Effect Of UV-B Treatment On Flowering Of Litchi (Litchi Chinensis Sonn.) And Its Mechanisms

Litchi（Litchi chinensis Sonn.）is an economically important ever-green fruit in southern China.Litchi floral initiation is influenced by environmental factors including temperature,soil moisture and photoperiod.UV-B can serve as a signal to regulate plant metabolism,establishment of morphogenesis and development,but the effect of UV-B on litchi flowering is not clear.In the present study,potted‘Feizixiao’litchi plants were treated with UV-B in a green house,to investigate the influence of UV-B treatment on litchi flowering.The main results were follows:1,Under room temperature（30℃）,exposure of litchi plants to different intensities of UV-B（intensity of 118 m W/m² or 710 m W/m²,8 hours per day）did not alter the SPAD value,photosynthesis and POD enzyme activity in leaves.Under flowering-inductive low temperature（15℃,35d）,the stomatal conductance,transpiration rate,and net photosynthetic rate of litchi leaves decreased,while the intercellular CO₂ concentration increased.The combination of low temperature and UV-B（intensity of 60 m W/m² or 130m W/m²,8 hours per day）showed rare effect on litchi leaf morphology,SPAD,photosynthetic physiology and antioxidant system,but the intensity of 130 m W/m² UV-B increased the contents of H₂O₂ and soluble sugars content in leaves,and eventually increased the ratio of flowering plans and pure inflorescence to 94%and 93%,respectively,in comparison with 78%and 43%of that in the control.When the intensity of UV-B increased to 1200 m W/m²,the net photosynthetic rate of leaves was significantly decreased and litchi flowering was highly suppressed.However,the content of flavonoid and POD enzyme activity were increased.2,The expression of Lc FT1 gene in leaves determined litchi flowering.The expression of Lc FT1 were induced under flowering-inductive low temperature and decreased under high temperatures（25^oC）.The intensity of 130 m W/m²UV-B treatment promoted the expression of Lc FT1 and flowering without alteration of flowering time.However,higher intensity of UV-B（1200 m W/m²）strongly suppressed the expression of Lc FT1 and flowering.There were two UV-B receptors genes,Lc UVR8-1 and Lc UVR8-2,were cloned in litchi,which could be classified into RCC1 super family based on amino acid sequence analysis.However,UV-B treatment didn’t influence the expressions of these two receptor genes and COP1.In conclusion,under room temperature UV-B showed no obvious effect on litchi flowering;under flowering-inductive low temperature,UV-B showed dosage-dependent effect on litchi flowering,namely,moderate UV-B treatment promoting litchi flowering and intense UV-B suppressing flowering.UV-B treatment could regulate the expression of Lc FT1 rather than the Lc UVR8 genes,so the underlying molecular mechanism needs to be further studied.