Effect of Temperature and Preconditioning on Photoperiodic Response of Pharbitis nil.

In both short-day and long-day plants, endogenous rhythms apparently are involved in photoperiodic response. One line of the evidence is a rhythmic flowering response to cycle length (1, 3,4, 6,9), and another is the rhythmic effectiveness of light breaks given at different points in very long dark periods (2, 4,6,8, 10). Phlarbitis nil, strain Violet, is an extremely sensitive short-day plant. A single dark period of 16 hours is sufficient to induce a maximum flowering response, an(l further increases in the dark period do not change the response. A brief exposure to light may inhibit flowering when it is given 6 to 12 hours after the beginning of a long dark period, but has no effect at any other points (11). Thus, it seems that there has been no evidence to indicate that endlogenous rhythms were involved in the photoperiodlic response of Pharbitis nil. Most of the experiments with Pharbitis nil, however, have been done at optimal temperatures, i.e., at 25 to 300. At these temperatures, all plants develop terminal flower buds with a single dark period of 16 hours. Therefore, even if furtlher increases in the length of the dark period causecl fluctuations in the photoperiodic stimulus, the number of flower buds formed at optimuni temperatures miglht not reflect these fluctuations since the measure of the photoperio(lic response (number of flower buds) reaches an upper limit (fornmation of a terminal flower bud). It was thought worthwhile to investigate the photoperiodic response of Plharbitis nil at suboptimal temperatures. At lower temperatures, it was expected that a single long dark periodI wouldl not be enough to indluce the maximum flowering response, and a more detailed photoperiodic response might be recognize(l. The first half of the present experiments was (lesigned to investigate the effect of temperature on photoperiodic response of Phlarbitis nil exposed to very long dark periods. The last half of the experimleints wlas carried out at suboptimal temperatures, and designed to determine whether or not endogenous rhytlhmis are involved in the photoperiodic response.