ZHU Shi-ping ,HUANG Tao-jiang,YU Xin ,HONG Qi-bin ,XIANG Jin-song,ZENG An-zhong,GONG Gui-zhi ,ZHAO Xiao-chun

1 Citrus Research Institute,Southwest University/Chinese Academy of Agricultural Sciences,Chongqing 400712,P.R.China

2 National Citrus Engineering Research Center,Chongqing 400712,P.R.China

3 Fengjie Navel Orange Research Institute,Chongqing 404699,P.R.China

Abstract Selection of rootstock is very important for citrus production.Besides its major role on resistance,the rootstock also can affect fruit production and quality.Currently,the main concerns on selection of rootstock for citrus production are compatibility and resistance,due to less information on the impacts of rootstock to the performance of scion varieties.This study aims to provide information on performances of navel orange varieties on different rootstocks.Three late-ripening navel orange varieties (Citrus sinensis var.Powell,Chislett and Banfield) grafted on seven rootstocks (Swingle citrumelo (C.paradisi×P.trifoliata),Carrizo citrange (C.sinensis×P.trifoliata),X639 (C.reticulata×P.trifoliata),MXT (C.sinensis×P.trifoliata),Hongju (C.reticulata),Ziyang Xiangcheng (C.junos) and trifoliate orange (P.trifoliata)) were used as plant materials for comprehensive comparison of the performances on tree growth,fruit yield and quality in 21 scion-stock combinations.Investigation was carried out in these combinations in field nine years after planting.Vigorous growth of all the three late-ripening navel orange varieties was observed on Carrizo citrange with the largest canopy volume at 33.34 m3 and the highest yield at 29.43 kg per tree,but a low yield efficiency at 2.87 kg m-3.On the contrary,those on trifoliate orange had the smallest canopy volume at 10.79 m3 and the lowest fruit yield at 12.51 kg per tree,but the highest yield efficiency at 3.95 kg m-3.Rootstocks did not show significant effects on fruit size,fruit shape index,peel thickness and the edible rate of the fruits,but fruit quality was significantly affected by the rootstocks.Fruits from the trees grafted on trifoliate orange presented the best quality with significantly higher total soluble solids (TSS) content than those on Ziyang Xiangcheng and Hongju,and also the highest ratio of TSS/titratable acidity (TA).The TA content was observed from the fruits on X639 at 0.59 g 100 mL-1.Vitamin C (Vc) content of fruits on Hongju was the highest at 49.25 mg 100 mL-1.Growth vigor of the trees was positively correlated with fruit yield at an extremely significant level.The canopy volume was negatively correlated with yield efficiency,but positively correlated with compatibility index.Results of this study indicated that the rootstock has great impacts on the growth vigor of the tree,yield efficiency and quality of the fruit.In order to achieve good quality and yield efficiency for navel orange production,less growth vigor rootstock such as trifoliate orange is highly recommended.

Keywords:rootstock,late-ripening navel orange,growth vigour,yield,fruit quality

1.Introduction

Rootstock not only plays vital roles in improving the resistance to biotic and abiotic stresses of the plants (Zhenget al.2011；Kostopoulou and Therios 2014；Hippleret al.2016；Hussainet al.2018),but also affects the size of canopy,fruit yield and quality (Emmanouilidou and Kyriacou 2017；Continellaet al.2018),even the production of primary and secondary metabolites (Hemmatiet al.2018；Nabilet al.2018；Albrechtet al.2019).There are several types of rootstocks used in citrus production in China.Trifoliate orange(Poncirus trifoliata) is the most common rootstock.It has been widely used in citriculture in China for over 700 years,because of its good compatibility,cold tolerance and CTV resistance (Zhuet al.2013).Hongju (Citrus reticulata)is another important traditional rootstock mainly used for mandarins and middle-term-ripening oranges (Shen 2017).Ziyang Xiangcheng (C.junos) is a newly developed citrus rootstock with strong tolerance to both alkaline and acidic stresses (Liuet el.2008；Nong 2018).Since the 1980s,several hybrid rootstocks have been introduced from overseas such as Swingle citrumelo (C.paradisi×P.trifoliata)and Carrizo citrange (C.sinensis×P.trifoliata).

