The promoting effect of far-red light on bulb formation in the long day plant Allium cepa L.

Abstract

Bulb formation in the long-day plant Allium cepa L. var. Dorata di Parma was promoted by the addition of far-red light (FR) throughout the 18-h photoperiod. Higher far-red energy reduced the duration required for the initiation of bulbing. Neither a long photoperiod without added FR nor the addition of far-red to 10-h photoperiod induced bulb formation. Interruptions with red and blue lights also do not permit bulbing. The far-red effect varies with time showing a rhythm. Dichromatic irradiations with red and far-red are almost always promotive. The marginal effects of P fr P tot ratio 0.56 given for 4 h, starting at time 1:00 and 13:00 after the beginning of the light period could point to a rhythmic shift in the optimum P fr P tot requirement. The results implicate phytochrome in the promotion of bulbing by FR and it is concluded that, in Allium cepa L, the bulbing response to FR, is mediated by the ‘high irradiance reaction’ (HIR) of phytochrome.