Crop plants face different abiotic stresses during their growth cycle ( Leufen et al. , 2016 ); however, water deficit (WD) is considered the most deleterious among all abiotic stresses, that limits the plant growth and final productivity in agricultural crop systems worldwide ( Leufen et al. , 2016 ; Liu et al. , 2018 ; Ghaffari et al. , 2019 ). The current situation is becoming more alarming due to climate change where many parts of the world are facing WD challenges to sustain crop production to meet the growing needs of the food and feed ( Mancosu et al. , 2015 ). At present, the decline in surface and ground water resources is further aggravating WD scenario and is a major threat to crop production and food security across the globe ( Mishra & Singh, 2010 ). Decline in water resources could negatively affect the agricultural productivity through various mechanisms; for instance, reduction in turgor and water contents in the growing plant, as a result of imbalanced osmotic pressure and cell homeostasis ( Din et al. , 2011 ; Leufen et al. , 2016 ), production of reactive oxygen species (ROS) along with enhanced levels of malondialdehyde (MDA) in plant cells ( Nahar et al. , 2016 ). Such adverse changes cause cell membrane damages in plants ( Moussa & Abdel-Aziz, 2008 ; Cunhua et al. , 2010 ), resulting in lower final productivity in agricultural systems. However, inbuilt defense systems have been reported in growing plants to combat the negative effects of WD inclu­ding oxidative stresses ( Ghaffari et al. , 2019 ). Oxida­ti­ve defense systems include enzymatic antioxidants such as catalase (CAT), ascorbate peroxidase (APX), superoxidase dismutase (SOD) and peroxidase (POX), whereas non-enzymatic antioxidants include lower mo­­lecular weight compounds, such as vitamins (vita­mins C and E), β-carotene, uric acid, and glutathione (GSH), a tripeptide (l-ℽ-glutamyl-l-cysteinyl-l-glycine) that comprises a thiol (sulfhydryl) group ( Birben et al. , 2012 ; Ghaffari et al. , 2019 ) that allows the scavenging of superoxide radicals and H ₂ O ₂ .

Sugar beet ( Beta vulgaris L.) is the second most important sugar crop after sugarcane ( Iqbal & Saleem, 2015 ). Sugar beet production mainly depends on the precipitation and irrigation in many areas of world ( Fotouhi et al. , 2017 ; Ghaffari et al. , 2019 ). Although the sugar beet is relatively tolerant to adverse environmental conditions ( Fotouhi et al. , 2017 ); however, severe WD could result in major losses in sugar beet yield ( Chołuj et al. , 2014 ; Moosavi et al. , 2017 ; Ghaffari et al. , 2019 ). For instance, up to 30% loss has been reported in arid, and semi-arid regions ( Ober, 2001 ; Mansuri et al. , 2018 ). In a previous study ( Ghaffari et al. , 2019 ), we observed 35% yield reduction under severe WD when sugar beet was grown under field conditions. Fotouhi et al. (2017) reported an increase in sugar percentage under WD compared to well water supplies; however, WD caused significant yield losses, which might be due to the reduction in root yield (RY) ( Mahmoud et al. , 2018 ).

Different agronomic and molecular approaches are being applied to mitigate the detrimental effects of WD on growing plants ( Hasanuzzaman & Fujita, 2011 ; Alam et al. , 2014 ; Ghaffari et al. , 2019 ). Foliage applications of different osmoprotectants are being considered as potential strategies to mitigate WD effects under various climatic conditions ( Gholami Zali & Ehsanzadeh, 2018 ; Ghaffari et al. , 2019 ). Jasmonic acid (JA) and methyl jasmonate (MeJA), commonly referred as jasmonates, are considered as osmoregulators and play crucial role in regulating environmental stresses ( Muñoz-Espinoza et al. , 2015 ; Pazoki, 2015 ). Alam et al. (2014) concluded that JA is an important signaling molecule and shows plant responses responsible in a wide range of morphological, physiological, and biochemical mechanisms. Keeping in view the importance of sugar beet and increasing events of drought stress, the current study was planned to evaluate the effects of foliar application of JA on different growth stages (sugar beet growth stage BGS 16-32, BGS 19-34 and BGS 19-36) of sugar beet plants in imparting the WD tolerance under field conditions. The specific purposes of this study therefore were to elucidate the possible roles of JA in WD tolerance, with special reference to the antioxidant defense as well as to some biochemical parameters in sugar beet exposed to WD. We hypothesized that enhanced enzymatic antioxidant system due to foliar JA applications could result in better crop growth, root yield and white sugar content (WSC) in sugar beet.

