Introduction

For European plum, nearly 50 plum varieties have been studied by researchers in the last decades (Kajtár-Czinege 2018a; Hrotkó 1999; Grzyb and others 2010, Achim et al. 2010; Botu et al. 2010). These rootstocks have different effects on tree growth, yield, rootstock–variety compatibility, longevity, disease and pest resistance, adaptation to climate and soil, and fruit quality (Hrotkó 1999; Kajtár-Czinege 2018b; Mészáros et al. 2015; Grzyb et al. 2010; Achim et al. 2010; Pedersen 2010). In Hungary, the plum rootstock usage is rather simple, 95–97% of nurseries use Myrobalan seedlings, because this rootstock produces nice, strong trees that are suitable for plum production for processing. The Myrobalan rootstocks are also suitable for traditional semi-intensive vase canopy harvested for processing industry. Nowadays, 50% of the plum crop are used for processing industry, making brandy and jam, and small quantities of dried prunes are also produced.

The other half of the crop is sold in fresh markets with much better profitability. For the orchards with modern, intensive spindle trees producing hand-picked fruit for fresh consumption, it would be necessary to use rootstocks low- or medium vigorous rootstocks. Our investigation target was to test the effect of a few promising new rootstocks on growth and productivity of plum varieties.

In the last few years, about 10 rootstocks have been tested in Hungary (Kajtár-Czinege 2018b; Kajtár-Czinege et al. 2022; Hrotkó et al. 1998; 2002). Among them, there are also rootstocks with different growth vigour, not all of them adopt to the semi-arid/arid continental climate of the Carpathian Basin (Kajtár-Czinege et al. 2022; Kajtár-Czinege 2018b; Hrotkó et al. 1998, 2002; Hrotkó and Magyar 2006; Magyar and Hrotkó 2006).

The correct choice of rootstock is a very important aspect for plum production, which depends on several factors. When choosing the crop, the following aspects should be considered: the market purpose (processing or fresh consumption); the cultivation system (intensive or semi-intensive); the climatic and soil conditions. Choosing the right rootstock–variety combinations is even more important, as studies have shown that plum rootstocks do not uniformly produce strong or weak growth. Each rootstock–variety combination behaves differently under different site conditions. While the Fereley rootstock with the ‘Topper’ cultivar shows weak growth, the ‘Toptaste’ cultivar shows strong growth (Kajtár-Czinege et al. 2022). A similar observation was made by Grzyb and Sitarek (2007) with Jaspy and GF655/2 on ‘Hanita’ and ‘Węgierka Dąbrowicka’ or by Meland (2010) with six cultivars on four rootstocks.

Vitality and lifespan are also important issues in the choice of the rootstock (Kajtár-Czinege et al. 2022). The lifespan of each rootstock and variety combination can determine the economics of plantations. The vitality and the condition of trees can be enhanced in different ways, such as root fertilizers, rhizosphere bacteria, mycorrhizal fungi, Trichoderma atroviride fungi, granulated cattle manure and other organic fertilizer granules (Károly and Király 2017). The rootstocks also influence the tree height growth, thickening, branching and yield (Nečas et al. 2023; Meland 2010). Rootstocks also affect fruit yield and fruit quality. The effect of rootstocks on fruit yield has been studied by several researchers, such as Grzyb and Sitarek (2006, 2007), Grzyb and Rozpara (2012), Radovic et al. (2022), Botu et al. (2010), Pedersen (2010). In addition, the effect on crop quality of the rootstocks has also been studied, as in Grzyb and Sitarek (2006), Radovic et al. (2022), Botu et al. (2010), Sottile et al. (2010). Fruit weight and fruit flesh thickness are positively influenced by the rootstock St Julien A (Zezulová et al. 2022). Fruit quality is also determined by the tendency of the fruit set, which is only identified as a varietal trait and not particularly influenced by the rootstock. Some cultivars (‘Jojo’, ‘Topper’) are more prone to twin fruit set than others (‘Presenta’, ‘Čačanska lepotica’) (Király and Kiss 2020).

The aim of our study was to investigate the effect of different rootstocks on the fruit set, yield and specific yield of different varieties and fruit quality.

Materials and Methods

The study was carried out in Kecskemét, Hungary, in Central Europe (Fig. 1). The climate of Kecskemét is semi-arid, with a distinct continental influence, which has become drier and more extreme in recent years. The average precipitation is 550 mm, averaged over 50 years, but there were years when the precipitation was less than 400 mm (OMSZ (National Meteorological Service) 2022).

