Failure of Photoreversible Control of Flowering in Pharbitis nil

Flowering of seedlings of Japanese morning-glory, Pharbitis nil Chois., grown in nutrient solution or in soil is inhibited by far-red and repromoted by red irradiations at the beginning of 16-hour dark periods and inhibited by both red and far-red irradiations at the middle of such dark periods. Older plants are not responsive to red or far-red irradiations at the beginning of the dark period, but, at the middle, red inhibits flowering and far red repromotes it. 2. Seedlings irradiated at the beginning of the dark period flower if at least one cotyledon is either not irradiated with far red or is irradiated with red after far red. Flowering of seedlings following irradiation at the middle of the dark period occurs only if one cotyledon is not irradiated. 3. Action spectra for control of flowering by irradiation of cotyledons were measured at the start and near the middle of inductive dark periods adequate to promote flowering. The energies required at the beginning of the dark period for half-saturation of the far-red and red responses are similar in value to those required for reversible control of flowering of other photoperiodic plants near the middle of inductive dark periods. 4. Approximately the same energies are required for control of flowering by red radiation at the beginning and near the middle of the inductive dark periods. The flowering response potentiated by red radiation, however, is not reversible by far-red radiation near the middle of the dark period and is opposite to the response upon the onset of darkness. About 4 hours in darkness are required for the plant to change from the one to the other type of response.