ECOLOGY OF PLANTS 249
a higher light intensity than did low elevation, high latitude
plants even when preconditioned under the same conditions.
Light Quality All visible light is effective in the syn-
thesis of chlorophyll except wavelengths above 680 m
(Sayre, 1928). Chlorophyll has a maximum absorption peak
in the blue-violet (430 m ) and a secondary maximum in the
red (660 m ), and chlorophyll b has a maximum absorption
peak in the blue (455 m ), and a secondary maximum in the
orange-red (640 m ) (Salisbury and Ross, 1969). Hoover
(1937) reported that wheat ( Triticum sp.) has a maximum
rate of photosynthesis at 655 m in the red and a second-
ary maximum at 440 m in the blue. Warburg and Negelein
(1923) gave a general curve for photosynthesis showing a
maximum rate in the red and a secondary maximum in the
blue. It is now known that photosynthesis actually consists
of two photosystems which work together to complete the
light reactions which result in the production of ATP and
NADPH. Photosystem I has an absorption peak at 700 m
and Photosystem II has an absorption peak around 672 m
(Salisbury and Ross, 1969).
Phototrophic responses (movements in response to light)
are chiefly due to wavelengths in the blue-violet, 400–490 m ,
but there may be some response to UV light (Johnston, 1934).
The most efficient light quality to interrupt the dark
period to prevent a short-day photoperiodic response is in
the wavelength range of 620–640 m .
Effects of Daylength The response of organisms to
the relative lengths of the light and dark periods is termed
photoperiodism. Garner and Allard (1920) discovered this
phenomenon while working with Maryland Mammoth
Tobacco ( Nicotiana tabacum ). They extended the work to
many other species and found that flowering in many plants
is regulated by the relative lengths of the light and dark peri-
ods. They classified plants into three categories on the basis
of their photoperiodic flowering responses: (1) short-day
plants—those plants which flower only when the day length
is below a certain critical maximum, (2) long-day plants—
those which flower only when the day length is above a cer-
tain critical minimum, and (3) day-neutral plants—those in
which flowering is not determined by day length. Another
category has been added, the intermediate-day plants—
those which flower only between two critical day lengths.
Larsen (1947) found that the strains of Andropogon sco-
parius (little bluestem) from the area south of 36N latitude
were intermediate-day plants because they failed to flower
on a 13-hour photoperiod and also on a 15-hour photope-
riod, but flowered well between those daylengths.
It is now known that numerous phenomena in plants are
regulated by day length in many species, bulb formation,
type of vegetative growth, dormancy, leaf abscission, seed
germination, etc. The timing mechanism is still obscure, but
it seems to involve a pigment called phytochrome which was
discovered and isolated by Borthwick and his associates at
the USDA laboratories in Beltsville, Maryland (Borthwick,
Hendricks, and Parker, 1952). They found that phytochrome
exists in two forms which can readily be converted from one
form to another. One form has an absorption peak in the red at
about 660 m and is designated as P r. The other form has anabsorption peak at a longer wavelength of red (730 m μ ) and
is designated as P fr , for far-red. When P fr absorbs light at 730
m it is converted to P r , and when P r absorbs light at 660
m it is converted to P fr. In addition to these rapid changes
brought about by light, there is a slow conversion of P fr to P r
in darkness. In sunlight and broad spectrum artificial light the
conversion of P r to P fr predominates. The P fr form appears to
be inhibitory to the flowering process, so darkness is impor-
tant to eliminate all or part of this inhibitory form. Salisbury
(1958) feels that the plant possibly synthesizes a postulated
flowering hormone only during darkness also. Borthwick and
Hendricks (1960) reported that the basic difference in long-
and short-day plants may be in the ratio of P r to P fr required
to initiate the flowering or other response.
The presence of photoperiodic ecotypes has been well
documented in many geographically wide-ranging species
of plants. Olmsted (1944) was the first to demonstrate such
ecotypes in his work with Bouteloua curtipendula (sideoats
grama). He found that plants of this species from southern
Texas and southern Arizona were generally short-day or
intermediate-day plants, with an upper critical photope-
riod for flowering between 14 and 16 hours. These strains
flowered more vigorously on a 13-hour than on a 9-hour
photoperiod. The North Dakota strains consisted largely
of long-day plants with a critical photoperiod of about
14 hours. The strains from Oklahoma, Kansas, and New
Mexico included mostly long-day individuals although the
length of the critical photoperiod decreased with decrease
in latitude of origin.
Larsen (1947) found that only strains of Andropogon
scoparius (little bluestem) originating south of 36N latitude
produced visible inflorescences on a 14-hour photoperiod.
The northern strains flowered on a 15-hour photoperiod but
not the southern strains. The northern strains were apparently
long-day plants because they were able to initiate flowers
on a long photoperiod but were inhibited from flowering on
13- and 14-hour photoperiods. As was previously indicated
at the beginning of this section on effects of day-length, the
strains originating south of 36N latitude were apparently
intermediate-day plants.
Vaartaja (1959) collected seeds from 19 genera of trees,
38 species and 82 origins of widely different latitudes in
Europe and North America. Plants derived from these seeds
were grown in uniform conditions except for photoperiod.
Photoperiods of 12, 14, 16, and 18 hours were used with the
total light energy being identical in all cases. He found that
the farther north the origin of the strain, the longer was the
critical daylength below which height growth of seedlings
was retarded. Moreover, the more northerly the source, the
shorter was the critical dark period which induced dormancy
in several species. In some plants from the far north, photo-
periods that inhibited elongation permitted great increases in
weight of stems and roots, and in numbers of buds. Cambial
growth thus would continue later in the growing season than
elongation.
Irgens-Moller (1957) collected approximately 100 plants
(2–6 year old) of Pseudotsuga menziesii (douglas-fir) from
each of seven locations in Oregon at elevations of 60 feet toC005_001_r03.indd 249C005_001_r03.indd 249 11/18/2005 10:20:11 AM11/18/2005 10:20:11 AM