Salinity Tolerance of Cleopatra Mandarin and Carrizo Citrange Citrus Rootstock Seedlings Is Affected by CO2 Enrichment during Growth

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 m m NaCl and grown at either ambient CO 2 (360 μL·L -1 ) or elevated CO 2 (700 μL·L -1 ) in similar controlled environment greenhouses for 8 weeks. Elevated CO 2 increased plant growth, shoot/root ratio, leaf dry weight per area, net assimilation of CO 2 , chlorophyll, and water-use efficiency but decreased transpiration rate. Elevated CO 2 decreased leaf Ca 2+ 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 CO 2 level. Neither salinity nor elevated CO 2 affected leaf chlorophyll fluorescence (Fv/Fm). Carrizo had higher Fv/Fm, leaf gas exchange, chlorophyll, N, and Ca 2+ than Cleopatra. Salinity-induced decreases in leaf osmotic potential increased leaf turgor especially at elevated CO 2 . The increase in leaf growth at elevated CO 2 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 CO 2 than at ambient CO 2 in Cleopatra but not in Carrizo. Elevated CO 2 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 CO 2 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 eCO 2 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 CO 2 were not only species dependent, but also involved whole plant growth and allocations of Na + and Cl-.