Sunki Mandarin vs Poncirus trifoliata Hybrids as Rootstocks for Pera Sweet Orange

Obtaining new rootstocks that have resistance to biotic and abiotic factors is one of the main goals of breeding programs for citrus. This study evaluated the performance of 42 hybrids of Sunki mandarin (Citrus sunki Hort. ex Tanaka) vs. Poncirus trifoliata cv. Rubidoux, as rootstock for Pera sweet orange. The experiment was conducted in Colômbia, São Paulo, Brazil, in randomized blocks with three replications in a spacing of 6.0 m x 3.5 m. The trees were seven years old and the experiment was conducted without irrigation. We quantified the variables height, diameter and canopy volume, and production of plants, besides the physico-chemical analysis of fruits. After the natural period of drought in the region, the trees were evaluated for resistance to drought, with scale ranging from 1 to 3. Plants were also evaluated for symptoms of citrus sudden death and compatibility canopy/rootstock. Differences were observed in height, diameter and canopy volume of Pera sweet orange grafted on 42 hybrids of Sunki mandarin x Poncirus trifoliata (TSxPT). Differences were observed in the physico-chemical characteristics of fruits of Pera sweet orange grafted on different hybrids TSxPT. Different degrees of drought tolerance were observed and six hybrids were resistant. The hybrids TSxPT 245 and 254 showed incompatibility with variety of Pera sweet orange.


Introduction
Hybrids from crosses of Sunki (Citrus sunki hort.ex Tanaka) or Cleopatra (C.reshni hort.ex Tanaka) mandarins with Poncirus trifoliata (L.) Raf. are called citrandarins, in which we want to gather the advantages presented by the mandarins, as lower susceptibility to citrus decline, to exocortis viroid and calcareous soils, with those of P. trifoliata, as resistance to citrus tristeza virus, to root rot caused by Phytophthora, frost, and induce the formation of smaller plants (Blumer & Pompeu Junior, 2005;Pompeu Junior & Blumer, 2009, 2011).
Today, with the emergence and spread of Huanglongbing (HLB) in the State of São Paulo, Brazil, smaller citrus plants are desirable because they have some advantages over larger ones.Smaller plants may increase the efficiency of inspection and hence the control of pests and diseases, reduce production costs, increase security at harvest, produce more fruit per cubic meter of canopy and allow high planting densities, enabling greater yield per area (Pompeu Junior, 2001;Pompeu Junior & Blumer, 2009).
In the State of São Paulo, most citrus orchards are not irrigated and flowering usually occurs between the months of August and September, a period of lower rainfall, making it necessary to use rootstocks tolerant to drought, an important characteristics of Rangpur lime (C.limonia Osbeck), conferring earliness and high yield to canopy varieties grafted on it (Pompeu Junior, 2005), however, the Rangpur lime is susceptible to citrus decline (Rodriguez et al., 1979) and citrus sudden death (Fernandes & Gimenes-Bassanezi, 2001).
With so many adversities in citrus, obtaining new rootstocks for diversification in orchards, is one of the main objectives of the breeding programs of citrus.The Centro APTA Citros 'Sylvio Moreira'/Instituto Agronômico is developing a breeding program of rootstocks having as goal to maintain and extend the studies of selection of new rootstocks, either by obtaining hybrids or the use of existing accesses of the Active Germplasm Bank of citrus.
This study aimed to evaluate the performance of 42 hybrids of Sunki mandarin vs Poncirus trifoliata cv.Rubidoux as rootstock for Pera sweet orange [C.sinensis (L.) Osbeck].

