Environmental and Experimental Botany 40 (1998) 173 – 178 Solute incompatibility with glutamine synthetase in water-stressed Populus nigra Richard Sibout, Gilles Guerrier * Biologie des Ligneux, EA 1207 (Associe ´ INRA), UFR Faculte ´ des Sciences, Uniersite ´d’Orle ´ans, BP 6759, 45067 Orleans Cedex 2, France Received 29 January 1998; received in revised form 14 April 1998; accepted 17 April 1998 Abstract When poplar leaves were submitted to water-stress for 18 h (corresponding to a 0.1 M mannitol application), the activity of glutamine synthetase (EC 6.3.1.2) decreased by about 50%. The ability of several solutes accumulated by stressed poplars (glucose, putrescine, salicin, spermidine, sucrose, trigonelline) to affect in vitro the GS activity has been compared with that of some classical compatible solutes (mannitol, sorbitol, proline). Mannitol, salicin and trigonelline (in the range of 0.1–0.2 M) were drastic inhibitors of GS, while glucose and sucrose did not interfere with GS activity and might partially alleviate the inhibition of GS activity induced by mannitol in vitro. Proline and putrescine (25 mM) were the most effective in alleviating the inhibitory effect of mannitol; since a 0.2 M concentration of each one inhibited GS, proline could not be referred to as a compatible solute. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Poplar; Compatible solutes; Enzyme stability; Glutamine synthetase; Water stress; Populus nigra 1. Introduction Short rotation intensive cultures of fast growing trees, such as poplar plantations, constitute an alternative of agricultural land. However, the cli- matic and/or anthropogenic shifts in water availability are responsible for the fall in poplar biomass production and their enhanced sensitivity to the vascular disease Dothichiza (Soule `res, 1992). Therefore, the physiological processes and the genetic controls that limit plant growth must be researched in a program of poplar breeding for water-stress tolerance. The effect of water-stress on tree productivity originated in leaf level by decreases in net assimi- lation rates, diversion of the normal flow of elec- trons in illuminated chloroplasts, and chloro- plastic synthesis of various organic cytocompat- ible osmolytes (Smirnoff, 1993; Foyer et al., 1994; Dreyer, 1997; Gaudille `re, 1997). Since the ad- aptive response involving both the osmotic ad- justment and the protection of cell components may develop rapidly (Turner and Jones, 1980; * Corresponding author. Tel./fax: +33 2 38417093; e-mail: gilles.guerrier@univ-orleans.fr S0098-8472/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0098-8472(98)00032-X