Chloride Versus Sulfate Salinity Effects on Alfalfa Shoot Growth and Ionic Balance

Abstract

Abstract Information on Cl vs. SO 4 salinity effects on alfalfa ( Medicago sativa L) dry matter yield (DM) and cation‐anion balance is limited. Consequently, we compared Cl and SO 4 salinity effects on shoot DM and ionic balance for Archer and Ladak varieties of alfalfa. A modified, flowing Hoagland solution, buffered with CaCO 3 , was the control: electrical conductivity (EC) = 0.7 dS m ‐1 . Chloride or SO 4 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, SO 4 , NO 3 and H 2 PO 4 ) 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 SO 4 solutions. As EC increased, DM decreased equally for both varieties in iso‐conductive Cl and SO 4 solutions. Solution π decreased, shoot H 2 PO 4 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 SO 4 solutions. In Cl solutions shoot Cl exceeded the toxic level of 282 mmol c kg ‐1 . The DM was correlated positively with π, shoot H 2 PO 4 , and C‐A , and negatively with shoot Cl and SO 4 . We conclude that (i) iso‐conductive Cl or SO 4 salinity depress DM equally, but isoosmotic SO 4 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 SO 4 , 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 SO 4 competition in saline environments.