Materials and Methods
The work was conducted at Muriti farm property of the Fischer group, in the municipality of Colômbia, São Paulo State, Brazil, geographic coordinates 24º 17' S and 48º 24' W, altitude of 480 meters and climate type aw, according to the Köppen classification (Ortolani, Júnior, & Alfonsi, 1991).The average annual rainfall is 1429.1 mm and the average annual temperature is 23.8°C, with an average maximum temperature equal to 30.7°C and minimum of 17.0°C (CEPAGRI, 2011).
The experiment was arranged in randomized complete block design.It was implemented in 2003, in a spacing of 6.0 m x 3.5 m, initially consisting of 111 hybrids of Sunki mandarin vs P. trifoliata cv.Rubidoux (TSxPT) grafted with Pera sweet orange with three replications and conducted without irrigation.Forty-two of these TSxPT hybrids were selected for purposes of evaluations in this work.
The height and diameter of the tree canopy were determined from measurements using a ruler graduated in centimeters.We calculated the volume of the canopy using the function (Mendel, 1956): We evaluated also the fruit yield (kg tree -1 ) and calculated the values of productivity (kg m -3 ), in the years 2007 and 2010.
For physical and chemical analysis, samples of five fruits were collected of each plant in the second week of October 2009 and sent to the Quality and Postharvest Laboratory of Centro APTA Citros 'Sylvio Moreira'/Instituto Agronômico, Cordeirópolis, SP.Measurements of height (H) and diameter (D) of the fruits were made by direct reading of each sample, with the help of ruler graduated in centimeters, and from these values, we calculated the relation H/D of the fruit.The total mass of the fruits was measured in grams, and the characteristics of juice were obtained as follows: the juice yield was determined after crushing of five fruits of each sample in extractor OIC model OTTO 1800 andcalculated using the relation between juice/fruit mass and expressed as a percentage, the soluble solids ( o Brix)content was obtained by direct reading using refractometer B & S model RFM 330, , the juice acidity was determined by titrating an aliquot of 25 mL of juice to pH 8.2, using a solution of sodium hydroxide (NaOH) with normality of 0.3125 N, and phenolphthalein as indicator.The ratio was calculated using the relation between soluble solids: acidity and the technological indexwas obtained according Di Giorgi et al. (1990) and expressed in Kg of total soluble solids (TSS) per box with 40.8 kg of fruit In the years 2007 and 2010, after about 90 days of drought in the region of the experiment, the plants were evaluated for tolerance to drought, assigning scores ranging from 1 to 3, and score 1 to plant showing highly susceptibity, score 2 for moderately susceptible plant and score 3 to tolerant plants.
We also evaluated the compatibility between the scion variety (Pera) and each rootstock studied (TSxPT hybrids), identifying incompatible in combinations that had a line of gum or necrosis observed after removal of the bark of the trunk, in the region of grafting (Nauriyal, Shannon, & Frolich, 1958).It is known that Pera sweet orange is incompatible with trifoliate orange and some of its hybrids (Pompeu Junior, 2005).
The resistance of citrus sudden death was evaluated by withdrawing part of the cortex, evaluating the presence of symptoms characteristic of the disease, ie, yellow-orange color in the internal tissues of the rootstock below the graft region, contrasting with the color of the inner tissue of the bark of the canopy.These tissues yellowish correspond for the most part to the phloem vessels that are obstructed and degenerate (Gimenes-Fernandes & Bassanezi, 2001   There were differences in the yield of the plants in both years of evaluation.The highest fruit yield of Pera sweet orange were observed in plants grafted on hybrid TSxPT 155, 142, 54, 92, which had the cumulative production above 135 kg tree -1 (Table 2).Cristofani-Yaly et al. ( 2007) also observed that the hybrid TSxPT 54 was one of the most productive.The correlation between the volume of plant canopy and fruit production in 2010 was 0.68 (P <0.05) (Figure 2), showing that the most productive plants also showed higher canopy volumes.Plant yield was calculated in kg of fruit per cubic meter of canopy and an average of two years of evaluation, the values ranged from 5.0 to 12.4 kg m -3 (Table 2).Significant differences were observed for the variables mass (g), height (cm) and diameter (cm) of fruit, juice yield (%) and number of fruits per 40.8 kg box (Table 3).For the height/diameter of fruits no differences were observed.
For fruit mass values were between 180 g (TSxPT 166) and 289 g (TSxPT 121).For juice yield values were between 42% (TSxPT 38) and 56% (TSxPT 299). ) Volume (m 3 ) Figure 2. Correlation between production (kg tree -1 ) and canopy volume (m 3 ) of Pera sweet orange plants grafted on 42 hybrids of Sunki mandarin x Poncirus trifoliata (Colômbia, SP, Brazil, 2010), We observed significant differences among the rootstocks for acidity, Brix, ratio and total soluble solids per box with 40.8 kg of fruit (Table 4).Di Giorgi et al. (1994) report that for Pera sweet orange, the fruit must be harvested when normally meet acidity levels between 0.6 to 0.9%, achieving this stage largest concentrations of soluble solids and lower rates of degradation of fruit that causes the loss of their commercial value both for fresh fruit market or for processing in the industry.Thus, it was found that during assessment of internal qualities of the fruit (October/2009), the juice acidity ranged from 0.58 (TSxPT 121) to 1.29 (TSxPT 128), and most part of the canopy / rootstock combinations studied were within the standards set for harvest (Table 4).
For the values of ratio, Viégas (1991) reports that the range between 11 and 14 is ideal for industrialization and that these levels the acidity are still adequate to maintain the quality and conservation of juice after processing.In this study the majority of canopy / rootstock combination showed ratio values within the ideal range, however, some hybrids TSxPT (42,121,142,152,190 and 248) induced early ripening of the fruits of Pera sweet orange, with values above 15.5.Moreover, the hybrids TSxPT 68, 110, 128, 136 and 143 had values of ratio below 10, providing late maturation of the fruits of Pera sweet orange (Table 4).

Table 1 .
). Height, diameter and canopy volume of plants of Pera sweet orange grafted on hybrids of Sunki mandarin x Poncirus trifoliata.Colômbia, SP, Brazil, 2007-2010 Means followed by same letter do not differ by Scott-Knott test (P ≤ 0.05).

Table 3 .
Physical characteristics of the fruits of Pera sweet orange grafted on hybrids of Sunki mandarin x

Table 4 .
Chemical characteristics of the fruits of Pera sweet orange grafted on hybrids of Sunki mandarin x Poncirus trifoliata.Colômbia, São Paulo, Brazil, 2009