Growth and gas exchange parameters of Citrus plants stressed with different salts

Valencia orange scions ( Citrus sinensis (L.) Osbeck) budded to either Cleopatra mandarin (Citrus reticulata) or Poncirus trifoliata ([L.] Raf.) rootstocks were treated with 60 mmol/L of different salts (NaCl, KCl and NaNO3) in order to distinguish the specific effects of each ion on growth and gas exchange parameters. The chloride salts markedly reduced plant growth in both scion-root stock combinations whereas NaNO3 had very little effect. Poncirus trifoliata accumulated relatively more Cl- in leaves and less in roots than Cleopatra mandarin. In contrast, the sodium concentration in leaves was higher in Cleopatra mandarin than in Poncirus trifoliata. The chloride and sodium contents in leaves of both scion-rootstock combinations were depressed when 30 mmol/L Ca(NO3)(2) was added to the culture solution. Both chloride salts (KCl and NaCl) caused a similar reduction in photosynthesis and stomatal conductance, whereas NaNO3 had no detectable effects on these parameters. Addition of calcium nitrate considerably increased growth and the gas exchange parameters in plants exposed to chloride salts and also reduced ion uptake and transport. Taken together, these results indicate that the salinity effects on growth and gas exchange parameters apparently are induced by the external chloride supply.