Plants are exposed to different environmental stresses and drought is one of the most important abiotic stresses influencing plant growth and development (Seleiman et al. 2021; Aliyari Rad et al., 2021; Jamshidi Zinab et al. 2022). Drought stress due to waste of water by evapotranspiration or depletion of available soil water adversely affects the growth of plants (Shafiq et al. 2014) and limits plant growth and yield in arid and semiarid regions (Kalantar Ahmadi et al. 2015a; Sun et al. 2013).

Rapeseed is a major oilseed crop, which is cultivated globally and ranks as the third largest oilseed crop worldwide, just behind palm and soybean (FAO 2020). Rapeseed seed yield is affected by the events prior to and during the flowering stage, but the reproductive period is more sensitive to drought stress (Khodabin et al. 2020). Plant growth depends on water uptake from the soil by roots and transfer to other plant parts (Mohammadi Alagoz et al. 2022). The reduction in growth characteristics when plants are subjected to drought stress could be due to the negative effects of drought stress on physiological and biochemical activities (Heshmat et al. 2021). Plants are susceptible to water-deficit stress during the flowering stage and exposure to drought stress during this stage causes fewer flowers and silique and flower abortion (El-Sabagh et al. 2017; Farahani et al. 2019; Zhu et al. 2021). A short period of drought stress can limit rapeseed seed yield during reproductive growth (Shirani Rad et al. 2021). Moreover, drought stress adversely influences silique formation, which ultimately results in a reduction in seed yield (Johnston et al. 2002). Environmental conditions play a key role in the produced oil content and quality (Zhu et al. 2021), and the effect of drought stress on reducing oil content and quality is well known (Shahsavari and Dadrasnia 2016; Eyni-Nargeseh et al. 2022). The composition of rapeseed fatty acids includes 66% monounsaturated, 27% polyunsaturated, and 7% saturated fatty acids (Safavifard et al. 2018) and determines oil quality. Fatty acid composition is highly affected by environmental conditions, management operations, and length of the grain-filling period (Pritchard et al. 2000; Enjalbert et al. 2013; Mokhtassi-Bidgoli et al. 2022).

Exogenous application of plant growth regulators (PGRs) such as ascorbic acid could substantially mitigate the negative effects of environmental stresses on plant growth and seed yield (Kalantar Ahmadi et al. 2015a; Shafiq et al. 2014). Ascorbic acid is a substantial metabolite involved in plenty of cellular processes including cell division and expansion (Pignocchi and Foyer 2003), enzymatic and nonenzymatic reactions (Kalantar ahmadi et al. 2015b; Smirnoff and Wheeler 2000), and photosynthesis and flowering (Barth et al. 2006). Some reports have indicated that ascorbic acid is an important factor involved in defense mechanisms and exogenous application of ascorbic acid is an effective agronomic method to enhance stress tolerance in different plants (Munir et al. 2013; Ahmad et al. 2012). A significant increase in proline, pigments, growth, and seed yield of rapeseed using foliar application of ascorbic acid (200 g l−1) was reported by Sakr and Arafa (2009). Improved seed yield caused by applying ascorbic acid can be attributed to increased leaf area, photosynthesis rate, and dry matter. Although the effect of ascorbic acid on fatty acids is not clearly understood, its application led to an increase in linolenic acid and a reduction in oleic acid (Bybordi 2012).

Salicylic acid is an important phenolic compound affecting various physiological aspects of crops (Hayat et al. 2010; Singh et al. 2017). Salicylic acid promotes uptake and translocation of nutrients (Khan et al. 2003), photosynthesis rate, and growth (Farhangi-Abriz and Ghassemi-Golezani 2016), as well as contributing to drought tolerance (Kalantar ahmadi et al. 2015b) through enhancement of leaf pigments, carboxylase activity of rubisco (Singh and Usha 2003), and scavenging of reactive oxygen species (ROS) due to an increase in the activity of antioxidant enzymes (Ghassemi-Golezani et al. 2019; Kalantar ahmadi et al. 2015b). Improvements in seed yield and oil quality of crops by foliar spray of salicylic acid under drought-stress conditions were reported by Ghassemi-Golezani et al. (2019) and Razmi et al. (2017). Moreover, oil content and fatty acids such as linoleic acid, linolenic acid, stearic acid, and total unsaturated fatty acids of rapeseed are influenced by interaction effects of salicylic acid and drought stress (Estaji and Niknam 2020).

