Classification and Adaptation: -As regards their photoperiodism,
plants have been classified by GARNER and ALLARD (1920, 1923) into short
day, long-day and day-neutral types. While this grouping is not fully ac
ceptable in the light of our present evidence on plant development as afPro.
1.-Original Maryland Mammoth tobacco plants with whirls photoperrodisn was discovered.
Plant on left grew in unlighted greenhouse (short days). Plant on right grew in electrically lighted
greenhouse (long days). Winter, 1919-Photo by GARNER and ALLARD.
FIG. 2--Original Rural - _ hears plants with which
photoperiodism was first demonstrated. Vegetative
plants on left grew outdoors cinder natural long days.
Reproductive plants on right grew outdoors under arti
ficially shortened 10-hour days. Summer, 1920.-Photo
by GARNER and ALLARD.
fected by light duration, it has been adopted by most investigators and is
used widely.
In short-day plants flowering is induced experimentally by relatively
short photoperiods, usually 10 hours or less; in long-day plants by photo
periods of 14 hours or more; and in so-called "neutral" plants by 10-18
hours of light duration or even continuous illumination (HARVEY, 1922).
Examples of short-day plants are: Some tobaccos and soybeans, Chrysanthemum,
Salvia, Poinsettia, Cosmos and many other of our spring and fall
flowers. Of long-day plants there are: Spring varieties of cereals, spinach,
lettuce, radish, potato, Rudbeckia and other summer flowers. Of neutral
plants we have: Tomato, dandelion, buckwheat, cotton and some squashes
and cucumbers.
Recently ALLARD (1938) and GARNER (1940) have added a fourth
group, of which there seem to be a few representatives, designating it
intermediates. They seem to flower at a day length of intermediate dura
tion (12-14 hours), but are inhibited in reproduction by day lengths either
above or below this length. Examples of such plants are: Mikania scandens,
Phaseolus polystachyus, Eupatorium torreyanum and Saccharum spon
taneum.
To lengthen the light period experimentally, GARNER and ALLARD
found electric light of comparatively low intensity consistently effective for
initiation or inhibition of reproduction or vegetation, depending on the type
of plant treated. This has been fully verified by other investigators and is
good evidence that, in general, intensity of the supplementary light used
is not a factor in photoperiodism. Very weak light, of course, cannot be
used, throughout the photoperiod for a large number of days without great
disturbance in growth and development of plants.
Photoperiodism is an important factor in the natural distribution of
plants. In their native habitats plants are adapted in various degrees to
a variety of environmental factors, including the diurnal duration of light.
They could not persist long in a specific region or extend their range unless
the environment is favorable for some form of reproduction (POTAPENKO,
1945). Numerous observations have shown that genera, species and va
rieties have developed photoperiodic responses which enable them to adjust
the time of flowering and fruiting (seed production) to definite seasons,
characterized by certain lengths of day. Thus we have more or less typical
spring, summer and fall blooming types of plants. The literature on this
subject is voluminous, of which the following papers may be cited as exam
ples: ADAMS, 1923; ALLARD, 1932; DARROW, 1934; DOROSHENKO, 1927;
DOROSHENKO and RASUMOV, 1929; MCCLELLAND, 1928; MAXIMOV, 1929;
TINCKER, 1925; KUZNETSOVA, 1929; LUBIMENKO and EGLOVA, 1927;
ALLARD and ZAUMEYER, 1944; ALLARD and GARNER, 1940 and BÜNNING,
1943. Some synchronization to the seasonal photoperiod is remarkably
close (SMITH, 1941).
In general, plants that have originated in southern or tropical latitudes
will require short days for flowering, while those of more northern lati
tudes, roughly to the north of 60°, are long-day plants. When the latter are
moved too far south, they will not produce blossoms. When carried farther
north, they will still be reproductive, because of an increased photoperiod
and despite the shortened season. But southern plants are more difficult
to adapt to northern latitudes where, due to the much longer days, they will
continue to grow vegetatively till killed by frost.
