Refereed Manuscript
Proc. Fla. State Hort. Soc. 125:50–55. 2012.
Salinity Tolerance of ‘Valencia’ Orange Trees on Allotetraploid Rootstocks
Jude W. Grosser*1, Ahmad A. Omar1, 2, Julie A. Gmitter1, and James P. Syvertsen1 1University of Florida/IFAS, Citrus Research and Education Center, Lake Alfred, FL 33850 2Zagazig University, College of Agriculture, Biochemistry Department, Zagazig 44511, Egypt
Additional index words. abiotic and biotic stress tolerance, conventional breeding, somatic hybridization, tetrazyg rootstock, tree-size control A significant effort at UF/CREC has been the production of allotetraploid rootstock selections by somatic hybridiza- tion, and more recently by conventional breeding at the tetraploid level using selected somatic hybrid parents. In a greenhouse study, we determined responses to salinity stress (50 mM NaCl ~4,400 ppm TDS) of well-fertilized 1-year-old ‘Valencia’ orange trees on 18 new allotetraploid citrus rootstock selections and on the relatively salt sensitive Carrizo citrange rootstock with and without salinity stress for comparison. There were six replicate trees per treatment. After 5 months, all trees were visually rated from 1 to 10 for appearance in terms of phytotoxicity symptoms and leaf loss. Leaf chlorophyll was estimated using a SPAD meter on fully expanded mature leaves, which were then briefly rinsed in deionized water, oven dried, and used to determine leaf Na+ and Cl– concentrations. Overall, leaf Na+ and Cl− were strongly related but low SPAD values and low appearance ratings were more strongly correlated to high leaf Cl− than to high leaf Na+. Three allotetraploid zygotic hybrids from crosses of allotetraploid somatic hybrids (“tetrazyg”) rootstock selections (denoted O3, O4, and S11) were able to exclude Na+ and Cl− ions from ‘Valencia’ leaves better than most other rootstocks while maintaining good growth with no phytotoxic symptoms. These three tetraploid hybrid rootstock selections merit further evaluation of salinity tolerance and horticultural performance in the field.
Rootstocks have significant impacts on citrus tree productivity crosses of selected somatic hybrids (Grosser et al., 2003, 2007), and fruit/juice quality, and new rootstocks with improved toler- are now playing a key role in this effort. ance/resistance to biotic and abiotic stresses are needed to sustain Somatic hybrid allotetraploid citrus rootstocks have continu- the profitability of citriculture (Castle, 1987). Recently, rootstock ously shown the ability to reduce tree size, even from combina- susceptibility to common abiotic stresses such as drought, cold, tions of vigorous diploid parents (Grosser et al., 2011). Although and salinity have been observed to increase citrus tree susceptibil- the dwarfing mechanism in citrus rootstocks has been linked to ity to huanglongbing (HLB, citrus greening disease) (personal competition between vegetative and reproductive growth (Lliso observations of Jude W. Grosser from multiple field trials), a newly et al., 2004), tetraploid citrus rootstocks can have relatively introduced disease that threatens the entire Florida citrus industry thick fibrous roots that can limit water and nitrogen uptake that (Chung and Brlansky, 2005). Salinity is becoming increasingly are at least part of the underlying mechanism limiting tree size important in coastal flatwoods groves in Florida, especially dur- (Syvertsen et al., 2000). Since tetraploid cells generally exhibit ing the dry season when the salinity levels in canals and shallow larger size and thicker cell walls than diploid cells (Allario et wells used for irrigation can reach 2000 ppm NaCl (Boman and