Agronomic performance of 'Valência' orange combined with 13 rootstocks in the state of Rio Grande do Sul, Brazil

The objective of this work was to evaluate the agronomic performance of 'Valência' sweet orange (Citrus sinensis) tree, in combination with 13 rootstocks cultivated in soil and climate conditions of the Brazilian state of Rio Grande do Sul. The following agronomic variables were evaluated: scion height and volume, plant production, production efficiency, productivity, fruit diameter, fruit mass, peel mass, juice yield, soluble solids content, titratable acidity, soluble solids:acidity ratio, ascorbic acid, peel color, antioxidant capacity, and phenolic compounds. A randomized complete block experimental design was used, with three replicates and nine plants per plot, in a two-factor arrangement (13 rootstocks and the 2015 and 2016 harvests). The following rootstocks stood out: TSKC × TRDP-023 for its juice yield (54.0 and 51.7%), soluble solids:acidity ratio (8.66 and 6.99), and ascorbic acid (49.3 and 51 mg 100 g-1); and the citrandarin 'San Diego', for its higher phenolic-compound content (106.4 and 74.9 100 g-1 gallic acid), coloration (76.2 and 75.9 ho), and ascorbic acid (42.2 and 44.7 mg 100 g-1). The TSKC × TRDP-026 and TSKC × CTQT1434-004 rootstocks induce canopy formation and productive precocity in 'Valência' orange trees grown in the southern region of the state of Rio Grande do Sul state, in the first few years of growth.


Introduction
Brazil occupies a notable position within global citriculture, with an annual production of 16 million tonnes. Sweet oranges [Citrus sinensis (L.) Osbeck] are particularly notable; and Brazil accounts for 30% of its global production whose primary focus is juice export (IBGE, 2014). 'Valência' is the main sweet orange cultivated on a global level (Fundecitrus, 2017), and it is highly adapted to the Brazilian state of Rio Grande do Sul (Cunha Sobrinho et al., 2013). Citrus production in the South of Brazil amounts to approximately 1.654 million tonnes, out of which 504 thousand tonnes (27%) come from Rio Grande do Sul (IBGE, 2014).
Various agronomic characteristics are influenced by the rootstock chosen, such as fruit productivity and quality, nutrient absorption, synthesis and use, plant size and longevity, and tolerance to drought and salinity (Schäfer et al., 2001). Rootstock choice also affects the content of bioactive compounds in citrus fruits (Legua et al., 2014). These compounds exhibit antioxidant properties that are beneficial to human health.
Rootstocks are commonly used in fruit farms for their favorable attributes to scions. However, there is a limited diversity available for citriculture rootstocks within Southern Brazil. The hybrid lemon 'Cravo' (C. limonia Osbeck) holds a notable position in domestic citriculture when used as a rootstock. In Rio Grande do Sul, the main rootstock use is the trifoliate orange [Poncirus trifoliata (L.) Raf.], as it shows and excellent adaptability to the local climate conditions, reduces the size of the scion, increases planting density, and is resistant to diseases, such as gummosis caused by Phytophthora. In addition, it improves fruit productivity and quality (Cunha Sobrinho et al., 2013;Carvalho et al., 2016). Nonetheless, this rootstock species is susceptible to drought. This problem is most evident both in years with drought stress and in very sandy soils, leading to production fluctuations. In some cases, the use of inappropriate rootstocks for a given scion can result in incompatibility issues or gum formation on the graft line, leading to minimally productive and short-lived orange trees (Pompeu Junior & Blumer, 2014).
The use of hybrid rootstocks is a method to increase the distribution of favorable characteristics within the available citric genetic diversity in Southern Brazil. For instance, hybrid citrandarin rootstocks are used due to their improved ability to tolerate citrus decline and sudden death, immunity to the tristeza virus, and resistance to gummosis (Pompeu Junior & Blumer, 2014).
Diversifying the variety of scions and rootstocks is significantly important for reducing risks. Fruit farms that restricts the use of the varieties that are available to them are subject to natural, biotic stress (in the form of plagues and diseases), as well as abiotic stress (mainly relating to climate).
The objective of this work was to evaluate the agronomic development of 'Valência' orange trees, combined with 13 rootstocks cultivated in climate and soil conditions of the Brazilian state of Rio Grande do Sul.

