This document discusses light and temperature effects on plants grown in high tunnels. It covers several topics: light intensity and photosynthesis; carbon dioxide levels; light color; shading issues; and temperature effects on plant development, stem elongation, photosynthesis, and flowering. For each topic, it provides data on different plant species' responses and optimal conditions. The overall focus is on understanding the light and temperature requirements of various crops to maximize growth and yield in high tunnel environments.

1. Light and
Temperature
Effects in High
Tunnels
• Light Intensity and Photosynthesis
• Carbon dioxide (CO2)
• Light color
• Shading issues
• Temperature effects on:
– Development, stem elongation, photosynthesis and
flowering.
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2. © 2009 Regents of the University of Minnesota

3. © 2009 Regents of the University of Minnesota

4. Light and
Temperature
Effects in High
Tunnels
• Light Intensity and Photosynthesis
• Carbon dioxide (CO2)
• Light color
• Shading issues
• Temperature effects on:
– Development, stem elongation, photosynthesis and
flowering.
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5. How much light can a plant use
for photosynthesis?
+ CO2 + H2O C2H + O2
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6. SOUTHERN GROWERS
NORTHERN GROWERS
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7. Response to increasing
light intensity
(irradiance).
Units are in umol m-2 s-1
Multiply umol m-2 s-1 by 5 to get footcandles.
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8. Variation in photosynthetic responses of
different species to increasing light intensity
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12. What we learned
• Species differed in how much light saturates
photosynthesis.
• Species studied showed photosynthetic saturation
between 200 and 600 umol m-2 s-1 (1,000-3,000
footcandles).
• When crops are spaced close, lighting levels should
be based on light intensity at lower leaf levels.
• By all accounts, tomato and pepper are high light
requiring plants, i.e. saturate at 600 umol m-2 s-1
(3000 footcandles).
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13. How much light is getting
to your plants?
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14. 750 ft-c
January Daily Light Integrals
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15. 3700 ft-c
April Daily Light Integrals
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16. In general, light penetration into a
greenhouse varies from about 30-85%.
60% light transmission is very common.
Single glass is the highest (85-90%),
followed by Exalite and single poly (65-
75%), following by double poly (45-60%).
This is without condensation!
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18. 3700 ft-c x 0.45 = 1,665 ft candles
(333 umol m-2 s-1)
April Daily Light Integrals
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19. SOUTHERN GROWERS
NORTHERN GROWERS
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20. Increasing DLI versus total flower bud number
10 moles/day
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21. Light and
Temperature
Effects in High
Tunnels
• Light Intensity and Photosynthesis
• Carbon dioxide (CO2)
• Light color
• Shading issues
• Temperature effects on:
– Development, stem elongation, photosynthesis and
flowering.
© 2009 Regents of the University of Minnesota

22. Response to increasing
carbon dioxide (CO2).
Units are in umol m-2 s-1
Multiply umol m-2 s-1 by 5 to get footcandles.
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27. How much light can a plant use
for photosynthesis?
+ CO2 + H2O C2H + O2
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28. © 2009 Regents of the University of Minnesota

29. What we learned . . .
• Species differed in how much CO2 they could
utilize under our conditions (300 umol m-2 s-1).
• Photosynthesis of some species is saturated at
lower CO2 levels (600 ppm; Rieger Begonia,
Poinsettia), while photosynthesis on other
species saturated at higher CO2 levels (<1000
ppm; cyclamen, impatiens, tomato, pepper).
• High tunnel crops are likely CO2 starved! High
light with limited CO2 is useless!
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30. Light and
Temperature
Effects in High
Tunnels
• Light Intensity and Photosynthesis
• Carbon dioxide (CO2)
• Light color
• Shading issues
• Temperature effects on:
– Development, stem elongation, photosynthesis and
flowering.
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36. What we know . . .
• Any leaf filtering increases leaf size, increases
stem elongation, and decreases flower
number.
• It is desirable to have short plants, that are
well spaced to maximize leaf area per plant
and limit shading.
• Spacing plants too close reduces yield,
increases labor/management costs.
© 2009 Regents of the University of Minnesota

37. Light and
Temperature
Effects in High
Tunnels
• Light Intensity and Photosynthesis
• Carbon dioxide (CO2)
• Light color
• Shading issues
• Temperature effects on:
– Development, stem elongation, photosynthesis and
flowering.
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38. Shade Cloth Issues
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39. SOUTHERN GROWERS
NORTHERN GROWERS
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41. Why do we use shade cloth?
• Limit heating in the greenhouse!
• In general, we have been finding that any shading
that reduces light levels below 3000 footcandles (600
umol m-2 s-1) is detrimental to yield!
• Shading selection should be based on light level at
plant level!
• Shading selection/management will change if
covering materials age and light transmission is
reduced over time.
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42. © 2009 Regents of the University of Minnesota

43. We routinely over-shade in
greenhouses and high tunnels!
The best shading materials are
materials that we can change the
% shading over time such as:
1) spray on shading
2) having different levels of light
screening.
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45. Open roof greenhouses allow for
maximum lighting for photosynthesis, little
depletion of CO2, and maximum cooling.
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48. Over-shading is often worst than
no shading!
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49. Take Home Messages
• Get a light meter!
• Don’t over-crowd!
• Find out how much CO2 is in your high tunnels! High
light with little CO2 is useless!
• Consider shading screens with high light transmission
if needed that are pulled only on certain days and at
certain times of the day! Also consider spray shading
compounds.
• Realize that poly transmission decreases over time
and that your shading management should change
as well! © 2009 Regents of the University of Minnesota

