Role of phytochrome in photoperiodic regulation of bulbing and growth in the long day plant Allium cepa

Plant elongation in Allium cepa L. cv. Dorata di Parma was stimulated by end‐of‐day far‐red radiation, while the same treatment was ineffective with respect to bulbing response. It was concluded that the Pfr‐dependent reactions which control bulbing are completed during the long daily light period (18 h). Day breaks of 3 h fluorescent white light, in the middle of the inductive photoperiod were inhibitory to bulbing. Repeated brief far‐red irradiations could substitute for continuous far‐red irradiations lasting 3 h in the middle of the photoperiod. Red light alone or applied immediately after each far‐red irradiation inhibits bulb formation.