Abstract. Drought is one of the most important abiotic stresses that affects plant growth and development. Polyamines (PAs) can have an active key role in improving plants tolerance to various stresses. Based on global ecological conditions, plants growing in about one third of the potentially viable lands such as Iran are exposed to drought stress during growing season. The objective of the present study was to investigate the enhancing effects of foliar application of Putrescine (PUT) on short-term tolerance against water stress in ornamental sunflowers. This study was conducted in the experimental greenhouse of the Department of Horticultural Sciences and Landscape, Ferdowsi University of Mashhad, Iran. Plant materials used in this study were the F1 seeds of ornamental sunflower (Helianthus annus L. cv. Sunbright kids). The experiment was conducted as a factorial (2 × 3) in complete randomized block design with four replications and 8 plants in each replicate. Plants were irrigated at 40 and 80% field capacity (FC). Foliar spray of PUT (0, 75 and 150 mg l-1) was applied twice during plants growing period. Water stress at 40% FC as compared with 80% FC, significantly affected reproductive and vegetative characteristics of plants. The effects of exogenous application of PUT (75 and 150 mg l-1) in comparison with control, significantly increased leaves number per plant, leaves fresh and dry weight, leaves area, stems fresh and dry weight, stems diameters at soil surface, plants height, roots fresh and dry weight, roots area, number of flowers, flowers longevity, seeds number per head and seeds weight. In this study, there was no significant difference between the effects of PUT levels on most of the reproductive and vegetative characteristics of sunflower plants under water stress conditions. The results confirmed that foliar application of PUT protected plants against drought stress and increased specially flowers retention. Therefore, to improve parameters of the studied plants under drought stress, the commercial application of PUT (75 mg l-1) is recommended for this cultivar.

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