In general,the selection and use of rootstock are mainly based on its compatibility,soil conditions of orchard and tradition of local citriculture.Most of the rootstocks were only used to a certain type of scion varieties,for example,Ziyang Xiangcheng and Hongju are used mainly for mandarins and middle-term-ripening oranges in China (Shen 2017；Liuet al.2019).Swingle citrumelo,Carrizo citrange,X639(C.reticulata×P.trifoliata) are important and widely used rootstocks for juice processing oranges in United States of America.Effects of rootstock on the performance of sweet orange varieties have been reported.Chunet al.(2010) compared the fruit quality of Jincheng sweet orange(C.sinensis) on 11 different rootstocks and found that Sunchusha mandarin (C.reticulata),Carrizo citrange and C35(C.sinensis×P.trifoliata) were the best rootstocks.Carrizo and trifoliate orange (Rich 16-6) demonstrated significant effects on growth of tree,fruit yield and quality of Hamlin sweet orange (C.sinensis) (Zhenget al.2010).

In China,middle-term-ripening citrus maturing from November to December,contributes approximately 57% of the citrus production.This has been a problem of excessive citrus production supplied to the market during this period of time.Increasing the production of late-ripening citrus to produce more fresh citrus from January to June are very important for supplying citrus all year around to fresh fruit market,and maintaining the sustainable development of citrus industry.The production of late-ripening citrus is mainly in Southwest China.Selection of suitable rootstocks for this region is important to ensure the good production and quality of the citrus fruit.

In this study,the performance of three late-ripening navel orange varieties,Powell,Chislett and Banfield,grafted on seven different citrus rootstocks were investigated to provide information for selection of rootstocks and for the production of late-ripening navel oranges.

2.Materials and methods

2.1.Plant materials

In October 2006,20 trees of each combination of the three late-ripening navel orange varieties with seven rootstocks were prepared by bud-grafting the scion onto one-year-old rootstock seedlings.In October 2007,14 uniform nursery trees of each scion-stock combination were planted in field in Fengjie County,Chongqing,China.Fengjie is located beside the Yangtse River with an average annual temperature of 17.7°C,1 470.3 hours of sunlight,and 918.5 mm of precipitation.The altitude of orchard is 342 m,the pH of soil is 5.8.The density of planting is 3.0 m×4.0 m.The three late-ripening navel orange (C.sinensis) varieties are:Powell,Chislett and Banfield.The seven rootstocks are:Swingle citrumelo (C.paradisi×P.trifoliata),Carrizo citrange(C.sinensis×P.trifoliata),X639 (C.reticulata×P.trifoliata),MXT (C.sinensis×P.trifoliata),Hongju (C.reticulata),Ziyang Xiangcheng (C.junos) and trifoliate orange (P.trifoliata).

2.2.Determination of vegetative growth of the tree and fruit yield

Maturity of three late-ripening navel orange varieties is at late March to early April.To study the performances of plants at full bearing stage with stable production,the investigation was conducted in 2016,approximately nine years after planting.The circumferences of trunk were measured at 5 cm above and below the graft union,and termed as Csand Cr,respectively.The compatibility index(CI) was calculated with the equation CI=Cs/Cr.The height of the tree and diameters in both parallel and perpendicular directions of the tree to the row were measured as H,Dland Dr,respectively.The shape of the citrus tree is close to a half prolate spheroid,thus the tree volume was calculated with the equation:V=(π/6)×H×Dl×Dr(Zekri 2000).The yield of fruit per tree (Y) was measured as kilograms per tree (kg per tree) at maturation (April 2,2018).The yield efficiency (YE) was calculated with the equation:YE=Y/V,to represent the weight (in kg) of fruits per volume (in m3) of the tree (Cantuarias-Avileset al.2010).The parameters of each scion-stock combination were measured with 10-14 trees,with each tree as a biological repeat.

