JXB Advance Access originally published online on September 15, 2006
Journal of Experimental Botany 2006 57(14):3697-3706; doi:10.1093/jxb/erl121
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RESEARCH PAPER |
Salinity tolerance of ‘Valencia’ orange trees on rootstocks with contrasting salt tolerance is not improved by moderate shade
1University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850, USA
2Universidad de Córdoba. Dpto. Agronomía, Apdo. 3048, E-14080 Córdoba, Spain
3Centro de Edafologia y Biologia Aplicada del Segura, CSIC, Campus Universitario de Espinardo, Espinardo, E-30100 Murcia, Spain
* To whom correspondence should be addressed. E-mail: JMSN{at}ufl.edu
Received 17 February 2006; Accepted 14 July 2006
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Abstract |
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The effects of shading in combination with salinity treatments were studied in citrus trees on two rootstocks with contrasting salt tolerance to determine if shading could reduce the negative effects of salinity stress. Well-nourished 2-year-old ‘Valencia’ orange trees grafted on Cleopatra mandarin (Cleo, relatively salt tolerant) or Carrizo citrange (Carr, relatively salt sensitive), were grown either under a 50% shade cloth or left unshaded in full sunlight. Half the trees received no salinity treatment and half were salinized with 50 mM Cl– during two 9 week salinity periods in the spring and autumn interrupted by an 11 week rainy period. The shade treatment reduced midday leaf temperature and leaf-to-air vapour pressure deficit regardless of salinity treatments. In non-salinized trees, shade increased midday CO2 assimilation rate (ACO2) and stomatal conductance, but had no effect on leaf transpiration (Elf). Shade also increased leaf chlorophyll and photosynthetic water use efficiency (ACO2/Elf) in leaves on both rootstocks and increased total plant dry weight in Cleo. The salinity treatment reduced leaf growth and leaf gas exchange parameters. Shade decreased Cl– concentrations in leaves of salinized Carr trees, but had no effect on leaf or root Cl– of trees on Cleo. There were no significant differences in leaf gas exchange parameters of shaded and unshaded salinized plants but the growth reduction from salinity stress was actually greater for shaded than for unshaded trees. Shaded trees on both rootstocks had higher leaf Na+ than unshaded trees after the first salinity period, and this shade-induced elevated leaf Na+ persisted after the second salinity period in trees on Carr. Thus, shading did not alleviate the negative effects of salinity on growth and Na+ accumulation.
Key words: Cl–, CO2 assimilation, Na+, stomatal conductance, water use efficiency
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Introduction |
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Secondary salinization from irrigation water is a growing worldwide problem as more than 6% of agricultural land has become saline (Ghassemi et al., 1995). Since subtropical citrus is susceptible to freezing temperatures and poor soil drainage, citrus is grown in warm areas with high evaporative demand and where irrigation is required to produce maximum yield (Levy and Syvertsen, 1981). In these areas, many soils and sources of water contain high amounts of salts that can inhibit the growth and yield of salt-sensitive citrus (Murkute et al., 2005).
All commercial citrus trees are grafted on rootstocks which can regulate the amount of Cl– and/or Na+ accumulated in foliage (Levy and Syvertsen, 2004). For example, the citrus rootstock Cleopatra mandarin is considered to be a Cl– excluder, whereas Carrizo citrange is considered to be a Cl– accumulator but a Na+ excluder. Na+ exclusion in rootstocks may depend on removing Na+ from xylem by an exchange of Na+ for K+ (Walker, 1986). The Cl– ion is considered to be a more important limitation than Na+ on citrus growth and yield (Bañuls et al., 1997). The accumulation of Cl– and, thus, relative salt tolerance, has been linked to plant growth, water use (Castle and Krezdorn, 1975; Syvertsen et al., 1989), and transpiration (Moya et al., 1999, 2003). The relationship between Cl– accumulation and water use in citrus is not universal, however, because when water use was reduced during growth at elevated CO2, leaf Cl– was reduced only in relatively salt-sensitive Carrizo seedlings but not in relatively salt-tolerant Cleopatra (García-Sánchez and Syvertsen, 2006).