Materials and Methods
The experiment was performed in Monte Bonito (31º40'47"S, 52º26'24"W, at 60 m altitude), a district of Pelotas county, in the state of Rio Grande do Sul, Brazil. 'Valência' orange scions, combined with 13 rootstocks, were evaluated during the 2015 and 2016 harvests. Twelve-month-old seedlings of 40 cm height were planted in 5.0 m x 4.0 m spacing and analyzed at four and five years of age. The local soil is moderately deep, with a medium texture sand in the horizon A and clay in the horizon B, and it is classified as Argissolo Vermelho Amarelo (Santos et al., 2006), i.e., Oxisol. According to the Köppen-Geiger classification, the region exhibits a Cfa climate, indicating a humid subtropical climate with hot summers. Climactic data were obtained in an Embrapa experimental station near the fruit farm. The area has 1,590 mm mean annual precipitation, 18.4°C mean annual temperature, and 78.8% relative annual air humidity (INMET, 2016).
A randomized complete block experimental design was employed, with three replicates and nine plants per plot, and a bifactorial arrangement with 13 rootstocks over the 2015 and 2016 harvests. The three central plants of each block were evaluated. The rootstocks consisted of hybrids selected by the program for the improvement of Citrus genetics of Embrapa Mandioca e Fruticultura, in Bahia state, Brazil (Table 1).
The following vegetative characteristics were evaluated for: scion height (H), measured with a graduated ruler from the intersection point of stalk and root until the scion apex, expressed in meters; scion volume (V), obtained by the formula V= π/6 x H x (d1 x d2), provided by Fallahi & Rodney (1992), in which Pesq. agropec. bras., Brasília, v.54, e00461, 2019 DOI: 10.1590/S1678-3921.pab2019.v54.00461 d represents the diameter of the scion measured both longitudinally (d1) and perpendicularly to the line of the plant (d2); plant productivity, obtained by counting and weighing the collected fruit (kg per plant); production efficiency, corresponding to the number of fruit per unit of scion volume, obtained with the formula EP = (number of fruit × fruit mass) / V (expressed in kg m -3 ); and fruit farm productivity, obtained from fruit mass and production area (Mg ha -1 ).
Fruit were collected at the maturation stage, at the ready-for-sale point 10 on the scale adapted from Barbasso et al. (2005), which randomly considers the four quadrants of the scion. The collected fruit were placed in cleaned and disinfected plastic boxes and then transported to the laboratory of the Universidade Federal de Pelotas. There, fruit undergone a pre-selection, and damaged or otherwise affected fruit were discarded. In this way, fruit were effectively standardized. From the selected fruit, 20 ones from each replicate were assigned to analysis, which totaled three replicates per combination of rootstocks and scions.
The following physical characteristics were evaluated: fruit diameter (mm), measured with a digital caliper; mean mass (g) of whole fruit and peel, measured with a semi-analytical balance; juice yield (%), determined by taking the ratio of the whole fruit mass and pulp mass; peel color, identified with a Minolta CR-300 colorimeter (Konica Minolta, Ramsey, NJ, USA) and described using the Cielab color space; and chromatic tonality (hº), calculated as the arctangent of b*/a* and expressed in degrees.
The following chemical characteristics were evaluated: soluble solids content (°Brix), which was quantified using a PAL-1 digital refractometer (Atago, Bellevue, WA, USA); titratable acid, determined by titrating 10 mL of fruit juice, added to 90 mL of distilled water at 8.1 pH, with a digital burette containing a 0.1 N solution of sodium hydroxide, and expressed as a percentage of citric acid, in line with AOAC (Horwitz & Latimer Jr., 2005); the ratio of soluble solids to titratable acids, obtained by following the analytic manual of Instituto Adolfo Lutz (Zenebon et al., 2008); and ascorbic acid content, quantified using the official AOAC method (Horwitz & Latimer Jr., 2005) by oxidative titration with 2,6-dichlorophenolindophenol to the point of the initial conversion of the solution to pink (mm ascorbic acid 100 g -1 sample (Jacobs, 1958;Leme Junior & Malavolta, 1950).
The following phytochemical characteristics were evaluated: antioxidant capacity (µg Trolox equivalent g -1 juice), determined by spectrophotometry using a method adapted from Brand-Williams et al. (1995); and total phenolic compounds (mL gallic acid equivalent 100 g -1 juice, quantified using the Folin-Ciocalteu reagent in line with the method described by Swain & Hillis (1959). Data were analyzed for normality and homoscedasticity, using the Shapiro-Wilk and Hartley tests, respectively. Afterward, data were subjected to the analysis of variance at 5% probability. When significant, the Scott-Knott test was performed, at 5% probability, in order to compare the grouping of the means.
The trifoliate hybrid HTR-207 showed the lowest height (1.37 m). This indicates the potential of this species to be used as a dwarfing rootstock, thereby allowing of fruit farms with a greater plant density than usual ones. These kind of rootstocks are of significant interest due to the practicality they offer for fruit farm management (Pompeu Junior & Blumer, 2009).
