Uptake, Transport, and Concentration of Chloride and Sodium in Three Citrus Rootstock Seedlings

Seedlings of three citrus rootstocks, sour orange (Citrus aurantiumL.; SO), Carrizo citrange (Citrus sinensis L., Osbeck x P. trifoliata L. Ref.; CA), and Macrophylla (Citrus macrophylla Wester; MA) were grown in nutrient solution and exposed for 60 days to different levels of salinity (0, 10, 20, 40, or 80 mM NaCl). The effects of the salinity treatments on growth, uptake, transport, and accumulation of Cl− and Na+ ions, in leaves and roots, were studied. Rootstock differences in leaf Cl− and Na+ concentrations were less evident at low salinity (10–20 mM NaCl) and clearly observed at higher external NaCl (40–80 mM NaCl). The most salt-tolerant rootstock was sour orange (SO), which experienced less growth reduction even when it accumulated high concentrations of Cl− and Na+ in the leaves. In addition, SO maintained the uptake and transport rates of ions similarly during the experiment, at all salinity levels. However, in Macrophylla (MA) and Carrizo (CA), at 40 and 60 days, the uptake and transport rates were greatly reduced for both ions, values being close to those of the control plants. CA and MA had high transport rates of Cl− and Na+ at low and moderate salinity, leading to a rapid influx of ions to the shoot and a reduction of growth. The analysis of ion accumulation and uptake rates within the leaves and roots indicated the absence of a direct correlation between vigor and salt tolerance at high salinity. The net transport rate to leaves in the first 20 days seemed to be the main factor regulating Cl− and Na+ concentrations in the leaves.