Study On Growth Difference And Dwarfing Mechanism Of Various Citrus Rootstock/Scion Combinations

Citrus is the most important fruit tree in the world.It is widely planted in the southern part of China.‘Shatangju’mandarin(Citrus reticulata Blanco cv.Shatangju),also known as ‘Shiyueju’,is native to Sihui city,Guangdong province and is one of the main cultivated varieties in South China.Most fruit trees are propagated by grafting,rootstock have a significant effect on the growth of fruit tree,however,the underlying mechanism by which rootstocks affect the growth of citrus scion are still unclear.In this study,‘Shatangju’ mandarin was used as the experimental material,which was grafted onto 13 different rootstocks: Sunki mandarin(Citrus reticulata Blanco),Carrizo citrange [Citrus sinensis(L.)Osbeck × Poncirus trifoliata(L.)Raf.],Ichang papeda(Citrus ichangensis Swingle),Trifoliate orange [Poncirus trifoliate(L.)Raf],Canton lemon(Citrus limonia Osb.),Goutou orange(Citrus aurantium L.),Sour orange(Citrus aurantium L.),Red tangerine(Citrus reticulata Blanco),Rough lemon(Citrus jambhiri Lush),Pomelo(Citrus maxima(Burm.)Merr.),Fragrant orange(Citrus Junos Sieb.ex Tanaka),‘Flying dragon’ trifoliate orange [P Poncirus trifoliate(L.)Raf.var.monstruosa],Shatangju’mandarin(Citrus reticulata Blanco cv.Shatangju).The vigor of different rootstock/scion combinations were observed and the dwarfing mechanism was clarified from physiological and molecular levels.The main results are as follows:1.Trees with the Rough lemon,Fragrant orange,Pomelo rootstock have the larger scion height,shoot length,rootstock diameter,scion diameter,and trunk cross-section area(TCSA).The rootstock/scion ratio of ‘Flying dragon’trifoliate orange,Ichang papeda,Pomelo rootstock were significantly higher than the others.In conclusion,Rough lemon,Fragrant orange,Pomelo can be used as vigorous rootstocks,while Ichang papeda,Trifoliate orange,‘Flying dragon’trifoliate orange can be used as dwarfing rootstocks in our study.2.The trees grafted on the vigorous rootstocks have higher net photoassimilates(A)and water use efficiency(WUE).Total chlorophyll was significantly higher in the leaf of ‘Shatangju’mandarin on Rough lemon,Pomelo,Trifoliate orange rootstock and lower on Ichang papeda,‘Flying dragon’ trifoliate orange,and self-rooted rootstock.The net photoassimilate rate(A)of the ‘Shatangju’mandarin grafted on the Fragrant orange was larger,while on the Ichang papeda and the Sunki mandarin were lower.Scions on the Fragrant orange,Sour orange,self-rooted rootstock were recorded higher stomatal conductance(Gs),while the lower Gs were found in the leaves of ‘Shatangju’mandarin grafted on Canton lemon,Red tangerine,Pomelo rootstock.The maximum water use efficiency(WUE)was recorded in leaves of shoots grafted on Rough lemon rootstock followed by Pomelo,Trifoliate orange,Red tangerine rootstock,the leaves of shoots grafted on to Carrizo citrange,‘Flying dragon’ trifoliate orange,Sunki mandarin,Canton lemon had lower WUE values.Correlation analysis showed that the net photosynthetic rate(A)and water use efficiency were positively correlated with the tree height.3.Vigorous rootstock can increase the content of soluble sugar and reducing sugar in the leaves of the scion.Trees on Fragrant orange,Canton lemon,Pomelo,Sour orange,Carrizo citrange,and self-rooted rootstock have higher leaf soluble sugar content than on dwarfing rootstock ‘Flying dragon’ trifoliate orange and Ichang papeda.There is higher leaf reducing sugar content on vigorous rootstock Fragrant orange,Rough lemon and Pomelo and lower leaf reducing sugar content on less vigorous rootstock Ichang papeda and Sunki mandarin.Leaf starch content is much higher on Rough lemon than on Ichang papeda and Goutou orange.4.The correlation analysis revealed that SOD,CAT,and total phenolic are positively correlated with the tree height,while the negative correlation was found between POD and the scion heigh.‘Shatangju’Mandarin had the higher activity of superoxide dismutase(SOD)on Fragrant orange and Rough lemon,while it was lower on Sunki mandarin and Ichang papeda.Significantly higher peroxidase(POD)activity in ‘Shatangju’Mandarin grafted on dwarfing rootstock Ichang papeda and Trifoliate orange than on Pomelo and Rough lemon.Phenol compounds were higher in ‘Shatangju’Mandarin on Fragrant orange than on Carrizo citrange and Trifoliate orange.5.Compared with the vigorous rootstock,trees on the dwarfing rootstocks have lower IAA levels and higher ABA levels.The indole acetic acid(IAA)levels in the scion on the vigorous rootstocks Rough lemon,Pomelo,Fragrant orange were higher,while the abscisic acid(ABA)levels were higher in Trifoliate orange,‘Flying dragon’ trifoliate orange and the lowest ABA content was observed in Rough lemon.The highest IAA: ABA ratio was observed in ‘Shatangju’mandarin/Rough lemon combination followed by ‘Shatangju’mandarin/Fragrant orange and ‘Shatangju’mandarin/Pomelo combinations and the Trifoliate orange confers the lowest IAA: ABA ratio.The concentration of IAA in the shoot and IAA: ABA ratio showed a positive correlation with vigor,while the ABA content had a negative correlation with vigor.6.The gene expressions of IAA synthesis and transport pathways was significantly down-regulated on the ‘Shatangju’ mandarin grafted on the dwarfing rootstock.The auxin synthesis genes YUC1,YUC3,YUC6,and the auxin transport genes LAX3,PIN4,PIN6 were highly expressed in the ‘Shatangju’ mandarin grafted onto vigorous rootstocks(Rough lemon,Pomelo,Fragrant orange).The higher auxin synthesis gene expression levels in the vigorous trees contribute to auxin accumulation in the scion,which may promote the plant growth.The lower expression levels of auxin transport genes LAX3,PIN4,PIN6 were suggested to limit the transport of auxin from the shoots to roots.On the contrary,the ABA synthesis genes NCED6,NCED8 expression levels were higher in the dwarfing rootstocks and lower in the vigorous rootstocks,which is negatively correlated with the tree vigor.