Hypoxia affects root sodium and chloride concentrations and alters water conductance in salt-treated jack pine (Pinus banksiana) seedlings

Abstract

The effects of NaCl were studied in 6-month-old jack pine (Pinus banksiana Lamb.) seedlings growing in solution culture under hypoxic (approximately 2 mg l⁻¹ O₂) and well-aerated (approximately 8 mg l⁻¹ O₂) conditions. The results showed that hypoxia led to further reduction of stomatal conductance (g _s) in plants treated with 45 mM NaCl. This effect was likely due to a reduction in root hydraulic conductance by both stresses. When applied individually or together, neither 45 mM NaCl nor hypoxia affected cell membrane integrity of needles as measured by tissue electrolyte leakage. Hypoxia did not alter shoot Na⁺ and Cl⁻ concentrations in NaCl-treated plants. However, root Na⁺ concentrations were lower in NaCl-treated hypoxic plants, suggesting that hypoxia affected the ability of roots to store Na⁺. Hypoxia also induced root electrolyte leakage from NaCl-treated and control plants. The higher root Cl⁻ concentrations compared with Na⁺ and the positive correlation between root Cl⁻ concentrations and electrolyte leakage suggest that Cl⁻ played a major role in salt injury observed in jack pine seedlings. Roots of well-aerated plants treated for 1 week with NaCl contained almost two-fold higher concentration of total non-structural carbohydrates compared with plants from other experimental treatments and these concentrations decreased in subsequent weeks. We suggest that under prolonged hypoxic conditions, roots lose the ability to prevent Cl⁻ uptake resulting in the increase in root Cl⁻ concentration, which has damaging effects on root cell membranes.