2.3.Determination of morphological and quality parameters of fruit

Fruits were harvested on April 2,2017.Ten fruits were collected from different directions of the tree as one sample.Three samples were randomly harvested from each scion-stock combination as three biological repeats.Fruit quality was determined according to the standards of GB/T 8210-2011.The fruit weight,the equatorial diameter and the height of fruit were termed as Gw,Geand Gh,respectively,therefore the fruit shape index (FSI) was calculated with the equation:FSI=Ge/Gh.The thickness of the peels was measured with vernier calipers at position of equatorial cut.The edible rate was the proportion of the fruit excluded the peel.The total soluble solids (TSS) content of the juice was measured by a temperature-compensated digital refractometer (ATAGO,Japan) and expressed as a percent.The titratable acid (TA) was determined by titrating 10 mL of juice dilution (10× dilution) with 0.1 N NaOH(pH=8.1).The amount of vitamin C (Vc) was measured with 2,6-dichlorophenolindophenol sodium salt titration.A total of 0.5 mL juice and 4.5 mL 1% oxalic acid were titrated with 0.08 g L-12,6-dichlorophenolindophenol sodium salt.The invert sugar (Sugar) was determined with potassium sodium tartrate and copper sulfate.To make invert sugar solution,10 mL of juice,40 mL of ddH2O and 5 mL of 5%HCl were added into 100 mL volumetric flask and kept in thermostatic water bath at 80-90°C for 30 min,and then one drop of phenolphthalein indicator was added.40% sodium hydroxide was added to neutralize the HCl,and then ddH2O was used to make up the volume to 100 mL.Invert sugar solution was titrated against Fehling mixture containing both potassium sodium tartrate and copper sulfate using methylene blue as an indicator.The parameters were measured in three replicates.

2.4.Data analysis

The data were prepared with Microsoft Excel.Statistical analysis was performed using SAS8.0 atP＜0.05.The significance of differences among the data was determined with Duncan's multiple-range test.The correlation matrixes of the different variables was achieved by the Pearson method using corr.test function of CRAN (Comprehensive R Archive Network,package psych) and plotted using ggcor function of CRAN (package ggplot2).

3.Results

3.1.Tree canopy and CI

Fig.1 Tree canopy of three late-ripening navel orange varieties grafted on seven different rootstocks.Values with different lowercase letters are significantly different among the seven rootstocks at P＜0.05.Error bars show SE of the means of replicates.

Growth of trees was significantly affected by different rootstocks (Fig.1).For Powell navel orange,trees on Carrizo citrange demonstrated the most vigorous growing at 11.24 m3of canopy volume,followed by Ziyang Xiangcheng,MXT,and X639 with 9.51,9.21 and 8.21 m3of canopy volume,and trees on Swingle citrumelo and Hongju were in medium size of 7.64 and 6.41 m3,while those on trifoliate oranges had the smallest canopy of 3.19 m3.For Chislett,tree canopy on Carrizo citrange was the largest.However,there was no significant difference among the rootstocks of Ziyang Xiangcheng,MXT,X639,Swingle citrumelo,and Hongju,but the canopy on trifoliate orange was significantly smaller than those of all the other rootstocks.For Banfield,trees on Ziyang Xiangcheng had the largest canopy at 12.73 m3,followed by Carrizo citrange,Hongju,MXT,Swinge citrumelo,and X639,and canopy on trifoliate orange was the smallest among all the rootstocks with significant differences.Over all,for the three late-ripening navel orange varieties,Carrizo citrange was the most vigorous rootstock to promote the vegetative growth of the tree,followed by Ziyang Xiangcheng,MXT,Hongju,Swingle citrumelo,and X639.The smallest canopy was observed from the trees on trifoliate orange,which was only 32.4% of Carrizo,34.1 and 40.9% of Ziyang Xiangcheng and Hongju,respectively.

CI,the ratio of the trunk circumferences between scion and rootstock (scion-stock),indicating relative growth of the trunks between scion and rootstock.To some extent,it is one of the parameters to determine the compatibility(Bisioet al.2003).In this investigation,seven rootstocks showed significant differences on compatibility with the three late-ripening navel orange varieties (Fig.2).The total index of Hongju with the three late-ripening navel orange varieties was 2.74,significantly higher than those of other rootstocks,followed by Ziyang Xiangcheng,Carrizo citrange and MXT.That of X639 was medium,followed by trifoliate orange.While the total index of Swingle citrumelo was the least at 1.97,significantly less than that of the other rootstocks.