al., 2011), tree size control may also be a partial incompatibility Stover, 2002). The primary goal of our rootstock breeding pro- response between tetraploid rootstocks and diploid scion at the gram at UF/IFAS has been the packaging of numerous required graft union (Lee, 1988). In addition, citrus tetraploid rootstocks biotic and abiotic tolerance/resistance factors together with wide have been shown to be more tolerant to salinity stress than their soil adaptation, high productivity, and the ability to control tree corresponding diploids (Saleh et al., 2008). Many of the allotetra- size (Ananthakrishnan et al., 2006; Grosser and Chandler, 2000; ploid rootstock selections created in our program have also been Grosser et al., 2000, 2003, 2004, 2007, 2011; Grosser and Gmit- greenhouse screened for tolerance to the Diaprepes/Phytophthora ter, 1990, 2005, 2010; Medina-Urrutia et al., 2004). Smaller trees complex (Grosser et al., 2003, 2007), and are in the process of are desirable to reduce harvesting costs, maximize the efficiency being screened in the field for tolerance/resistance to citrus blight, of cold-protection, and may now be needed for new ACP (Ad- tolerance to HLB (J.W. Grosser, unpublished information), and vanced Citrus Production; Schumann et al., 2009) systems that as reported here, salinity tolerance. have potential to minimize the impact of citrus canker and HLB Citrus trees are one the most salt sensitive perennial horticul- (Stover et al., 2008). Allotetraploid somatic hybrids produced tural crops (Mass, 1996). Salt damage is usually manifested as by protoplast fusion (Grosser and Gmitter, 2010), and sexual reduced growth, leaf burn and defoliation, and is associated with allotetraploid hybrids (tetrazygs) produced from conventional accumulation of toxic levels of Cl− and Na+ in leaf cells (Levy and Syvertsen, 2004; Storey, 1995). Since the ability of citrus trees to Acknowledgments. We thank Southern Gardens Citrus Nursery for propagation tolerate salinity mostly depends on rootstocks (Maas, 1993) the of the test trees. production and identification of improved rootstocks with better *Corresponding author; phone: (863)-956-8680; email: [email protected] capability to cope with root zone salinity should improve citrus
50 Proc. Fla. State Hort. Soc. 125: 2012. salinity tolerance. The relatively good salt resistance of citrus NaCl salt solution and on 5 Feb., the salinized treatment of 50 rootstocks such as Rangpur lime and Cleopatra mandarin is as- mmol NaCl began and was continued every 3 d thereafter until sociated with their capacity to limit the accumulation of Cl− in the end of the experiment in June. The 50 mmol NaCl + fertilizer scion leaves (Syvertsen et al., 2010). Such Cl− exclusion traits had an EC ~6.3 dS·m–1 and a TDS ~ 4400 ppm. There were six are heritable and can be transmitted to breeding progeny (Sykes, additional non-salinized trees on standard, Cl− sensitive, CZ that 1992). Trees on other rootstocks, like Poncirus trifoliata (L.) Raf. received no salinity treatment (CZ0) and six salinized CZ trees and its hybrids like Carrizo citrange, accumulate high levels of (CZS) for comparison. The experimental design was a completely Cl− in leaves and so are considered to be salt sensitive, but these random design with 20 treatments, 19 salinized rootstocks and the rootstocks also demonstrate a capacity to restrict Na+ transport to non-salinized CZ0, with six replicate trees in each treatment. In the shoot (Garcıa-Sanchez and Syvertsen, 2006). Thus, the Na+ February, all trees were treated once with imidacloprid (Admire excluding trait is also heritable (Walker, 1986). Pro, Bayer