50. Take Home Messages
• Consider retractable roof high tunnels to
maximize light/CO2/temperature for optimal
plant growth.
© 2009 Regents of the University of Minnesota

51. Light and
Temperature
Effects in High
Tunnels
• Light Intensity and Photosynthesis
• Carbon dioxide (CO2)
• Light color
• Shading issues
• Temperature effects on:
– Development, stem elongation, photosynthesis and
flowering.
© 2009 Regents of the University of Minnesota

52. Response to increasing
temperature.
Units are in degrees Celsius
Multiply times 1.8 plus 32 to get units in
Fahrenheit.
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58. What did we learn?
• Species differed in how temperature affected
photosynthesis.
• The optimal temperature for photosynthesis
varied from low temperature optima crops
(59oF; Rieger begonia) to medium
temperature optima (68oF; New Guinea
impatiens) to high temperature optima (76oF;
gerbera, tomato, pepper) under our
experimental conditions.
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59. Rate of Plant Development
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64. Optimum leaf unfolding rate of
many plants occurs around 76-
84oF. When temperatures exceed
84oF, leaf unfolding slows and yield
will be reduced!
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65. How does temperature effect stem
elongation?
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70. Sensitivity of stem elongation to
temperature varies within a day/night
cycle.
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72. Variation in Daily Temp Sensitivity of Stem Elongation
During the Day
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73. Does temperature effect
flowering?
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74. Arabidopsis after 8 d Temperature Exposures
20 C 24 C 28 C 32 C 36 C 40 C
Warner, R. Studies on high temperature effects on flower development. PhD Thesis, Department of
Horticultural Science, University of Minnesota, St. Paul, MN USA.
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75. -These data suggest that the
window for inhibition of
flowering may be smaller than
we thought.
-These data also suggest that
there is a cumulative effect and
how temperatures were
provided was irrelevant.
Rather, it was an accumulation
of degree-hours that was
important (>32C).
Warner, R., and J.E. Erwin. 2005. Naturally-occurring variation
in high temperature induced floral bud abortion across Arabidopsis
thaliana accessions. Plant, Cell and Environ, 28:1255-1266.
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76. Heat stress
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77. 68 ºF 86 ºF
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78. © 2009 Regents of the University of Minnesota

79. In general, your leaf temperature is
5-7oF warmer than the air
temperature on sunny days.
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80. New Guinea Impatiens ‘Celebration Orange’
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81. Does the length of the high
temperature exposure make a
difference in how long or much
photosynthesis is depressed?
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82. N.G. Impatiens ‘Divine White’
2 Days After a 1 or 2 hour 95oF Exposure
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83. © 2009 Regents of the University of Minnesota

84. Cooling leaves in the middle of
the day on sunny days can
increase photosynthesis! Why?
By cooling leaves. . . . .
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85. Overhead
irrigation
increases
photosynthe
sis in the
middle of
the day.
This occurs
presumably
through leaf
cooling.
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86. Fog Evaporative Cooling
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www.truefog.com

87. Take Home Messages
• Buy an infrared thermometer ($75).
• When you let your night temperatures drop and
allow day temperatures to get hot, you INCREASE
stem elongation.
• Consider dropping temperatures during the first 2-3
hours to no lower than 55oF for tomatoes/peppers
and 45-50oF for spinach and other leafy crops.
• Manage high tunnel environments to achieve as
close to 76-80oF LEAF temperatures on bright days as
possible!
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88. Other Research Areas
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92. Potted Plants?
Garden Plants?
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94. 0 ppm 600 ppm
Marigold
Fast-drying
(86 F/ 45% RH)
Afternoon
Slow-drying
(59 F/ 85% RH)
Morning
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95. Airborne interplant signalling for plant
defence
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96. Other Airborne Signals?
volatile profile from
undamaged Alnus
volatile profile from
beetle-infested Alnus
From Tscharntke et al. 2001.
Biochem. Syst. Ecol. 1025–1047.
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97. Jasmonates
►Methyl
Watercress
jasmonate elicits
defense
responses, just
like jasmonic
acid.
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99. K. glaucescens
K. manginii
K. uniflora
9 10 11 12 13 14 15 hrs
Photoperiod (hrs)
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100. Green Roofs
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106. Liquid culture
Seed germination
Meristemoid induction in liquid culture In vitro multiplication
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107. Bailey Endowed
Chair for Nursery
Crops Research
Todd and Barbara
Bachman Chair for
Marketing of
Horticulture Crops
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108. Additional Special Thanks
• Participants in the FRA and
the Young Plant Center
• USDA-ARS, SAF, Lin
Schmale, and you for your
support through the
National Floriculture and
Nursery Research Initiative
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109. Industry Acknowledgements
► MNLA Foundation
► American Floral Endowment
► Gloeckner Foundation
► Altman Plants, Inc.
► Oro Farms/Florida Specialty
Plants
► Nurseryman’s Exchange
► Wagner’s Greenhouse
► Pleasant View Gardens
► Smith Greenhouses
► Sakata, Syngenta, Goldsmith,
Ball Horticultural
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110. © 2009 Regents of the University of Minnesota