Potential rootstocks for Valencia sweet orange in rain-fed cultivation in the North of São Paulo, Brazil

1 Universidade Estadual de Santa Cruz – UESC, Ilhéus, BA, Brazil 2 Embrapa Mandioca e Fruticultura, Cruz das Almas, BA, Brazil 3 Estação Experimental de Citricultura de Bebedouro, Bebedouro, SP, Brazil 4 Universidade Federal do Recôncavo Baiano – UFRB, Cruz das Almas, BA, Brazil 5 Universidade de São Paulo – USP, Piracicaba, SP, Brazil 6 Universidade Estadual Paulista – UNESP, Jaboticabal, SP, Brazil


INTRODUCTION
Brazil is the major producer of orange [Citrus sinensis (L.) Osbeck] in the world (FAO, 2016). Production is most prominent in the State of São Paulo (SSP), which produced 279 million orange boxes (40.8 kg/box) in 2014/2015, with the North region of the state responsible for 63.8 milion boxes amounting to 22.9% of SSP production .
Rangpur lime [C. limonia (L.) Osbeck] rootstock has several agronomic characteristics which made it the most used rootstock in the country: tolerance to tristeza and drought, high number of polyembryonic seeds per fruit, compatibility with most scion varieties, early bearing and high productivity for all scions grafted to it, and fair fruit quality. It also has important limitations, such as susceptibility to blight, citrus nematode, citrus sudden death (CSD), as well as moderate susceptibility to gummosis from Phytophthora spp. Castle, 2010).
The scion used was Valencia IAC sweet orange. The hybrids and citrus species evaluated as rootstocks are listed on Table 1 Fruit quality was evaluated in 2011 to 2013, and 2015. Ten fruits were randomly picked on the medium canopy height around the trees in each plot. Samples were collected in October/November, whenever fruits presented typical mature appearance, as this is the usual harvesting period for Valencia in the locality (Nonino, 1995), and the following factors were evaluated: weight, diameter and height of the fruits, total soluble solids (SS) measured in a refractometer According to Passos et al. (2007) and Vasconcelos & Araújo (1975); 2 Numbers of the identifying codes regarding to introduced germplasm are given as the original registration number received in Brazil, while hybrids obtained by Embrapa follow the Citrus Breeding Program own nomenclature.
The Valencia orange trees grafted on TSKFL x CTARG -028,  TSK x TRSW -311, CLEO x TRSW -295, CLEO x TRSW -71158,  CLEO x TRSW -224, TSK x TRSW -308, CTC 25,  CLEO x TRSW -287 and TSKC x LHA -010 did not differ from those grafted on Santa Cruz Rangpur lime, being classified in the group of the highest average annual production per tree (Tables 3). Rootstocks LVK x LCR -017, LVK x LCR -048 and Sacaton citrumelo presented the lowest average annual production per tree of the 27 rootstocks evaluated. For the quality attributes, the rootstocks TSK x TRSW-308 and Thomasville 1439 were superior to Santa Cruz Rangpur lime regarding all variables analyzed, including lower acid (Table 4). CRC 3551 CN trifoliate orange, TSKC x LHA -004, TSKFL x CTARG -028 and TSKC x LHA -010 had similar results with the exception of lower ratio as titratable acidity was the same of Rangpur lime. Rootstocks TSKFL x CTARG -028 and TSKC x LHA -010 were also highlighted for being in the group of the most productive together with Santa Cruz Rangpur lime rootstock (Table 4).
The mean values of soluble solids varied from 9.89 to 11.06 ºBrix, with the mean of the years (2011 to 2015, except 2014) of 10.62 ºBrix. Except for rootstocks TSKC x LHA -004, LVK x LCR -030, TSKFL x LRM -011, LVK x LCR -048 and Sacaton citrumelo, the other rootstocks were superior to Rangpur lime (10.3 ºBrix), varying from 10.4 to 11.1 ºBrix. Conversely, the highest values of soluble solids were obtained in the year 2015, reaching a mean of 11.4 ºBrix, without difference among the rootstocks in this year nor in 2011 and 2013 (Table 5).
The experimental planting design was randomized blocks with four replications and four plants per plot, with 27 rootstocks. Analysis of variance was performed by plots subdivided in time, and for grouping of the means of treatments, the Scott-Knott multiple comparison test was used at 5% probability. Scott-Knott uses a hierarchical cluster analysis to partition treatments into distinct groups with no overlapping as with Duncan's Multiple Range Test (Scott & Knott, 1974). Statistical analyses were performed using the program AgroEstat (Barbosa & Maldonado Junior, 2015).

