Performance of european pear ‘Shahmiveh’ grafted onto different rootstocks

Introduction

After grape and apple, pear is the world’s third most important temperate fruit with a world production of 23.5 million metric tons (FAO, 2012). European pear (Pyrus communis L.) is cultivated commercially throughout the temperate zones of the world and in Iran, and even in the tropical highlands of Guatemala (Cruz-Castillo, Rodríguez-Bracamontes, Vásquez- Samtizo, & Torres-Lima, 2006) and Mexico (Sánchez-Cervantes, Cruz-Castillo, & Inurreta-Aguirre, 2013).

The proper choice of scion-rootstock can influence vegetative growth, quality and quantity of fruit production, plant bearing habit, precocity, and plant tolerance to pest and diseases in unfavorable conditions. Dwarfing rootstocks of pear are widely cultivated in orchards because they have superior advantages over seedling rootstocks such as a high tolerance level to a calcareous soil in semi-arid conditions, high-density planting capacity as well as yield performance and fruit quality (Ikinci, Bolat, Ercisli, & Kodad, 2014). The interaction effects of scion-rootstock and their adaptability to the soil and climatic condition are the key elements of successful commercial production in a new orchard. However, there is insufficient information about the behavior of rootstocks in different environmental conditions. Especially in Brazil, researchers aspire to investigate the behavior of European pear cultivars and Quince rootstocks (Machado, Rufato, Bogo, Kretzschmar, & Mario, 2013).

Iran is a vast country with various climatic conditions and a wide range of temperature. Pear orchards are located over a large area in Iran, including the north to northwest, west, and south central regions. Since seed-based propagation in most traditional Iranian orchards was very common in the past and pears have widespread gametophytic self-incompatibility, there is a high genetic diversity in Pyrus communis and the opportunity for finding commercial cultivars with superior traits (Najafzadeh & Arzani, 2015; Sanzol, 2010; Gharehaghaji et al., 2014). There are some local pears (Pyrus communis L.) in Iran, including ‘Shahmiveh’, ‘Peyghambary’, ‘Sardrood’, ‘Dargazy’, ‘Natanzy’ and ‘Domkaj’ (Arzani, 2002). ‘Shahmiveh’ is the dominant pear cultivar in Iran. This cultivar is similar to the ‘Williams’ and ‘Bartlett’ pear in terms of size and shape (Hedrick, 1995 as cited by Kalbasi-Ashtari, 2004), with a crisp texture, very good taste and slight aroma (Kalbasi-Ashtari, 2004). ‘Sebri’ pear is one of the latest maturing pear cultivars and is grown mainly in Esfahan and Mashhad. Owing to their high fruit quality and long storage life, ‘Natanzy’ (originated from Pyrus communis L.) and ‘Sebri’ (originated from Pyrus serotine Rehd) are the most precious local cultivars in Iran with a high market price of about 7 times more than other pear cultivars (Davarynejad and Davarynejad, 2004).

Due to the self-incompatibility of most pear cultivars (Davarynejad and Davarynejad, 2004), cross-pollination is necessary for Iranian orchards (Davarynejad et al., 1996). Accordingly, choosing the endemic pear cultivars with high compatibility to the rootstock and environmental condition would exert a great positive impact on the commercial production of pear orchards. Therefore, this study was conducted to scrutinize the behavior of Iranian 7- and 8-year-old endemic pear ‘Shahmiveh’ as a scion on different rootstocks. Experimental rootstocks include clonal Quince (Q. A, Q. B, Q. C, and PQBA29), local pear ‘Konjuni’, and also wild generative seedling in the arid region of Isfahan. The vigor, vegetative growth, and the main production traits (e.g., flowering and cropping indices) were examined to find the proper combination.

Materials and methods

The experiment was conducted in 2008 in East-West rows at a spacing of 4 . 3 m with three replications at the Agricultural Research Center of Kabutarabad, Isfahan, Iran, geographically located at 32° 31’ N, 51° 51’ E and 1545 m above sea level. According to the Köppen- Geiger system, Isfahan is classified as an arid or desert-like climate with an average annual temperature of 15.6 °C and average annual precipitation of 125 mm. Despite its altitude, Isfahan has hot summer days and moderate nights. In the winter, days are mild while nights can be very cold.