In the early 1990s, it was reported that applying methanol is a way to improve yield and reduce water demands in crops (Nemecek-Marshall et al. 1995). Methanol plays a vital role in preventing increased photorespiration induction in stressed plants (Moran et al. 1994) and reducing crop water requirements (Madhaiyan et al. 2006). Among the solutions to diminish the adverse effect of drought stress on crops, foliar application of methanol is to reduce photorespiration and increase CO2 fixation, which lead to improved crop yield under drought stress, and use of methanol as a carbon source has been reported to achieve this goal (Nonomura and Benson 1992; Abbasian et al. 2016). Some researchers have shown that application of methanol resulted in an increase in growth (Dewez et al. 2003), chlorophyll content, leaf weight and relative water content (Vojodi et al. 2017), acceleration of maturity (Ramirez et al. 2006), and change in the activity of enzymes (Kalantar Ahmadi et al. 2015a).

Rapeseed is one of the most important oilseed crops in many parts of the world and implementation of crop management practices has great importance to improve its seed yield and oil quality. Hyola401 is a spring commercial cultivar that is commonly cultivated by Iranian farmers and can potentially produce an acceptable oil content and yield under water-limited and head-stress conditions in Khuzestan province, Iran. Over recent years, the water shortage crisis has increasingly affected the agricultural sector in Iran, especially in Khuzestan province due to its hot and dry climate. It is always a great challenge for farmers to irrigate fall crops at the end of the growing period and spring crops at the beginning of the growing period. Withholding irrigation would be considered a management strategy to save water and allocate it to spring crops. On the other hand, the negative effects of withholding irrigation on quantitative and qualitative yields should be minimized. Since the PGRs involved directly or indirectly impact quantitative and qualitative characteristics and mitigate the adverse effects of environmental stresses on plants, the aim of this research was to investigate the beneficial effects of foliar application of ascorbic acid, salicylic acid, and methanol on seed yield, oil content, and fatty acid composition of rapeseed under optimal water supply and drought-stress conditions.

Materials and Methods

Experimental Site

The present field experiment was conducted in Safiabad Agricultural and Natural Resources Research and Education Center located in Khuzestan province in southwest Iran (82.9 masl, 48° 26′ E, 32° 16′ N) over 2 years (2015–2016 and 2016–2017). The soil characteristics are shown in Table 1. The climatic data including minimum and maximum temperatures and rainfall are provided in Fig. 1.

Table 1 Characteristics of physical and chemical properties of soil used in the experiment
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Fig. 1
figure 1

Daily climatic parameters (air maximum and minimum temperatures and rainfall) over the growing periods (2015–2016 and 2016–2017) of rapeseed in Dezful

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Experimental Design

A split-plot arrangement of treatments was used based on a randomized complete block design with three replications. Main plots were three levels of irrigation including full irrigation after 70 mm evaporation from a class A evaporation pan (control), restricted irrigation from the flowering stage (BBCH scale: 61) until the end of the growing season, and restricted irrigation from the appearance of siliques (BBCH scale: 75) until the end of the growing season. Subplots consisted of 10 levels of foliar application of PGRs (100, 200, and 300 mg l−1 ascorbic acid; 100, 200, and 300 µM salicylic acid; 10, 20, and 30% [w/v] methanol; and foliar application of distilled water as control). Hereafter, the foliar applications of PGRs are abbreviated to AsA100, AsA200, AsA300, Sa100, Sa200, Sa300, Me10, Me20, Me30, and control, respectively. It should be noted that the PGRs were applied at two stages of rosette (BBCH scale: 30) and opening of the first flower (BBCH scale: 61; Weber and Bleiholder 1990). PGRs used were manufactured by Merck, Darmstadt, Germany.