Fig. 1
figure 1

Orchard location

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The soil type of the study site is a sandy soil with poor nutrient supply capacity and low humus content. The pH-KCl [−] 8.01; the soil cohesion index (KA) 28; all water-soluble salts [m/m%] < 0.02; CaCO3 [m/m%] 2.5; humus [m/m%] 0.67; NO3− -N+NO2−-N [mg/kg]; P2O5 [mg/kg] 344; K2O [mg/kg] 61.

Kecskemét is located on the sandy plateau between the River Danube and the River Tisza. Its soil was formed from the alluvium of the Danube. Kecskemét has a warm-dry continental climate. The annual sunshine is 2040 h. The annual mean temperature in the production area (averaged over 50 years) is 11 °C. The mean temperature in January is −1 °C, the mean temperature in July is 21 °C and the mean temperature in the growing season is 18 °C (OMSZ 2022).

The trees were planted in 2.5 × 1.5 m spacings. During the first few years we trained the trees to a slender spindle crown by pruning and tying off the canes and shoots to make the plantation productive as soon as possible. The first tie-offs were done in a horizontal position after planting, before bud break or shoots. The second tie-downs were in July–August, when the shoots start to turn woody and senesce. Shoots or twigs were tied to bags filled with sand. The trees were tied in 2010 and 2011, but not in 2012. The trees were grown to a height of 3.2 m. At this height, the top of the trees was pruned back to a flat lateral to stop vertical growth.

From the rootstocks and varieties tested, 12–12 trees were planted per combination in a random block arrangement. Planting was carried out in 2010 with 1‑year-old crown cuttings from a German nursery. The varieties and rootstocks were evaluated in two blocks for comparability. In block A, two rootstocks, the traditional Myrobalan seedling and a vegetatively propagated rootstock, St Julien A, were evaluated with three varieties (‘Čačanska lepotica’, ‘Jojo’ and ‘Toptaste’). In the block B two of the varieties (‘Toptaste’, ‘Topfive’) were evaluated on three rootstocks, namely St Julien A, St Julien GF655/2 and Fereley.

Measurement Methods

Our database includes trunk diameter, yield from 2010–2016. From these we calculated the trunk cross-sectional area, which is an indicator of growth vigour. The trunk cross-sectional area was calculated from the trunk diameter.

$$\text{Trunk cross-sectional area TCSA}\colon(\mathrm{cm}^{2})=(1/2\text{trunk diameter})^{2}*\uppi$$

The data on tree growth have already been evaluated in our previous article (Kajtár-Czinege et al. 2022), and the results obtained there were used here to calculate the specific yield indices.

Cumulative yield over the 7 years, the cumulative yield efficiency (CYE) as cumulative yield per trunk cross-sectional area was calculated, which indicate the yield potential related to trunk cross-sectional area (TCSA).

$$\text{Cumulative yield}\colon \mathrm{CY}(\mathrm{kg}*\text{tree}^{-1})=\Upsigma \mathrm{Y}_{\left(2010-2016\right)}$$
$$\text{Cumulative yield efficiency}\colon \mathrm{CYE}[\mathrm{kg}*({\mathrm{cm}^{2}})^{-1}]=\mathrm{CY}*{\text{TCSA}_{2016}}^{-1}$$

The precocity index (PI) (Bujdosó et al. 2019) was calculated, where the first 3 years’ yields were cumulated and divided by the total cumulated crop of 7 years, then multiplied by 100 to get a percentage. This index refers to the proportion of tree yields in the first 3 years of the study compared to the total yield per tree.

$$\text{Precocity index}\colon \mathrm{PI}(\mathrm{{\%}})(\mathrm{Y}_{2010}+\mathrm{Y}_{2011}+\mathrm{Y}_{2012})[\left(\mathrm{CY}_{\text{Total}}\right)^{-1}]100$$

Fruit quality parameters were also measured, such as average fruit weight, °Brix, flesh firmness, seed weight. The average fruit weight was calculated from individual measurements of 25 fruits and averaged. The °Brix values were obtained by qualitative data collection on six fruits per tree, i.e. six samples were taken randomly from each tree, resulting in 72 fruit samples per rootstock–variety combination, data collection was performed in 2012 and 2013. Data from the 2 years were averaged.