Iranian local pear ‘Shahmiveh’ (European pear: P. cummunis) seedlings were used as the scion and grafted onto six different one-year-old rootstocks, which were cultivated in late winter 2008 and grafted in late summer of the same year. The examined rootstocks were clonal Quince A (Q. A), Quince B (Q. B), Quince C (Q. C) and PQBA29 as dwarfing rootstalks, in addition to the local pear cv. ‘Konjuni’, and local wild P. communis seedling obtained by sexual propagation method (generative seedling).

Most pear cultivars have self-incompatibility and self-pollinating species produce more fruits near a compatible pollinizer. Therefore, we used ‘Natanze’ and ‘Sebri’ cultivars as the pollinizers for ‘Shahmiveh’ pear cultivar. Trees were pruned and trained based on the espalier system.

The orchard had a clay loam soil with a pH of 7.5. Experimental trees were furrow irrigated at 7- to 10- day intervals in response to the plant’s water needs. Potassium sulphate and urea (each 50 g per year age of each pear tree) were applied as the soil fertilizers.

Growth habit

After leaf abscission in December, trunk growth was measured to estimate vigor, water uptake efficacy of rootstocks (Goldhamer & Fereres, 2004), and graft-compatibility. Using a digital caliper, we measured the trunk diameters (Visser, 1964) at 3 points, including 20 cm above, 5 cm under, and on the graft union. Then, they were reported as the mean trunk diameters of 7- and 8-year-old pears.

Tree height (TH) and current season shoot length (CSHL) were also recorded at the end of the growing season (of dormant plants) and reported as the mean growth rate of 7- and 8-year-old pears. Tree height increment was measured as the distance from the ground up to the plant apex.

Leaf area

Leaf length (LL) and width (LW) were measured using a 20 cm ruler.

Flowering characteristics

The flowering season was in late March. Before the full bloom, the number of flowers per inflorescence and the number per current season shoot were randomly recorded. After 14 days of full bloom, the number of fruit set was noted to estimate the number of abscised flowers on the current season shoots. Finally, they were reported as the mean flowering rate and abscission rate of two years.

Fruiting and yield

Fruit set occurred approximately after April 9 and harvesting time was in September. After one month of fruit set, the number of fruits on current season shoots was estimated. The percentage of fruit set was also calculated based on the following formula (Westwood, 1988):

For each tree, the number of fruits was counted. Using a precision balance, we weighed all of the fruits and calculated the mean fruit weight based on yield per tree (kg·tree-1) and yield per plot (kg·36m^-2). Ultimately, they were reported as the mean cropping rate of two growing seasons.

Statistical analysis

The experiment used a randomized complete block design (RCBD) with three replications. Each replication (block) included 6 plots (one treatment in each plot) with 6 trees in each plot. After having been checked for normality, the data were analyzed for statistical significance, using Statistical Analysis System software (2013). Duncan’s multiple range test (P < 0.01) was used for mean separation.

Results and discussion

Graft viability and compatibility

PQBA29 and Q. A rootstocks showed a better status in graft viability percentage compared to Q. C and Q. B rootstocks. Apart from Q. B and Q. C, results showed a high rate of grafting success in the first growing season (data not shown). Among different rootstocks, ‘Shahmiveh’ pear cultivar on Q. B had the lowest leaf dimensions, number of flowers per current season shoot, number of fruits per current season shoot, percentage of fruit set and the highest incompatibility symptoms, which reduced leaf size and productivity (Table 2 and 4).

Growth habit

The aging process had a significant influence on trunk diameter, current season shoot length (CSSL), and leaf width (LW). Similarly, rootstock selection had a substantial effect on trunk diameter, CSSL, and leaf length (LL) (Table 1).

Table 1.. Variance analyzes of growth of ‘Shahmiveh’ pear grafted onto different rootstocks during two years (2013 and 2014).

Table 2. ‘Shahmiveh’ pear growth data, average of two years of rootstock selections.

Trees on dwarfing rootstocks had significantly (P ≤ 0.001) lower scion and rootstock diameters; however, in the grafting point they had no significant differences in relation to the generative seedling. While all recorded trunk diameters of ‘Konjuni’ were at the highest point, the lowest scion diameter was obtained by grafting on PQBA29 (Table 2). The use of Q. A as a rootstock highly reduced CSSL (Table 2). Although generative seedling had the longest CSSL, it had the lowest number of fruit per current season shoot (Table 2 and 4).

