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To gain insight into salinity tolerance of citrus, we studied growth, leaf, and root Cl^– concentrations and physiological responses of 5-month-old seedlings of the citrus rootstock Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliate L.] grown in a greenhouse in three different substrates: Candler sand soil, Floridana sandy clay soil, or a commercial soilless peat/perlite/vermiculite potting media. Plants were kept well-watered with a complete nutrient solution plus either no salt (control) or 50 mM NaCl for 9 weeks. Without salinity, substrate type did not affect total plant growth although there were differences in shoot/root dry weight ratio and mineral nutrient relationships attributable to substrate. Predawn leaf water potential, midday CO₂ assimilation, and leaf water use efficiency were highest in seedlings grown in the soilless peat. The salt treatment decreased leaf and root growth, reduced leaf Ca²⁺, and increased leaf K⁺ concentration in all the three substrates. Overall, plant growth was negatively related to leaf Cl^–. Leaf growth reductions were least in Candler-grown seedlings and greatest in Floridana soil as Cl^– concentrations were lowest in Candler sand and highest in Floridana soil. Leaf Na⁺ was also highest in Floridana seedlings. In contrast to salt ions in leaves, roots of salinized seedlings in Candler sand had the highest Na⁺ and Cl^– concentration. Salinity reduced net gas exchange of leaves similarly in all three substrates. Salinity reduced both leaf water potential and osmotic potential such that leaf turgor was increased. Thus, salinity-induced reductions in growth and net gas exchange were not the result of loss of turgor but more likely resulting from toxic ion accumulation in leaves. Based on the relative rankings of leaf growth and leaf Cl^– concentrations, Carrizo seedlings from Candler sand had the highest salt tolerance and those grown in Floridana soil had the lowest salt tolerance. Substrate type should be considered when characterizing plant growth and physiological responses to salinity.

Three-month-old citrus rootstock seedlings of the Cl- excluder Cleopatra mandarin (Citrus reticulata Blanco) and the Cl- accumulator Carrizo citrange [C. sinensis (L.) Osb. × Poncirus trifoliata L.] were fertilized with nutrient solution with or without additional 50 mm NaCl and grown at either ambient CO₂ (360 μL·L^-1) or elevated CO₂ (700 μL·L^-1) in similar controlled environment greenhouses for 8 weeks. Elevated CO₂ increased plant growth, shoot/root ratio, leaf dry weight per area, net assimilation of CO₂, chlorophyll, and water-use efficiency but decreased transpiration rate. Elevated CO₂ decreased leaf Ca²⁺ and N concentration in non-salinized Cleopatra. Salinity increased leaf Cl- and Na⁺ in both genotypes. Carrizo had higher concentrations of Cl-but lower Na⁺ in leaves than Cleopatra. Salinity decreased plant growth, shoot/root ratio, net gas exchange, water use, and root Ca⁺² but increased root N in both genotypes regardless of CO₂ level. Neither salinity nor elevated CO₂ affected leaf chlorophyll fluorescence (Fv/Fm). Carrizo had higher Fv/Fm, leaf gas exchange, chlorophyll, N, and Ca²⁺ than Cleopatra. Salinity-induced decreases in leaf osmotic potential increased leaf turgor especially at elevated CO₂. The increase in leaf growth at elevated CO₂ was greater in salinized than in nonsalinized Carrizo but was similar in Cleopatra seedlings regardless of salt treatment. In addition, salinity decreased water-use efficiency more at elevated CO₂ than at ambient CO₂ in Cleopatra but not in Carrizo. Elevated CO₂ also decreased leaf Cl- and Na⁺ in Carrizo but tended to increase both ions in Cleopatra leaves. Based on leaf growth, water-use efficiency and salt ion accumulation, elevated CO₂ increased salinity tolerance in the relatively salt-sensitive Carrizo more than in the salt-tolerant Cleopatra. In salinized seedlings of both genotypes, Cl- and Na⁺ concentration changes in response to eCO₂ in leaves vs. roots were generally in opposite directions. Thus, the modifications of citrus seedling responses to salinity by the higher growth and lower transpiration at elevated CO₂ were not only species dependent, but also involved whole plant growth and allocations of Na⁺ and Cl-.

Two-month-old citrus rootstock seedlings of Cleopatra mandarin (CM) and Carrizo citrange (CC) were fertilized with nutrient solution, with or without additional 50 mM NaCl, and grown under either ambient CO₂ (aCO₂, 360 ppm) or elevated CO₂ (eCO₂, 720 ppm) for 8 weeks. Elevated CO₂ increased plant growth, shoot: root ratio, net assimilation of CO₂, leaf chlorophyll, and water use efficiency (WUE), but decreased plant water use. Salinity decreased growth, shoot: root ratio, net gas exchange and water use. Neither salinity nor eCO₂ affected leaf chlorophyll fluorescence (Fv/Fm), but CC had higher Fv/Fm, leaf gas exchange, chlorophyll, N and Ca than CM. Although salinity increased leaf Cl and Na in both genotypes, CC had higher leaf Cl, but lower Na than CM. Salinity-induced decreases in leaf osmotic potential increased leaf turgor, especially at eCO₂. There were no interacting effects of eCO₂ and salinity on plant growth, but salinity decreased WUE more at eCO₂ than at aCO₂ in CM; but not in CC. Elevated CO₂ decreased leaf Cl and Na in CC, but tended to increase both ions in CM leaves. Patterns of Cl and Na responses in roots generally were in opposite direction to their respective responses in leaves. Thus, the modifications of citrus seedling responses to salinity by higher growth and lower water use at eCO₂ were not only species dependent, but also involved whole plant allocations of Na and Cl.

