Photoperiodic Responses of Certain Greenhouse Annuals as Influenced by Intensity and Wavelength of Artificial Light Used to Lengthen the Daylight Period

Abstract

IntroductionTwo types of experimental method have been employed by investigators in the field of photoperiodism. In the one case the experiments have been conducted during the long days of summer and the natural daylength used for the long day treatment, with the short days produced by moving the plants into dark chambers after a certain number of light hours. In the second type of experiments the investigations have been carried on during the short days of winter and the natural daylength used for the short day period, the long days being produced by the addition of artificial radiation.It might be questioned as to whether the results of the two experimental methods are comparable and whether both involve the same group of physiological processes, since the conditions of radiation for the two differ so widely. That those conditions used in the greenhouse during the winter do not simulate the conditions under which the plant would respond during its natural growing season must be recognized. This is especially true in regard to light intensity and spectral energy distribution.In the first paper published by GARNER and ALLARD (1) on the effect of the photoperiod on the flowering of plants, it was shown that typically long day plants became reproductive and short day plants assumed the vegetative state when they were subjected to naturally short winter days artificially lengthened with relatively low intensity incandescent lamp radiation. TINCKER (14) obtained similar results with several long and short day plants when he used 5 foot candles of visible radiation to supplement the natural daylight period. TINCKER (15) conducted experiments

Experiments on Photoperiod in Relation to the Vegetative Growth of Plants

“…The promotive effect of long daily photoperiods on vegetative growth does not appear to be directly attributable to increased duration of CO2 assimilation (11). Plants grown under conditions of long day made up of a short photoperiod of natural light supplemented by light of exceedingly low intensity exhibit the vegetative vigor typical of plants on a long day rather than that of planits on a short day regime (13).…”

Photoperiodic Response of Certain Long and Short Day Plants to Filtered Radiation Applied as a Supplement to Daylight

“…When germinated in a shorter day of 91 hours, the response is as indicated above. 4. Salvia from clonal cuttings, with all plants in flower when placed under days lengthened by wavebands indicated, reverted to the vegetative state under red and white radiation.…”

“…There seems to be a maximum peak of response in the neighborhood of 650 to 700 mp (4) and possibly a second lesser peak at wavelengths shorter than 450 m,u.…”

The Effect of Various Wavebands of Supplementary Radiation on the Photoperiodic Response of Certain Plants

“…RASU3IoV (8) (10). The plant material was harvested, weighed and dried at 1000 C. in a forced draft thermostatieallv controlled drying oven for about 14 hours.…”