Acta Sci. Pol. Hortorum Cultus, 21(4) 2022, 75–81 THE INFLUENCE OF FOUR ROOTSTOCKS ON THE GROWTH, YIELD AND FRUIT QUALITY OF TWO PLUM CULTIVARS

The influence of three clonal rootstocks (‘Pixyʼ, ‘Fereleyʼ and ‘St. Julien Aʼ) along with seedlings of My robalan (control) on growth, yield and fruit quality of plum cultivars ‘Čačanska Najboljaʼ and ‘Čačanska Lepoticaʼ was studied in the Belgrade region (Serbia) for the six year-period (2013–2018). In comparison to control, clonal rootstocks have shown a significant effect on the decrease of trunk cross-sectional area. The lowest vigor was found in trees on the ‘Pixyʼ rootstock, then on ‘Fereleyʼ and ‘St. Julien Aʼ. Yield per hectare on trees grafted on clonal rootstocks was higher for 62–82% in ‘Čačanska Najboljaʼ, and for 26–27% in ‘Čačanska Lepoticaʼ compared to Myrobalan. In cultivar ‘Čačanska Najboljaʼ significantly higher fruit weight compared to control was obtained in the trees on the ‘Fereleyʼ and ‘St. Julien Aʼ rootstocks. The effects of rootstocks on the contents of soluble solids and total acids in the fruit were not significant. All three studied clonal rootstocks showed better results than standard Myrobalan, and can be recommended for establishing intensive plum plantations with higher planting density.

The aim of this study was to examine the influence of three clonal rootstocks and one seedling rootstock on the tree vigor, productivity and fruit quality of two table plum cultivars ('Čačanska Najboljaʼ and 'Čačanska Lepoticaʼ).

MATERIALS AND METHODS
Plant material. The study was conducted in the plum orchard at the Experimental Station "Radmilovac" of the Faculty of Agriculture in Belgrade (Serbia). During the six-year period (2013-2018) the influence of a seedling rootstock (Myrobalan as a control) and three clonal rootstocks ('Pixyʼ, 'Fereleyʼ, and 'St. Julien Aʼ) were studied on two table plum cultivars ('Čačanska Najboljaʼ and 'Čačanska Lepoticaʼ).
The orchard was planted in spring of 2010. Planting distance is 4 m between rows and in the row different distances were applied depending on the rootstock vigor: 2.3 m for Myrobalan seedling, 2.0 m for 'Fere-leyʼ and 'St. Julien Aʼ and 1.7 m for 'Pixyʼ. Training system is the Spindle. Standard cultural practices were applied, including drip irrigation. Every variant (cultivar/rootstock) was represented by six trees (two replications with three trees).
Methods. As a vigor indicator, trunk cross-sectional area (TCSA) was used. TCSA was calculated on the basic of trunk circumference measured at 20 cm above the graft union. As for indicators of productivity fruit set, yield tree -1 and cumulative yield efficiency (CYE) were examined. The fruit set was determined on the six selected branches (two replicates with three branches) of each cultivar/rootstock combination. Every branch contained at least 100 flowers. The fruit set was determined two weeks before harvest and it was calculated as a ratio of the number of fruits and the number of flowers, expressed in %. CYE was calculated as a ratio of the cumulative yield per tree for six years (2013)(2014)(2015)(2016)(2017)(2018) and the TCSA in the last year of investigation (2018), expressed in kg cm -2 .
Characteristics of fruit (fruit and stone weight, fruit dimensions and pedicel length) were determined by measuring of individual fruits on a sample of 60 fruits of each variant (10 fruits per tree). Fruit shape index was calculated using the formula: where: L -length; W -width; T -thickness. The soluble solids were determined using a refractometer (Pocket PAL-1, Atago, Japan). Total acids were determined by titration with NaOH and expressed as malic acid.
Statistical analysis. The results were processed statistically using the analysis of variance. The significance of differences between mean values was evaluated using Duncanʼs multiple range test for significance level of 0.05. Data analysis was performed using the statistical software package IBM SPSS Statistics 20 (SPSS Inc., Chicago, IL, USA).

