SUNFLOWER SEEDLINGS SUBJECTED TO INCREASING WATER-DEFICIT STRESS - OXIDATIVE STRESS AND DEFENSE-MECHANISMS

Sunflower seedlings (Helianthus annuus cv. Licia Stella) reached a mild, moderate and severe level of water deficit stress after 5, 8 and 11 days of soil water depletion, respectively. At a moderate level of water deficit stress, an elastic adjustment occurred. At the same time, in response to a minor osmotic potential (psi(pi)) and an intermediate rate of water potential (psi(w)) decrease, glutathione level increased and enzyme activities related to the ascorbate/glutathione cycle were induced. At severe water deficit stress, the efficiency of this defence mechanism fell, oxidative processes intensified and soluble protein content decreased further. The results of the present experiments suggest that during water deficit stress the water status of the plants plays a key role in the activation of defence mechanisms.