The compatibility of seven rootstocks among the three late-ripening navel orange varieties were significantly different.The CI of Powell on Hongju was the highest at 0.89,followed by Ziyang Xiangcheng,Carrizo citrange,MXT,X639,trifoliate orange and Swingle citrumelo.However,the CI of Hongju was not significantly different from that of Ziyang Xiangcheng,but significantly higher than those of the other five rootstocks.The CI values of trifoliate orange and Swingle citrumelo were significantly lower than those of the other five rootstocks.

For Chislett,the CI of Hongju was also the highest at 0.93 among seven rootstocks with significant differences.CI values of both trifoliate orange and Swingle citrange were the same at 0.69,significantly lower than those of other five rootstocks.For Banfield,the CI of Hongju was 0.92,followed by Ziyang Xiangcheng,Carrizo citrange,MXT and X639.The CI of Swingle citrumelo and trifoliate orange were 0.66 and 0.70,respectively,significantly lower than those of all the other rootstocks.

3.2.Fruit yielding and yield efficiency

Fig.2 Compatibility index of three late-ripening navel orange varieties grafted on seven different rootstocks.Values with different lowercase letters are significantly different among the seven rootstocks at P＜0.05.Error bars show SE of the means of replicates.

Rootstocks significantly affected the fruit yield (Fig.3).Trees on trifoliate orange had significantly lower yield than those on the other six rootstocks.However,the differences on fruit yield among the other six rootstocks were not significant.Three scion varieties demonstrated different yield potential on different rootstocks.For Powell,fruit yield on Carrizo was the highest at 29.07 kg per tree,followed by Ziyang Xiangcheng,Swingle citrumelo,MXT and X639,which were between 20.82-27.05 kg per tree,but without significant difference among them.The yield on trifoliate orange was the lowest at 11.46 kg per tree,significantly lower than those on the above five rootstocks,but not significantly less than that on Hongju at 17.03 kg per tree.For Chislett,the yield on trifoliate orange was the lowest at 14.21 kg per tree,significantly lower than that on other rootstocks,which were 27.5-29.5 kg per tree.There was no significant difference among the other six rootstocks.For Banfield,the yield on X639 was the highest at 34.32 kg per tree,but without significant difference with Carrizo citrange and Swingle citrumelo.The yield on Hongju,Ziyang Xiangcheng and MXT was medium at 26.77,25.98 and 24.06 kg per tree,respectively,significantly lower than the previous three rootstocks.The yield on trifoliate orange was the lowest at 11.87 kg per tree,significantly lower than that on all the other rootstocks.Over all,for the three lateripening navel orange varieties,the yield on Carrizo was the highest at 88.29 kg per tree,followed by X639,Ziyang Xiangcheng,Swingle,MXT and Hongju between 73.3 and 86.72 kg per tree without significant difference,while that on trifoliate orange was the lowest at 37.54 kg per tree,significantly lower than that on all the other six rootstocks.

Yield efficiency is an important parameter to determine the production of fruits in a defined space.The significant effect on yield efficiency from rootstock to scion was observed.Trifoliate orange presented the highest yield efficiency with an overall value for the three late-ripening navel orange varieties at 11.85 kg m-3,not significantly different from that of Swingle citrumelo and X639 at 9.72 and 10.31 kg m-3,respectively,but significantly higher than that of Carrizo,MXT,Ziyang Xiangcheng and Hongju between 8.58 and 8.89 kg m-3(Fig.4).Yield efficiency of Powell on trifoliate orange was the highest at 4.25 kg m-3,without significant difference from Ziyang Xiangcheng,X639,Swingle citrumelo and Hongju,but significantly higher than those on Carrizo and MXT.For Chislett,the yield efficiency on X639 was the highest at 3.85 kg m-3,but not significantly higher than that on the other six rootstocks.Yield efficiency of Banfield on trifoliate orange was the highest at 4.31 kg m-3,followed by X639 at 3.44 kg m-3,without significant difference,but both of them were significantly higher than that on the other five rootstocks.Yield efficiency of Ziyang Xiangcheng was the lowest at 2.19 kg m-3,significantly lower than those on the other rootstocks.