Experiment Details

Each experimental plot consisted of eight 6‑m lines with an interrow distance of 30 cm and a density of 80 plants m−2. According to the results of soil analysis and fertilizer recommendations, 200 kg ha−1 potassium sulfate, 150 kg ha−1 triple superphosphate, and 390 kg ha−1 urea were applied. All potassium sulfate, triple superphosphate, and one third of the urea were used before planting, and the remaining urea was applied equally at two stages of stem elongation (BBCH scale: 30) and flowering (BBCH scale: 61). Hyola401 genotype was sown on 05 November in two study years. Weeds were controlled by applying 2.5 l ha−1 Botizan star at the cotyledon stage (BBCH scale: 09) and by hand weeding as needed during the growth period. Irrigation intervals were adjusted on the basis of 70 mm water evaporation from the class A evaporation pan under standard conditions and the quantity of the water used was 80% of the evaporated water. The water volume entering the field was measured by water meter. Accordingly, the full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting applications received 4480, 2800, and 3360 m3 ha−1 in the first year and 3920, 2240, and 2800 m3 ha−1 in the second year, respectively. The irrigation dates of experimental treatments are provided in Table 2.

Table 2 Irrigation dates and amounts of experimental fields used in the model calibration and validation steps
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Agronomic Traits

To calculate seed yield, four center rows in each plot were separately cut off at the physiological maturity stage (BBCH scale: 89) in an area of 6 m2. Ten plants were randomly selected from each plot, and the number of siliques per plant, seed number per silique, and thousand-seed weight were measured in three replications.

Oil Content and Fatty Acid Composition

To measure oil content, 5‑g seed samples were selected from each treatment (three replications) and nuclear magnetic resonance (NMR) was used to estimate the oil content (minispec mq20; Bruker, Bremen, Germany; International Standard [ISO] 5511 1992) at the laboratory of the Seed and Plant Improvement Institute, Karaj, Iran. Fatty acid methyl esters obtained from the oil samples were analyzed using gas chromatography (GC; Agilent Technologies, Santa Clara, CA, US) according to the method proposed by Azadmard-Damrichi et al. (2005): 2 ml of 0.01 M NaOH in methanol was added to the oil sample dissolved in 0.5 ml hexane. Thereafter, it was kept in a water bath at 60 °C for 10 min. In the next stage, B trifluoride in methanol (20% of BF3 in methanol) was mixed with it and left in the water bath at 60 °C for 10 min. Afterwards, samples were cooled under the water and 2 ml of sodium chloride (20% w/v) and 1 ml hexane were added. Mixing completely, the hexane layer which contained the fatty acid methyl esters was separated by centrifugation. The fatty acid methyl esters were analyzed by GC according to the method described by Azadmard-Damirchi and Dutta (2006). Glucosinolate was measured using a spectrophotometer (model UV2100, Company Unike, USA) equipped with a 50‑m CP-Sil 88 capillary column, an inner diameter of 0.25, and 0.2 µm of a static thickness (Makkar et al. 2007). All materials used were manufactured by Merck, Germany.

Statistical Analysis

After conducting Bartlett’s test and supplying the homogeneity of the test variance in each year, the combined analysis of variance was conducted using SAS software (version 9.2; SAS Institute, Cary, NC, US). The least significant difference (LSD) was applied to separate the means of the main effect and the significant interactions were assessed by the LSD test by the slicing method at P < 0.05. The significance of linear and quadratic regression models was tested with polynomial orthogonal contrasts. The regression models are given when orthogonal contrasts were significant at the P ≤ 0.05 probability level.

Results

Combined analysis of variance revealed that the simple effect of year was significant for all studied traits, except oil content. Simple effects of irrigation regime and foliar spray of PGRs were significant for oil content. The interaction of year × irrigation regime was significant for number of siliques per plant, number of seeds per silique, seed yield, and stearic acid, linolenic acid, and glucosinolate content. Seed yield and palmitic acid were affected by the interaction of year × foliar spray of PGRs. Combined analysis showed that the interaction of irrigation regime × foliar spray was statistically significant for the number of siliques per plant, number of seeds per silique, thousand-seed weight, seed yield, and palmitic acid, stearic acid, linolenic acid, and glucosinolate content. Amounts of oleic, linoleic, and erucic acids were impacted by the three-way interaction of year × irrigation regime × foliar spray of PGRs (Table 3).