Reduced photosynthesis in well-watered but salt-stressed citrus leaves has been associated with the specific toxicity of Cl– and/or Na+ rather than with osmotic stress (Bañuls and Primo-Millo, 1992; Levy and Syvertsen, 2004). Reductions in CO2 assimilation (ACO2) have been attributed to a direct biochemical inhibition of photosynthetic capacity (Lloyd et al., 1987b; García-Sánchez and Syvertsen, 2006) which was more important than reduced stomatal conductance (gs) in limiting ACO2.
Salinity stress often occurs in conjunction with flooding, drought, and/or high temperature stress. Shade can improve the physiological response of the plants to drought (Duan et al., 2005) or to excess boron stress (Sotiropoulos et al., 2004) compared with unshaded plants. In citrus, 50% shade screens reduced excessively high leaf temperatures and leaf-to-air vapour pressure differences (D) at midday such that ACO2, gs, and photosynthetic leaf water use efficiency were increased above that of unshaded leaves (Syvertsen et al., 2003). Thus, salt stress should be reduced by shade. Increases in midday gs by shade were accompanied by decreases in D, however, such that leaf transpiration and whole plant water use were unchanged (Jifon and Syvertsen, 2003a, b). If chloride uptake and transport in citrus are indeed linked to water use (Moya et al., 1999, 2003) and shade has little effect on water use, we hypothesized that growing trees under shade should have little affect on leaf Cl– concentration under salinity stress.
‘Valencia’ orange trees grafted on two rootstocks with contrasting salinity tolerance, Cleopatra mandarin (relatively salt tolerant) and Carrizo citrange (relatively salt sensitive), were tested to determine their physiological responses to salinity in sun and shade. These responses should yield insights into mechanisms of salinity tolerance in citrus. Since salinity problems in Florida citrus normally occur only in the relatively dry spring and autumn irrigation periods (Syvertsen et al., 1989), the salinity treatment was applied in spring and autumn with an intervening non-saline summer rainy period that leached any accumulated salts from the soil.
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Materials and methods |
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Plant culture and treatments
The study was carried out at the University of Florida/IFAS Citrus Research and Education Center, Lake Alfred, FL (28.09° N, 81.73° W; 51 m a.s.l.). Two-year-old ‘Valencia’ orange (Citrus sinensis L. Osbeck) trees grafted on Cleopatra mandarin (Cleo, C. reticulata Blanco) or Carrizo citrange (Carr, C. sinensis L. OsbxPoncirus trifoliata L.) were used in this experiment. Twenty uniform trees on each rootstock were grown outdoors in plastic containers (5.0 l) filled with native Candler fine soil sand. The trees were watered three times per week with 1.0 l of soluble fertilizer solution (9N–2P–9K), (NO3)2Ca and iron-chelate (6%) with an N concentration of 66 mg l–1. The 1.0 l volume of nutrient solution was enough to achieve leaching from the bottom of all containers.
The shade treatment was applied from April to November 2003 by placing shade screens on top of 2.2-m-tall PVC frames constructed over the trees. Shade screens were Aluminet-50 (Polysack Plastic Industries, Nir Yitzhak, Israel), a spectrally neutral, aluminized polypropylene shade screen with a mesh size of 6x3 mm, which transmits about 50% of incident photosynthetically active radiation (PAR). Ten trees on each rootstock were placed under the shade and 10 trees served as an unshaded control. Two salinity treatments, 0 and 50 mM Cl– (NaCl and CaCl2; 3:1), were evaluated on five trees on each rootstock in both full sun under the shade. The salinity treatment was begun at the same time as the shade treatment but the salinity was applied during two 9 week dry periods, 23 April to 24 June and 18 September to 21 November. At the beginning of each period, the salinity treatment was added in increasing increments of 15 mM Cl– per day during two consecutive days (to avoid osmotic shock) and on the third day, salinity was increased by 20 mM Cl– to reach the final concentration of 50 mM Cl–. Although the shade treatment was maintained during the intervening typical summer rainy period (25 June to 17 September), the previously salinized trees were irrigated only as necessary with the standard nutrient solution without salt. The experimental design was a two rootstocks (Cleo and Carr)xtwo salt concentrations (0 and 50 mM Cl–)xtwo light intensities (unshaded and 50% shade) factorial design with five replicate trees in each treatment.
