Chloride versus sulfate salinity effects on alfalfa shoot growth and ionic balance

Information on Cl vs. SO₄ salinity effects on alfalfa (Medicago sativa L) dry matter yield (DM) and cation-anion balance is limited. Consequently, we compared Cl and SO₄ salinity effects on shoot DM and ionic balance for Archer and Ladak varieties of alfalfa. A modified, flowing Hoagland solution, buffered with CaCO₃, was the control: electrical conductivity (EC) = 0.7 dS m^-1. Chloride or SO₄ salts of K, Ca and Mg were added to the control to get iso-EC solutions (2-11 dS m^-1). Shoot cations (Ca, Mg, K, and Na) and their sum (C), anions (Cl, SO₄, NO₃ and H₂PO₄) and their sum (A), and DM were measured. Organic anions (C-A) were calculated in mmol(c) kg^-1. The calculated hydroponic osmotic potentials (π) were from 1.4 to 1.7 times lower in Cl than in iso-conductive SO₄ solutions. As EC increased, DM decreased equally for both varieties in iso-conductive Cl and SO₄ solutions. Solution π decreased, shoot H₂PO₄ declined below its critical level of 65 mmol(c) kg^-1, shoot C stayed constant, shoot A increased; and therefore, shoot C-A decreased. The shoot C-A was lower in SO₄ solutions. In Cl solutions shoot Cl exceeded the toxic level of 282 mmol(c) kg^-1. The DM was correlated positively with π, shoot H₂PO₄, and C-A, and negatively with shoot Cl and SO₄. We conclude that (i) iso-conductive Cl or SO₄ salinity depress DM equally, but iso-osmotic SO₄ is more depressive; (ii) the yield declines are probably due to any one or a combination of low water potential, toxic shoot Cl and possibly SO₄, and deficiencies of shoot P and organic anions; (iii) neutral organic solutes were probably responsible for osmotic adjustment, since total ionic charges (2C) stayed constant; and (iv) P deficiency occurred despite high solution P, due to Cl or SO₄ competition in saline environments.