Effects of NaCl and Na₂SO₄ on Growth and Solute Composition of Highbush Blueberry (Vaccinium corymbosum)

Abstract

Hardwood cuttings from the field-grown blueberry cultivar Blue Crop were grown in nutrient solution with added NaCl(0-200 mol m^-3) or Na₂S0₄ (0-50 mol m^-3). After 5 weeks' growth, leaf fresh weight was greater on either salt than on no salt, and was maximal at 20 mol m^-3 NaCl or 25 mol m^-3 Na₂SO₄. Leaf dry weight increased in proportion to external salinity for both salts. Leaf damage was severe at concentrations greater than 75 mol m^-3 NaCl or 50 mol m^-3 Na₂SO₄.

In the presence of NaCl, Na⁺ and Cl^- concentrations in the leaves were 3-4 times greater than initial solution concentration. Growth in the presence of NaCl was accompanied by loss of K⁺ in leaves (from 94 to 36 μmol g FW^-1), stems (from 48 to 20 μmol g FW^-1) and roots (from 88 to 12 μmol g FW^-1). In the presence of Na₂SO₄, leaf Na⁺ concentration increased to 2-3 times greater than initial solution concentration. Na⁺ uptake and loss of K⁺ were less in plants grown on Na₂SO₄ than on NaCl solutions containing equimolar concentrations of sodium. In Na₂SO₄ solutions, SO₄^2- in stems and roots accumulated approximately to initial external concentration. SO₄^2- concentration in leaves was 1.5-2.0 greater than initial external levels.

In plants grown in 20-50 mol m^-3 NaCl, non-significant slight increases in the foliar concentrations of glycinebetaine, choline and proline were measured (maximum total of 8 μmol g FW^-1 for all metabolites). However, sucrose (2-fold), glucose (3-fold), fructose (4-fold), sorbitol (4-fold) and malate (7-fold) concentrations were significantly enhanced. The total increase in concentration of all measured sugars was approximately 100 μmol g FW^-1, while malate concentrations increased by approximately 15 μmol g FW^-1.

Our data suggest that blueberries are poor Na⁺ and Cl^- excluders and appear to accumulate these ions in leaves. Although growth is enhanced initially on the levels of salt tested, and accumulation of ions and sugars is presumably sufficient for osmotic adjustment, leaf damage eventually occurs. This is probably due to the toxic effects of accumulated Na⁺ and Cl^- in leaves and/or to the severe reduction of K⁺ levels.