Gas exchange and water relations
Gas exchange and water relations were measured on selected clear days near the end of each salinity period. Measurements were made on two leaves chosen from the mid-shoot area of each plant, giving 10 replicate leaves per treatment. Net assimilation of CO2 (ACO2), stomatal conductance (gs), and leaf transpiration (Elf) were determined with a Li-Cor portable photosynthesis system (LI-6200; Li-Cor Inc., Lincoln, NB, USA) equipped with a well-stirred 0.25 l leaf chamber with constant-area inserts (9 cm2). Leaf temperature (Tlf) was measured using the thermocouple inside the gas exchange cuvette. Intercellular CO2 concentration (Ci) and photosynthetic water use efficiency (ACO2/Elf) were calculated based on the equations of von Caemmerer and Farquhar (1981). All gas exchange measurements were made in the morning from 10.00 h to 12.00 h. The measurement conditions within the cuvette are included in the Results section below. Leaf water potential was measured near midday (13.00 h to 14.00 h) using a Scholander-type pressure chamber (PMS instrument, Corvallis, OR, USA; Scholander et al., 1965) on leaves similar to those used for net gas exchange.
Chlorophyll analysis
After gas exchange measurements, two leaf discs (0.45 cm2 each) were sampled from the same leaves avoiding major veins. Chlorophyll was eluted from the discs by submerging them in 2 ml of N,N-dimethylformamide in the dark for at least 72 h. The amount of absorbance was read at 647 nm and 664 nm with a Shimadzu UV-vis spectrophotometer (Model UV2401PC, Shimadzu, Riverwood Drive, Columbia, MD, USA) and used to calculate leaf chlorophyll concentrations according to equations of Inskeep and Bloom (1985).
Leaf ion concentration and growth parameters
At the end of each salinity period, five leaves per tree were used to analyse leaf Cl– and Na+ concentrations. Leaves were briefly rinsed with deionized water, oven-dried at 60 °C for at least 48 h, weighed, and ground to a fine powder. Samples were extracted with a 0.1 N solution of nitric acid and 10% acetic acid. Chloride concentration was measured using a silver ion titration chloridometer (HBI Chloridometer; Haake Buchler, Saddle Brook, NJ, USA). Leaf Na+ concentration was determined by a commercial laboratory (Waters Agricultural Laboratory, Camilla, GA, USA). At the end of the experiment, root Cl– and Na+ concentrations were also analysed as above. Plants were harvested and total dry weights of leaves, stem, and roots were determined. Total leaf area was measured using a leaf area meter (Li-3000; Li-Cor).
Statistical analysis
Analysis of variance used two rootstocksxtwo shade levelsxtwo salinity levels and five replicate plants per treatment. Treatment means were separated by Duncan's multiple range test at P <0.05 using the SPSS statistical package (SPSS, Chicago, IL, USA). Linear regression was used to describe relationships between selected variables and analysis of covariance was used to compare slopes of relationships.
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Results |
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Tree growth and leaf chlorophyll
In non-salinized Cleo, shading increased total plant dry weight (TPDW) by increasing leaf dry weight, but in non-salinized Carr, TPDW was similar in shaded and unshaded trees (Table 1). Salinity reduced TPDW and leaf dry weight of trees on both rootstocks but reductions in leaf dry weight were greater in unshaded Carr (42%) than in unshaded Cleo trees (24%). In addition, salinity reduced root dry weight in shaded and unshaded Carrizo, and in shaded Cleo but not in unshaded Cleo. Leaf dry weight of shaded trees on Cleo, however, was reduced similarly (about 59%) by salinity to that of Carr. Root dry weight was reduced by salinity in all the trees except in unshaded Cleo trees. Leaf dry weight/area was decreased by shade but was only decreased by salinity in unshaded Carr trees.
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After the first 9 weeks of salinity treatment, chlorophyll concentrations in leaves were decreased by salinity (Table 2). Leaf chlorophyll a, b and total chlorophyll, followed similar patterns with respect to treatments. The shade treatment increased leaf chlorophyll in trees on both rootstocks although not significantly so for Carr (rootstockxshade significant at P <0.05).
