Interrelations between growth and photosynthesis of salt bush (atriplex halimus L.) Grown in saline media

Plants of A. halimus were grown in culture solutions to which NaCl or Na2S04 was added to different concentrations. The osmotic potential of the sap, growth, morphology, and leaf gas exchange (transpiration and photosynthesis) were studied. At least three different effects of salinity were identified: (1) an increase of leaf area and succulence induced by relatively low concentrations of salt in the growth medium. This resulted in an increase of the leaf area available for transpiration and photosynthesis (and hence growth); (2) an increase of stomatal resistance to water vapour loss and CO₂ uptake. This tended to reduce both transpiration and photosynthesis per unit leaf area; (3) changes in mesophyll resistance to CO₂ uptake (r). Relatively low concentrations of NaCl reduced r_m, but high NaCl concentrations (i.e. external osmotic potentials lower than -9 atm) and all concentrations of Na₂SO₄ brought about an increase in r_m. In addition to the above effects, salinity may have caused a hormonal imbalance as was indicated by the increased sprouting of lateral buds. These responses to salinity tend to counteract one another in their overall effect on growth. Which factor will become dominant is determined by the level and type of salinity, and also, possibly, by climatic conditions.