Physiological Responses of Linseed Seedlings to Iso Osmotic Polyethylene Glycol, Salt, and Alkali Stresses

The aim of this study was to evaluate the physiological responses of linseed (Linum usitatissimum L.) seedling growth after establishment to osmotic, salt, and alkali stresses and to elucidate the adaptive mechanisms involved. Stresses were generated by exposure for 10 d to different iso-osmotic (-0.2, -0.5, -0.8 MPa) solutions of PEG 6000 (osmotic stress), 1: 1, NaCl/Na2SO4 (salt stress), and 1: 1, NaHCO3/Na2SO4 (alkali stress). The RGR decreased as osmotic potential decreased; decreases due to alkali stress were much greater than those due to osmotic stress or salt stress. This was demonstrated by greater reductions in water content and root system activity. Photosynthetic activity and pigment indices were nearly unchanged with increasing osmotic and salt stresses, but were severely inhibited under alkali stress. This implies that alkali stress may be mediated through Na+ uptake accumulation to toxic levels, leading to a decrease in photosynthetic pigments and damage to the photosynthetic apparatus. Massive Na+ influx may be the main cause of damage from alkali stress. The alkali stress causes precipitation of phosphate and metal ions, which causes a sharp decrease in ionic activity and in the free concentrations of various other ions. We found that proline synthesis decreased osmotic potential, remedied the shortage of inorganic anions, and maintained stability of the intracellular pH. Betaine contributions to osmotic adjustment were small because its ratio to total organic matter was very low. The accumulation of organic acids may be a central component of the adaptive mechanism by which ionic balance is maintained under alkali stress.