PHOTOPERIODISM, the term employed to designate the response of organisms to length of day, with special reference to plants. Its possible application to animal life has not yet been adequately investigated, though there is some evidence of its connection with bird migration. So far as known at present, sun light may affect plant growth in three ways, in virtue of its corn position, including visible and invisible radiation, in virtue of its intensity and its daily durations.
It has been found that, for the sake of convenience, plants may be considered to fall into three great groups, depending upon the initiation of sexual reproductive expression and flowering in response to different day-lengths. In one group, termed the short day plants, day-lengths of 12 hours or less are most favourable to the initiation of sexual reproduction. In a second group, the long-day plants, lengths of day in excess of 12 hours, amounting in some instances to continuous illumination are most favourable for reproduction and flowering. A third group includes plants which attain sexual expression independently of any particular length of day. Such plants have been called indeterminate types, because they show no distinctive flowering response to either long or short days.
By the use of appropriate dark. ventilated chambers, or houses in which plants may be placed, cutting off the early morning and later afternoon light of the long days of summer, it has been found practical to study the behaviour of plants as affected by daylight exposures much shorter than the normal. During the short days of winter, greenhouses suitably heated have been used, with electric illumination afforded the plants at sunset to increase the daily illumination periods to the desired degree. The be haviour of responsive plants has been consistent under the two conditions, according as they are normally long-day, short-day, or indeterminate types of plants. Poinsettia is an example of a typical short-day plant, because the initiation of reproduction does not take place until the seasonal day-length has fallen below 12 hours. Buckwheat is an indeterminate type of plant, and Oswego tea or bee balm (Monarda didyma), a garden orna mental, demands the long midsummer days for flowering.
The distinctive response of the different species of plants ap pears to have much to do with their normal ranges in many in stances, at least it is one of the factors in the environmental complex, which must also be considered in any study of natural distribution. Likewise the geographical limits of some of our
cultivated crops probably depend to a greater or less extent upon this factor. Biloxi soy beans, for instance, a crop fruiting success fully in the Gulf States, cannot be grown for seed in the Wash ington region (latitude 39°), because they do not flower until the September length of day is reached. As a result, unfavourable temperatures with frost ensue and kill the plants while the pods are still immature.
As originally defined, photoperiodism refers to the normal 24 hour period of time, a constant condition which holds everywhere on the earth except at the poles. Breaking the normal daily duration of light during the long days of summer with a period of darkness of one to several hours in the middle of the day does not appear to produce any fundamental effect upon the normal reproductive development of typical long- or short-day plants.
In addition to its effects upon sexual reproduction day-length is known to modify composition, growth and development of plants in various ways, including tuberization, branching habit, root and rhizome development, stature, dormancy, leaf fall, senescence, re juvenescence and cleistogamy.
In any analysis of plant behaviour, either in the field or in the laboratory, the factor of photoperiodism must be considered as operative in the environmental-complex. Just as plants are known to have their specific requirements with respect to temperature, moisture, light-intensity, etc., it is known that they have definite length-of-day requirement for promoting the different phases of vegetative expression of flowering and fruiting. The character and extent of these expressions, the seasonal behaviour, stature, longevity, etc., may be more a matter of response to length of day than anything else. This principle has probably been uncon sciously applied in the agricultural operations of man with his various economic and ornamental plants, just as the requirements of temperature, light-intensity, soil, moisture, etc., have been met. Definite recognition of the principle will unquestionably aid greatly in a more intelligent interpretation of plant behaviour, and prove a factor of no small importance in the successful intro duction of plants from one region into another. The phenomena of photoperiodism were brought to the attention of the scientific world in 192o by workers in the U.S. Department of Agricul ture.
See K. F. Kellerman, Quarterly Review of Biology (1926).