Table 3 Combined analysis of variance for studied traits as affected by irrigation regime and foliar application of plant growth regulators over two years (2015–2016 and 2016–2017) in Dezful, Iran
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Number of Siliques per Plant

The means comparison of the two-way interaction of year × irrigation regime revealed that the highest number of siliques per plant was with optimal irrigation, averaging 51.1 and 70.8 in the first and second years, respectively. Rapeseed plants grown under restricted irrigation from the flowering stage had the lowest number of siliques per plant, averaging 46.5 and 59.6 in the first and second years, respectively (Table 4). Means comparison of the two-way interaction of irrigation regime × foliar application of PGRs showed that while the maximum number of siliques per plant (71) was detected in foliar spray of Sa200 under the full irrigation regime, the highest number of siliques per plant under restricted irrigation from flowering (46.83) and silique setting (63.16) stages was with foliar spray of AsA300 (Table 5).

Table 4 Means comparison of the two way-interaction of year × irrigation regime for study traits of rapeseed in 2015–2016 and 2016–2017
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Table 5 Means comparison of the two way-interaction of irrigation regime × plant growth regulators for study traits of rapeseed during two study years (2015–2016 and 2016–2017)
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Number of Seeds per Silique

The means comparison of the interaction of year × irrigation regime showed that the highest number of seeds per silique was obtained from the optimal irrigation regime in both years (27.0 and 32.3 in first and second years, respectively; Table 4). The means comparison of the two-way interaction of irrigation regime × foliar application of PGRs indicated that the highest number of siliques per plant (32.04) was observed with foliar spray of AsA100 when the rapeseed plants were fully irrigated, while the Sa300 and AsA200 foliar spray treatments produced the maximum number of seeds per silique, with averages of 32.88 and 29.59 under restricted irrigation from flowering and silique-setting stages, respectively (Table 5).

Thousand-Seed Weight

The means comparison of the simple effect of year showed that thousand-seed weight was 3.74 and 3.56 g in the first and second years, respectively (data not shown). The means comparison of the interaction effect of irrigation regime × foliar spray of PGRs illustrated that the AsA200, Me30, and Sa200 foliar spray treatments produced the maximum thousand-seed weight, averaging 4.22, 4.28, 4.20 g when rapeseed plants were grown under optimal water supply, restricted irrigation from flowering, and restricted irrigation from silique setting, respectively (Table 5).

Seed Yield

Averaged across irrigation regimes, rapeseed seed yield was 2915 and 3199 kg ha−1 in the first and second years, respectively; the highest seed yield was obtained from the full irrigation regime in both years (Table 4). The interaction of year × foliar spray revealed that the highest seed yield belonged to foliar spray of AsA300 in the first (3483.33 kg ha−1) and second (3681.66 kg ha−1) years (Table 6).

Table 6 Means comparison of the two way-interaction of year × plant growth regulators for seed yield and palmitic acid of rapeseed
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The results of means comparison illustrated that the rapeseed seed yield responded differently to foliar spray of PGRs under three irrigation regimes, so that the highest seed yield was found in AsA300, Me30, and AsA300 treatments (4493.33, 2667.77, and 3884.72 kg ha, respectively) under full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting, respectively (Table 5).

Oil Content

The results showed that oil content was higher in the optimal irrigation regime (44.57%) compared to restricted irrigation from flowering (40.43%) and silique-setting (41.97%) stages (data not shown). The simple effect of foliar spray of PGRs on oil content showed that the maximum (43.61%) and minimum (40.64%) oil content belonged to the Sa300 and control treatments, respectively (Fig. 2).