RESULTS AND DISCUSSION
Influence of rootstocks on tree vigor. Trunk cross-sectional area (TCSA) is considered as the most important indicator of tree vigor. Cultivar 'Čačanska Najboljaʼ grafted on all three clonal rootstocks had statistically significantly lower TSCA values compared to control rootstock (Myrobalan) - Figure 1A. The highest TCSA in cultivar 'Čačanska Najboljaʼ was on Myrobalan seedling (122.3 cm 2 ). Compared to control, reduction of TCSA was highest on 'Pixyʼ (21%), then on 'Fereleyʼ (10%) and 'St. Julien Aʼ (5%). Significant differences in TCSA between rootstocks were also found in cultivar 'Čačanska Lepot-icaʼ (Fig. 1B). The highest TCSA in the last year of the study was found on Myrobalan seedling rootstock (68.9 cm 2 ). Reduction of TSCA compared to control was for 31% on 'Pixyʼ, 16% on 'Fereleyʼ and 9% on 'St. Julien Aʼ rootstock. Radović, M.M., Milatović, D.P., Zec, G.N., Boškov, D.D. (2022). The influence of four rootstocks on the growth, yield and fruit quality of two plum cultivars. Acta Sci. Pol. Hortorum Cultus, 21(4), 75-81. https://doi.org/10. 24326/asphc.2022.4.8 Higher values of TCSA for all rootstocks were found in cultivar 'Čačanska Najboljaʼ compared to cultivar 'Čačanska Lepoticaʼ. Our results are in accordance with the previous findings [Sosna 2002, Blažek et al. 2004]. On the other hand, Blažek and Pištěková [2012] reported higher values of TCSA for the combination 'Čačanska Lepoticaʼ/Myrobalan compared with our results. Results of low vigor of the clonal rootstock 'Pixyʼ are in accordance with the previous findings [Sosna 2002, Kosina 2004, Sitarek et al. 2004. This rootstock showed sugnificantly lower values of the vigor compared to 'St. Julien Aʼ, which is in agreement with the results of Sosna [2006]. According to Botu et al. [2002] the value of TCSA for the rootstock 'Pixyʼ was 35% lower in comparison to seedling rootstock Myrobalan in environmental condi-tional of Romania. The same authors reported that the clonal rootstock 'St. Julien Aʼ influenced the decrease in value of TCSA by 29% on average. Comparing with these results, we obtained lower vigor reduction for both rootstocks, especially for 'St. Julien Aʼ. These differences in vigor decrease may be caused by environmental conditions, yield and cultural practices, such as fertilization and irrigation.
Influence of rootstocks on fruit set. Fruit set is one of the most significant indicators of fruit trees productivity [Glišić et al. 2012, Nikolić et al. 2012]. Results of fruit set of plum cultivars 'Čačanska Najbol-jaʼ and 'Čačanska Lepoticaʼ grafted on four different rootstocks are presented in Table 1.
Fruit set in the cultivar 'Čačanska Najboljaʼ ranged from 2.4% in 2017 on Myrobalan seedling rootstock   The percentage of fruit set depends mostly on the self-compatibility of cultivars. 'Čačanska Lepoticaʼ was classified as a self-compatible cultivar, while 'Čačanska Najboljaʼ was classified as self-incompatible cultivar [Nikolić and Milatović 2010]. This is one of the reasons for the higher fruit set in the cultivar 'Čačanska Lepoticaʼ. Cultivar 'Čačanska Najboljaʼ in some years had good and in some years moderate fruit set. This mostly depends on weather conditions during the flowering.
Based on fruit set of European plum cultivars, Neumüller [2011] gives the following classification: low (under 10%), middle (10-20%), high (20-40%) and very high (above 40%). According to this classification, the cultivar 'Čačanska Najboljaʼ can be classified in the group of middle fruit set, while the cultivar 'Čačanska Lepoticaʼ belongs the group of high fruit set. Our results of fruit set are consistent with the values reported by other authors. Namely, Surányi [2006] found the average fruit set in the interval of 10.9% to 44.4% in 21 cultivars of plum. Fruit set of six plum hybrids in the study of Glišić et al. [2012] ranged from 7.6% to 30.6%.

