ORIGINAL PAPER Drought stress mitigation by foliar application of salicylic acid and their interactive effects on physiological characteristics of fennel (Foeniculum vulgare Mill.) genotypes Ehsan Askari • Parviz Ehsanzadeh Received: 16 September 2014 / Revised: 24 December 2014 / Accepted: 29 December 2014 Ó Franciszek Go ´rski Institute of Plant Physiology, Polish Academy of Sciences, Krako ´w 2015 Abstract This study was aimed at characterization of physiological responses of fennel to water deficit and exogenous application of salicylic acid (SA). Six fennel genotypes namely ‘Urmia’, ‘Hamadan’, ‘Kerman’, ‘Shi- raz’, ‘Birjand’, and ‘Yazd’ were subjected to two levels of irrigation (irrigation after 35 and 75 % depletion of avail- able soil water) and three levels of SA (0.0, 0.5, and 1.0 mM). Leaf water potential, relative water content (RWC), proline, total soluble carbohydrates, chlorophyll a (Chl a), b (Chl b), total (Chl a ? b), a/b (Chl a/b) and carotenoids contents, catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD) specific activities along with seed yield per plant and seed essential oil content were measured. Water deprivation left significant effects on all characteristics, i.e., in contrast to the anti- oxidative enzymes activities and seed essential oil content, the rest of the attributes were decreased significantly with water deficit. Drought-tolerant genotypes (‘Yazd’ and ‘Shiraz’) exhibited higher accumulation of osmotic solutes and carotenoids along with higher leaf water potential, RWC, and antioxidant enzymes activities (CAT, APX, and SOD) than drought sensitive genotypes (‘Hamadan’ and ‘Birjand’). SA affected the activities of antioxidant enzymes and increased water potential, RWC, leaf osmo- lytes, chlorophyll and carotenoids, and seed essential oil content; the extent of the increases appeared greater when fennel was grown under drought. All the above effects helped the seed yield of fennel genotypes to increase with increase in SA level and led to the conclusion that drought- induced harmful effects on fennel could be alleviated by exogenously applied SA. Keywords Drought  Antioxidative enzymes  Water potential  Salicylic acid (SA)  Essential oil Introduction Water deficit is an important problem for crop production in arid and semi-arid areas of the world. Osmolytes such as proline and soluble carbohydrates accumulate in the plant cells to renovate cell turgor during drought stress. Proline plays a key role in osmotic adjustment and also keeps the cells safe by scavenging reactive oxygen species (ROS) (Mittler 2002), whereas carbohydrates are employed for maintaining metabolism and saving energy under drought (Khalid et al. 2010). In addition, the capability of a plant to synthesize more chlorophyll under water shortage may improve drought tolerance (Pinhero et al. 2000). Drought stress is known to trigger oxidative stress in plant tissues by increasing reactive oxygen species (Kadkhodaie et al. 2014). Chloroplasts are the main organelles producing the reactive oxygen species (ROS), such as the hydrogen per- oxide (H 2 O 2 ), superoxide radical (  O 2 – ), singlet oxygen ( 1 O 2 ), and hydroxyl radical (HO  ) during photosynthesis (Asada 1992). Drought stress induces a significant reduc- tion in photosynthesis. ROS cause membrane lipid perox- idation and chlorophyll degradation. To scavenge ROS, plants employ specific mechanisms, including activation of antioxidant enzymes and non-enzymatic antioxidants, such as carotenoids, glutathione, ascorbic acid, and proline (Mittler 2002). Drought tolerance has been found to be Communicated by A. Gniazdowska-Piekarska. E. Askari  P. Ehsanzadeh (&) Department of Agronomy and Plant Breeding, College of Agriculture, Isfahan University of Technology, Isfahan, PO Box 84156-8311, Iran e-mail: ehsanzadehp@gmail.com 123 Acta Physiol Plant (2015) 37:4 DOI 10.1007/s11738-014-1762-y