Fig. 2
figure 2

Means comparison of foliar spray of plant growth regulators for rapeseed oil content. AsA1, AsA2, and AsA3: 100, 200, and 300 mg l−1 ascorbic acid; Sa1, Sa2, and Sa3: 100, 200, and 300 µM salicylic acid; Me1, Me2, and Me3: 10, 20, and 30% (w/v) methanol; control distilled water

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Palmitic Acid

The response of rapeseed to foliar spray of PGRs in both years was different in term of palmitic acid content, so that the Sa200 and Me30 treatments had the highest values of palmitic acid in the first and second years, with averages of 4.26 and 3.98%, respectively (Table 6). The means comparison of the interaction of irrigation regime × foliar spray of PGRs revealed that foliar spray of Me30 had the highest content of palmitic acid under optimal irrigation and restricted irrigation from flowering (3.79 and 4.64%, respectively), while the Sa200 foliar spray treatment produced the highest palmitic acid content (3.83%) when irrigation was restricted from the silique-setting stage (Table 5).

Stearic Acid

The results of means comparison indicated that stearic acid content was higher in the first year as compared to the second year under three irrigation regimes; the highest value was found in rapeseed plants that experienced restricted irrigation from the flowering stage in the first and second years (2.16 and 1.92%, respectively; Table 4). The foliar spray of PGRs had different influences on stearic acid content under three study irrigation regimes; the AsA300, Sa300, and AsA100 treatments produced the highest stearic content, averaging 1.79, 2.19, and 1.53%, when full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting regimes were applied, respectively (Table 5).

Oleic Acid

The oleic acid content responded differently to foliar spray of PGRs under three irrigation regimes in two study years. Averaged across irrigation regimes and foliar spray of PGRs, the oleic acid content was 46.76 and 51.04% in first and second years, respectively (Table 7). The maximum oleic acid content (averaged across years and foliar sprays of PGRs) was obtained from the well-watered irrigation regime, averaging 57.53%, and decreases of 29 and 18% were detected under restricted irrigation from flowering and silique setting, respectively (Table 7). Averaged by years, spraying Sa300, Sa200, and AsA300 had the highest oleic acid content, averaging 58.69, 41.71, and 49.54% under full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting, respectively (Table 7).

Table 7 Means comparison of the two way-interaction of irrigation regime × plant growth regulators for study traits of rapeseed
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Linoleic Acid

Different responses were found for the three studied irrigation regimes when PGRs were sprayed on rapeseed plants. Averaged by irrigation regime and foliar sprays of PGRs, linoleic acid content was 19.08 and 20.84% in the first and second years, respectively (Table 7). The highest linoleic acid content (averaged across years and foliar sprays of PGRs) was recorded in the well-watered irrigation regime, averaging 21.82%, and decreases of 16 and 9% were observed under restricted irrigation from flowering and silique setting, respectively (Table 7). Averaged across years, spraying Sa300, AsA300, and Me30 produced the highest linoleic acid content, averaging 22.50, 18.96, and 20.31%, under full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting, respectively (Table 7).

Linolenic Acid

The results of comparing means showed that the highest linolenic acid content belonged to restricted irrigation from the silique-setting stage in the first (7.32%) and second years (5.87%), respectively (Table 4). Although the rapeseed plants responded differently to foliar spray of PGRs under irrigation regimes in term of linolenic acid content, the AsA300 foliar spray treatment maximized the linolenic acid content under full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting c, averaging 4.8, 6.44, and 7.58%, respectively (Table 5).

Erucic Acid

Means comparison of the three-way interaction of year × irrigation regime × foliar spray of PGRs revealed that the erucic acid content was different for PGRs foliar spray treatments under irrigation regimes in both years (Table 7). Averaged by irrigation regimes and foliar spray of PGRs, the erucic acid content was 0.057 and 0.029% in the first and second years, respectively (Table 7). Averaged across years and foliar spray of PGRs, the lowest erucic acid content was recorded in the optimal irrigation regime, averaging 0.0408%, and decreases of 11.02 and 10.78% were observed under restricted irrigation from flowering and silique setting, respectively (Table 7). Averaged across years, spraying Me30 produced the minimum erucic acid content, averaging 0.036, 0.038, and 0.040% under full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting, respectively (Table 7).

