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UNDER DROUGHT STRESS, FOLIAR APPLICATION OF PUTRESCINE ENHANCES PHOTOSYNTHETIC ACTIVITIES, STOMATAL CLOSURE AND DROUGHT TOLERANCE IN ORNAMENTAL SUNFLOWER ( HELIANTHUS ANNUS L.) PLANT

. Plants’ resistance to drought stress largely depends on their polyamine contents. The present study aims to investigate the enhancing effects of foliar application of Putrescine (Put) on short-term tolerance against drought stress in ornamental sunflowers ( Helianthus annus L. cv. Sunbright Kids). Plants were irrigated at 40 and 80% field capacity (FC). Foliar spray of Put (0, 75 and 150 mgl -1 ) was applied twice during the growing period. Water stress at 40% FC, compared to 80% FC, significantly affected plant physiological characteristics. While the exogenous application of Put, in comparison with the control, significantly increased chlorophyll b, total chlorophyll, carotenoids, soluble carbohydrate, leaf potassium (K) and nitrogen and root K contents, photosynthetic rate, membrane stability index and photosystem II quantum florescent efficiency (F v /F m ), it significantly decreased stomatal conductance of CO 2 (g s ) and transpiration rate (T r ). The results confirmed that the foliar application of Put protected plants against drought stress by promoting plants RWC, MSI, Chl and Pn. Accordingly, the reduction in stomatal g s and K + content may lead to stomatal closure and reduction in T r . Stomatal images are in agreement with g s and T r reduction. It was concluded that in order to improve the sunflower plants tolerance to drought stress, the foliar application of commercial Put (75 mgl -1 ) is recommended for this cultivar.