Within specific groups of plants there may exist great variability as to
their responses to definite photoperiods, even if attention is paid only
to flowering and not to the earlier or later stages of the reproductive cycle.
Thus within the genus Phaseolus, to three species of which (P. vulgaris,
P. lunatus and P. coccineus) belong the beans commonly grown in the
United States, there is a large number of varieties that are either short
day or day-neutral. But the Scarlet Runner bean, a variety of P. coccineus,
is a long-day plant. Since most of the varieties of P. vulgaris and P. luna
tus are day-neutral, they can be and are grown over a considerable range
of latitudes. In fact, much of the varietal improvement in these two spe
cies has been associated with day-neutral characteristics (ALLARD and
ZAUMEYER, 1944). Analogous and possibly even more extreme situations
are exhibited by soybeans (BORTHWICK and PARKER, 1939; RUDORF and
SCHRöCK, 1941) and many other cultivated species (BELJDENKOVA, 1940;
GOODWIN, 1941; LAIBACH, 1940; ALLARD, 1941).
Among wild plants, or plants that have been domesticated but little, simi
lar conditions seem to exist. Of the several instances that could be cited,
it would seem to be sufficient to refer here to two recent studies. OLM
STED (1944-1945) has investigated, both in the field and laboratory, the
photoperiodic response of twelve geographic strains of side-oats grama
grass (Bouteloua curtipendula). They were found to differ in their re
sponses to the length of day, showing the existence of definite photoperiodic
types within the species. Strains from southern Texas and Arizona con
sisted largely of short-day or intermediate plants, those from Oklahoma,
Kansas and Nebraska included numerous long-day individuals, and those
from North Dakota were made up chiefly of long-day plants. It is con
cluded that most specimens, within the twelve strains investigated, probably
can develop flowers best on photoperiods existing in their native habitats.
Whether this wide variability in response to the photoperiod is due to more
or less stable inherited characters or is the result of heterozygocity, is not
certain from the evidence presented. A similar situation seems to exist
in other groups of plants (RASUMOV, 1937; KIRICENKO and BASSARSKAJA,
1937).
The adaptability to length of day of various species of the potato has
been reported by DRIVER and HAWKES (1943). Though most wild South
American potatoes were found to be short-day plants, certain clones of
Solanum andiginum, S. curtilobum and S. tenuifilamentum appeared day
neutral, and one clone in each of the first two species reacted as long-day
plants.
From these and other instances it is quite apparent that the present
classification of plants on the basis of reactions of some members of the
group, as regards sexual reproduction, to more or less definite photoperiods
is not very appropriate.
In this connection there are other aspects of the problem of classifica
tion that should be taken into consideration. Very few species, excepting
possibly those in equatorial regions and in the case of some desert, polar or
high altitude ephemerals, seem to be adapted throughout their length of
sexual reproduction, from flower bud inception to seed maturity, to the
same photoperiod or a combination of photoperiod with other environmental
factors. In many plants flowers are initiated at one time of the year (one
photoperiod) and their further development to anthesis takes place at an
other. It is highly probable that reproduction may commence by exposing
experimental plants continuously to specific lengths of day but further de
velopment of floral organs may be curtailed or inhibited under this particu
lar photoperiod. (BORTHWICK and PARKER, 1939; MURNEEK, 1939).
Disregarding equatorial regions, in nature most plants probably have ad
justed themselves not to uniform diurnal periods of light but to continu
ously changing ones. It has been shown that many so-called short-day
plants are really short-day -> long-day plants as regards their response to
photoperiods. Similarly quite a number of long-day plants are, in fact,
short-day -> long-day types. Hence, paradoxical as it may appear, there
does not seem to be an essential difference between the two groups (WHYTE
and OLJHOVIKOV, 1939).
CAJLACHJAN (1933) is of the opinion that classification of cereals into
spring and winter types is unjustifiable in view of the fact that in any
large collection strains exist within varieties, secured from different lati
tudes, that may be arranged in series, from spring to winter forms. Using
certain species of Poa, Digitalis, Trifolium, Pyrethrum and Hyoscyamus as
examples, KRIER (1941) claims that there is no clear-cut distinction be
tween winter and spring annuals, biennials and perennials. One type can
be converted into the other under environmental conditions as determined
by geographic location.