Glucosinolate

The data presented in Table 4 show that glucosinolate content was significantly affected by the two-way interaction of year × irrigation regime; the highest glucosinolate content belonged to the restricted irrigation regime from flowering stage, averaging 9.14 and 6.45 µmol g−1 in the first and second years, respectively (Table 4). When compared to the full irrigation regime, the restricted irrigation from flowering and silique-setting regimes boosted glucosinolate content; by contrast, foliar spray of PGRs significantly decreased glucosinolate content as compared to the control treatment. The lowest glucosinolate content was achieved from the Me30, Sa300, and Sa200 foliar spray treatments under full irrigation, restricted irrigation from flowering, and restricted irrigation from silique setting, with averages of 3.07, 6.97, 4.89 µmol g−1, respectively (Table 5).

Discussion

The lower rapeseed seed yield in the first year than the second year could be attributed climatic parameters such as rainfall and minimum and maximum temperatures. In other words, different climatic conditions during the flowering and seed-filling periods in the two years of the experiment (Fig. 1) and their impacts on yield components led to a significant difference in seed yield between the two study years. Crop seed yield is a function of yield components, which are influenced by environmental factors (i.e., temperature and rainfall) as well as agronomic practices (i.e., irrigation, sowing date, plant nutrition, etc.). Drought stress caused by restricted irrigation leads to an increase in the ratio of respiration to photosynthesis (Aliyari Rad et al. 2021) and drought stress in the reproductive stages such as flowering and silique setting leads to falling flowers and siliques due mainly to the lower supply of photosynthetic materials. Obviously, the harmful effects of drought stress on growth, yield components, and grain yield depend on the intensity and time at which the stress occurs (Eyni-Nargeseh et al. 2022; Mokhtassi-Bidgoli et al. 2022). Applying PGRs could mitigate adverse effects of environmental stresses and improve crop growth and yield (Ijaz et al. 2019; Ganj-Abadi et al. 2021). Increased growth caused by applying ascorbic acid under stress and non-stress conditions is due to increased cell division and elongation (Athar et al. 2008), and considering this, it seems that the positive effect of ascorbic acid on growth led to an increase in the number of siliques (Kamal et al. 2017). The current findings specified that increasing the salicylic acid concentration under the optimal irrigation regime resulted in an increase in the number of siliques per plant, while a low concentration of salicylic acid was more effective under restricted irrigation from the silique stage (Table 5). Exogenous application of salicylic acid improves growth, physiological traits, yield components, and seed yield (Aftab et al. 2011; Fariduddin et al. 2003). In some studies, it was reported that applying methanol caused an increase in rapeseed seed yield by increasing the number of siliques (Zbiec et al. 2003). Methanol increases the number of siliques by influencing different metabolic pathways such as growth and development and activation of genes involved in jasmonic acid biosynthesis (Gout et al. 2000), and improvement of remobilization leads to an increase in seed weight (Paknejad et al. 2009).

Water supply is important during flowering and early growth of siliques, and drought stress during these stages affects carbohydrate reserves and seed development, and causes seed abortion (Mohammad et al. 2007). Occurrence of drought stress in the reproductive stage of rapeseed decreases seed yield and yield components (Jamshidi Zinab et al. 2022). Basically, the reduction in the number of seeds per silique can be attributed to the negative effect of water-limited conditions on pollination and the fertility of flowers. Nevertheless, application of foliar spray of PGRs such as salicylic acid, ascorbic acid, and methanol under water-limited irrigation positively affects crop growth and yield.

Increasing seed weight can be due to enhanced photosynthesis and distribution of assimilates entering seeds along with a reduction in drought stress because of ascorbic acid. Dolatabadian et al. (2010) declared that foliar application of ascorbic acid (150 mg l−1) decreased the adverse effects of drought stress and improved growth and seed yield under optimal and drought-stress conditions. Furthermore, application of salicylic acid had a positive effect on thousand-seed weight under different irrigation regimes and this beneficial influence may be due to more remobilization to seeds during the seed-filling period. On the other hand, closure of stomata and reduced transpiration are induced by salicylic acid under drought-stress conditions (Zhang et al. 2020). The reaction of thousand-seed weight to methanol was positive under restricted irrigation from flowering and silique stages, and this result is in agreement with other research (Mirakhori et al. 2009; Rehman 2018).