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Measurement conditions and leaf gas exchange
Shade reduced PAR about 58%, and, at the end of the first 9 weeks of salinity treatment (spring salinization), the mean leaf temperature at midday in unshaded trees was 37.5 °C compared with 34.6 °C in shaded trees (Table 3). Shade also lowered D compared with unshaded leaves. There were significant differences in the two-way interaction shadexsalt on ACO2, gs, and ACO2/Elf. In the non-saline treatment, ACO2, gs, and ACO2/Elf were significantly increased by shade. Salinity reduced ACO2, gs, and ACO2/Elf in both unshaded and shaded leaves, but reductions were greater in leaves on Cleo than Carr. In salinized trees, however, the already reduced gas exchange responses were not affected by shade. Leaf transpiration rate (Elf) was not affected by rootstock or shading but, in shaded Cleo leaves, Elf was reduced by salinity. Salinity increased Ci in leaves on both shaded and unshaded Cleo trees and on shaded Carr trees compared with the non-salinized treatment. Shade increased Ci in salinized trees on Carr, but not significantly so in trees on Cleo.
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At the end of the second salinity period (autumn salinization), shade again reduced midday leaf temperatures by about 2 °C from about 34.6 to 32.4 °C, and average D from about 3 kPa to 2.4 kPa (Table 4). Overall, stomatal conductance and Elf were lower during this autumn sampling day than during the previous spring sampling day as environmental conditions changed and leaves aged. Again, shading tended to increase ACO2, gs, and ACO2/Elf of non-saline trees on both rootstocks. The salt treatment reduced these net gas exchange characteristics for trees on both rootstocks except for the gs of unshaded Carr trees, which was not significantly reduced by salinity. Shading increased gs and ACO2/Elf of salinized Carr trees. Leaf transpiration rate was not affected by shading, but salinity decreased Elf of shaded and unshaded Cleo trees and that of unshaded Carr. Salinity consistently increased Ci for trees on both Cleo and Carr. Shade increased Ci in salinized trees on Cleo, but not in trees on Carr.
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Pooling gas exchange measurements across all treatments revealed that high leaf temperature increased D and decreased gs (Fig. 1). The slope of gs versus Tlf appeared greater for the non-saline than for the salinized trees but these slopes were not significantly different when tested by analysis of covariance at P >0.05. ACO2 and ACO2/Elf both increased linearly with increasing gs and the slopes were greater (P <0.05) in the non-saline treatment than in the saline treatment (Fig. 2). Although Ci was greater for salinized than for the non-saline treatment, the relationship between Ci versus gs was not significant as Ci was similar over a wide range of gs values.
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Chloride and sodium concentration in leaves and roots
Leaves on Carr trees had higher leaf chloride concentrations than those on Cleo after both salinization periods, but these differences were not significant for shaded trees after the second salinization period (Fig. 3). At the end of the spring salinization, leaf Cl– concentration was greater than at the end of the autumn salinization that followed the leaching rainy period. Shade decreased leaf Cl– in salinized Carr at the end of both salinization periods. At the end of the autumn salinization, shaded Carr trees had significantly lower root Cl– concentration than unshaded trees.
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Leaves of salinized shaded trees had higher leaf Na+ concentration than those of unshaded trees on both rootstocks at the end of the spring salinization treatment (Fig. 3). Overall, leaf Na+ and Cl– were lower at the end of the autumn salinization than after the spring salinization. Leaves on Carr trees had a significantly lower leaf Na+ concentration than those on Cleo at the end of the spring salinization under shade and at the end of the autumn salinization in full sun. Shade increased leaf Na+ in salinized trees except in Cleo at the end of the autumn salinization. There was no rootstock or shade effect on root Na+ concentration.
Leaf water potential
At the end of the both salinization periods, leaf water potential was consistently decreased by salinity in leaves on both rootstocks under both light treatments (Fig. 4). Shade had no significant effect on leaf water potential at the end of the spring salinity period but, at the end of the autumn salinity period, shade decreased leaf water potential in all trees except non-salinized trees on Cleo.