CropScience, Research Triangle Park, NC) at the In this study we examined the salinity tolerance of ‘Valencia’ recommended rate for pest control and all trees were sprayed orange trees on 18 allotetraploid genetically diverse citrus rootstock every other week with 1% oil or soap solution. selections developed in our program, which are in various stages Tree evaluations. The effects of salinity on tree size, chloro- of evaluation. All of these selections produced adequate nucellar phyll content, and accumulation of Cl− and Na+ ions in leaves were seed for standard nursery propagation. Although not pre-selected determined after 5 months of salinization (June). All trees were for salinity tolerance, based on their parentage we expected to visually rated from 1 to 10 with a rating of 10 representing the find some salinity tolerance in these hybrids. largest, healthiest appearing trees with no leaf loss or phytotoxic symptoms and a rating of 1 was given to trees that had lost all Materials and Methods leaves. Using fully expanded mature leaves from the mid-shoot area on each tree, an index of leaf chlorophyll was estimated using Plant material. Rootstock liners from all selections were a SPAD meter (SPAD-502, Minolta Corp., Ramsey, NJ; (Jifon et grown from seed and bud grafted with commercial ‘Valencia’ al., 2005). After tree harvest on 5 June five mature leaves from [Citrus sinensis (L.) Osbeck] sweet orange in a commercial cit- each tree were briefly rinsed in deionized water, and oven dried rus nursery. Rootstock test material included 18 tetraploid citrus at 60°C for at least 48 h. Dried leaves were ground to a powder rootstocks (n = 6 trees), including 15 “tetrazygs,” three somatic and leaf Na+ and Cl– concentrations, expressed as a percentage hybrids, and also trees on standard diploid Carrizo citrange (CZ; of dry weight, were determined in a commercial lab (Waters [Poncirus trifoliata (L.) Raf. × C. s.] for comparison (Table 1). Agricultural Lab, Camilla, GA). The “tetrazyg” hybrids were produced from crosses that included Data analysis. Analysis of variance and Duncan’s multiple either ‘Nova’ mandarin hybrid (Citrus reticulata Blanco) + Hi- range tests (SAS Institute, Inc. Cary, NC) were used to compare rado Buntan (zygotic) pummelo (Citrus grandis (L.) Osbeck) genotype means at P < 0.05. Linear regressions (SigmaPlot, or sour orange (Cirus aurantium L.) + rangpur (Citrus jambhiri Systat Software, Inc., San Jose, CA) were used to investigate Lush) somatic hybrid as the female parent, and either Cleopatra relationships between selected variables. (Citrus reshni Tanaka) + Argentine trifoliate orange (Poncirus trifoliata) (Orange Series), sour orange + Palestine sweet lime Results and Discussion (Citrus limettiodes Tan.; Blue Series), Cleopatra + Sour orange (SR × SH series), sour orange + Carrizo citrange (Green Series), Visual rating. The overall visual ratings of ‘Valencia’ grown or Succari sweet orange (Citrus sinenesis L.) + Argentine trifoliate on O1 and O19 tetraploid were lower than all other rootstocks as orange somatic hybrid (White Series) as pollen parents. they suffered the most salinity induced defoliation (Table 2). In Salinity experiment. This experiment was carried out in a contrast, Valencia grown on O16, O14, O4, O3, and S11, tetraploid greenhouse from Jan. to June 2010 at the UF/IFAS Citrus Re- rootstock were less affected by salt stress than the other rootstocks search and Education Center, Lake Alfred (lat. 28° N, long. 82° as their average visual rating did not differ significantly from the W; elevation 51 m). One-hundred twenty uniform, 1-year-old non-salinized CZ0. Average visual rating of