Foliar spraying of AsA300 led to increased seed yield under the well-watered irrigation regime and restricted irrigation from the silique stage by 40 and 45%, respectively. It seems that reducing stress intensity and an increased rate of application of ascorbic acid can prevent a further decrease in seed yield (Fig. 3a). Ascorbic acid would increase crop seed yield through a positive effect on respiration activities, cell division, growth, and enzyme activities (Smirnoff 2011). Application of salicylic acid at low concentration (Sa100 treatment) was more effective in terms of seed yield compared to a high concentration (Sa200 and Sa300 treatments) under optimal irrigation and restricted irrigation from the silique stage, while rapeseed plants reacted positively to a high concentration of salicylic acid in restricted irrigation from flowering stage and showed an increasing trend upon increasing the salicylic acid concentration from 100 to 300 μm (Fig. 3b). Although the use of salicylic acid in high concentrations can have an inhibitory effect on plant growth (Fariduddin et al. 2003), an appropriate concentration boosts the seed yield by improving growth and physiological processes and remobilization of photosynthetic materials (Grown 2012). The regression analysis results showed that increasing the concentration of methanol from 10 to 30% linearly increased the rapeseed seed yield and this increase was higher with restricted irrigation from flowering than in the other two irrigation regimes (Fig. 3c). The increase in seed yield caused by application of methanol can be attributed to its positive role in enhancing chlorophyll concentration and plant photosynthesis. Moreover, increasing sucrose production improves plant turgor pressure and therefore reduces the sensitivity of methanol-treated plants to water-limited stress (Zbiec et al. 2003).

Fig. 3
figure 3

Regression analysis of the response of rapeseed seed yield to foliar application of plant growth regulators (a ascorbic acid, b salicylic acid, and c methanol) at different concentrations under three irrigation regimes (IR1 optimal water supply, IR2 restricted irrigation from flowering, and IR3 restricted irrigation from silique setting) in two study years (2015–2016 and 2016–2017)

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Oil content in oilseed crops is a key trait that is influenced by environmental and management factors (Kalantarahmadi and Daneshian 2023). High oil accumulation in seeds is related to expression of genes involved in fatty acid biosynthesis (Liu et al. 2014). Drought stress at flowering and silique-setting stages negatively affects oil content due to reducing seed capacity for absorbing assimilates and converting them into oil. Overall, the results of this experiment are in line with previous studies based on reduction of oil content due to a decrease in the photosynthesis assimilation rate and remobilization (Elferjani and Soolanayakanahally 2018; Safavifard et al. 2018). Foliar spray of Sa300 led to an increase in oil content by 7.3% compared to distilled water. As previously discussed, PGRs improve various parameters of plant growth under environmental stress and non-stress conditions and ultimately positively affect crop quantity and quality. Regression analysis shows that the application of all three PGRs increased rapeseed oil content with a linear trend under the three irrigation regimes (Fig. 4a, b, c).

Fig. 4
figure 4

Regression analysis of the response of rapeseed oil content to foliar application of plant growth regulators (a ascorbic acid, b salicylic acid, and c methanol) at different concentrations under three irrigation regimes (IR1 optimal water supply, IR2 restricted irrigation from flowering, and IR3 restricted irrigation from silique setting) in two study years (2015–2016 and 2016–2017)

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Foliar spray of salicylic acid increases chlorophyll content, leaf area (Ghassemi-Golezani et al. 2018), and raises transportation of photosynthetic materials (Grown 2012), leading to higher seed yield and oil content (Mabudi Bilasvar et al. 2022; Ullah et al. 2012). Salicylic acid also enhances the accumulation of oil by increasing the proteins involved in the transcription of synthetic oil genes (Wu et al. 2014). The current findings clearly indicate an increase in oil content following the use of methanol (30%), ascorbic acid (300 mg l−1), and salicylic acid (200 µm). Also, by using PGRs, it was observed that the harmful effects of drought stress were somewhat neutralized, particularly by methanol at 30% (Fig. 4). In this regard, Pastori et al. (2003) declared that ascorbic acid is an important soluble antioxidant and acts as a coenzyme in reactions by which carbohydrates, fats, and proteins are metabolized. Kalantar Ahmadi et al. (2016) showed that foliar spray of methanol, salicylic acid, and ascorbic acid improved the oil content of canola under drought-stress conditions. Methanol increases turgor pressure, photosynthesis, sugar content (Zbiec et al. 2003), and oil content (Yazdi Far et al. 2015).

