Plant growth , yield and fruit quality of Clementine mandarin selections under subtropical climate in Brazil

1Universidade de São Paulo. Piracicaba-SP, Brazil. E-mails: srsilva@usp.br; tatiana.cantuarias@gmail.com 2Embrapa Mandioca e Fruticultura, Cruz das Almas-BA, Brazil. E-mails: eduardo.girardi@embrapa.br; magno.santos@embrapa.br; eduardo. stuchi@embrapa.br Abstract In Brazil, few mandarin (Citrus reticulata, Blanco) cultivars are being currently exploited, limiting the availability of fresh fruit in the local market. This study was aimed to investigate the performance of 15 Clementine mandarin selections and 3 hybrids, seeking for alternative varieties for production under subtropical conditions. Fifteen Clementine selections, namely: ‘Oroval-SRA335’, ‘Tomatera-SRA535’, ‘Reina-SRA534’, ‘Bruno-SRA531’, ‘Nules-SRA389’, ‘Ragheb-SRA386’, ‘Oroval-Y45’, ‘2-Kr-Monreal’, ‘Commune-SRA88’, ‘Commune-SRA85’, ‘Caffin-SRA385’, ‘De Nules-VCR’, ‘Commune-SRA92’, ‘Clemenules-EECB128’ and ‘Marisol-EECB126’; three hybrids (‘Clemelin-IVIA335’, ‘Caçula-1’ and ‘Caçula-3’) and the standard commercial varieties ‘Ponkan’ and ‘Cravo’, grafted on ‘Swingle’ citrumelo [C. paradisi Mafad. cv. ‘Duncan’ x Poncirus trifoliata (L.) Raf.], were evaluated in Bebedouro, São Paulo State, Brazil, from 2004 to 2008. The experimental plot was planted in 2001 with the 20 genotypes disposed in a randomized block design, with three replications and two plants per plot. The ‘Cravo’ variety and the ‘Clemelin-IVIA335’ hybrid presented the smallest plant size. The selections ‘Cravo’, ‘Caffin-SRA385’, ‘Clemelin-IVIA335’, ‘Marisol-EECB126’ and ‘Ragheb-SRA386’ had the smallest cumulative yield. Fruits of ‘Caçula-1’ had the highest soluble solids content and those of ‘Caçula-3’ showed the highest juice content. Among the Clementine selections, ‘Bruno-SRA531’, ‘Clemenules-EECB128’, ‘De Nules-VCR’ and ‘Reina-SRA534’ reached earlier maturity. In terms of yield efficiency, ‘Ponkan’ trees performed similarly or outperformed the other selections, with larger fruits and earlier ripening. The ‘Reina-SRA534’ selection outstood due to its high fruit yield and quality, with medium fruit size and early bearing, while ‘De Nules VCR’ and ‘Tomatera-SRA535’ had high yield, low alternate-bearing and fine fruit quality, despite of their large plant size. Good performance of Clementine selections and hybrids encourages further studies on cultural practices for growing this group of mandarins under subtropical climatic conditions.


Introduction
The global production of mandarins in 2016 reached 32.8 million t (FAO, 2018).Brazil occupied the sixth position in the world production rank, with nearly 1,000,000 t, harvested from approximately 50,000 ha, being the State of São Paulo the main producer, accounting for 35% of the national production (IBGE, 2018).
The local market of mandarins is limited to a few varieties.Mandarin 'Ponkan' (Citrus reticulata Blanco) is the most planted and is the most preferred variety by Brazilian consumers, because of its pleasant taste and ease of peeling, with low acidity and high sugar content.'Murcott' tangor [C.reticulata Blanco x C. sinensis (L.) Osbeck] is the second most planted variety, followed in smaller proportions by 'Cravo' (C. reticulata) and 'Montenegrina' mandarins (C.deliciosa Tenore) (PIO et al., 2005).
'Ponkan' mandarin is harvested from May to July, while tangor 'Murcott' is harvested from August to November (CEAGESP, 2014a).However, the 'Murcott' mandarin has numerous seeds and is more difficult to peel (PIO et al., 2005), being also highly susceptible to brown spot caused by Alternaria alternata (SOLEL et al., 1997;PERES et al., 2003).
'Clementines' (C.clementina Hort.ex Tanaka) have been poorly exploited in Brazil.They present low susceptibility to Alternaria brown spot, variable fruit size and variable skin color, depending on the variety, with a firm and adherent skin, that peels off easily, and fruits with a few or no seeds at all when grown in isolated plots (HODGSON, 1967).In Brazil few studies have been published regarding the production and fruit quality of Clementine selections (PIO et al., 2006;SILVA et al., 2009).