In a study performed in the Bahia state, França et al. (2016) analyzed 'Tuxpan Valência' orange trees at four and eight years of age which had been combined with 14 rootstocks. They measured 2.20 m mean height, ranging from 1.40 m for TSKFL × CTSW-049 hybrid rootstock to 2.80 m for 'Indio' citrandarin rootstock.
The most rootstocks studied herein increased the scion volume over the evaluation period, with the most notable performance in the 2016 harvest attributed to HTR-206 and TSKC × CTTR-028.
Rootstocks TSKC × TRDP-026, HTR-206, TSKC × CTTR-028, and TSKC × CTQT1434-004 provided the most favorable fruit productivity values per plant in their fifth year of age, with values of 14.6, 17.4, 14.0, and 16.8 kg per plant (Table 3), respectively. This may be the result of the climactic conditions in the South region of Brazil during the evaluated period, as the mean temperature and rainfall were 18.4ºC and 1,590 mm, respectively. However, in their study of 'Tuxpan  (Table 3). These results suggest that these rootstocks induce precocity in fruit production. In addition, the hybrids TSKC × TRDP-026, HTR-206, HTR-208, and TSKC × TRDP-023 showed higher-production efficiency values specifically in the 2016 harvest. This is most likely the result of greater vegetative growth during the 2015 season, which led to greater production in the following year.
However, the first harvest showed no difference for productivity between the rootstocks due to plant precocity, and had an average value of 2.0 Mg ha -1 . The rootstock TSKC × CTQT1434-004 provided 3.0 Mg ha -1 , which is the highest-productivity value (Table 3).
In the first season, juice yield varied between the different rootstock combinations (Table 4), with TSKC Values followed by equal letters, lowercase in the columns and uppercase in the rows, do not differ, by the Scott-Knott's test, at 5% probability. (2) F, analysis of variance. ns Non significant. *, **Significant at 5 and 1% probability, respectively.
Total soluble solids varied significantly across the evaluation years (Table 4). In the first year, the highest values were provided by TSKC × TRDP-023 (11.8ºBx) and the citrandarin 'San Diego' (11.4ºBx), whereas in the second year, the highest values were observed for HTR-207 (10.4ºBx), which meet the minimum requirements for total soluble solids. In a study performed in Paraná state, Brazil, Auler et al. (2008) found 10.4 and 11.9ºBx when using 'Cravo' lemon and 'Troyer' citrange, respectively, as rootstocks for 'Valência' oranges. However, França et al. (2016) found 10.8 and 11.3ºBx which were the highest values for TSKC × CTSW-028 and TSKFL × CTTR-017, respectively, over the course of four harvests in their aforementioned study.
Alternatively, França et al. (2016) found acidity values below 1% in their aforementioned study. Likewise, Auler et al. (2009) found values lower than 1.2% in 'Valência' oranges produced in the northeastern Paraná state, in the first three harvest years of their experiment.
Variations were found across the different samples for the ascorbic acid content, whose highest values were observed for TSKC × CTRK-001 and TSKC × TRDP-023, with 50 mg 100 g -1 juice ( Table 5).
The relative values of antioxidant capacity and phenolic compound content decreased from the first to the second harvest (Table 5). The number of fruit per plant affected the orange quality, as a consequence of the source/ drain relationship that characterizes the distribution of photoassimilates to all fruit. The rootstocks HTR-207, TSKC × TRDP-023, and citrandarin 'San Diego' showed higher-antioxidant capacity than the other combinations across both harvests, with total productions of 5.1, 11.0, and 8.5 kg per plant, respectively, in 2016. The overall mean value of antioxidant activity across all tested combinations was 151.36 Trolox g -1 juice.
There was a significant variation for phenolic content between the different combinations (Table 5). TSKC × CTRK-001 and citrandarin 'San Diego' were associated with an elevated amount of bioactive compounds in the first harvest year. In a study of 'Clemenules' tangerines, Legua et al. (2014) found that the rootstock plays an important role in the determination of the concentration of phenolic compounds. In particular, they obtained 60 mg gallic acid equivalent 100 g -1 when using the 'Volkameriano' lemon as a rootstock. However, in the 2015 harvest, TSKC × CTRK-001 and citrandarin 'San Diego' had 102.7 and 106.4 mg gallic acid equivalent 100 g -1 . This shows that these rootstocks provide to 'Valência' orange the capacity to produce greater quantities of phenolic compounds, increasing the health potential that fruit can offer due to their antioxidant activity. Values followed by equal letters, lowercase in the columns and uppercase in the rows, do not differ by the Scott-Knott test within 5% probability.
2. The rootstock TSKC × TRDP-023 provides fruit with high levels of soluble solids, titratable acid, color, and ascorbic acid; and the rootstock citrandarin 'San Diego' offers fruit with improved color and phenolic compound content for 'Valência' oranges in early stages of production, under the soil and climactic conditions of Rio Grande do Sul.