Plant Physiol. 1944 October; 19(4): 638-648.
L. L. Danielson


Most contemporary studies of photoperiodism in plants have dealt with species which normally flower either in long or short day conditions. There is great experimental convenience and clarity of response in the employment of species which can be qualitatively distinguished as reproductive in a short photoperiod and vegetative in a long photoperiod or vice versa. Phenomenal improvement in our knowledge of the physiology of growth and reproduction has attended the study of such clearly defined types in the two decades following the enunciation of the basic concept of photoperiodism by GARNER and ALLARD (8).

Scientific interest has centered in the nature of the metabolic factors concerned with the shift from vegetative to reproductive habit of the plant under the stimulus of long or short photoperiod, due largely to the need for improved control over reproduction. Interpretation of the phenomena of flowering and fruiting require a clearer understanding of the vegetative processes that precede them. Recognition of this fact has recently stimulated numerous studies of the physiology of growth in its entirety from germination to maturation. A comprehensive study of soybeans by MURNEEK (15) employs this approach.

A somewhat neglected group of plants from the standpoint of growth under contrasted daylength are those which cannot be clearly classified as either long or short day types in terms of flowering response. Yet by virtue of their deviation from the typical long or short day flowering response, the day neutral and photoperiodically indeterminate species promise to contribute considerably to an understanding of light as a factor in plant development.

The commercial strain of Cucumis sativus commonly known as the small gherkin was grown in three photoperiodic environments, predetermined in a series of preliminary experiments, to determine the developmental physiology of a species known to be indeterminate in its flowering response to photoperiod. Experimental procedures were arranged so as to provide a continuous growth record in order to identify qualitative differences in structure and quantitative variations in composition correlated with the contrasted photoperiods employed.

Fig. 2. Representative 95-day-old plants and roots from eight- (A), twelve- (B), and sixteen-hour (C) diurnal photoperiod groups.