248 ECOLOGY OF PLANTS
and scattered by various components of the atmosphere. As
a result, only about 50% of the total radiation reaches the
earth on the average. This figure may be greater than 70%
in dry regions with little cloud cover and lower than 40%
in tropical rainy areas (Slatyer, 1967). Obviously, on very
cloudy days the energy reaching the surface of the earth
may be very low. Of the solar energy reaching the earth,
about 40–45% is still present in the visible range.
Radiation in the visible range is most important in
affecting organic processes in plants, so the rest of this dis-
cussion will be concerned primarily with visible light. The
chief processes in plants which are affected by light are:
(1) chlorophyll synthesis, (2) photosynthesis, (3) phototro-
pism, (4) photoperiodism, and (5) transpiration.
Light Intensity Effects. Many plants are able under
most conditions to develop well in less than full sunlight.
Red kidney beans ( Phaseolus vulgaris ) grow and fruit well
in growth chambers in a light intensity of 700–1000 ft-c.
Intensity of light has a definite effect on the distribution of
plants. Many ferns grow well under trees because such ferns
attain their maximum rate of photosynthesis at light inten-
sities considerably below full sunlight. Species which char-
acteristically grow in shaded locations are able to remain
alive at very low light intensities. Redwood ( Sequoia sem-
pervirens ) seedlings normally grow in the deep shade of
the forest floor and they are able to grow at intensities of as
little as 100 ft-c (Bonner and Galston, 1952). Pinus taeda
(loblolly pine) and Pinus echinata (shortleaf pine) seed-
lings are found in great profusion along road cuts and other
open areas in the southeastern United States, but they fail
to survive under dense forest canopies. Many hardwood
seedlings such as those of Quercus alba (white oak) and
Quercus rubra (red oak) survive nicely under such cano-
pies and thus replace the pines in succession. Kramer and
Decker (1944) found that photosynthesis in loblolly pine
increased with light intensity up to the highest intensity
used which was almost that of full sunlight. On the other
hand, photosynthesis in white oak and red oak reached a
maximum at one-third or less of full sunlight and showed
slight decreases at higher light intensities. These results
indicate that lack of sufficient light for photosynthesis may
be a significant factor in the failure of pine seedlings to
become established under forest stands. Seedlings of many
other climax forest dominants such as Fagus grandifolia
(beech) and Acer saccharum (sugar maple) have very low
light requirements for photosynthesis also (Daubenmire,
1959). After a long series of studies, Blackman (1950) con-
cluded that the distribution of Scilla nonscripta (bluebell) in
English forests is determined primarily by light intensity.
Björkman and Holmgren (1963) investigated the pho-
tosynthetic response to light intensity of sun and shade
plants of Solidago virgaurea (golden-rod). They found that
the shade plants had a higher chlorophyll content per unit
leaf area in weak light (3 10 4 ergs cm^2 sec^1 ) than in
strong light (15 10 4 ergs cm^2 sec^1 ) and more than did
sun plants in weak light. On the other hand, the sun plants
had lower concentrations of chlorophyll in weak light than
strong light. The gross rate of photosynthesis (apparent rateplus respiration rate) in sun plants at light saturation was
25.5 2.0 mg CO 2 dm^2 h^1 when the plants were grown
in strong light (same as above) prior to testing whereas
it was only 17.9 0.6 mg CO 2 dm^2 h^1 in shade plants
grown in strong light. The gross rate of photosynthesis at
light saturation was higher in the shade plants than in the
sun plants when the plants were grown in weak light prior
to testing, 2.93 0.08 in shade plants versus 2.31 0.04
in sun plants. The difference of the means in the plants
grown in low intensity was highly significant statistically.
The authors concluded that the photosynthetic behavior
was consistent with light intensities prevailing in the natural
environment indicating that the behavior was a result of a
genetic adaptation to the habitat. Hybridization experiments
supported this conclusion.
Björkman (1966) reported on additional comparative
studies of sun and shade plants of several species. Plants
of shaded habitats consistently reached a higher percent-
age of the maximum photosynthetic rate at light saturation,
when tested in a low light intensity of 10^4 ergs cm^2 sec^1
(Table 3). Plantago lanceolata (plantain) was found only
in open meadows and Lamium galeobdolon (henbit) only
in dense beech forests. Björkman found that the activity
of Photosystem I (far red, 700 m ) in Solidago virgaurea
was about the same when grown in weak or strong light,
regardless of the original habitat. On the other hand, the
action of Photosystem II (625 m used) was impaired in
shade plants grown in high light intensity and in sun plants
grown in low light intensity. The Emerson enhancement
effect (increased photosynthesis from combination red–far
red light over sum of photosynthesis in each light quality
separately) was drastically reduced in shade plants grown
in high light intensities.
Milner and Hiesey (1964) studied the photosynthetic
responses to light intensity of several races of Mimulus car-
dinalis (monkey-flower) collected from an elevation of 45 m
to 2220 m in California. They found that the saturating light
intensity for the races increased with elevation of their origi-
nal habitats. At 0C, the saturating light intensity varied from
290 to 570 ft-c for different races, and at 40C from 3500
to 5900 ft-c. Light intensity, of course, generally increases
with elevation. Mooney and Billings (1961) in their study
of ecotypes of Oxyria digyna found that the high elevation,
low latitude plants attained photosynthetic light saturation atTABLE 3
Photosynthesis at a low light intensity of 10^4 ergs cm^2 sec^1 in percent
of the photosynthetic rate at light saturationaSpecies Sun plants Shade plantsPlantago lanceolata 8 —
Solidago virgaurea 9 17
Rumex acetosa 11 16
Geum rivale 14 16
Lamium galeobdolon — 25
a From Björkman (1966).C005_001_r03.indd 248C005_001_r03.indd 248 11/18/2005 10:20:11 AM11/18/2005 10:20:11 AM