the relatively salt ‘Valencia’ orange trees grafted in June 2009 on the 19 different sensitive CZS also did not differ from CZ0 even though there rootstocks (Table 1) at Southern Gardens Citrus Nursery (Tren- were large differences in salt ion concentrations between CZS ton, FL) were grown in 3-L citripots filled with a well-drained and CZ0 (see below). commercial soilless media containing a mixture of peat/perlite/ SPAD greenness index. The SPAD index for ‘Valencia’ vermiculite at 3:1:1 by volume with added slow release fertil- trees on salinized CZ (CZS) was numerically higher than for izer. Trees were about 40 cm tall when transported to CREC in trees on non-salinized CZ0. However, there were no significant Jan. 2010. Trees were grown in a 50% shaded greenhouse under differences between CZS and CZ0 nor among the B2, W1, S6, natural photoperiods and were kept well watered. Throughout B1, O4, O3, S11, and S18 tetraploid rootstocks with the highest the experiment period, maximum daily photosynthetically active SPAD readings (Table 2). The rootstocks O1 and O19 had the radiation (PAR; LI-170; LICOR, Inc., Lincoln, NE) at plant level lowest SPAD index. was about 800 µmol·m–2·s–1, average day/night temperature was Ion concentration. For bearing citrus trees in the field, 38/25°C and relative humidity varied diurnally from 40% to 100%. leaf Cl− concentrations that exceed 0.7% are considered toxic On 29 Jan. 2010 and once per week thereafter, all plants were (Obreza and Morgan, 2008) so all leaf Cl− levels in this study watered to leaching with a 100 ppm N fertilizer solution (EC were excessively high (Table 2). The greatest Cl− concentration ~1.1 dS·m–1 and a TDS ~770 ppm) from a complete 7–2–7 Citrus accumulated in leaves of trees on O1 followed by those on O19, blend fertilizer plus iron. Salinity treatments were begun on six which both exceeded salinized CZS. Thus, trees on 7 of the other replicate trees for each rootstock on 1 Feb. 2010. To avoid osmotic 16 tetraploid rootstocks accumulated significantly less Cl− than shock, salinized trees received initially 20 mmol NaCl solution the diploid CZS, supporting the idea that tetratploids in general to leaching. Two days later, salinized trees received a 40 mmol are more salt tolerant then diploids (Salleh et al., 2008). The
Proc. Fla. State Hort. Soc. 125: 2012. 51 17
307 Table 1. Identification and description of the rootstock germplasm included in this study. Rootstock Code Germplasm Description Blue 1 B1 Tetrazygs*: two selections of ‘Nova’ mandarin** hybrid + Hirado Buntan (zygotic) pummelo Blue Series Blue 2 B2 (HBP) somatic hybrid x Sour Orange (S.O). + Palestine sweet lime (PSL) somatic hybrid Somatic hybrid***: ‘Changsha’ mandarin (C. Changsha+50-7 C Changsha+50-7 reticulata Blanco) + Trifoliate Orange 50-7 (P. trifoliata L.) Tetrazygs*: ‘Nova’** mandarin + HBP somatic Green 7 G7 Green Series hybrid/ Sour Orange (S.O) + Carrizo citrange somatic hybrid Somatic hybrid***: ‘White Grapefruit’ (C. WGFT+50-7 GF WGFT+50-7 paradisi Macf.) + Trifoliate Orange 50-7 Orange 1 O1 Orange 2 O2 Orange 3 O3 Tetrazygs*: eight selections of ‘Nova’ Orange 4 O4 mandarin** hybrid + HBP somatic hybrid x Orange Series Orange 13 O13 Cleopatra mandarin (C. reticulata Blanco) (C.r.) Orange 14 O14 + Argentine trifoliate orange (P.t.) somatic hybrid Orange 16 O16 Orange 19 O19 SR x SH99-6 S6 Tetrazygs*: three selections of Sour Orange (SO) SR x SH99-11 S11 SR x SH + 'Rangpur' (RP) somatic hybrid x Cleopatra (C. SR x SH99-18 S18 r.) + Sour orange somatic hybrid Somatic hybrid***: ‘Sour Orange’ + Trifoliate SO+50-7 SO SO+50-7 Orange 50-7 Tetrazygs*: ‘Nova’ mandarin** + HBP somatic18 White 1 W1 White Series hybrid x Succari sweet orange + Argentine18
