Hormones in Photoperiodic Flower Induction

The timing of the transition from vegetative to reproductive development is of great fundamental and practical interest. Physiological event is a subject to strict and precise control. Among the environmental factors, the most important is photoperiod defined as the following changes in the light and darkness duration in the daily cycle, while the endogenous control depends on hormones action. Due to the different photoperiodic sensitivity, the plants were divided into long-day plants (LDPs), short-day plants (SDPs), and day-neutral plants (DNPs). Mainly genetic and molecular approaches have led to the identification of various components in the photoperiodic pathway among which the key are photoreceptors necessary for light perception, and elements of the endogenous circadian clock. Moreover, the circadian clock also synchronizes changes in the level of hormones. In Arabidopsis thaliana, as well as in many other LDPs, gibberellins (GAs) promote flowering, whereas in SDPs, GAs do not affect or inhibit this process. The influence of other hormones on flower induction is also dependent on photoperiodic requirements. Numerous studies have shown that ethylene (ET) and jasmonates (JAs) usually inhibit generative development. In the case of abscisic acid (ABA), a dual role in photoperiodic flowering was also recognized. The unclear role of individual hormones in the induction of flowering results from the fact that they interact in many ways. It is indisputable, however, that these small molecules are an important element coordinating the transformations leading to the generative induction of plants. On one hand, photoperiod regulates hormone levels and affects their signal transduction pathways, while on the other hand, the steady balance between hormonal stimulators and inhibitors of flowering determines the activation or inhibition of gene expression associated with generative induction pathways.