Hormonal control of flower induction in litchi and longan

Abstract

As for many other fruit trees originating from subtropical climates, cool temperature is a key trigger of flower induction (FI) in litchi and longan. However, the expansion of fruit growing areas to lower, more tropical latitudes renders FI more unreliable, due to the lack of sufficient cool temperatures, and causes problems with alternate bearing behaviour. Fortunately, in the advent of inadequate low temperatures, it is now possible to force FI at least for longan (Dimocarpus longan Lour.) through the application of KClO3, allowing the production of off-season fruit. In contrast, there are with the exception of stem girdling currently no FI techniques for litchi (Litchi chinensis Sonn.). Consequently longan became an ideal model plant for our investigations on the hormonal regulation of FI. By comparing hormonal data from KClO3-induced field-grown longan trees with those of cool temperature induced litchi trees in growth chambers, we attempt to derive the essential hormonal changes for FI. In a first plant response, all inductive treatments/conditions resulted in a significant reduction of leaf photosynthesis. In KClO3induced longan as well as in litchi following cool temperature treatment a clear increase of cytokinin (CK) concentrations in buds is found with a concomitant reduction in concentrations of gibberellins (GAs) and auxin (IAA). Whether the raised levels of CKs in buds derived from elevated concentrations of CKs in bark and wood as a consequence of deconjugation or originated from de novo biosynthesis in the roots remains unclear. It can also not be concluded if GAs exerts their FI inhibiting effect directly or through crosstalk with IAA metabolism and therefore help to maintain the CK/IAA ratio at a level which favours floral development. However, knowing which sequence of hormonal events is necessary to induce flowering could help to develop new strategies of smart crop manipulations to improve FI and to achieve off-season production even in litchi.