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w (MPa) in leaves of Valencia orange trees grafted on Cleopatra mandarin or Carrizo citrange. Data are from 6 weeks and 9 weeks after the experiment started. Different letters within each figure indicate significant differences at P <0.05 (Duncan's test). |
Discussion |
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Effect of shading on non-salinized trees
Shading trees with 50% screen cloth reduced Tlf and D and resulted in higher ACO2, gs, and ACO2/Elf in non-salinized trees on both Cleo and Carr. Leaf transpiration, however, was affected little by shading. Although gs was higher in shaded leaves, the driving force for transpiration (D) was lower in shaded leaves and, thus, compensated for the higher gs such that Elf was not affected. Despite the significant correlation between gs and ACO2, the shade-induced increases in ACO2 occurred with no change or a slight increase in Ci (in the second set of gas exchange measurements), implying that gs was not the dominant limitation to ACO2 (Farquhar and Sharkey, 1982). High leaf temperatures were apparently more important than gs in limiting ACO2 because, if CO2 diffusion at low gs in unshaded leaves had been the major limitation to ACO2, a decrease in Ci would have occurred with the decrease in ACO2. Excessively high Tlf at high PAR can also lead to reductions of ACO2 in unshaded leaves due to an increase in photoinhibition (Jifon and Syvertsen, 2003b). Salinity also had a direct effect on the reduction of ACO2 rather than an indirect effect via lowered gs, as indicated by the consistent increases in Ci in salinized trees compared with non-salinized trees at the end of both salinity periods.
Similar responses of Tlf, D, and gas exchange parameters to shade occurred during both measurement periods in non-salinized trees on Carr. In trees on Cleo, however, there was no effect of shading on ACO2 at the end of the second period. Thus, after 31 weeks of shading, citrus leaves on Cleo may have acclimated to shade and reduced ACO2 (Syvertsen, 1984). In an experiment with young Murcott citrus trees, shading increased growth by increasing the leaf dry weight during a 3 month shade period (Medina et al., 2002). In the present study, TPDW increased in shaded trees on Cleopatra from increased leaf growth but shaded trees on Carrizo had a similar TPDW to those in full sun. Higher leaf gas exchange of shaded plants is typically only observed during the middle of the day, since PAR can be limiting in early morning and late afternoon (Medina et al., 2002; Syvertsen et al., 2003). Therefore, the positive effect of shading on ACO2 at midday may have been insufficient to increase the overall growth of trees on Carrizo. In addition, at the end of the experiment, leaves of trees on Carr had a lower leaf water potential in shade than in unshaded conditions. This decrease in leaf water status could have negated any effect of shade on growth of trees on Carr.
Effect of shading on salinized trees
At the end of both the spring and autumn salinization periods, ACO2 of salinized trees was not enhanced by shading, despite the fact that Tlf and D were consistently decreased. Stomatal conductance and Ci, however, were consistently increased by shade in trees on both rootstocks regardless of salinity. Thus, the salinity-induced decrease in ACO2 was not primarily due to stomatal constraints, but was more likely attributable to direct effects of Cl– or/and Na+ ion toxicity (Storey and Walker, 1999; García-Sánchez and Syvertsen, 2006). Leaf Cl– in salinized Carr was reduced by shade, but apparently not enough to affect ACO2. Shaded leaves had higher leaf Na+ concentration than unshaded leaves in trees on both rootstocks. Therefore, high Na+ could have been responsible for negating any ACO2 response in shaded trees. In salinized ‘Valencia’ orange trees grafted on Troyer citrange or Cleopatra mandarin, ACO2 inhibition by salinity was more readily attributable to Na+ toxicity than to Cl– toxicity (Lloyd et al., 1987a). In the present experiment, reduction of PAR by shading could be a limiting factor for photosynthesis in salinized citrus leaves, especially during the morning and afternoon. Non-optimal growth conditions can even decrease the light saturation point (Liaw and Chen, 1991) by as much as 50% in drought-stressed Plantago leaves (Mudrik et al., 2003).
Since shade increased leaf chlorophyll in trees on both rootstocks (Table 2), expressing ACO2 on a leaf chlorophyll basis (ACO2/chl) resulted in a non-significant effect of shade on ACO2/chl (data not shown) that previously was significant when ACO2 was expressed on a leaf area basis (Table 3). All other treatment effects on ACO2 were similar, regardless of whether expressed on a leaf area basis or on a leaf chlorophyll basis.