Fig.3 Fruit yield of three late-ripening navel orange varieties grafted on seven different rootstocks.Values with different lowercase letters are significantly different among the seven rootstocks at P＜0.05.Error bars show SE of the means of replicates.

Fig.4 Yield efficiency of three late-ripening navel orange varieties grafted on seven different rootstocks.Values with different lower case letters are significantly different among the seven rootstocks at P＜0.05.Error bars show SE of the means of replicates.

3.3.Fruit quality

The effects of seven rootstocks on fruit characteristics of the three late-ripening navel orange varieties were shown in Table 1.There was no significant difference on either average or individual value of fruit weight of three late-ripening navel orange varieties among different scionrootstock combinations,indicating that those rootstocks had no significant effect on fruit weight to the three scion varieties.

The variations of average values of fruit shape index,thickness of peel and edible rate were not significant among seven rootstocks.However,the effect of the seven rootstocks to individual scion on those parameters was significant.Fruit shape index of Powell navel orange on Swingle citrumelo presented the highest value at 0.96,which was significantly higher than that on MXT with the lowest value at 0.91.Fruit shape index of Chislett on Ziyang Xiangcheng was the highest at 0.97,while those on Swingle and Carrizo were the lowest both at 0.93 with significant differences.Fruit shape index of Banfield on Swingle citrumelo,X639 and trifoliate orange was the highest at 0.97,without significant difference to Ziyang Xiangcheng and Hongju at 0.94 and 0.95,respectively,but significantly higher than the lowest value on MXT at 0.92.The thicknesses of fruit peel of Powell on X639 and Swingle were 0.63 and 0.62 cm,significantly thicker than those on Ziyang Xiangcheng and MXT at 0.54 and 0.56 cm,respectively,but without significant differences from that on other three rootstocks.Fruit peel of Chislett on Ziyang Xiangcheng was the thickest at 0.63 cm,while that on MXT was the thinnest at 0.51 cm,but the difference was not significant.Also,there was no significant difference on the peel thickness of Banfield among the seven rootstocks.There were no significant differences on edible rate of Powell fruit among the rootstocks.Chislett fruit on Swingle citrumelo had the highest edible rate at 72.9%,while that on Ziyang Xiangcheng was the lowest at 68.35%,significantly lower than that on Swingle citrumelo.Banfield fruit on X639 had the lowest edible rate at 67.56%,significantly lower than that on the other rootstocks at 70.33-72.29%.

Table 1 Comparisons on fruit quality of three late-ripening navel orange varieties grafted on seven different rootstocks

The quality of fruit,presented by the parameters of TSS,sugar,TA,TSS/TA,sugar/TA and Vc was significantly affected by rootstocks.Overall value of evaluation indicated that fruits from the tree on trifoliate orange possessed the highest content of TSS at 12.61%,while that on Ziyang Xiangcheng and Hongju were the lowest at 11.55 and 11.52%,respectively.Fruits of Powell navel orange on trifoliate orange had the highest TSS at 12.87%,while TSS of which on X639,MXT and Hongju were 11.53,11.7 and 11.57%,respectively,significantly lower than that on trifoliate orange and Carrizo,but not significantly different from Swingle and Ziyang Xiangcheng at 12 and 11.97%,respectively.For Chislett fruits,TSS on Carrizo was the highest at 12.27%,on Ziyang Xiangcheng was the lowest at 10.77%,with significant differences.For Banfield fruits,TSS on trifoliate orange was the highest at 12.77%,but not significantly different from that on Swingle citrumelo,Carrizo citrange,X639 and MXT at 12-12.63%.TSS on Hongju and Ziyang Xiangcheng were significantly lower than that on the other rootstocks at 11.9 and 11.3%,respectively.

Sugar content was greatly affected by the rootstocks.In general,sugar contents of fruits on trifoliate orange and Carrizo citrange were significantly higher than those on Ziyang Xiangcheng and X639,and higher than those on the other three rootstocks,but without significant difference.With all three scion varieties,sugar contents of fruit on both trifoliate orange and Carrizo citrange were the highest among the seven rootstocks,significantly higher than the lowest appeared in the fruits on X639,Ziyang Xiangcheng and Hongju.