Oil composition is determined by genetic and environmental conditions (Shirani Rad et al. 2021) and agronomic operations such as drought stress (Mokhtassi-Bidgoli et al. 2022; Feizabadi et al. 2021; Shiranirad et al. 2023), fertilization (Amiri-Darban et al. 2020), and application of PGRs (Ijaz et al. 2019; Estaji and Niknam 2020). Water-deficit stress would cause a shortening of the lipid accumulation phase, some detriments to enzymatic activities including oleate desaturase (Ebrahimian and Bybordi 2012), and decrease unsaturated acids (Flagella et al. 2004). In a study by Safavifard et al. (2018), oleic acid content was reported to be decreased when irrigation was restricted from flowering and silique-setting stages. Similarly, Ebrahimian and Bybordi et al. (2012) declared that oleic acid and linoleic acid contents were reduced when sunflower plants were subjected to drought stress.

PGRs such as salicylic acid (Ullah et al. 2012) and ascorbic acid (Ali-Shahhat et al. 2014) can improve the oil quality of rapeseed under drought-stress conditions. Foliar spray of ascorbic acid, salicylic acid, and methanol significantly reduces lipid peroxidation and malondialdehyde of rapeseed under drought-stress conditions (Kalantar Ahmadi et al. 2015a). The enhancement in linolenic acid with application of ascorbic acid might be related to acceleration of the biosynthesis pathway of linolenic acid (Ali-Shahhat et al. 2014). It seems that the antioxidant properties of ascorbic acid reduce the negative effects of drought stress and improve the conditions necessary to improve the accumulation of linoleic acid. The role of salicylic acid in alteration of unsaturated fatty acids can be mentioned as it increases the activity of oleoyl-phosphatidylcholine D12 desaturase. The current findings are in agreement with Ganj-Abadi et al. (2021), who reported that use of salicylic acid led to increased palmitic acid, while Noreen and Ashraf (2010) declared that palmitic and stearic acids were not affected by salicylic acid. The present findings are in line with Ali-Shahhat et al. (2014) regarding increased stearic acid as a result of using ascorbic acid. Emam et al. (2011) declared that ascorbic acid caused a decrease in palmitic and stearic acids. Increasing linolenic acid content under drought-stress conditions and application of salicylic acid is supported by a report from Mabudi Bilasvar et al. (2022).

The contents of erucic acid and glucosinolate are considered two important traits to evaluate the quality of rapeseed oil and cake press. In this research, although drought stress resulted in a slight increase in the amount of these two traits and foliar spraying of PGRs reduced their contents, the contents of both traits were within a safe range. Eyni-Nargeseh et al. (2022) concluded that drought stress increases the amount of erucic acid and glucosinolate for rapeseed genotypes, while Ganj-Abadi et al. (2021) reported a decrease in these traits under salicylic acid application.

Conclusion

The results showed that seed yield and fatty acid composition reacted differently to irrigation regimes and foliar spray of PGRs. Foliar spray of salicylic acid (300 µm) in the optimal irrigation regime and restricted irrigation from flowering as well as ascorbic acid (300 mg l−1) in restricted irrigation from the silique setting stage were more effective in increasing the oleic acid content. Foliar spray of salicylic acid (300 µm) in the optimal irrigation regime, and ascorbic acid (300 mg l−1) and salicylic acid (300 µm) in restricted irrigation from the flowering stage were more effective for increasing linoleic acid content. Overall, the increase of unsaturated fatty acids in reaction to growth regulators improves the nutritional value of oil. Further experiments with a combination of growth regulators under different irrigation regimes are required to investigate the response of rapeseed in terms of seed yield and oil quality.