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

Abstract

AbstractInformation 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 mmolc 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 mmolc 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 mmolc 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 isoosmotic 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.