Rootstocks significantly affected the titratable acidity(TA) of scion fruits.MXT and X639 had the greatest effect on TA of the fruit with the highest and lowest values at 0.67 and 0.59 g 100 mL-1,respectively.Seven rootstocks demonstrated significantly different effect on TA to individual scion variety.No significant difference was found on TA of Powell fruits among rootstocks.Chislett fruits on Swingle citrumelo had the highest TA value at 0.65 g 100 mL-1and on X639 was the lowest at 0.55 g 100 mL-1,with significant differences.For Banfield,TA value of fruits on MXT was the highest at 0.68 g 100 mL-1,but without significant difference among MXT,Swingle citrumelo,Carrizo citrange,Ziyang Xiangcheng and trifoliate orange,while that on X639 and Hongju was significantly lower at 0.57 g 100 mL-1.

The ratio of TSS to TA (TSS/TA) is an important character of the fruit representing the fruit flavor.Similar to TSS and TA,rootstocks showed a strong influence to the TSS/TA of scion fruit.The overall values of TSS/TA of the three scions on trifoliate orange and MXT were the highest at 21.19 and lowest at 18.12,respectively,with significant differences.For Powell fruits,TSS/TA values on trifoliate orange and MXT were the highest at 21.54 and the lowest at 16.92,respectively with significant differences,while there was no significant difference among the other rootstocks.For the fruits of Chislett,TSS/TA values on trifoliate orange and Swingle citrumelo were the highest at 21.24 and lowest at 18.23,respectively,with significant differences.For Banfield,TSS/TA values of fruits on X639 and Hongju were the highest at 20.97 and the lowest at 18.16,respectively,but without significant differences.

The ratio of Sugar to TA,Sugar/TA of three scion varieties on trifoliate orange was the highest at 17.18,significantly higher than those on Swingle citrumelo,MXT and Ziyang Xiangcheng.For Powell,Sugar/TA of fruits on trifoliate oranges was the highest at 17.13,significantly higher than the lowest at 13.46 on MXT.However,there was no significant differences for Sugar/TA of fruit among the different rootstocks with Chislett and Banfield.

The highest and lowest contents of Vc were observed in fruits of the three varieties on Ziyang Xiangcheng and Swingle citrumelo at 49.25 and 44.31 mg 100 mL-1,respectively,with significant differences.The lowest values were found in all the three varieties on Swingle citrumelo.The highest contents of Vc varied among the three varieties,as Powell on Ziyang Xiangcheng at 47.9 mg 100 mL-1,Chislett on Hongju at 50.85 mg 100 mL-1and Banfield on X639 at 51.25 mg 100 mL-1.The significant differences were only presented between the highest and lowest rootstocks for all the three varieties.Comparisons of the parameters such as fruit size,TSS,TA,peel thickness,edible rate,sugar content and Vc content clearly indicated fruit quality of the three late-ripening navel orange varieties grafted on trifoliate orange was the best,that on Hongju was the worst,while that on the other fourPoncirushybrids and Ziyang Xiangcheng was medium.

3.4.Correlation among the parameters

Fruit yield (Y) was found extremely and significantly associated with the size of tree canopy with correlation coefficient at 0.81,which means the larger the tree volume,the higher the fruit yield (Fig.5).Among the three lateripening navel orange varieties on the seven rootstocks,the relationship between the two parameters was extremely significant at 0.98,0.70 and 0.87,respectively.However,the volume of canopy was negatively correlated to the YE at an extremely significant level of -0.61.Significance of this relationship was different among the three varieties.The canopy volume was extremely significantly related to the YE in Powell navel orange at -0.83,but not significant in Banfield and Chislett,with correlation coefficient at -0.65 and -0.45,respectively.In the view of overall values of three late-ripening navel orange varieties,the extremely significant correlation existed between canopy volume and CI at 0.6.However,for individual scion variety,this correlation only presented in Banfield navel orange at 0.88 at an extremely significant level,but not significant in Chislett and Powell at 0.61 and 0.30,respectively.

Fig.5 Correlation analysis of the parameters on horticultural characteristics.A,three navel orange varieties overall.B,Powell.C,Banfield.D,Chislett.Y,yield；YE,yield efficiency；CI,compatibility index；FW,fruit weight；FSI,fruit shape index；PT,peel thickness；TSS,total soluble solids；TA,titrable acid；Vc,vitamin C；SugTAR,Sugar/TA；TSSTAR,TSS/TA；EDR,edible rate.＊and ＊＊ represent the correlation of the parameters at P＜0.05 and P＜0.01,respectively.