This research was performed in Chaharmahal-Bakhtiari province in Iran at two different locations (please see Ghaffari et al. , 2019 for complete details). The climatic growth conditions at the experimental lo­ca­tions during whole crop growth season are presented in Table 1. Monogerm Castile sugar beet rhizomania resistant seeds (SesVanderHave, Tienen, Belgium) were planted in late May 2015 on a clay loam soil ( Ghaffari et al. , 2019 ). A completely randomized block design with three replicates was arranged as split-plot. Factor one was kept in main plots comprising of normal irrigation (control: 100% water requirement of plant), mild WD (75% water requirement of plant) and severe WD (50% water requirement of plant), whereas the second factor included no JA foliage applications as control, low JA (5 µM) and high JA (10 µM) applications which were maintained in the subplots. The planting density was 10 plants m ^-2 with 50 cm apart rows, whereas the subplots were 3 m × 4 m in width and length. WD was imposed 75 days after sowing when crop attained about 16-20 leaves (BGS 19-35). JA foliage spray treatments were applied three times in two-week intervals on foliage parts of sugar beet plants in respective plots and first spray was carried out after 45 days after planting. Three JA applications were applied at BGS 16-32, BGS 19-34 and BGS 19-36, respectively (BBCH Scale: https://www.politicheagricole.it/flex/AppData/WebLive/Agrometeo/MIEPFY800/BBCHengl2001.pdf ). Plots related to con­trol treatments were sprayed with water at time of JA foliar applications. WD treatments were determined ba­sed on maximum allowable depletion by adopting the method developed by Allen et al. (2000) and explained in our previous report ( Ghaffari et al. , 2019 ).

Growing plants show various symptoms to express drought stress effects; e.g ., electrolyte leakage and MDA contents occur commonly due to plant cell membranes damage ( Poonam et al. , 2013 ). The estimated MDA value is commonly utilized to measure the lipid peroxidation as an index of the membrane damage caused by drought ( Lin & Kao, 2000 ). The lower membrane stability index is a consequence of higher oxidative stress under drought stress ( Nazarli et al. , 2014 ). Majid & Akbar (2006) reported that MeJA mitigated the ROS effects in maize seedlings under WD; they showed that MeJA lowered the ratio of membrane fatty acids, and targeted free radicals. Imbalance between ROS production and antioxidant concentration resulted from stress conditions in cells, may lead to damage to membranes and increase in membrane permeability. However, ROS are not only produced as a toxic by-product but also plays a role in plant response to stress conditions. These responses to stress include hormone signaling, stomatal regulation, and gene activation ( Sharma et al. , 2012 ). Increased ROS levels under drought stress are reported to initiate the protective mechanisms in growing plants; however, such mechanisms are further enhanced due to phytohormones such as JA to mitigate the drought stress effects ( Nazarli et al. , 2014 ).

JA foliar applications significantly ( p <0.05) reduced the cell membrane damage by down regulating the MDA contents and H ₂ O ₂ levels in WD compared to normal irrigation treatment (Fig. 2a, 2b). The effects of JA foliar application were more pronounced under WD conditions compared to normal irrigation where high JA (10 µM) foliar applications resulted in up to 22% reduction in MDA contents compared to controlled JA application under severe WD (Fig. 2a). Similarly, H ₂ O ₂ levels decreased by 45% under mild and by 8% under severe WD compared to normal irrigation sugar beet plants under high JA foliar applications (Fig. 2b). JA may be used in large-scale production systems because of its low cost and high production of secondary metabolites. The present study indicates that JA applications increased antioxidant enzymes activities whether plants were under stressed or non-stressed conditions, by supplementing the ROS scavenging mechanism (Fig. 1-2). However, the effects of JA on enzymes activities were more pronounced under WD conditions. Higher accumulation of antioxidative enzy­mes in the plants may lead to increased resistance against water stress ( Kadkhodaie et al. , 2014 ). Addi­tionally, we have observed an enhanced drought tolerance in growing plants due to JA applications that reduced the ROS production and lipid peroxidation (MDA and H ₂ O ₂ ) on both study sites (Fig. 2ab, 2cd). Additionally, a reduction in ROS production and lipid peroxidation (MDA and H ₂ O ₂ ), and enhanced drought tolerance in plants under stress conditions (Fig. 2a, 2b). It appears that JA increases CAT, SOD and POX activities and reduces the toxic effects of free radicals or quenched ROS, thereby it plays an important role in signal transduction pathway in oxidative stress ( Norastehnia & Nojavan-Asghari, 2006 ). These findings are in agreement with those of Shan & Liang (2010) , who reported that JA increased the transcript levels of APX and CAT in Agropyron cristatum under water stress. It was also observed that foliar application of JA reduced the deleterious effects of WD on the growth of sugar beet crop. Anjum et al. , (2011) and Mahmood et al. (2012) reported significantly positive effects of JA in reducing the water losses and upregulating the drought tolerance in growing plants. In addition, under drought, JA may act as an inducer of signal that leads to the upregulation of ascorbate and glutathione metabolism leading to water stress tolerance ( Shan & Liang, 2010 ). Many studies have shown the positive effects of JA in mitigating the deleterious effects of drought on growing plants. Exogenous application of JA may cause an increase in compatible osmolites levels, activity of antioxidant enzyme and level of betaine aldehyde dehydrogenase protein in plants grown under drought conditions ( Gao et al. , 2004 ).

