285 ISSN 1021-4437, Russian Journal of Plant Physiology, 2020, Vol. 67, No. 2, pp. 285–293. © Pleiades Publishing, Ltd., 2020. Progesterone and Salicylic Acid Elevate Tolerance of Poa pratensis to Salinity Stress E. Sabzmeydani a , S. Sedaghathoor a, *, and D. Hashemabadi a a Department of Horticulture, Rasht Branch, Islamic Azad University, Rasht, Iran *e-mail: sedaghathoor@yahoo.com Received March 15, 2019; revised July 27, 2019; accepted August 1, 2019 Abstract—Kentucky bluegrass (Poa pratensis L.) was exposed to salinity in the factorial study with 2 factors including salinity [irrigation with tap water and saline water with the EC (Electrical Conductivity) of 2, 4, and 6 dS/m] and progesterone and SA (no hormone, 1 mg/L progesterone, 10 mg/L progesterone, 1 mM SA, 3 mM SA, 1 mg/L progesterone + 1 mM SA) applied in four replications. The results showed that the increase in salinity resulted in the loss of N and Ca. Also, Cl and Na accumulated in the leaves. Salinity reduced relative water content to 78%. It increased electrolyte leakage, too. Salinity impaired shoot and root weight, plant height, and the number of tillers. In contrast, the treatments of progesterone and SA mitigated the harmful effects of salinity. These two regulators increased relative water content of leaves, N, Ca, and K contents, shoot and root fresh weight and the number of tillers in salinity-affected plants. Keywords: Poa pratensis, electrolyte leakage, progesterone, relative water content, salinity, storage factor DOI: 10.1134/S1021443720020132 INTRODUCTION Smooth meadow-grass, or Kentucky bluegrass, (Poa pratensis L.) is a grass from the family Poaceae. It is used as cold-season grass in the design of green spaces and golf fields because of its dwarfness and del- icate form. This species can have extensive applica- tions in environmental stresses such as drought, low- temperature, and salinity. This grass is regarded as an invader in natural ecosystems; it reduces the biodiver- sity by changing the nitrogen cycle and ecosystem functions [1]. In recent years, salinity stress has been the major limiting factor of plant growth in most parts of the world due to the increased level of salinity in soils and the lack of proper water resources for agricultural uses [2]. The increase in soil salinity is a serious environmental threat to most plant species, especially non-halo- phytes. It has been reported that about 20 percent of arable lands are influenced by salinity and it is pro- jected that with the increase in the area of saline soils, 50 percent of arable lands will be abandoned [3]. Nitrogen is a major element in stressful conditions as it plays a key role in plant adaptation in salinity stress through reducing nitrogen-containing osmopro- tectants like proline, glycine betaine, protein, and polyamine. As a result, plant tolerance of salinity is improved. In addition, intra-cellular nitrogen balance can be involved in enhancing water use efficiency, CO 2 uptake, enzyme activation, chlorophyll synthesis, and osmotic adjustment and may have an antagonistic effect on the accumulation of sodium and chlorine [4]. Salicylic acid (SA) is an important signal that stim- ulates the defense system in stressful conditions. Extensive studies in recent years have proven the key role of SA in improving the plants exposed to environ- mental stresses like salinity [5]. SA application to salinity-exposed plants of Lolium perenne and P. pratensis reduced seed germination. Steroids are lipophilic compounds with low molec- ular weight with multiple functions in the growth and development processes of the plants and animals just like signal molecules. Numerous studies have focused on the impacts of exogenous application of these hor- mone compounds at different stages of germination, development, and flowering and their effect on improving stress tolerance [6, 7]. Steroid hormones include brassinosteroids and sex steroid hormones including progesterone, estrogen, androgen, and tes- tosterone [6, 8]. Some studies have mentioned their positive effects on alleviating the stress. Filek et al. [9] reported that this hormone binds to protein receptors in the cell membrane and enhances wheat tolerance of low temperatures. They, also, stated that progesterone has a structure similar to sterols and can be directly associated with the fluidity of cell membranes. The results of Shpakovski et al. [10] research indicate a truly fundamental relationship of the steroids biosyn- thesis and their regulation systems of plants and ani- mals. The researchers have succeeded in increasing RESEARCH PAPERS