Influence of rootstocks on yield.
The average yield per hectare in the cultivar 'Čačanska Najboljaʼ was the highest on the rootstock 'Pixyʼ (23.5 t), then on the clonal rootstocks 'Fereleyʼ (22.7 t) and 'St. Julien Aʼ (20.9 t), while it was the lowest on Myrobalan seedling rootstock (12.9 t) - Table 2. Among years, maximum yield was obtained on the rootstock 'Fereleyʼ in 2013 (53.5 t ha -1 ), and the minimum yield was found on Myrobalan rootstock in 2017 (5.9 t ha -1 ). The average yield on all three clonal rootstocks was significantly higher compared to the Myrobalan seedling rootstock (control). The average yield per hectare in the cultivar 'Čačanska Najboljaʼ grafted on clonal rootstocks was higher for 82% on 'Pixyʼ, 76% on 'Fereleyʼ and 62% on 'St. Julien Aʼ compared to control.
The average yield per hectare in the cultivar 'Čačanska Lepoticaʼ was the lowest on Myrobalan seedling rootstock (23.0 t), while it was the highest on the rootstock 'Fereleyʼ (29.1 t). Among years, maximum yield was obtained on the rootstock 'Pixyʼ in 2016 (60.1 t ha -1 ), and minimum yield was found on Myrobalan in 2018 (12.9 t ha -1 ). Differences in average yield between control rootstock (Myrobalan) and all three clonal rootstocks were statistically significant. Compared to the control, the yield per hectare was higher for 26% on the rootstocks 'Pixyʼ and 'St. Julien Aʼ, and for 27% on the rootstock 'Fereleyʼ. Higher values of the yield were recorded in the cultivar 'Čačanska Lepoticaʼ (27.5 t ha -1 for on average for all rootstocks) than in the cultivar 'Čačanska Najboljaʼ (20.0 t ha -1 on average).
Higher yield on clonal rootstocks can be partly explained by their influence on lower vigor of grafted   cultivars. Because of that, more nutrients are available for flower bud development. The other possible reason is higher fruit set in some cultivar/rootstock combinations. According to Radović et al. [2016], clonal rootstocks influenced the increase in the number of flower buds on the fruiting branches, especially on the 'Fereleyʼ rootstock. Grzyb and Sitarek [2006] and Ogašanović et al. [2011] stated the positive influence of the rootstock 'Fereleyʼ on increasing the yield of grafted cultivars. Our results of yield on the clonal rootstock 'Pixyʼ are in accordance with previous reports [Kosina et al. 2000, Sosna 2002]. However, under our enviromental conditions, the yield of the cultivar 'Čačanska Najboljaʼ was lower than under the conditions in Czech Republic [Blažek et al. 2004]. Cumulative yield efficiency (CYE) was lowest on combination 'Čačanska Najboljaʼ/Myrobalan (0.53 kg cm -2 ) - Figure 2. Highest value was obtained in combination 'Čačanska Lepoticaʼ/'Pixyʼ (2.47 kg cm -2 ) and it was almost five times higher. In both studied cultivars CYE was significantly higher on all three clonal rootstocks compared with Myrobalan seedling rootstock.
Cumulative yield efficiency was higher in the cultivar 'Čačanska Lepoticaʼ (2.20 kg cm -2 on average for all rootstocks) compared with the cultivar 'Čačanska Najboljaʼ (0.76 kg cm -2 on average). The resons for higher values for 'Čačanska Lepoticaʼ cultivar are lower vigor on one side, and higher yield on the other side. The results obtained for CYE in our research are within the previously reported ranges Sosna 2002, Magyar and Hrotkó 2006, Świerczyński and Stachowiak 2009.

Influence of rootstocks on fruit characteristics.
One of the most important pomological properties of cultivar is fruit weight. Fruit weight of cultivar 'Čačanska Najboljaʼ ranged from 52.5 g on control rootstock to 56.8 g on the medium vigorous clonal rootstock 'Fereleyʼ (Tab. 3). Significantly higher fruit weight was obtained on rootstocks 'Fereleyʼ and 'St. Julien Aʼ compared with control (Myrobalan). Also, stone weight was significantly higher in clonal rootstocks compared to control. Fruit shape index was largest on rootstock 'Fereleyʼ indicating its inluence on more elongated fruit shape. Our results for fruit size of 'Čačanska Najboljaʼ cultivar were similar to those obtained by Sosna [2006], and higher compared to results of Blažek et al. [2004] and Kosina [2004].
Cultivar 'Čačanska Lepoticaʼ had the lowest fruit weight on clonal rootstock 'Pixyʼ, while the highest value was on medium vigorous rootstock 'St. Julien Aʼ. However, differences among rootstocks were not significant. This is in line with results of Sitarek et al. [2007] and Meland [2010]. Our values for fruit size of 'Čačanska Lepoticaʼ cultivar were similar or slightly lower than the results of other authors [Blažek et al. 2004, Blažek and Pištěková 2012, Mészáros et al. 2015.

CONCLUSION
All studied clonal rootstocks ('Fereleyʼ, 'Pixyʼ and 'St. Julien Aʼ) had a positive effect on reducing the vigor and increasing the yield of plum cultivars 'Čačanska Najboljaʼ and 'Čačanska Lepoticaʼ. The lowest vigor was found in trees grafted on 'Pixyʼ rootstock, followed by 'Fereleyʼ and 'St. Julien Aʼ. The yield per hectare was significantly higher on trees grafted on clonal rootstocks compared to Myrobalan seedling rootstock. The highest yield in the cultivar 'Čačanska Najboljaʼ was obtained on the rootstock 'Pixyʼ, and in the cultivar 'Čačanska Lepoticaʼ on the rootstock 'Fereleyʼ. Based on the results obtained, it can be concluded that all three clonal rootstocks showed better results than Myrobalan, and can be recommended for establishing intensive plum plantations with higher planting density. Among the rootstocks tested, the best results in terms of productivity and fruit quality were obtained on the 'Fereleyʼ rootstock.

SOURCE OF FUNDING
This work was realized as a part of the project No. 19/6-020/961-91/14 financed by the Ministry of Science and Technological Development of the Republic of Srpska. It was also supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Contract No. 451-03-68/2020-14/200116