Water stress is likely the most important factor that adversely affects plant growth and development. In our study two inbred lines of sunflower (Helianthus annuus L.) with contrasting behavior to water deficit, the inbred lines B59, sensitive, and B71, tolerant, were subjected to water stress and compared in terms of growth parameters, relative water content, photosynthetic pigments and osmo-compatibles contents. Mannitol treatment generated a moderate water stress which resulted in a relative water content decline in both lines. In aerial part of B59 and B71 stressed-seedlings, a dramatic reduction of osmotic potential was observed, although in B71 the osmotic potential was 1.8-fold lower than in B59. The B71 line recovered the osmotic potential to control values after 8 d of re-watering. Several morphological parameters were altered by the water stress. Shoot and root relative fresh weight decreased in both lines under water stress, although B71 showed a minor drop. However, the root was less affected than the shoot in stress condition and recovery period. Water stress was affected in a greater proportion shoot dry weight (DW) than root DW in B59 and B71 lines. Nevertheless, a significant increase in root DW/shoot DW ratio was observed in stressed seedlings. A higher reduction in the individual leaf area was observed in B71 line under stress. The basal chlorophyll content was higher in the tolerant line B71 than in the sensitive B59, and carotenoids showed a high content in B71 control and stressed seedlings. Under water stress, the aerial and root proline content increased in the tolerant B71 line. Glycinebetaine decreased significantly in the aerial part of both lines in response to water stress indicating that this compound is not the main osmo-regulator. In conclusion, B71 has suitable mechanisms to enable it to respond more effectively to moderate water stress.