Light Rquirements for Photoperiodic Sensitivity in Cotyledons of Dark‐grown Pharbitis nil

Abstract

AbstractThe light requirements for induction of flowering by a long dark period were investigated in dark‐grown seedlings of Pharbitis nil Chois, cv. Violet. The cotyledons bcame photoperiodically sensitive to a 24 h dark period by two 1 min red irradiations (6.3 μmol m−2 S−1) separated by a 24 h dark period. The reversibility of the effect of brief red irradiations, and the effectiveness of low energies of red irradiation suggest the involvement of phytochrome in the induction of photoperiodic sensitivity. Partial de‐etiolation occurred after these brief periods of red irradiation but the seedlings were not capable of net CO2 uptakeeven 7 h after the start of the main light period that followed the critical dark period. A changing response to the duration of the priod of darkness given between the two short red irradiations showed the the correct phasing of an endogenous photoperiodic rhythm is needed for the attainment of photoperiodic snsitivity.