Effect Of Prohexadione-calcium On The Vegetative Growth,Fruits,Flowering,Endogenous Hormones And Expression Of Ga-related Genes In ’Fuji’ Apple(Malus Domestica Borkh.)

The vegetative growth of‘Fuji’apple cultivar with an area accounting for more than65%of the total cultivated area of apple in China is vigorous,which causes a higher cost to prune,a poorer lighting conditions of canopy and a mechanized operation difficulty,so it is an urgent need to regulate terminal shoots of‘Fuji’in production.Prohexadione-calcium（Pro Ca）has been proven to regulate the shoots growth,fruits in apple abroad but fewer studies on its effect on the vegetative growth,fruits and return flowering have been reported in China.Moreover,the mechanism of its action is incompletely understood.So different concentration treatments were applied as foliar sprays to dwarf self-rooted‘Fubrax Red Fuji’/M9-T337 trees at 3,14,35 and 56 DAPF（Days after petal fall）.Endogenous hormone content,the expression of GA-related genes of leaves in long terminal shoots,shoot growth,fruit quality,yield and return flowering were analyzed.RNA-sequencing analyzed the expression levels of flowering and GA-related genes of terminal spur buds and q RT-PCR validated DEGs related to flowering and GA.This study provided a valuable insight to regulate the vegetative growth of‘Fuji’apple and explored the mechanism of Pro Ca action.The main results are as follows:1.During the growing season,spraying Pro Ca effectively inhibited the growth of shoots compared to control and with the best effect of spraying time at the average terminal shoots length was about 12 cm at 3 DAPF.The average lengths of terminal shoots at one spray of 500 and 750 mg/L Pro Ca,two sprays of 125 and 250 mg/L Pro Ca and four sprays of 62.5,125 and 250 mg/L Pro Ca were small.Moreover one spray of 500 and 750 mg/L Pro Ca and four sprays of 250 mg/L Pro Ca showed the highest reduction and the average terminal shoot length had a decrease of about 40%compared with that（50.2 cm）of the control.The differences of average bourse shoot growth among the Pro Ca treatments were small,with an average length of about 23 cm,which had a decreasing of about 34%compared with control.The results also showed that Pro Ca had very slight effect on fruit yield and quality.Both single high concentration in early stage and multiple low concentration accumulation could inhibit the vegetative growth of apple.2.During the growing season,two applications of Pro Ca of 250 mg/L on 3 and 14DAPF significantly reduced the content of GA₁₊₃,IAA and ZR,but increased the content of ABA in long terminal shoot leaves in at least one time point.And there was no significant effect on the content of GA₄₊₇.The regulation of terminal shoot growth by Pro Ca was mainly achieved by reducing the content of GA₁₊₃.3.The expression patterns of GA-regulated genes showed that Pro Ca with two applications of 250 mg/L down-regulated MdGA3ox12 transcript level in long terminal shoot leaves at 10,20,30,40 and 50 DAPF and down-regulated MdCPS,MdGA20ox1,MdGA20ox2,MdGA2ox1,MdGA2ox2,MdRGL1a,MdSLY1 transcripts in at least one time point.4.The RNA-sequence of terminal spur buds revealed that Pro Ca spraying twice significantly upregulated the expression of GA2ox2 at 30 DAF（days after full bloom）,downregulated the expression of RGL1 at 50 DAF and upregulated the expression of GA2ox8 at 70 DAF notably,the differential expression multiples of all DEGS（different expression genes）were below 2.2 and the expression levels of other GA-related genes were no significant difference.5.With compared to control,Pro Ca spraying twice significantly upregulated the expression of GI and downregulated the expression of one of AP2 and TSF at 30 DAF.Besides,Pro Ca spraying twice significantly upregulated the expression of GI and SPL5,downregulated the expression of one of SOC1,FLK and one of AP2 at 50 DAF and also significantly upregulated the expression of FLD,downregulated the expression of one of CO and SPL5 at 70 DAF.The multiples of differential expression for all DEGS were below1.6.With recording of the return flowering rate,we conclude that Pro Ca had no obvious facilitation on flowering.In conclusion,for comprehensive evaluation effects of prohexadione-calcium on Fuji apple shoots,yield,fruit quality,return flowering and the economic benefits,spraying 1-2 times of 125 mg/L and 250 mg/L prohexadione calcium at 3,14 DAPF was recommended in vigorous Fuji apple orchards in the Loess Plateau.