Therefore, the horticultural performance of fifteen Clementine mandarin selections, three hybrids and the 'Ponkan' and 'Cravo' varieties grafted onto Swingle citrumelo, was evaluated under the subtropical climatic conditions of São Paulo State, in Brazil, with the aim of selecting alternative mandarin cultivars for increasing fruit supply into the local market.

Materials and Methods
The experiment was conducted in the county of Bebedouro, in the northern region of São Paulo State, Brazil (20°53'16" S, 48°28'11" W, with 601 m altitude).Climate is Köeppen's Cwa, i.e., subtropical highland climate (Figure 1).The experimental plot was planted in February 2001 in a 6.0 m × 3.0 m spacing (555 trees per ha), on a haplustox soil -a deep dark red oxisol, eutrophic, endoalic, with a moderate A horizon with clayey texture (38% clay) [pH (CaCl2) = 5.7; CEC = 55 mmol c dm -3 in the 0-20 cm of upper surface soil layer].The plot was drip irrigated since 2006, after five years from planting.Field evaluations were concluded in 2008, at the seventh year after planting.
Dolomitic limestone was applied before installing the experiment, (70% acid neutralizing capacity in order to increase soil base saturation to 70%) and it was reapplied when the soil base saturation reached 60%.The mean annual fertilization rates applied per tree from 2001 to 2008 were 213 g of Nitrogen, 121 g of P2O5 and 119 g of K2O.The plants were grown under the standard recommended practices for mandarin cultivation in Brazil, with no fruit thinning and pruning.Weeds were controlled by mechanical cutting of the inter-row natural vegetation (Brachiaria spp.), supplemented by herbicide applications along the tree rows.
The following variables were measured: plant height (PH), assessed from the ground to the top of the tree; mean canopy diameter (CD), calculated by averaging tree width measured in parallel (D l ) and perpendicular ]}, where n = total number of years evaluated, and a 1 , a 2 , ..., a n-1 , a n = production in the corresponding year (STENZEL et al., 2003).
Fruit quality was assessed in samples of 10 fruits per plot that were annually collected in May, according to the visual maturation and softness.In these samples, the following variables were determined: fruit weight, diameter and height; total soluble solids content (TSS), measured with a refractometer (Palette PR-101, ATAGO, Tokyo, Japan); total acidity (TA), obtained by titration with sodium hydroxide (0.3125 N); ratio, calculated as TSS/ TA and juice yield (%), determined in an extractor (Otto 1800, OIC, Limeira, São Paulo, Brazil).
The trial was set following a randomized block design with 20 treatments (genotypes), three replications and three trees in each plot.Results were submitted to the analysis of variance and means were grouped by the Scott-Knott test (P ≤ 0.05).Data that did not meet normality and variance homogeneity assumptions were submitted to an arcsine square root transformation.Statistical analyses were performed using the program AgroEstat (BARBOSA and MALDONADO JR., 2015).
In this study, 'Ponkan' trees were 3.57 meterhigh, smaller than the average tree height of 4.50 meters recorded for this variety by Pio et al. (2006) in Capão Bonito, southern São Paulo State, under cooler and wetter weather conditions.In another study conducted in the Guilan province, in Iran, 'Ponkan' mandarin trees were also taller, but had smaller diameter than the 'Cadoux' clementine (ABEDI-GHESHLAGHI and FIFAEI, 2012).Scion/rootstock combinations resulting in small and highly productive plants are more suitable for establishing highdensity plantings that allow easy-to-pick orchards.For this purpose, mandarin genotypes with smaller tree size would be definitely more appropriate and should be preferred.
When analyzing the cumulative yield -CY -in the 2004-2008 period (Table 2), the genotypes were classified into three different groups : a) highest yielding selections (98.6 < CY < 156.6 kg plant -1 , equivalent to 54.8 to 87.0 t ha -1 ), including the 'Ponkan' and all the other evaluated genotypes, except for the 'Caffin-SRA385' mandarin; b) selections with intermediate CY (65.6 < CY < 89.0 kg plant -1 , equivalent to 36.4 to 49.4 t ha -1 ), including the 'Clemelin-IVIA335' hybrid, the 'Rhageb SRA386' and 'Marisol-EECB126' selections and the 'Cravo' standard variety, and c) the 'Caffin-SRA385' selection, that showed a very low CY during the evaluated period, with only 16.5 kg plant -1 (9.1 t ha -1 ), and its first harvest recorded only in 2006 (five years after planting).Under Mediterranean climatic conditions in Corsica, France, the 'Carrizo' citrange, used as rootstock for the 'Commune-SRA92' mandarin, induced the highest mean yield (62.00 kg plant -1 ) along a 9-year time period (HUSSAIN et al., 2013), being this value smaller than the yields observed in this study for all selections in the first five harvest seasons.
