LEVELS OF LIGHT ON EARTH AND SOME ASSOCIATED BIOLOGICAL RESPONSES*

Column 1 lists different environmental conditions with different light levels [1]; column 2, the spherical photon flux, the number of photons (400 to 700 nm) intercepted by a sphere of unit cross-section per unit time [1]; column 3, biological responses associated with these light levels; and column 4, notes and references for the listed biological responses.

Environmental Condition Spherical Photon Flux
(photons m2s-1)		 Biological Response Notes and References
Clear Midday
~1021
 Saturation of photosynthesis in "sun-loving" plants
(continuous exposure)		 Light level is species dependent [2]
Cloudy Midday
~1020
 Saturation of photosynthesis in "shade-loving" plants
(continuous exposure)		 Light level is species dependent [2]
Sunset
~1019
 Increase in Gonyaulax circadian period
(single 4 hr red light pulse)		 Threshold for bioluminescence rhythm [3]
Early Twilight
~1018
 Seed Germination (LFR)
(1 sec red light pulse)		 Threshold for Low Fluence-Response [4]
Late Twilight
~1017
 -

-

Midnight
clear, full moon
~1016
 -

-

Midnight
clear, new moon
~1015
 Human color vision Threshold corresponding to ~10-2 milliLamberts [5]
Midnight
cloudy, new moon
~1014
 Seed germination (VLFR)
(1 sec red light pulse)		 Threshold for Very Low Fluence-Response [4]
-

~1013
 -

-

-

~1012
 Phototaxis in Dictyostelium slugs
(continuous white light exposure)		 Threshold level for NC4 strain [6]
Threshold for AX2 strain is ~100-fold lower [7]
-

~1011
 -

-

-

~1010
 Human Vision
(1 millisec green light exposure)		 Threshold corresponding to ~10-6.5 milliLamberts [5, 8]
-

~109
 Phototropism in Phycomyces
(continuous blue light exposure)		 Threshold level [9]
-

~108
 -

-

-

~107
 Adaptation acceleration in Phycomyces
(35 min red light pulse)		 Threshold Level [10]

References
[1] Based on measurements in Erlangen, Germany given in [K.M. Hartmann, 1995, International Symposium on Weed and Crop Resistance to Herbicides. Cordoba, Spain.].
[2] F.B. Salisbury & C.W. Ross, 1985, Plant Physiology. Wadsworth, Inc.
[3] T. Roenneberg & J.W. Hastings, 1991, Photochem Photobiol 53, 525.
[4] K. M. Hartmann & W. Nezadal, 1990, Naturwissenschaften 51, 681.
[5] P. Buser & M. Imbert, 1995, Vision. MIT Press.
[6] K.L. Poff & D.-P. Hader, 1984, Photochem Photobiol 39, 433.
[7] D.-P. Hader & A. Haser, 1991, Bot Acta 104, 200.
[8] S. Hecht et al., 1942, J Gen Physiol 25, 819.
[9] K. Bergman et al., 1969, Bacteriol Rev 33, 99; P.A. Ensminger et al., 1990, Photochem Photobiol 51, 681.
[10] P. Galland et al., 1989, Photochem Photobiol 49, 485; X.-Y. Chen et al., 1993, Photochem Photobiol 58, 425.

*from Life Under the Sun (by Peter A. Ensminger), a collection of photobiology essays, to be published by Yale University Press.

Peter A. Ensminger, ensmingr@npac.syr.edu
last updated July 7, 1998