In three separate experiments, the growth and water use of salinized citrus rootstock seedlings and grafted trees were modified using different growth substrates, elevated CO₂, or 50% shade screen under field conditions. By reanalyzing previously published data, we tested the hypothesis that salinity tolerance in citrus can be characterized as the ability to maintain low levels of leaf Cl⁻ accumulation through high plant growth and high water use efficiency (WUE) under saline conditions. Well-irrigated salinized seedlings of the relatively salt-sensitive Carrizo citrange [Carr (Citrus sinensis × Poncirus trifoliata)] were grown in sand, clay, or a peat-based soilless media. Salinity stress reduced plant growth and water use. Leaf Cl⁻ concentration was negatively related to plant growth, but leaf Cl⁻ increased with transpiration rate in low-saline treatments. In a second experiment using salinized seedlings of the relatively salt-tolerant Cleopatra mandarin [Cleo (Citrus reticulata)] grown along with Carr seedlings with or without elevated CO₂, leaf Cl⁻ was negatively related to growth and to shoot/root dry weight ratio, but was positively related to water use such that leaf Cl⁻ was negatively related to leaf WUE. In a third experiment using salinized 2-year-old ‘Valencia’ orange (C. sinensis) trees grafted on Cleo or Carr rootstocks and grown with or without shadecloth, leaf Cl⁻ was positively related to leaf transpiration as both were higher in the spring than in the fall, regardless of rootstock or shade treatment. Overall, leaf Cl⁻ was positively related to water use and was negatively related to leaf WUE. High growth, low water use, and consequently, high WUE of salinized citrus were related to low leaf Cl⁻. Such relationships can be used as indicators of salinity tolerance.

The effect of annual defoliation over two consecutive years on fruit yield, juice quality, leaf size, and number was examined in 11-year-old `Hamlin' and 13-year-old `Valencia' orange [Citrus sinensis (L.) Osb.] trees. Removal of up to 50% of the leaves in late November had no effect on fruit number, fruit weight, fruit yield, soluble solids yield, juice °Brix, and °Brix: acid ratio of juice in `Hamlin' oranges. In `Valencia' oranges, removal of up to 50% of the leaves in late March also did not affect °Brix or the °Brix: acid ratio of the juice, but decreased fruit yield and soluble solids yield. Leaf size was reduced by removal of 50% of the leaves in both cultivars. Removal of up to 50% leaves in late November had no significant influence on net CO₂ assimilation (aCO₂) of the subsequent spring flush leaves in early May in `Hamlin' oranges, whereas aCO<sub>2</sub> of `Valencia' spring flush leaves in early May increased linearly with increasing levels of defoliation in late March. The results indicate that fruit yield, fruit quality, leaf size, and number were not negatively impacted when annual defoliations did not exceed 25% of the total canopy leaf area for `Valencia' and `Hamlin' oranges for two consecutive years. Overall, in whole `Hamlin' or `Valencia' orange trees, fruit weight increased linearly with increasing ratio of leaf area to fruit, suggesting that fruit enlargement depends on available photosynthate and can be limited by leaf area.

Previous work on citrus trees has shown that an interstock, grafted between the rootstock and scion combination, not only can improve tree growth, longevity, fruit production, and quality, but also can increase salinity tolerance. This research was designed to evaluate flooding responses of 2-year-old ‘Verna’ lemon trees [Citrus limon (L.) Burm.; VL] either grafted on ‘Sour’ orange (C. aurantium L.; SO) rootstock without an interstock (VL/SO) or interstocked with ‘Valencia’ orange (C. sinensis Osbeck;VL/V/SO) or with ‘Castellano’ orange (C. sinensis Osbeck; VL/C/SO). Well-watered and fertilized trees were grown under greenhouse conditions and half were flooded for 9 days. At the end of the flooded period, leaf water relations, leaf gas exchange, chlorophyll fluorescence parameters, mineral nutrition, organic solutes, and carbohydrate concentrations were measured. Leaf water potential (Ψ_w), relative water content (RWC), net CO₂ assimilation rate (A_CO2), and stomatal conductance (g _S) were decreased by flooding in all the trees but the greatest decreases occurred in VL/V/SO. The C_i/C_a (leaf internal CO₂ to ambient CO₂ ratio), F_v /F_o (potential activity of PSII) and F_v /F_m (maximum quantum efficiency) ratios were similar in flooded and non-flooded VL/SO and VL/C/SO trees but were decreased in VL/V/SO trees by flooding. Regardless of interstock, flooding increased root calcium (Ca), iron (Fe), copper (Cu), and manganese (Mn) concentration but decreased nitrogen (N) and potassium (K) concentration. Based on the leaf water relations, gas exchange, and chlorophyll parameters, ‘Verna’ lemon trees interstocked with ‘Valencia’ orange had the least flooding tolerance. Regardless of interstock, the detrimental effect of flooding in ‘Verna’ lemon trees was the leaf dehydration which decreased A_CO2 as a result of non-stomatal factors. Lowered A_CO2 did not decrease the leaf carbohydrate concentration. Flooding decreased root starch in all trees but more so in VL/V/SO trees. Sugars were decreased by flooding in roots of interstocked trees but were increased by flooding in VL/SO roots suggesting that the translocation of carbohydrates from shoots to roots under flooded condition was impaired in interstocked trees.