Salt injury to plants with special reference to cations versus anions and ion activities

Summary In contrast with the toxicities of sulfate and chloride salts added to substrates, the anions SO₄ and C1 were not injurious when accumulated without leaf burning by cotton and tomato plants from atmospheres enriched with SO₂ or HC1 gases. The foregoing results are discussed in terms of cationenzyme interactions which appear to represent at least a major cause of salt toxicity. Although anions are largely unreactive with enzymes it has long been observed that chloride salts in soil solutions are far more toxic than sulfate salts. Five of seven species have shown nearly equal growth repressions on substrates with 100 me/1 of C1 salts versus 200 me of SO₄ salts, each added as 50 per cent Na. The ion activities of the two solutions were equal and the sum of cations in the plant saps were similar. The osmotic differentials (average about 10 atm) between the expressed tissue fluids and these substrate solutions were remarkably uniform within species. It is projected that the downward transport of salts via the phloem provides for root concentrations which supply ions to the xylem and thereby control the uptake of substrate salts.