By considering two stages of reproduction only, flowering and fruiting,
EGUCHI (1937) recognizes the following classification, some representa
tive plants for each being given by LOEHWING (1939).
Optimal periods for:
Flowering: - Fruiting: - Representative species: -
Short Long Strawberry, Cineraria
Long Long Oxeye daisy, Spring barley
Long Short Physostegia virginiana, Boltonia latisquama
Short Short Soybeans, Cosmos bipinnatus
Long Day-neutral Phlox paniculata
Short Day-neutral Late rice varieties
Day-neutral Short Chrysanthemum osticum
Day-neutral Long
Spinach, wheat
Day-neutral Day-neutral Pepper, early rice, buckwheat
Whether many plants, in flowering and fruiting are as closely adjusted
to the light period as would seem to be indicated here, may be questioned,
for often changes in length of day merely delay flower development. Then
there is considerable evidence extant also that the age of the plant deter
mines in a large measure its sensitivity to length of day CAJLACHJAN,
1936; PURVIS and GREGORY, 1937; BORTHWICK and PARKER, 1938; MOSHKOV,
1939; MIROLJUBOV, 1940). All this seems to suggest the necessity of
revision of our conception of photoperiodism.
The period from flower initiation to their full development (anthesis)
should receive at least an equal if not greater consideration than the time of
floral inception. Flower differentiation does not always lead to their
macroscopic development, certainly not in equal numbers (MURNEEK,
1939). The histological analysis by BORTHWICK and PARKER (1938)
suggests this, and their further studies on the effects of the photoperiod on
development of the Biloxi soybean (PARKER and BORTHWICK, 1939) dem
onstrate that when plants, with initiated floral primordia, were transferred
to long, 16-18 hour photoperiods, no flowers opened, and when photoperiods
above 13 hours were given no fruit was formed. NIELSEN (1942) found
that even 10 cycles of short photoperiods resulted in a high percentage of
degenerated microspores in the Biloxi soybean.
In view of these facts, and the thought-provoking statement by GREGORY
(1936) that "the problem of photoperiodism may be considered not as con
cerning conditions leading to flower formation but as concerning failure to
flower," it would seem to be desirable that in studies of photoperiodism,
as it affects sexual reproduction and metabolism, the whole reproductive
cycle should be followed in detail both by observation and histologically.
As far as the writer is aware this procedure was started for the first time
with the soybean, var. Biloxi (MURNEEK and GOMEZ, 1936) and has sub
sequently been used successfully by other investigators (HAMNER and
BONNER, 1938; BORTHWICK and PARKER, 1938; SNYDER, 1940; MANN,
1940, etc.).
Selection and breeding of plants for adaptability to localities of certain
photoperiods has been in progress for a number of years. The testing of
species, varieties and strains was started by GARNER and ALLARD (1920)
and has been continued by numerous other investigators. Unconsciously
growers have been doing it for a very long time, especially in comparative
tests of types and varieties of various economic plants. In this connection
emphasis should be placed on the importance of conducting selection by ex
posing plants to near the critical length of day for flower initiation and de
velopment, for there will be revealed the greatest degree of variation in
time of flowering (ALLARD and ZAUMEYER, 1944).
Phenotypic adaptation is adaptation of the individual, but there is also
genotypic adaptation of successive generations. In adaptability to a locality,
therefore, not only the environment but also the endogenous rhythm of the
selected plants must be taken into account. These do not always coincide.
Though we do not know much about the specific mechanism of this "inner
rhythm," it probably originated as a result of natural selection of random
mutations, but segregation and recombination or other gene mechanisms
may be operative (LUBIMENKO, 1939; BUNNING, 1943). Breeding plants
for photoperiodic adaptability has been successful in several instances
(MUNERATI, 1931
again thanks for sharing such great knowledge! :thumbs: :booya:
