Growth and Development in NaCl-Treated Plants. I. Leaf Na⁺ and Cl^- Concentrations Do Not Determine Gas Exchange of Leaf Blades in Barley

Abstract

Plants of Hordeum vulgare cv. Clipper were grown in gravel culture in solutions containing 0, 100 or 175 mol m^-3 NaCl from the seedling stage until anthesis. When salt was first applied, leaf 4 was elongating within the sheath, and leaf 7 had yet to appear on the growing apex. These two leaves were sampled most intensively. Relative to the controls, salinity reduced the final area of all leaves progressively with higher node of insertion, and in the 175 mol m^-3 NaCl treatment the reduction was more than twice that in 100 mol m^-3 NaCl. Because leaves were at very different stages of development when salt was applied, the progressive reduction inferred there was no evidence of a particular sensitivity to salinity at different stages of leaf expansion. The peak CO₂ exchange rate (CER), which occurred shortly after ligule emergence of each leaf was reduced far more in leaf 4 than in higher positions. Indeed, CER averaged throughout the leaf's life, was actually increased by salinity in upper leaves which initiated and expanded during the salt treatment. However, the reduction in leaf area was not offset by this increase in CER. The integration of these adjustments in area per leaf and CER resulted in a reduction in carbon fixation per leaf of about 30 and 60%, by the 100 and 175 mol m^-3 treatments respectively for all leaves. There was no obvious relation between CER and Na⁺ and Cl^- concentrations in leaf 7, and between CER and Na+ concentrations in leaf 4. Regression analysis showed that chloride and sodium ions in the leaves accounted for 54% of the statistical variation in CER in leaf 4 but for an insignificant proportion of the variation in leaf 7. As leaf 7 is presumably no different from leaf 4 in its ionic response, we conclude that the high leaf concentrations of Na⁺ and Cl^- did not directly affect the rate of photosynthesis.