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Salinity, CO2 and Citrus Trees
Reference
Syvertsen, J. and Levy, Y.  2005.  Salinity interactions with other abiotic and biotic stresses in citrus.  HortTechnology 15: 100-103.

What was done
The authors review what is known about salinity stress in citrus trees and how it may be modified by atmospheric CO2 enrichment.

What was learned
Syvertsen and Levy note that rapidly growing plants almost always use more water than slower growing plants, and that, "in citrus, many vigorous rootstocks that produce fast-growing trees also tend to have poor salt tolerance (Castle et al., 1993)," possibly because they accumulate more salt in their tissues because of their greater uptake of water.  When growing plants in CO2-enriched air, however, plant stomatal conductance and water use are often decreased at the same time that net photosynthesis and growth are increased, so that, in the words of the two scientists, "elevated CO2 almost always leads to higher water use efficiency as it disconnects rapid tree growth from high water use."  Consequently, as they explain, "if salt uptake is coupled with water uptake, then leaves grown at elevated CO2 should have lower salt concentrations than leaves grown at ambient CO2 (Ball and Munns, 1992)."

So, do things really work that way?  "As expected," Syvertsen and Levy continue, "all citrus rootstock species studied increased growth and water use efficiency in response to elevated CO2 that was twice ambient," and they say that generally, but not always, "the salinity-induced accumulation of sodium (Na+) in leaves was less when seedlings were grown at elevated CO2 than at ambient CO2."  One exception, where Na+ accumulation was not affected by elevated CO2, was Rangpur lime (Citrus reticulata); but they report that this citrus variety is already relatively salt-tolerant, and that another variety of this same species (Cleopatra mandarin) had lower leaf chloride (Cl-) concentrations in CO2-enriched air than in ambient air.

What it means
All citrus trees that have been tested to date have exhibited increased growth rates and water use efficiencies when growing in CO2-enriched air.  In addition, they generally experience less salinity stress than when grown in lower-CO2 ambient air.  Consequently, the ongoing rise in the atmosphere's CO2 concentration bodes well for the future vitality and productivity of earth's many varieties of citrus trees, which in turn bodes well for humanity (see Idso et al., 2002).

References
Ball, M.C. and Munns, R.  1992.  Plant responses to salinity under elevated atmospheric concentrations of CO2Australian Journal of Botany 40: 515-525.

Castle, W.S., Tucker, D.P.H., Krezdorn, A.H. and Youtsey, C.O.  1993.  Rootstocks for Florida Citrus: Rootstock Selection, the First Step to Success.  University of Florida, Institute of Food and Agricultural Science, Gainesville, Florida, USA.

Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D.  2002.  The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice.  Agriculture, Ecosystems and Environment 90: 1-7.

Reviewed 6 April 2005