Department of Biology, Guru Nanak Dev University, Amritsar-143005
Calendula officinalis L., an ornamental herb belonging to the family Compositae is shown to be a qualitative long day plant with an exceptionally low photoperiodic requirement of 6 h 30 min in a 24 h cycle. However, the period of inflorescence initiation did not differ quantitatively with an increase in the length of inductive photoperiod from 6 h 30 min to 24 h. The increase in the length of photoperiod beyond 12 h promoted extension growth but did not affect leaf differentiation.
