BETAINES AND FREE AMINO-ACIDS IN SALT STRESSED VITROPLANTS AND WINTER RESTING BUDS OF POPULUS-TRICHOCARPA X DELTOIDES

Organic solutes, in particular glycine betaine and proline, have been detected as osmoprotective compounds in many microorganisms and herbaceous species. However, for woody plants, very little information is available on mechanisms of adaptation to salt stress. In the present study, effects of NaCl treatment on betaine and free amino acid contents in a clone of Populus trichocarpa x deltoides micropropagated vitroplants were analyzed using HPLC and silicate plate chromatography. The application during 12 days of 50 to 200 mM NaCl to vitroplants cultured on Murashige and Skoog medium (Murashige and Skoog 1962. Physiol. Plant. 15: 473-497) led to a progressive decrease of growth, leaf senescence, abscission, and apical necrosis. Populus trichocarpa x deltoides was characterized by the absence of glycine betaine and/or proline accumulation. However, trigonelline was found to increase in vitroplants subjected to 100 mM NaCl. Trigonelline was also present in dormant buds harvested in natural conditions and missing in active buds. In vitroplants, the content of some amino acids was strongly modified by salt stress. A progressive accumulation of alanine and gamma-amino butyrate was particularly significant in roots, whereas the relative concentration of glutamine was strongly enhanced in leaves. Leaf content of glutamate and ornithine attained maximum in the presence of 100 and 150 mM NaCl concentrations, and then decreased. Arginine and serine pools were not significantly modified by salt treatment. The variations in vitroplants were of small amplitude compared to those observed in mature poplars where winter rest was associated with a very high arginine level and with a disappearance of nearly all other free amino acids. The results are discussed in relation to previous data obtained with herbaceous species and in relation to some metabolic pathways in poplars where trigonelline could be considered as a sensitive stress indicator.