trifoliate orange (P.t.) somatic hybrid trifoliate orange (P.t.) somatic hybrid Standard commercial diploid rootstock (C.s. x Carrizo (+ Salt) CZS Carrizo citrangeStandard commercial diploid rootstock (C.s. x Carrizo (+ Salt) CZS Carrizo citrange P.t.) ; salinized P.t.) ; salinized StandardStandard commercial commercial diploid rootstockdiploid rootstock; non- ; non- CarrizoCarrizo (no (no Salt) Salt) CZ0CZ0 CarrizoCarrizo citrange citrange salinizedsalinized *Tetrazygs:*Tetrazygs: Origin Origin from from crosses crosses of allotetraploid of allotetraploid somatic somatic hybrids hybrids **Nova**Nova mandarin: mandarin: Fina Fina ‘Clementine’ ‘Clementine’ and Orlandoand Orlando ‘tangelo’ ‘tangelo’ (Duncan (Duncan grapefruit grapefruit X Dancy X Dancy tangerine)tangerine) made made by by F.G. F.G. Gardner Gardner and andJ. Bellows J. Bellows in 1942 in and1942 released and released in 1964 in(Saunt, 1964 1990 (Saunt,). 1990). ***Somatic***Somatic hybrid: hybrid: Obtained Obtained via protoplastvia protoplast fusion fusion 308 308 lowest Cl− concentrations of course occurred in non-salinized Cl− levels, trees on O1 and O19 along with those on O13 accumu- CZ0 trees followed by B1, SO, O4, O3, S18, and S11, which lated highest Na+ levels. The extreme contrast in responses of O4 were not significantly higher than CZ0. Since salinity tolerance and O3 vs. O1, O13, and O19, all progeny from the same cross, in citrus has been linked with leaf Cl− concentrations, ranking of demonstrates the genetic power of breeding at the tetraploid level. these genotypes on the basis of leaf Cl− was the most important Crosses of allotetraploid somatic hybrid parents are expected to parameter we studied. It was remarkable that salinized trees on maximize genetic variability in progeny for selection, since up these six tetraploid rootstocks did not accumulate more Cl− than to four different alleles are possible at any given locus. trees on non-salinized CZ0. Leaf Na+ accumulation can also be SPAD index readings were positively correlated with over- important, as evidenced by the fact that SO, O4, O3, S18, and all visual ratings as high greenness certainly contributed to a S11 also had the lowest Na+ levels. Again, similar to high leaf high visual rating (Fig. 1A). Leaf Na+ and Cl− concentrations
52 Proc. Fla. State Hort. Soc. 125: 2012. 19
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Table 2. Mean (+ SE, n = 6) visual rating, SPAD index, leaf chloride and sodium concentrations (% Dr Wt) of ‘Valencia’ (Val) orange trees on the rootstock genotypes (see Table 1 for abbreviations) when grown under salinity stress (50 mM NaCl ~ 4,400 ppm TDS) for 5 months. All genotypes and mean values are ranked by leaf chloride concentration in descending order. Visual rating Cultivar Code (1-10)a SPAD index Chloride (%) Sodium (%) Val/Orange-1 O1 2.33±0.21 Jb 39.83±1.22 G 6.19±0.17 A 2.33±0.12 A Val/Orange-19 O19 3.33±0.67 JI 30.20±1.59 H 4.23±0.33 B 1.04±0.17 BCDEF Val/CZc (+ Salt) CZSc 8.67±0.21 AB 56.00±2.22 AB 3.58±0.42 BC 0.95±0.07 BCDEFGH Val/Orange-13 O13 5.33±0.56 GHI 43.33±2.64 FG 3.40±0.38 BCD 1.26±0.13 B Val/WGFT+50-7 GF 5.50±0.56 GHI 48.50±0.99 BCDEF 3.36±0.18 BCD 0.98±0.07 BCDEFG Val/Blue-2 B2 5.50±0.76 GHI 50.50±2.22 ABCDEF 3.07±0.37 CDE 1.25±0.21 BC Val/Orange-16 O16 7.83±0.17 BCDE 44.33±0.67 EFG 2.84±0.73 CDEF 0.81±0.14 DEFGHI Val/Orange-14 O14 7.67±0.42 BCDE 44.83±1.87 DEFG 2.80±0.14 CDEF 0.74±0.13 FGHI Val/Orange-2 O2 5.67±0.88 GH 49.33±2.53 ABCDEF 2.73±0.20 CDEF 1.16±0.11 BCD Val/Changsha+50-7 C 5.00±1.46 FGH 44.75±7.12 DEFG 2.70±0.53 CDEF 0.48±0.11 I
