Water relations in Azospirillum-inoculated wheat seedlings under osmotic stress

Azospirillum has been shown to improve coleoptile growth in seedlings growing in darkness under osmotic stress. However, the changes in water relations that may occur in this experimental system have not yet been studied. Two-centimetre long Triticum aestivum cv. Buck Pucará and Triticum durum cv. Balcarceño-INTA seedlings were inoculated with viable or autoclaved (control) Azospirillum brasilense Sp. 245 bacteria, at approximately 108 cells per seedling. Three days after inoculation, seedlings were exposed to osmotic stress by immersing their roots in 20% polyethylene glycol 6000 for up to 72 h. Germination and seedling growth were at 20°C in darkness. Shoots were excised after 72 h of stress, and water-status parameters were determined through pressure–volume analyses. While osmotic potential at full turgor remained constant, Azospirillum-stimulated growth in Buck Pucará seedlings was accompanied by significant decreases in osmotic potential and relative water content at zero turgor, in volumetric cell wall modulus of elasticity, and in absolute symplastic water volume and by a significant rise in apoplastic water fraction parameters. Except for a constant volumetric cell wall modulus of elasticity, similar results were obtained with Balcarceño-INTA seedlings. However, bacterial growth promotion was evident only in the less tolerant cv. Buck Pucará. Turgor at low water potential was higher in inoculated seedlings in both wheat cultivars under osmotic stress. These results are consistent with a better water status in Azospirillum-inoculated wheat seedlings under water stress, where both effects on cell wall elasticity and (or) apoplastic water are evident.