Plants of the 'Nules SRA389', 'Ragheb-SRA386', 'Oroval-Y45' and 'Caffin-SRA385' selections together with the 'Caçula-1' and 'Caçula-3' hybrids and the 'Cravo' and 'Ponkan' standard varieties had very high ABI values (between 0.53 and 0.72), while the other selections showed lower ABI values (from 0.28 to 0.46) (Table 2).High ABI observed in this study may be partially explained by the fact that no fruit thinning was performed.In the other hand, low ABI values, as those observed on most of the Clementine selections evaluated in this study, were also reported by Abedi-Gheshlaghi and Fifaei (2012) and by Pio et al. (2006).In the State of Paraná, in the southern region of Brazil, 'Ponkan' trees cultivated without irrigation onto seven rootstocks, different from 'Swingle' citrumelo, also reached low ABI values (between 0.28 and 0.48), depending on the rootstock (STENZEL et al., 2003).
Mean values of ratio in the juice followed the same trend as TA (Table 3).The 'Ponkan' standard variety stood out with its significantly higher ratio (20.42), being the earliest ripening variety, and was followed by a group including 'Reina-SRA535', 'Marisol-EECB126', 'De Nules VCR', 'Caffin-SRA389' and the 'Cravo' standard variety,with 13.15 < ratio < 15.41.All the remainder selections showed significantly lower ratio values (9.21 < ratio < 12.70).Citrus fruit quality is highly influenced by the climate, particularly for mandarins (REUTHER, 1973).These results confirm that typical subtropical climatic conditions, as those existing in the Bebedouro region (Figure 1), are not restrictive for commercial mandarin production, and adequate fruit yield and quality may be achieved with some selections.Nonetheless, the outstanding performance of the 'Ponkan' variety confirms its better adaptation to the local climatic conditions.
Eight years after planting, no symptoms of graft incompatibility were observed in the twenty studied scion/rootstock combinations, confirming that 'Swingle' citrumelo is a suitable rootstock for the various Clementine mandarins evaluated in this study, under the local soil and climatic conditions.
Under milder climatic conditions in the southern region of Brazil, 'Ponkan' trees grafted on seven different rootstocks had high fruit yield, but lower fruit juice content and ratio than those observed in this study (STENZEL et al., 2003).For 'Ponkan' and 'Nules-SRA389', Pio et al. (2006) found similar TSS, TA, and ratio than those observed in this study.
Except for their smaller fruit size and weight as compared to the 'Ponkan' and 'Cravo' standard varieties, most of the Clementine selections evaluated in this study overcame the quality standards established for the Brazilian market (CEAGESP, 2014b).For both standard varieties, minimum juice content of 35% to 40%, 9.0 °Brix SS and 9.5 ratio have been established.Fruit thinning could improve fruit size and weight of the Clementine selections grown under subtropical climate, attending the internal market requirements of larger fruit size (GONZATTO et al., 2016).
According to the United Nations normatives, all the selections evaluated in this study reached appropriate fruit size and ratio for export, i.e., minimum equatorial diameter > 35 mm and ratio > 7.0, but only a few selections developed the adequate juice percentage of 40% (UNECE, 2014).Most of the evaluated selections were early-bearers (Table 3).This observation corroborates the results obtained by Silva et al. (2009).The authors also evaluated most of these selections in other orchards in the Bebedouro region, and found significant variations in the optimum harvest date among varieties, with 'De Nules VCR' and 'Bruno-SRA531' being the earlier bearers and earliest ripening selections.Both varieties may be harvested starting in February, while the 'Clemelin-IVIA335' is harvested last of all, by September.
Fruit of the 'Caçula-1' hybrid had high juice content and acidity, as well as the highest TSS content.The 'Caçula-3' hybrid had similar attributes, except for its lower TSS, which could result in a later harvest period, probably with no flesh crystallization, due to its higher juice content (Table 3).Pio et al. (2000) also noted good fruit quality of the 'Caçula-3' and 'Caçula-4' hybrids, with the chance of developing seedless fruits.However, ABI was high for both of these hybrids (Table 3).
'Ponkan' mandarin grafted on Swingle citrumelo outstood when compared to the other mandarin selections of this study, due to its higher yield efficiency and the production of larger and sweeter fruit, which are more suitable for the internal market.However, some of the Clementine selections evaluated in this study also showed good performance.plant materials; thanks also to the Estação Experimental de Citricultura de Bebedouro for providing the experimental area and technical and administrative support, and to Prof. Dr. Luiz Carlos Donadio (Unesp/FCAV), who introduced the evaluated selections in the EECB.
Clementine' and 'Cravo' as parentals, performed by Prof. Dr. Luiz Carlos Donadio in 1972 at the Universidade Estadual Paulista "Júlio de Mesquita Filho" (Unesp/FCAV), in Jaboticabal,São Paulo State, Brazil.These Clementine selections did not present Alternaria brown spot symptoms on leaves and fruits in previous studies at the
*Fruits were annually harvested in May.