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

Abstract

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-.