EFFECT OF OSMOTIC AND IONIC STRESSES ON PROLINE AND ORGANIC-ACID CONTENTS DURING IMBIBITION AND GERMINATION OF SOYBEAN SEEDS

Time course accumulations of organic acids and proline were measured in soybean (Glycine max Merr, cv. Maple Arow) seeds. Seeds were soaked and germinated either in control Knop solution (C solution), in buffered C (from pH 4 to pH 7.8), or in C supplemented with -0.11 MPa isoosmotic salts (KCl, NaCl, NH4Cl, CaCl2, mannitol). Solute analyses were carried out during imbibition (from 0 to 12 h) or 5 days after radicle emergence. Soaking seeds as well as seedlings (at the stage of radicle emergence) were transferred from C into NaCl or mannitol media, or from NaCl into C solution. During imbibition, KCl, NaCl and mannitol caused a 50% increase of the organic acid pool (mainly succinate), while proline increased only slightly. Conversely, pH solutions did not exert changes in organic acids, proline levels decreasing only at a pH value about 8. NaCl-treated seeds previously soaked for 1 h in C solution displayed increased organic acid contents compared with seeds continuously soaked in C solution. Yet, no change in organic acids was detectable in seeds soaked in NaCl solution or transferred into C after 1 or 6 h of exposure to NaCl. In NaCl-imbibed seeds, the proline synthesis began after a 7-h lag phase and was independent from external osmolarity. Growth of NaCl-treated seedlings decreased by 55% compared with C seedlings. Dry matter production, organic acid and proline levels remained comparable either in seedlings stressed by 1-h imbibition in NaCl before transfer into control solution, or in salt tolerant seedlings imbibed in control medium before transfer into NaCl medium. The relationships between proline and organic acid accumulation, NaCl-sensitivity of seeds and NaCl-tolerance of seedclings are discussed.