Unshaded salinized trees on Cleo had lower leaf Cl– than trees on Carr (Fig. 3), supporting salinity-tolerance differences attributable to these rootstocks (Levy and Syvertsen, 2004). This well-known regulation of leaf Cl– concentration in citrus leaves has been associated with leaf transpiration and total water absorbed per plant (Moya et al., 1999, 2003), shoot:root ratio (Storey and Walker, 1999), and efficiency of the root system for limiting Cl– uptake (Storey and Walker, 1999). In this experiment, the higher exclusion of Cl– from shoots in trees on Cleo than on Carr was more likely related to the ability of roots to restrict the movement of Cl– since their shoot:root ratio, leaf dry weight, and leaf transpiration were similar. In addition, shade decreased leaf Cl– concentration in leaves on Carr without changing leaf transpiration. Thus, leaf Cl– concentration was not necessarily closely linked to water use.
The overall growth reduction by salinity in unshaded trees was greater for Carr than for Cleo trees (45% and 28%, respectively), and was related to their relative leaf Cl– concentrations. Such differences in effects of salinity on growth between trees on Cleo and Carr were consistent with earlier findings (García-Sánchez et al., 2002; García-Sánchez and Syvertsen, 2006). Growth reductions by salinity under shaded conditions, however, were greater than those in unshaded conditions and were similar for trees on Cleo and Carr (55–59%). Negative effects of high salinity on strawberry fruit yield (Awang and Atherton, 1995), bean plants (Helal and Mengel, 1981), or melons (Meiri et al., 1982) were also greater under shaded than in unshaded conditions. In the present study, the lower amount of growth of salinized trees under shaded conditions than in full sun could have been due to the greater increase in leaf Na+ concentration in shaded trees. This important effect of Na+ occurred in spite of the high amount of Ca2+ in the salinity treatment as high amounts of Ca2+ can mitigate the negative effects of Na+ (Gratten and Grieve, 1992).
Leaf Na+ concentration of salinized trees on Cleo tended to be higher than on Carr at the end of the spring salinization period, and this difference remained in unshaded trees at the end of the second period. Shade increased leaf Na+ in salinized Carr trees by the end of the second salinity period. Shading affected the Cl– and Na+ concentration in different ways since leaf Na+ concentration was higher for shaded trees on both Cleo and Carr, whereas leaf Cl– concentration was lower for leaves and roots of shaded trees on Carr. So as not to damage roots in the middle of the experiment, root Na+ was not sampled at the end of the first salinity period. The decrease in the leaf temperature by shade did not affect the final accumulation of Na+ in roots but must have increased root uptake, since Na+ transport to leaves was apparently increased. Since shade consistently increased root growth in non-salinized trees on both rootstocks, a greater uptake of Na in shaded trees could have occurred through a transient increase in root growth that was not measured at the end of the experiment.
In conclusion, citrus leaves growing in full sun experience high temperatures that decreased midday ACO2, gs, and water use efficiency. Lowering leaf temperature by shading increased chlorophyll, midday ACO2, gs, and ACO2/Elf but did not affect Elf. Shade did decrease Cl– concentrations in leaves of salinized Carr trees but shade had no effect on Cl– in Cleo. Salinity stress limited the positive effect of shading on net gas exchange of leaves on both rootstocks and negated any effects of shade on growth of ‘Valencia’ orange trees on Carr. In salt-stressed trees, growth was reduced more under shade than in full sun and leaf Na+ was increased more than 2-fold after the spring salinization period. Even though the overall amount of leaf Na+ after the autumn salinization period was lower, Carr shade leaves still had twice the Na+ as sun leaves. Root Na+ in Cleo tended to be higher than in Carr roots and root Na+ was higher than leaf Na+ in both rootstocks. Although root Na+ was not significantly affected by shade, salinized root Na+ was consistently reduced by shade. Thus, the redistribution of Na+ from roots to leaves under shade conditions may have been responsible for the increase in leaf Na+. This idea is supported by previous studies (García-Sánchez and Syvertsen, 2006) where patterns of changes in Na+ and Cl– occurred in opposite directions in roots and leaves.
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Acknowledgements |
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We thank Jill Dunlop and Eva Ros for their skilled assistance. This research was supported by a fellowship from the Ministerio de Educacion, Cultura y Deportes of Spain (AGL2003-08502-CO4-02/AGR) and the University of Florida Agricultural Experiment Station.
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