Introduction

Daylength is the most powerful environment factor in the regulation of the onset of flowering (Vince-Prue, 1975). Changes in photoperiod through the year provide an unambiguous index of seasonal progression. Through the ability to detect and respond to daylength, plants can tailor flowering to seasonal changes in climate, and can also ensure synchrony of flowering to facilitate outbreeding. Plants are classified as short-day plants (SDP), long-day plants (LDP), day neutral plants (DNP) or combinations of these. SDP flower in response to days shorter than a critical daylength, LDP flower in response to days longer than a critical daylength and DNP flower irrespective of daylength. Some plants show a requirement for particular sequences of daylengths or other combinations of the above basic types (Thomas & Vince-Prue, 1984). Photoperiodic requirements may themselves be the only determining factors, but frequently they are overlaid on other developmental or environmental factors, the most important being juvenility and vernalisation (Thomas, 1993).

Photoperiodic mechanisms

Daylength is generally accepted as being perceived by the leaves in both SDP and LDP, rather than in the apex where the transition to flowering occurs. When a permissive daylength is perceived, a semi-stable change in the properties of the leaves occurs. This can be demonstrated by the ability of such leaves to cause flowering when grafted to plants maintained in non-permissive daylengths (for example, Zeevaart, 1969). The change in the leaf is called induction and the molecular basis of the change is unknown.