Statistical Analysis

Analysis was performed by using the SPSS (Freund et al. 2021) program package. This is a statistical analysis software. The mean values for a studied property of rootstock–scion combinations were compared using analysis of variance (ANOVA) or two-sample t‑test (for two groups). When the t‑test confirmed significant difference within the mean values of the studied property across groups, the means were separated by using the modified Duncan’s multiple rank test and labelled them in the tables with a, b, c (combinations) or A, B, C (for varieties). The significance level for the comparison procedures was 0.05.

Results

Varieties and Rootstocks Tested in Block A

Growth Vigour and Cumulative Yield

Data of the growth vigour expressed in the TCSA are presented in Table 1. There is no difference in growth vigour of trees on different rootstocks. ‘Čačanska lepotica’ and ‘Jojo’ showed the same growth rate; however, trees of ‘Toptaste’ showed a significantly lower TCSA.

Table 1 Performance of growth vigour and cumulated yield (CY) in evaluation block A
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For cumulative yield of 7 years, similar results were obtained for the varieties as for growth vigour. However, results differed depending on the rootstock, with significantly higher values in cumulative yields measured for the trees on St Julien A rootstock than for the Myrobalan rootstock. The lowest cumulative yield (14.65 kg) over the 7 years was observed in the ‘Toptaste’/Myrobalan combination. On the other hand, the highest yield was produced by the trees of the combination ‘Jojo’/St Julien A, with 42.23 kg cumulative yield (Table 1).

Yield Efficiency and Precocity

There is no significant difference in the cumulative yield efficiency (CYE) of trees of varieties but the rootstock was found to be a significant factor. The lowest value of cumulative yield efficiency (kg/cm2) was calculated for the combination ‘Toptaste’/Myrobalan (Table 2), while the highest value was obtained for the combination ‘Toptaste’/St Julien A. For the other two cultivars (‘Čačanska lepotica’ and ‘Jojo’), trees on the St Julien A rootstock showed a tendency for a higher CYE compared to those on the Myrobalan rootstock.

Table 2 Cumulative yield efficiency (CYE) and precocity index (PI) in evaluation block A
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The precocity is an important factor for a quick return on orchard investment. The calculated precocity index (PI%) shows the proportion of the first 3 years related to the total cumulated yield of the trees. The higher this figure is in relation to the total yield, the higher the proportion of fruit produced in the first 3 years. Variety was the significant factor here. This suggests that ‘Toptaste’ turns to bearing earlier and has a significantly higher early fruit set than the other two varieties tested (Table 2).

The yearly mean crop of non-productive and productive years is also compared in (Table 3). Variety was found to be a significant factor in both cases. In the case of ‘Toptaste’/Myrobalan, low yields were observed in both the non-bearing and bearing years. On the other hand, in the productive years, the highest yearly crop was observed for the combination ‘Toptaste’/St Julien A. The initial low yield of ‘Čačanska lepotica’ is significantly higher in the bearing years compared to ‘Toptaste’, while ‘Jojo’ shows significantly higher yields in the non-bearing year compared to ‘Čačanska lepotica’, but no difference in the bearing years. The difference between Myrobalan and St Julien A is significant for both ‘Čačanska lepotica’ and ‘Jojo’.

Table 3 Average yields of non-productive and productive years in evaluation block A
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Fruit Quality

The results for fruit quality are presented in Table 4. All four indicators were significantly affected by the rootstock. For ‘Jojo’, there is a significant difference between rootstock based on the effect on fruit weight. The St Julien A rootstock produced significantly larger fruit (44.02 g) than the fruit (39.70 g) of the trees grown on the Myrobalan rootstock. Thus, ‘Jojo’/St Julien A not only produced more fruit, but also had larger fruit size. The fruit size of trees on Myrobalan rootstock is smaller or significantly smaller in ‘Jojo’ and ‘Čačanska lepotica’. For ‘Toptaste’ there is no difference in fruit size between rootstocks.

Table 4 Performance of fruit quality in evaluation block A
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°Brix value (°Bx) and flesh firmness are influenced by fruit ripeness. Although a given °Brix B° and flesh firmness is characteristic of the variety and may be slightly influenced by the rootstock, it is in fact characteristic of the state of ripeness. The values for dry sucrose content (B°) are quite different (Table 4), partly reflecting the ripeness at harvest. For flesh firmness, the St Julien A rootstocks had higher values (Table 4), i.e. firmer fruit than the Myrobalan rootstocks. This suggests that fruit ripening was earlier in the trees grown on the Myrobalan rootstock.

