Zinc-induced anti-oxidative defense and osmotic adjustments to enhance drought stress tolerance in sunflower (Helianthus annuus L.)

Abstract

Zinc (Zn) is an essential micronutrient for plant growth and development and plays a vital role in drought stress tolerance. To explore the physiological mechanisms of drought stress tolerance improved by exogenous application of Zn in sunflower, a hydroponic experiment was conducted. The present study comprised of two factors i.e. PEG-6000 induced drought stress for 0, 7, and 14 days and Zn treatments (0, 1, and 3 μM ZnSO4), arranged in a completely randomized factorial design. Exogenous application of Zn significantly boosted the photosynthetic pigments i.e. chlorophyll a (31.2 %), chlorophyll b (35.9 %), photosynthetic rate (41.7 %), and shoot (44.0 %) and root dry matter (38.7 %) of sunflower under PEG-6000 induced drought stress compared to the control without any Zn treatment. In comparison to control, antioxidant enzymes activities, i.e. ascorbate peroxidase, superoxide dismutase and catalase and non-enzymatic antioxidant i.e. ascorbic acid were significantly enhanced by 57.1, 26.6, 62.3, and 38.3 %, respectively while the malondialdehyde (MDA) contents reduced by 83.5 % with exogenous application 3 μM ZnSO4 under PEG-6000 induced drought stress. The same treatment enhanced osmoregulatory substances such as soluble proteins (79.5 %) and free proline contents (70.5 %) under PEG-6000 induced drought stress as compared to control. Relative water contents showed highly significant positive correlation with free proline (R2 = 0.95), soluble proteins (R2 = 0.98), superoxide dismutase (R2 = 0.91) and negative correlation with MDA contents (R2 = 0.91). It is concluded that exogenous application Zn enhanced anti-oxidative defense and osmotic adjustment and ultimately drought stress tolerance in sunflower (Helianthus annuus L.).