a.m. and p.m. light capture

– Inclined leaf orientation reduces leaf temperature and facilitates photosynthetic gas exchange

Dissertation Committee:

Dr. William K. Smith (advisor)

Dr. Ronald V. Dimock

Dr. Kathy Kron

Dr. Miles Silman

Dr. Tara Greaver

Field and Lab Assistants:

Matt Marenberg

Wreana Ward

Joseph White

Wyatt Allen

John Track

Funding:

National Science Foundation (NSF) –Research Coordinated Network (RCN)

Coastal Barrier Island Network (CBIN)

Wake Forest Research Fund

Department of Biology, Wake Forest University Vecellio Fund

Acknowledgements