TSS was extremely significantly correlated with sugar content at 0.95 with overall value of three varieties.The same strong interrelationship also presented between TSS/TA (TSSTAR) and sugar/TA (SugTAR).TA was negatively correlated with both TSSTAR and SugTAR at extremely significant levels for the three scion varieties.However,for the individual varieties,there were extremely significant differences or significant differences in Powell and Banfield,but not in Chislett.The thickness of the fruit peels (PT)was negatively correlated with the edible rate (EDR) at extremely significant level of -0.72 in overall value,and significant levels of -0.71 and -0.80 in Banfield and Chislett,respectively,but not significant in Powell at -0.66.The significantly negative correlation was also found between CI and TSS at -0.43.However,for the three varieties,the correlation between CI and TSS was only significant in Banfield at -0.78,but not in Chislett and Powell.

4.Discussion

The characteristics of tree and fruit are greatly affected by the rootstocks.In this study,the effects of rootstocks on the performances of horticultural characteristics and fruit production of scion varieties have been observed.For vegetative growth,Poncirushybrid rootstocks demonstrated stronger effect to the scion varieties.All three late-ripening navel orange varieties on Carrizo citrange possessed the largest canopy,significantly higher than those on Swingle citrumelo.This result is similar to the results observed in Okitsu satsuma mandarin (Cantuarias-Avileset al.2010)and Folha Murcha sweet orange (Cantuarias-Avileset al.2011),but opposite to that in Tarocco Scirè pigmented orange (Continellaet al.2018).Scions onPoncirushad a significant smaller canopy size than those on Swingle citrumelo,Carrizo citrange and otherPoncirushybrids.This observation was consistent with the results studied in Folha Murcha (Cantuarias-Avileset al.2011).Nodaet al.(2000)compared the IAA and the ABA levels in the new shoot and fibrous roots of the Eureka lemon grafted on the dwarfing trifoliate orange Flying dragon,the common trifoliate orange and the comparatively more vigorous rootstock Swingle and found that the vigorous rootstock induced higher IAA-ABA ratio in the new shoot.Llisoet al.(2004) reported that the reduced growth of Navelina sweet orange on the dwarfing rootstock #23 and FA418 was not caused by hormonal deficiency but by the enhanced reproductive development and fruit growth.Gaona-Ponceet al.(2018) found that the xylem vessels with greater area,lower density,greater fiber length and xylem radiuses contributed to the vigorous growth of Tahiti lime on Sour orange and Volkamer rootstocks.The dwarfing mechanism of trifoliate orange need further elucidation.

The compatibility was influenced by the genotypes of rootstocks.The closer genetic relationship to the scion variety,the better compatibility (Nitoet al.2005).Swingle citrumelo andPoncirusdemonstrated lower CI to all three late-ripening navel orange varieties than other rootstocks.The CI of the navel oranges grafted on Swingle citrumelo was the lowest,followed byPoncirus,indicating that trunks of Swingle andPoncirusgrew faster than that of navel oranges due to less compatibility caused by distant genetic relationship from scion varieties.Low CI (scionstock) of Swingle citrumelo with Navelina navel orange and Tongxianyou was also reported by Cruzet al.(2019) and Jianget al.(2004),respectively.The combination of Hamlin/Poncirusalso had lower CI compared to that of Hamlin/Carrizo citrange (Zhenget al.2010).

Effects on fruit production by rootstocks were similar to that of compatibility,butPoncirusgave the lowest fruit yield to all three late-ripening navel orange varieties.However,the effect on yield efficiency was opposite.Poncirusdelivered the highest yield efficiency,which was significantly higher than those of Carrizo citrange,MXT,Ziyang Xiangcheng and Hongju.Cantuarias-Avileset al.(2011) also reported that the cumulative yield of Folha Murcha sweet orange on dwarfing trifoliate Flying Dragon was significantly lower than that on both Swingle citrumelo and Carrizo citrange,but the yield efficiency was much higher.The results suggested that selection of rootstock is directly related to the model of planting.Ponciruscan be applied to dense planting,while the Carrizo citrange,MXT and Ziyang Xiangcheng are better for spars planting.