Among fruit quality parameters, significant differences in seed weight were observed between rootstocks for ‘Jojo’ and ‘Toptaste’ (Table 4). While Myrobalan rootstock trees produced smaller seeds (1.93 g; 2.36 g), St Julien A rootstock trees had significantly larger seeds (2.1 g; 2.46 g), perhaps due to the larger fruit size.

Evaluation of Varieties and Rootstocks in Block B

Growth Vigour and Cumulative Yield

There was no difference in the TCSA of ‘Toptaste’ and ‘Topfive’ evaluated in block B, and this was not significantly influenced by the three rootstocks tested (Table 5). However, there is a significant difference in the cumulative yield between the varieties and between the rootstocks. Trees of ‘Toptaste’ produced significant larger cumulative yield compared to ‘Topfive’. The cumulative yield of trees on different rootstocks showed significant differences. The lowest cumulative yield was obtained with ‘Topfive’/St Julien GF655/2. Similarly low cumulative yields were observed for the same cultivar but on St Julien A. Higher values for cumulative yield were measured for ‘Toptaste’/St Julien A (28.57 kg) and even higher (40.27 kg) for Fereley, which were significantly different from the values measured for the ‘Topfive’ combinations.

Table 5 Performance of growth vigour and cumulated yield (CY) in evaluation block B
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Cumualtive Yield Efficiency (CYE) and Precocity Index (PI)

There is a significant difference in CYE between ‘Toptaste’ and ‘Topfive’ (Table 6): ‘Toptaste’ has a higher cumulated yield per unit stem thickness. There are significant differences in CYE between the trees of ‘Toptaste’ on different rootstocks. The most efficient trees were on Fereley rootstock followed by St. Julien A, while much lower efficiency was calculated for trees on St. Julien GF 655/2. There are no significant differences in CYE of ‘Topfive’ trees on different rootstocks.

Table 6 Cumulative yield efficiency (CYE) and precocity index (PI) in evaluation block B
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The PI (Table 6) of ‘Toptaste’ trees is significantly higher than that of ‘Topfive’. Trees of ‘Toptaste’/St Julien GF655/2 and St Julien A showed higher PI than all the other scion/rootstock combinations, ‘Topfive’ showed low values for all three rootstocks.

The yearly mean crop of trees in the first 3 years was significantly higher of ‘Toptaste’ compared to ‘Topfive’ (Table 7). The trees on different rootstocks showed significant differences in early years, the highest crop of ‘Toptaste’ produced trees on St. Julien A, while the ‘Topfive’ trees on Fereley rootstock. There is no significant difference in the yield of cropping years of trees on different rootstocks.

Table 7 Average yields of non-productive and productive years in evaluation block B
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Fruit Quality

Table 8 presents the fruit quality parameters. ‘Toptaste’ produces significantly larger fruit than ‘Topfive’, but there is no significant difference in fruit size of trees on different rootstocks.

Table 8 Performance of fruit quality in evaluation block B
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The soluble sucrose content (°Brix) of ‘Toptaste’ fruits showed larger values than ‘Topfive’, and the soluble sucrose content of fruits harvested from trees on different rootstocks showed significant differences. ‘Toptaste’ on Ferelay and GF 655/2 produced °Brix values higher than on St. Julien A, while the soluble sucrose content of ‘Topfive’ fruits was higher on St. Julien A and GF 655/2 rootstocks compared to Fereley.

Firmness of fruit flesh of ‘Toptaste’ showed larger values than that ‘Topfive’. There are no differences in the fruit firmness of ‘Toptaste’ harvested from trees on different rootstocks, while fruits of ‘Topfive’ were less firm on St. Julien A rootstocks. Stone weight (Table 8) is also clearly a varietal trait and the difference in stone size between the two varieties is very evident, with ‘Topfive’ having a stone weight of around 1.22 g compared to 2.47 g for ‘Toptaste’.

