Effect of GA3 and ABA on cell ultra-structure of the secondary-burstingbuds in blackcurrant（Ribes nigrum）

Abstract:【Objective】Blackcurrant (Ribes nigrum L. ) is a small perennial shrub with good winter hardi⁃ness, but it has the problem of secondary bud burst on their stems occurring in late summer to autumn,which causes yield reduction by 10% to 20%. In order to elucidate the mechanism of secondary sproutingin black currant, cell ultra-structure of the secondary bursting buds in two different black currant varieties was observed and changes in response to exogenous GA3 and ABA treatments were analyzed. The studynot only provides understandings of the comprehensive revelation of secondary bud sprouting but also pro⁃vides artificial regulation in actual production.【Methods】Two different varieties,‘Yade’and‘Bajila’, inthe period of secondary bud burst were taken as materials.‘Yade’is high yielding but highly prone to secondary burst, while‘Bajila’is the opposite. After harvest, 30 mg·L^-1 GA3 and 50 mg·L^-1 ABA solutionswere sprayed onto the green leaves (on August 13th, 2016), plants sprayed with water were taken as thecontrol. Foliar spray was conducted until drip-off for two consecutive days. From the first day of treat⁃ment, the bud in the middle of the branch was collected. The buds with scales removed were observed forultra-structure under transmission electron microscopy.【Results】GA3 treatment significantly promotedsecondary bud sprouting. Under GA3 treatment, the phenomenon of plasmolysis in two varieties was disap⁃peared; plasmodesmata and endoplasmic reticulums differentiated earlier; the decomposition of starchgranules was faster; the number of mitochondria was increased; the large vacuoles formed and the electronic dense materials in vacuoles degraded. On the contrary, ABA treatment inhibited the secondary sprouting. Under the treatment, the phenomenon of plasmolysis occurred and the intercellular association waslimited. Endoplasmic reticulum differentiated later and its length was shorter; the decomposition of starchgranules was inhibited and their number increased; mitochondria were fewer and their structure was notclear. In the bud cells of‘Yade’, the electronic dense materials in the vacuoles decomposed completely,while in‘Bajila’bud, these materials maintained.【Conclusion】The occurrence of the secondary sprouting in black currant is a result of bud dormancy breaking. Exogenous GA3 treatment accelerates the release of dormancy and ABA treatment promotes dormancy maintenance. This study showed that when thecells were in the state of plasmolysis, the plasmodesmata differentiation failed to occur, and secondarybud burst would not take place; when plasmolysis disappeared, which is favorable for plasmodesmata dif⁃ferentiation, bud burst would occur. GA3 and ABA treatments had opposite effects on plasmolysis and plasmodesmata differentiation. Therefore, it was suggested that the plasmolysis and plasmodesmata formationwere the key to the secondary bud burst in black currant. There was a close relationship between decomposition of starch granules and the number of mitochondria. GA3 treatment promoted the decomposition ofthe electronic dense materials in the vacuoles and also promoted the the differentiation of endoplasmic reticulum, mitochondria and other organelles. ABA treatment slowed down the decomposition of electrondense materials in the vacuoles, which might be important for the resistance of black currant to poor environmental conditions in the fall or winter.