RESULTS
At seven years of age, the rootstocks presented differences for height, diameter and canopy volume, forming six groups of mean (Table 2). Of the 27 rootstocks evaluated, 11.1, 33.3 and 41.7% induced plants that were higher, larger and bulky, respectively, than those of trees on Santa Cruz Rangpur lime. Dancy mandarin rootstock was noteworthy for the induction of the largest trees, whereas Sacaton citrumelo rootstock resulted in the smallest Valencia trees.
The production per tree of Valencia orange from 2011 to 2015 varied with the year of evaluation, with usually increasing yields and most of the rootstocks presenting the highest production per tree in 2015 (Table 3).

DISCUSSION
The search for superior citrus rootstocks which impart lower plant size, greater productivity and increased fruit quality is needed for citrus cultivation around the world (Schäfer et al., 2001). Based on these characteristics, rootstocks CLEO x TRSW -71158, CLEO x TRSW -224, CTC 25, CLEO x TRSW -287 and TSK x TRSW -308 were the most efficient in terms of production efficiency, as they exhibited a smaller tree size and similar production per tree in relation to the reference rootstock, Santa Cruz Rangpur lime. In this work, rootstocks which were hybrids of Volkamer lemon x Rangpur lime induced semi-dwarfing and dwarfing canopy volume, which was not expected. Only Sacaton citrumelo was truly dwarfing (reduction to 25% of full size as represented by Santa Cruz Rangpur lime), according to the classification of Castle & Phillips (1977), even though it was previously described as normal size inducer, being equal or superior to 2.80 m of height (Arana et al., 2006). Ramos et al. (2015) evaluated the preliminary behavior of Valencia orange on 44 rootstocks and found that, among the non-trifoliate types, Rangpur lime induced the best production, similar to the results in this study. This is consistent with the general consensus that Rangpur is highly productive in rain-fed conditions .
Nine hybrid rootstocks of P. trifoliata induced fruit production per plant equivalent to Rangpur lime, and thus have great potential for use in the North region of SSP in rain-fed cultivation, while reducing risk of CSD. It must of Valencia on all rootstocks analyzed conformed with this range in most of the years evaluated.
Variation among rootstocks for soluble solids was only observed in 2012 (Table 4), a year in which drought conditions were present before and during harvest. Moderate water stress during maturation may sometimes enhance fruit quality (Aguado et al., 2012). The processing index values (2011-2015 means) recorded for rootstocks in this work were substantially lower than those reported by Di Giorgi et al. (1990) and by Nonino (1995), for the be highlighted that among the four rootstocks selected as best in the present work (Figure 1), three are citrandarins, which emphasizes their potential for rain-fed cultivation (Blumer & Pompeu Junior, 2005). The poor fruit production of trees with Dancy mandarin rootstock was also previously reported (Pompeu Junior et al., 2003). Trifoliate orange and its hybrids generally induced the scion to produce fruits with traits superior to those obtained for other rootstocks, as found by Bordignon et al. (2003). According to Koller (1994), the ideal ratio is between 10 and 16. The fruits Rootstocks that result in trees of smaller size, with high overall productivity and production efficiency are desirable for higher density orchards. Several rootstocks in this trial displayed significant dwarfing potential as they are plants of small size, efficient and productive that can be employed in high density orchards. In January 2017, trees were scouted and it was possible to observe some Valencia orange trees grafted on Santa Cruz Ranpur lime rootstock presenting visual symptoms that resemble CSD. Trees on LVK x LCR hybrids presented poor plant growth and general nutritional defficiency with more than 50% of tree death. Although trees on CLEO x TRSW -71158 performed well, they presented general chlorosis, shoot dieback and incompatibility symptoms in the graft union, such as crease and rootstock phloem yellowing, which may limit its use as rootstock of Valencia sweet orange. It was also observed general nutritional defficiency on TSK x TRSW -308 and the proeminent overgrowth of the scion trunk on TSKFL x CTARG -002 rootstocks. Trees grafted on Dancy mandarin were the most vigorous without any symptoms of biotic either abiotic stresses and no tree loss. Considering the overall tree appearance, stand and fruit load in comparison to a commercial block of Valencia orange grafted on Swingle citrumelo with the same age and similar management just next to the experimental plot, the main rootstocks are ranked in the following descending order: Swingle citrumelo > CTC 25 > TSKFL x CTARG -028 > CLEO x TRSW -287 > CLEO x TRSW -295 > Santa Cruz Rangpur lime with the other rootstocks with similar or inferior perfomance than Rangpur lime.

ACKNOWLEDGEMENTS
To Mr. Nelson, Aroldo and Marcelo Calegari, owners of Santa Maria Farm, for the technical support and availability of the area, to Embrapa Cassava & Tropical Fruits, for the financial support, to the Citrus Experimental Station of Bebedouro for the technical and financial support, and to CAPES-Embrapa, for the Doctorate scholarship of the first author.