Floral photoperiodism

Within less than two decades after the discovery of photoperiodism in 1920, the hypothesis developed that one or more specific flowering hormones are responsible for floral initiation (23,36-39). 

From the available data it can be tentatively concluded that the level and/or metabolism of endogenous cytokinins changes markedly, often transiently, at the time of floral transition in many plant species [1]. The trend of changes was, however, opposite in species with different photoperiodic requirements an increase in levels was recorded, for example, in leaves of the LDP Hyoscyamus and Nicotiana sylvestris and the SDP Begonia, whereas a decrease was observed in the SDP Xanthium and Chenopodium [see 5, 11, 16]. 

Heide, O. M., Sonsteby, A. Floral initiation in black currant cultivars (Ribes nigrum L.) Effects of plant size, photoperiod, temperature, and duration of short day exposure. Scientia Horticulturae 138, 1, 2012. 

Floral initiation is best studied at the microscopic level. The fact is, floral initiation often occurs and is unrecorded because abortion takes place or there is failure of the flower initials to develop to the macroscopic level where detection is easy with the unaided eye. There are many pieces of evidence that this is true. One was provided by Naylor (1941) with the photoperiodically sensitive cocklebur plant (Xanthium pennsylvanicum). He found that even when the plants were severely deficient in nitrogen or phosphorus or potassium they differentiated floral primordia after the leaves received an appropriate photoperiodic treatment. However, floral development did not occur. If dissection of stem apices had not been carried out, floral initiation would have gone undetected and the plants would have been recorded as vegetative. 

While low irradiance levels often limit the number of flowers developed, duration of the light period perhaps more frequently dominates in the floral development phase. In any event, an unsuitable carbohydrate nitrogen balance under either long or short photoperiods can limit the number of flowers that reach the pollination stage of development. 

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