Fruit quality was also obviously affected by rootstocks.For all the three scion varieties,TSS onPonciruswas the highest,significantly higher than those on both Hongju and Ziyang Xiangcheng,while that on the fourPoncirushybrids was medium.In previous studies,Poncirusalso demonstrated significantly strong effect to TSS in Okitsu satsuma mandarin (Cantuarias-Avileset al.2010),Hamlin sweet orange (Zhenget al.2010),Folha Murcha sweet orange (Cantuarias-Avileset al.2011),Common Clementine(Hussainet al.2013) and Shatangju tangerine (Liuet al.2016),but opposite in Jincheng (Chunet al.2010).In Shatangju tangerine fruits,Liuet al.(2015) suggested that the content of both TSS and sugar on trifoliate orange was higher than those on Canton lemon rootstock,and the activities of sucrose synthase was also higher.The sucrose synthase gene (SS2) and the sucrose transport gene (SUC3) expressed higher in fruits of the combination Shatangju/trifoliate orange than those of Shatangju/Canton.The ABA concentration was also higher in fruits of Shatangju/trifoliate orange.NCED1(9-cis-epoxycarotenoid dioxygenase gene),encoding the rate-limiting enzyme of ABA synthesis expressed also higher.These results indicated that rootstocks could affect sugar accumulation in fruits by the influence of the activities and the expression of the genes related to sucrose metabolism,sucrose transport and ABA levels.

The strong interrelationship between some parameters have been revealed in this study.The fruit yield was positively associated with the volume of canopy.On the contrary,the yield efficiency was negatively related to the volume of canopy.This agreed with the report of Zhenget al.(2010) from the study of performance of Hamlin sweet orange grafted on different types of rootstocks.In the report of Continellaet al.(2018),Tarocco Scirè grafted on C35,Furr and Carpenter possessed large canopy,high yield and low trunk cross-section area (TCSA),but high yield efficiency.Sauet al.(2018) reported the positive correlation between the canopy size and fruit yield,but the coefficient of correlation was not significant.In general,the combination ofCitrus/Ponciruspossess much lower CI value.The TCSA was significantly positively correlated to the fruit yield in the investigation of Sauet al.(2018).However,this correlation was not significant in our study.This may be due to the different scions and rootstocks used in the two studies.This study provided more detailed information on the effect of rootstocks to the performance of scions.The fruit production of scion-stock combinations could be predicted based on the canopy size.The interrelationship among the horticultural parameters is useful information for selection of rootstocks.

Although using trifoliate orange as the rootstock could harvest fruit with higher quality and grow trees with comparative dense planting to use the space with higher efficiency,its CI was low and it is incompatible to some varieties.The good fruit quality and vegetative growth of the fourPoncirushybrids showed their potential use for sweet oranges.The good compatibility and tolerance to alkaline stress of Ziyang Xiangcheng and toCitrus excocortis viroidof Hongju indicated their good acceptance by citrus growers in some regions.

5.Conclusion

Citrus rootstocks have strong effects on the horticultural,productive and qualitative performances of late-ripening navel orange varieties.The canopy was positively correlated with fruit yield and CI,but negatively to yield efficiency.Trifoliate orange,which had the lowest canopy volume and the highest yield efficiency,presented the highest TSS.The results indicated that trifoliate orangewould be the best rootstock for navel oranges in terms of fruit quality.The information of interrelationships among the horticultural,productive and qualitative parameters revealed in this study could help to predict the performances of citrus tree and select the most suitable rootstocks.

Acknowledgements

This research was financially supported by the National Key R&D Program of China (2018YFD1000101 and 2018YFD0201503),the earmarked fund for China Agriculture Research System (CARS-26),the Fundamental Research Funds for the Central Universities,China(XDJK2016B024),the National Citrus Engineering Research Center,China (NCERC),the earmarked fund for Chongqing Special & Economic Agriculture Research System on Late Maturation Citrus,China,and the Basic Research and Frontier Exploration Projects in Chongqing,China(cstc2018jcyjAX0400).