Discussion of the Results

When evaluating the growth (Kajtár-Czinege et al. 2022) and yielding characters of the four varieties on different rootstocks, one of our main conclusions is that strong or medium vigorous rootstocks (Myrobalan, St Julien A, St Julien GF 655/2, Fereley) may be suitable for intensive, hand-picked plantations under lowland climatic conditions in Hungary. However, there are varietal differences, because of the high cumulative yield of ‘Čačanska lepotica’, ‘Jojo’ and ‘Toptaste’ seem to be more productive under our site conditions than variety ‘Topfive’. The cumulative yields of ‘Topfive’ were significantly lower than the above combinations, giving cumulative yields of only less than 50%. Similar results were obtained in a study by Blažek and Pistékova (2009), who found that the ‘Topfive’ variety performed below average in yield and had poor growth vigour. In contrast, in Surányi’s (2019) studies ‘Topfive’ yielded properly in a traditionally trained orchard (36.5 kg/tree with 31–35 g fruit weight). This indicates the specific site condition requirements of this variety.

Several studies report that plum cultivars show significant differences in yield depending on the rootstock (Blazek and Pistekova 2012; Mészáros et al. 2015; Zezulová et al. 2022). Among the tested cultivars, the cultivars ‘Čačanska lepotica’ and ‘Jojo’ gave similarly high cumulative yields on St Julien A rootstock, with both cultivars having yield indices on this rootstock exceeding those of trees on Myrobalan rootstock; however, we found no difference between the two rootstocks in later full crop years. Hrotkó et al. 1998 and Magyar and Hrotkó 2006 in their studies on a similar site did not find significant differences in yield efficiency (CYE) between the rootstocks (Myrobalan and St Julien GF655/2) for the cultivar ‘Čačanska lepotica’, although the precocity of fruit set was not investigated. Our results confirm their opinion. The higher CYE of trees on St. Julien A and the higher PI indicate the advantage of this rootstock compared to the Myrobalan seedling.

The CYE of ‘Toptaste’ trees showed high values on St Julien A and Fereley rootstocks, while Myrobalan performed significantly below them. This confirms the opinion of Radovic et al. (2022) on Fereley and indicates again the individual performance of variety–rootstock combinations. In terms of fruit set, even St Julien GF 655/2 showed very early fruit set. The cultivar ‘Toptaste’ is more characterized by the formation of long fruiting branches, although the proportion of short fruiting spurs increased in St Julien and Fereley compared to the cultivar average (Kajtár-Czinege et al. 2022). It can be concluded that the size of the trees and the relatively evenly distributed fruiting wood proportions (Kajtár-Czinege et al. 2022) on the Fereley rootstock, which had the strongest growth at the site, led to the highest yield per tree on this rootstock.

The cultivar ‘Topfive’ is a medium-growing cultivar with predominantly short fruiting wood (Kajtár-Czinege et al. 2022), and its poor productivity is explained by lower specific yield indices. The highest yield efficiency (CYE) was obtained on the rootstock St Julien A.

Fruit characteristics showed differences in fruit size, which is more varietal character. Although a given °Brix and flesh firmness is characteristic of the variety and may be slightly influenced by the rootstock, it is in fact characteristic of the state of ripeness. The values for dry sucrose content (°Brix) are quite different, partly reflecting the ripeness at harvest. For flesh firmness, the St Julien A rootstocks had higher values, i.e. firmer fruit than the Myrobalan rootstocks. This suggests that fruit ripening was earlier in the trees grown on the Myrobalan rootstock. There is a marked difference in the dry matter content of the fruit between the two varieties (‘Toptaste’, ‘Topfive’), which is also a varietal characteristic, and although there is also a difference in the values between the rootstocks, this is more an indication of differences reflecting maturity stages.

Conclusion

Evaluating the yielding characters of the four varieties, our conclusions is that cultivars ‘Čačanska lepotica’, ‘Jojo’ and ‘Toptaste’ perform well and may be suitable for intensive, hand-picked plantations under lowland climatic conditions in Hungary, while ‘Topfive’ developed weak and less productive trees. In conclusion, considering the productivity and fruit quality indices, St Julien A proved to be a good rootstock for the cultivars ‘Čačanska lepotica’, ‘Jojo’ and ‘Toptaste’. Rootstock Fereley, although some trees died early in our experiments, showed very high yield potential with ‘Toptaste’, confirming the opinion of other authors. However, the vigour of St Julien GF655/2 rootstock was like St Julien A, and the productivity of trees was lower on this rootstock. The traditionally used rootstock Myrobalan is suitable for high yielding intensive plantations with highly productive cultivars but lags clonal rootstocks in precocity with some cultivars.