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

Sunflower seedlings (Helianthus annuus cv. Licia Stella) reached a mil d, moderate and severe level of water deficit stress after 5, 8 and 11 days of soil water depletion, respectively. At a moderate level of wa ter 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 increase d and enzyme activities related to the ascorbate/glutathione cycle wer e induced. At severe water deficit stress, the efficiency of this defe nce mechanism fell, oxidative processes intensified and soluble protei n content decreased further. The results of the present experiments su ggest that during water deficit stress the water status of the plants plays a key role in the activation of defence mechanisms.