Importance of Dark Period

For quite sometime the role of light period (photoperiod) was emphasised in flowering? However, based on certain experiments it was realised that it is the dark period that is more important than the light period for inducing a plant to flower. In fact, as early as 1912, from his experiments on flowering, Julien Tournois concluded that flowering occurred "not so much by shortening of day as by lengthening of nights". We describe below an experiment, which was done much later by Hamner and Bonner (1936) that proved this point. A short-day plant was taken that required 16 hours of darkness and 8 hours of light to flower. If the dark period was reduced, plants did not flower. Interestingly, if the dark period of 16 hours was interrupted by light, again there was no flowering. However, if the light period was interrupted by dark period, or if plant8 were kept less than 8 hours in light there was no effect on flowering. It was clear that altering the light period had no effect but if the dark period was less than 16 hours, the plants would not flower. A look at Figure will further clarify this experiment and confirm that dark period is more important for inducing plants to flower. This concept has now been well established. The role of dark period is to bring about some changes which trigger the development from the vegetative to the flowering state. The role of light period is to realise this change and help in bringing about maximum flowering.

Figure: Experiments to show that dark period is important for flowering. If dark period is interrupted (3) SDP do not flower but if light period is interrupted (4) or shortened (5) there is no effect on flowering in LDP