The results further revealed that the WSC increased under both water stress and foliar application of JA, which are in line with the results reported by Ghamarina et al. (2012) , who found 4.45%, 17.32% and 37.63% higher WSC at 25%, 50% and 75% drought stress treatments, respectively. Some studies have reported that higher applications of K result in higher sugar beet yield, sucrose percentage, water use efficiency, higher recoverable sugar yield and lowest impurities under drought stress ( Abdel-Motagally & Attia, 2009 ; Neseim et al. , 2014 ). Therefore, the increased WSC in our study with JA foliage application was related to the high K contents and reduced impurities as Na and α-N (Fig. 3), and increase in sugar recovery as water stress increased. In addition, JA probably increases stress-tolerance by increasing K absorption under stress conditions; as a result, more K accumulation occurs in the sugar beet root (Fig. 3). Reduction of RY and WSY under WD could be a result of the negative effect of WD on sugar beet growth parameters such as dry weight of plant and leaf area index, which were reflected on the lower relative growth rate and net assimilation rate ( Mahmoud et al. , 2018 ). Severe water stress causes a photosynthesis reduction, and in result would tend to reduce RY ( Ghaffari et al. , 2019 ). Despite the high WSC in severe water stress treatment, RY could not recover the low WSY as compared to the control. Therefore, the plants were not able to increase the potential of producing WSY. In this regard, the 50% water stress had the least WSY and decreased by 35% compared to the normal irrigation. In conclusion, it can be seen that the RY has a greater impact than sugar content on sugar yield ( Masri et al. , 2015 ). Chołuj et al. (2014) indicated that the RY and WSY were reduced 22% and 35%, respectively under drought, but there were significant differences between genotypes. It has been reported that water stress can adversely influence tap-root and WSY of sugar beet by about 15-50% depending on the severity and duration of drought during the vegetation season. Additionally, Chołuj et al. (2008) reported that long-term water shortage resulted in significant changes in sugar content, and slightly increase K, Na, and α-N contents in the tap-roots of sugar beet. In contrast, Hoffmann (2010) showed that sucrose accumulation in the beet tap-root was markedly reduced, whereas ions contents and compatible solutes were increased under drought. However, sucrose and other compounds contents were not affected by water supply, indicating a negative relationship between the content of sucrose and compatible solutes ( Hoffmann, 2010 ). Also, Topak et al. (2011) showed that sucrose, K, and Na contents were not significantly affected by drought stress. Whereas α-N was accumulated in beet root only under severe drought.

Application of JA may enhance the fruit quality that may be due to metabolites acting as signal transduction under drought stress. Nitric oxide, ROS, calcium, abscisic acid (ABA), ethylene, and salicylic acid have also been reported to be important mediators of plant growth and development during JA signal transduction and synthesis ( Ahmad et al. , 2016 ). Moreover, Rakwal & Komatsu (2005) demonstrated that treating leaf sheath of rice with foliar application of JA caused an increase in endogenous level of ABA. In addition, ABA accumulation trigger ABA-inducible gene expression, leading to closure of stomata and organ drop, therefore reducing water loss in the aerial part ( De Ollas et al. , 2013 ). These findings suggest the possible existence of a positive feedback regulatory system for JA biosynthesis and the possibility of cross-talk between JA signaling and other signaling pathways stresses.

In conclusion, the results of the present study indicate that JA could be an effective compound for enhancing drought tolerance in sugar beet crop. The studies re­garding the effects of JA in conferring drought as well as abiotic stress tolerances on quality of sugar beet are scarce. JA has been reported as an important signaling molecule that also interacts with other hormones or signaling molecules known to be important in stress signaling pathways. Our results showed that JA appli­cation reduced the oxidative stress and improved the biochemical adaptation in sugar beet under water stress. JA applications also resulted in higher K content that caused lower impurities and increased white sugar contents. In addition, JA increased the activity of antioxidant enzymes and tolerance under water stress conditions, as well as increased root yield and white sugar yield. It is worth noting that the reduction in root yield accompanying water deficit was not compensated by the increase in sucrose, sugar recovery percentage, and finally WSY decreased.