With subtle changes among rootstocks, grafting on Q. C provided trees with the lowest height (Table 2). Results showed that vigor and vegetative growth were highly affected by the rootstock selection. The scion and rootstock growth were markedly affected by year and type of rootstock (Table 1). The effect of rootstocks on LW dimension was insignificant, whereas the LL was markedly enhanced by the PQBA29 rootstock. Seemingly, the highest leaf dimension of PQBA29 is an indicator of its compatibility with the climatic and environmental condition of this arid region (Table 2).

Flowering characteristic

While having the lowest current season shoot length and scion diameter, PQBA29 had the highest number of flowers and fruits per current season shoot length (Table 2 and 4). After PQBA29, the highest number of fruits per current season shoot length and fruit percentage were observed on Q. A rootstock (Table 4). Although in PQBA29 the number of flowers per current season shoot was similar to Q. A and Q. C and its number of flowers per inflorescence was even lower than other rootstocks, PQBA29 flowers exhibited higher fruit formation rate and lower abortion rates (Table 4).

Cropping efficiency

A previous study of ‘Santa Maria’ pear cultivar revealed that pear seedling and PQBA29 had a better mineral uptake in highly calcareous soil in semi-arid condition. Ikinci et al. (2014) showed that the highest cumulative yield efficiency (kg·cm^-2) was observed on Q. C and PQBA29, respectively. Nonetheless, the highest cumulative yield (ton·ha-1) was recorded on PQBA29 and followed by Q. C at a density of 800 trees·ha-1 for both rootstocks (Ikinci et al., 2014). The highest number of fruits per current season shoot was obtained from PQA29 followed by Q. A and Q. C, respectively (Table 4). Year and rootstock had a significant influence on the number of fruits per current season shoot and fruit set percentage (Table 3). The cropping efficiency markedly improved in the second year of the experiment (data not shown). Considering the productivity, PQBA29 exhibited the highest fruit set followed by Q. A, Q. C, and ‘Konjuni’, respectively. In comparison to generative seedling, PQBA29, Q. A, and Q. C had a significantly higher number of flowers per current season shoot, number of fruits per current season shoot, and fruit set percentage (Table 4).

Table 3. Variance analyzes of flowering and fruit set of ‘Shahmiveh’ pear grafted onto different rootstocks during two years (2013 and 2014).

Table 4. ‘Shahmiveh’ pear fruiting and flowering data, average of two years collected data of different rootstock selections.

Results were consistent with those obtained by Rahmati, Arzani, Yadollahi, and Abdollahi (2015), who showed that P. communis ‘Williams Duchess’ and Asian pear cultivar ‘KS’10 produced the highest number of fruits on Q. A rootstock. The highest productivity of the PQBA29 and Q. A indicate that at a high density their performance would be strikingly better. By analyzing flowers and leaves in 2010-2012, it was found that PQBA29 samples had the highest nitrogen concentration compared to other rootstocks (Akbari, Ghasemi, Ebrahimpour, & Branch, 2014). Ikinci et al. (2014) showed that PQBA29 and Q. A on ‘Santa Maria’ had the highest nutrient uptake efficacy in highly calcareous soil in semi-arid climatic conditions. Similar to our results, PQBA29 presented the highest cumulative yield (Ikinci et al., 2014). It has been documented that there is a close relationship between leaf nitrogen concentration, canopy size, fruit development, crop sink strength, leaf age, and leaf distance from fruits with photosynthetic capacity (Kriedemann & Canterford, 1971; Reich, Walters, Kloeppel, & Ellsworth, 1995; Jackson, 2003). Hence, the higher N availability, leaf size, fruit size, and productivity of PQBA29 might correspond to its higher photosynthesis rate and adaptability to the environmental condition of this arid region.

Conclusions

In summary, PQBA29 grafted onto ‘Shahmiveh’ pear cultivar exhibited comparatively high productivity and compatibility symptoms, e.g., large leaf dimensions in addition to cropping efficiency or fruit formation rate, in the arid climatic condition of Isfahan in Iran. In a high-intensity orchard, the beneficiary effect of PQBA29 would be even more promising. Owing to its high nutrient uptake efficacy and adaptability in arid regions and calcareous soils, results suggest that this combination would be the best substitute for the pear seedling in Iran’s traditional orchards.