A Study On The Effect Of Shading On Budbreak In Litchi And Its Mechanisms

Bud dormancy is a cyclical activity in perennial plants for surviving adverse conditions,and subject to regulations by various factors.At present,the studies of bud dormancy focus on perennial deciduous trees,and few studies on tropical subtropical evergreen trees are available.The development of litchi buds can be divided into four stages: dormant stage(S1),breaking stage(S2),rapid growth stage(S3),and growth cessation stage(S4).Light is one of the most important factors in regulating bud dormancy with light intensity effect and photoperiod effect.Previous studies have shown that bud break inlitchi is strongly suppressed by canopy shading,which raises new questions: Which are the organs that senses the light signal,leaf,bud,or both? Is the effect of light sensed by leaves on the bud development a direct effect of the light signal or an indirect effect due to light effect on photosynthetic product supply? How do leaves and buds sense light signal and regulate bud dormancy/budbreak?This study examined the effect of bud shading and leaf shading on the changes in budbreak rate,and analyzed the changes in carbohydrates during budbreak and the expressions of photoperiod responding genes,Lc CO and Lc FT,and dormancy related gene,Lc SVP2...The main results are as follows:1.Both the leaf shading and bud shading treatments resulted in significant inhibition of budbreak in different cultivars,suggesting that litchi bud dormancy removal requires both buds and leaves to be exposed to light.2.Supply sugar and amino acid(2% sucrose+0.5% glutamic acid)failed to reverse the effect of leaf shading in suppression budbreak,suggesting that the effect was not a direct result of shortage of photosynthsate supply.3.With bud dormancy removal/budbreak,Lc CO and Lc FT2 in bud were significantly upregulated,while Lc FT1 was undetectable in all treatments and all stages.Lc CO in bud was more strongly expression in the morning than in the after.Expression of both genes inthe leaves displayed obvious diurnal patterns.The expression of Lc CO peaked 6-9 am and decrease in the afternoon and night;while the peak of Lc FT2 was about 3 h beyind that of Lc CO.Both leaf shading and bud shading significantly suppressed the expression levels of both Lc CO and Lc FT2 in bud,the suppression effect increasing with shading period.Leaf shading did not change the diurnal pattern of both genes in the leaves,but significantly reduced the range of diurnal variation.With VIGS that significantly reduced the expression levels of both genes,budbreak was delayed.The results suggested that photoperiod responding CO-FT pathway is involved in regulation of bud dormancy removal/budbreak in litchi.They are positive regulators in budbreak.4.With bud dormancy removal,Lc SVP2 that maintains bud dormancy was down-regulated.However,neither leaf shading nor bud shading showed significant effect on Lc SVP2 expression.The result suggests that the light regulation of bud dormancy/ budbreak is not Lc SVP2 dependent.