Effects Of Exogenous 24-epicastasterone On Apple Tree Growth And Flower Bud Differentiation

In the cultivation of‘fuji’apples,the difficulty in low floral bud rate,alternate and bieenial bearing affect its economic efficiency.Most studies are based on the application of growth regulators to promote flowering.Brassinolide which is defined as the sixth new type of phytohormone.It is highly efficient and non-toxic,while it plays a regulatory role in the growth links such as plant organ differentiation and flowering time.In the experiment,24-Epicastasterone（24-Epi）was used as the exogenous hormone treatment agent,meanwhile the‘Nagafu 2’was selected as the test material.The materials were treated with the concentration of 0.2 mg/L,0.4 mg/L,1 mg/L and different times at 35 days and 39 days after full bloom（DAF）.The growth of branches and leaves after treatment was investigated,and the soluble sugar content and endogenous hormone content of spur leaves were measured.q RT-PCR analysis was also performed for the related genes of sugars,hormones,flowering and BRs.To explore the effect of exogenous 24-Epi on the growth of apple branches and leaves and the physiological differentiation.The key results were as follows.1.Effect of exogenous 24-Epi on the tree growth.（1）The chlorophyll a and total chlorophyll content of‘Nagafu No.2’leaves increased noticeably compared with the control after 35 and 39 DAF treatments of different concentrations.It was 16.73 mg/g,a significant increase of 18.7%compared to the control.Also the total chlorophyll content of the leaves of this treatment was 23.68 mg/g,which was significantly increased by 16.1%compared to the control.However,the contents of chlorophyll b and carotenoid did not vary significantly between treatments.（2）The fresh weight of leaves in the 35 and 39 DAF 0.4 mg/L treatments was 66.67 g,the highest among the control and all treatments,and the dry weight of leaves was 25.80 g.（3）All treatments inhibited the growth of annual bourse shoots.Single treatment at 35 DAF,superimposed treatments at 35 and 39 DAF with concentration of 0.4 mg/L treatment resulted in similar growth of 19.63 cm and 20.59 cm,respectively,significantly lower than control and single treatment at 39 DAF.35 d and 39 d superimposed treatments increased the proportion of short shoots but were not significantly different from control.（4）The superimposed treatment of three concentrations for two times increased the expression of the gene Mdrbc L of Rubisco enzyme on the 40 DAF,and increased the expression of the gene Mdrbc S on the 40 and 80 DAF,which was beneficial to the progress of photosynthesis.（5）The treatment of 35 and 39 DAF increased BR synthase gene Md DWF4 expression at 40 and50 DAF,however decreased expression at 60 DAF.Lower concentrations of exogenous 24-Epi treatment increased gene Md DWF4 transcript levels at 60 and 70 DAF.Expression of Md BZR1,the key transcription factor in the BR signaling pathway,increased at 60 and 70DAF.2.Effects of exogenous 24-Epi on the flower bud differentiation.The flower formation rate in the next year was 54.1%after the treatment 35 and 39 DAF with concentrations of 0.4mg/L.（1）After the treatment with this concentration,the soluble sugars in the spur leaves was significantly affected.The contents of sucrose,fructose and glucose increased significantly,and sorbitol showed a trend of first increasing and then decreasing.The transcription levels of glucose metabolism-related genes Md SUSY1,Md SPS6,Md HXK1,Md FK4,and Md S6PDH were consistent with the changes in soluble sugar content.（2）The endogenous IAA content of the leaves after this treatment decreased at 40-50 DAF,increased to a peak at 60 DAF,and then decreased again.GA₃ increased and then decreased at 40-50 DAF.The content of ABA increased significantly at 60-80 DAF.The hormone-related genes Md ARF2,Md GA20ox1,Md NCED3,and Md ABF3 appeared to be differentially expressed in response to the treatments.（3）The treatment affected flowering gene expression.The flowering gene Md FT expression was significantly higher at 50 d,70 d and 80 DAF.Meanwhile,Md SOC1 expression was significantly higher at 50-80 DAF.The treatment inhibited the expression of the flowering repressor gene Md FLC-like at 40-60 DAF and the expression of Md SVP at 40-70 DAF was lower than control.In conclusion,exogenous 24-Epi is beneficial to the photosynthesis of‘Nagafu 2’tree.Simultaneously,it provides accumulation of nutrients for flower bud differentiation,and cooperates with various hormones to up-regulate the expression of flower-promoting genes and thus affect apple flowering rates.In practical production applications,the superimposed treatment of exogenous 24-Epi with the concentration of 0.4 mg/L can be carried out twice during the critical period of flower bud gestation after flowers fall,which is beneficial to photosynthesis and promotes tree growth and flower bud differentiation.