Val/Green-7 G7 7.00±0.45 DEFGH 46.83±1.85 CDEFG 2.69±0.40 CDEF 1.12±0.17 BCDE 21
Val/White-1 W1 7.00±0.58 DEFG 49.33±1.50 ABCDEF 2.55±0.12 CDEF 0.80±0.04 EFGHI Val/SRxSH99-6 S6 6.67±0.76 EFGH 52.67±2.63 ABCD 2.24±0.38 DEFG 0.89±0.08 CDEFGH Val/Blue-1 B1 6.67±0.33 EFGH 53.83±1.72 ABC 1.96±0.13 EFGH 0.99±0.07 BCDEFG Val/SO+50-7 SO 6.67±0.49 EFGH 48.67±2.86 BCDEF 1.95±0.25 EFGH 0.52±0.06 I 20 Val/Orange -4 O4 8.50±0.22 ABC 54.17±0.95 ABC 1.82±0.15 FGH 0.76±0.06 FGHI Val/Orange-3 O3 8.17±0.31 ABCD 56.83±3.38 A 1.78±0.39 FGH 0.60±0.13 HI Val/SRxSH99-18 S18 7.50±0.43 CDEF 51.00±2.78 ABCDEF 1.75±0.26 FGH 0.64±0.06 GHI Val/SRxSH99-11 S11 9.33±0.33 A 53.83±1.92 ABC 1.32±0.14 GH 0.78±0.06 EFGHI Val/CZc (no salt) CZ0c 8.67±0.21 AB 52.17±2.89 ABCDE 0.97±0.61 H 0.00±0.00* J a Trees rated 10 were the largest, healthiest appearing trees with no phytotoxic symptoms, trees rated 1 were completely defoliated. b Means followed by the same letter are not significant at P<0.05. * non-detectable c CZ, Standard Carrizo citrange included for comparison; 0 = no salinity, S = 50 mM NaCl. 21
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Fig. 1. Relationships between SPAD index vs. visual rating (A) and leaf chloride vs. sodium concentration (B) from ‘Valencia’ orange trees on diploid and tetraploid rootstocks (see Table 1 for abbreviations) after 5 months of growth under salt stress conditions. Each symbol is the mean (n=6) for each rootstock but linear regression lines were fit to the combined data (n=120) from trees on all rootstocks. (A) y = 2.5943x + 31.295 (r2 = 0.5643, P < 0.001) and (B) y = 2.21723x + 0.728 (r2 = 0.714, P < 0.001).
Proc. Fla. State Hort. Soc. 125: 2012. 53
310 of ‘Valencia’ leaves grown on different tetraploid and diploid be attributed to their breeding parents (Cleopatra and Rangpur), citrus rootstocks under salinity conditions were also correlated which are known to be chloride excluders (Cooper, 1961; Zekri positively (Fig. 1B). and Parsons, 1992). Our results suggest that development of Low to moderate NaCl salinity stress can stimulate chlorophyll improved citrus rootstocks with enhanced salinity tolerance can degradation, whereas higher salt concentrations more drastically be accomplished by combining diverse genetic backgrounds into affect chlorophyll synthesis (Santos, 2004). Although other min- individual hybrids. eral nutrients in leaves were not measured, the adverse effects The results obtained in this experiment suggest that promising of salinity on total chlorophyll in the leaves can be attributed to tetraploid, tree-size controlling citrus rootstock selections cur- reduced availability of water and nutrients, particularly magnesium rently under evaluation in field trials, may also exhibit enhanced (Castle and Krezdorn, 1975; Nijjar, 1985). Chlorophyll decreases salinity tolerance. These include new rootstock genotypes O3, O4, under stress due to the suppression of specific enzymes that are and S11. These three allotetraploid hybrid rootstock selections responsible for its synthesis (Murkute et al., 2006). Based on merit further evaluation of salinity tolerance and horticultural SPAD greenness, visual ratings and leaf Cl, we can conclude that performance in the field. O3, O4, and S11 were the best performing tetraploid rootstocks. Rootstock candidate SO produces dwarfing trees similar in Cl− and Na+ concentrations in leaves of ‘Valencia’ grown on size to trees on Flying Dragon rootstock, but with better yield and different tetrapolid citrus rootstocks under salinity stress were excellent fruit quality (Castle et al. 2004; Grosser et al., 2011). The negatively correlated with visual rating and SPAD index (Fig. rootstock candidates O3 and O4 appear to be the most promising 2). SPAD index readings were correlated more strongly with leaf selections in the group to date. These two tree-size controlling Cl− concentration than with leaf Na+ concentration, underscoring selections grow rapidly in the nursery and during the tree establish- the greater importance of leaf Cl− than leaf Na+ in determining ment phase in the grove, unlike the slow growing dwarfing Flying salinity tolerance in citrus rootstocks (Levy and Syvertsen, 2004) Dragon. The rootstock O3 has shown tolerance23 to the Diaprepes/ − and their hybrids. The low Cl accumulation in S11 and S18 can Phytophthora complex in greenhouse tests (Grosser et al., 2003),
317 Fig. 2. Relationships between leaf sodium (A) and chloride concentration (B) vs. visual rating Fig. 2. Relationships318 between leaf sodium (A) and chloride concentration (B) vs. visual rating and between leaf sodium (C) and chloride concentration (D) vs. SPAD index319 from and‘Valencia’ between orange leaf trees sodium on diploid (C) and and tetraploid chloride rootstocks concentration (see Table 1 (D)for abbreviations) vs. SPAD indexafter 5 monthsfrom ‘Valencia’of growth under salt stress conditions. Each symbol is the mean (n=6) for each rootstock but linear regression lines were fit to the combined data (n=120) from all rootstocks. A( ) y = 0.1667x + 2.0133 (r2 = 0.4543,320 P < 0.001);orange (B trees) y = 0.504xon diploid + 6.0453 and (r 2 tetraploid= 0.6286, P rootstocks< 0.001); (C) (seey = 5.5358x Table + 1 53.557 for abbreviations) (r2 = 0.1571, P= after0.0834) 5 andmonths (D) y =of 3.5907x + 58.219 (r2 = 0.4369, P < 0.001). 321 growth under salt stress conditions. Each symbol is the mean (n=6) for each rootstock but linear 54 322 regression lines were fit to the combined data (n=120) from all rootstocks. (A) yProc. = 0.1667x Fla. State + Hort. Soc. 125: 2012. 2 2 323 2.0133 (r = 0.4543, P<0.001); (B) y = 0.504x + 6.0453 (r = 0.6286, P<0.001); (C) y = 5.5358x 2 2 324 + 53.557 (r = 0.1571, P= 0.0834) and (D) y = 3.5907x + 58.219 (r = 0.4369, P<0.001). and both O3 and O4 are showing wide soil adaptation and tolerance Gmitter, C.X. Chen, and F.G. Gmitter. 2007. Continued development to blight and good early performance regarding yield and fruit of rootstocks tolerant of the Phytophthora/Diaprepes complex via quality in new field trials (J.W. Grosser, unpublished data). Less greenhouse screening. Proc. Fla. State Hort. Soc. 120:103–109. information is available regarding the performance of S11, as it is Grosser, J.W., J.H. Graham, C.M. McCoy, A. Hoyte, H.M. Rubio, D.B. a newer selection. The next step to validate the potential salinity Bright, and J.L. Chandler. 2003. Development of “Tetrazyg” rootstocks tolerant of the Diaprepes/Phytophthora complex under greenhouse tolerance in the selections identified above will be to establish conditions. Proc. Fla. State Hort. Soc. 116:262–267. replicated field trials in flatwood sites that are affected by high Grosser, J.W., V. Medina-Urrutia, G. Ananthakrishnan, and P. Serrano. salinity conditions. Finally, these results allowed for better sepa- 2004. Building a replacement sour orange rootstock: Somatic hybrid- ration and selection of salt tolerant genotypes produced from the ization of selected mandarin + pummelo combinations. J. Amer. Soc. same genetic background. For example, O3 and O4 were shown Hort. Sci. 129:530–534. to be tolerant/resistance to salt stress, while siblings O1 and O19 Grosser, J.W., P. Ollitrault, and O. Olivares-Fuster. 2000. Somatic hybrid- were shown to be very sensitive. 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