Both seed and foliar applications of priming agents have been found to enhance germination rate, uniformity, and metabolic activities, while also triggering defense responses against biotic and abiotic stresses, with positive impacts on physiological changes in plants (Raj and Raj, 2019). They have also been found to improve growth parameters in various plant species such as Daucus carota, Matricaria chamomilla (Kováčik et al., 2009), Aloe vera (Miri et al., 2014), Solanum lycoperscium (Patel and Rai, 2018), and Solanum melongena (Ali et al., 2019).
In the present research, a reduction in the number of seeds per head was observed in susceptible germplasm under the biotic stress of charcoal rot disease. The head diameter of the sunflower contributes as much as 55.56% toward sunflower seed yield and varied from 20 cm to 30 cm. Our results are in line with Skoric (2012), who found that the number of seeds per head was reduced in sunflower crops under different biotic and abiotic stresses, resulting in reduced sunflower yields. The range of head diameter was 20.33–31.63 cm in HA-259 with seed and foliar application of four priming agents (SA, GA, JA, AA), which showed great potential to enhance the yield. These findings match those of Rafi et al. (2015), who reported that the application of priming increased the shoot weight, root fresh and dry weight of sunflower (Helianthus annuus), chickpea (Cicer arietinum), okra (Abelmoschus esculentu), and peanut (Arachis hypogaea) as compared to unprimed plants. Our results about head diameter, achene weight, and the number of achene per head were also in line with the findings of Asghar et al. (2019), who confirmed that seed priming has a significant impact on the yield, head diameter, achene weight and achene per head of two sunflower genotypes (Hysun-33 and FH-331) as compared to non-primed control. A study using Pseudomonas fluorescens as seed priming against abiotic stress also observed physiological parameters that increased dramatically in primed plants compared to control (Pravisya et al., 2019).
The present study found that sunflower height increased significantly with seed and foliar application of priming agents at three concentrations in the presence of charcoal rot as compared to unprimed plants. Similar results were obtained by Shah et al. (2018) regarding seed priming with GA, which enhanced seed germination, plant height, stem length, and stem thickness of Lisianthus plants (Eustoma grandiflorum). Seed treatment with gibberellic acid also enhanced plant height in chickpea (Shariatmadari et al., 2017) under drought stress in glasshouse and field conditions. The results are confirmed by Zayan (2016), who primed the seed of okra plants with salicylic acid (50, 100, and 200 ppm), ascorbic acid (50, 100, and 200 ppm), Bion (200, 400, and 800 ppm) and humic acid (1000, 2000 and 4000 ppm) and observed the impact of priming agents on plant height and yield under charcoal rot stress in two consecutive years (2013–2014). The increase in physiological parameters and reduction in disease severity of charcoal rot found with increasing the concentration in primed plants as compared to unprimed is in line with the present findings. The same trend was observed by Noreen et al. (2017) with regard to the impact of salicylic acid on chlorophyll content, 100-grain weight, total grain weight, root and shoot tissue in wheat varieties under water stress and the improvement in all these parameters with a high dose of SA as compared to low and control. The findings of (Zayan, 2016) with regard to seed treatment with priming agents of Abelmoschus esculentus with wheat plants were similar to the current findings with sunflower plants, as the increase in physiological parameters was recorded at a maximum dose of each priming agent in both seed treatment and foliar application as compared to untreated plants. Similarly, Kalaivani et al. (2016) treated the seed of rice varieties with different concentrations (75, 50, 25, and 100 mg/L) of methyl salicylate (MeSA) to explore the effect on the physiology of plants and observed that seedling emergence was enhanced at a higher concentration when compared to lowers. Additionally, MeSA not only modified plant physiology but was also useful for crop production and protection. The results are further supported by the findings of Karthika and Vanangamudi (2013), who reported that maximum concentration of seed bio priming (phosphobacteria and Azospirillum) led to increased physiological parameters (dry matter, root shoot length) of maize compared to control.
Our present results are in line with the findings of Wahid et al. (2008), who found that the application of ascorbic acid, gibberellic acid, and salicylic acid has positive impacts on physiological parameters such as shoot and root length and the shoot and root dry weight of sunflower. Similarly, Noreen and Ashraf (2008) applied three doses of salicylic acid as a foliar application on two sunflower lines (Hisun-33 and SF-187) under abiotic stress and found an appreciable increase in root and shoot fresh and dry weight as compared to untreated plants.. Similarly, Dai et al. (2017) compared seed priming and hydro-priming with different chemicals including gibberellic acid to investigate the effect on shoot weight, root weight, and chlorophyll content in Glycine max seedlings. They found more increase in chlorophyll in seed priming as compared with hydropriming. Our results are also in line with those of Khan et al. (2018), who studied the effects of zinc and farm-yard manure at different concentrations (1.25, 2.44 and 5 mg kg − 1 and 1% and 2%) on chlorophyll content, grain yield, 100-seed weight, shoot fresh/dry length in Vigna radiata in the presence of M. phaseolina and found that physiological parameters increased at maximum concentration.
The present study demonstrated that seed priming with AA improves physiological parameters, including chlorophyll content of sunflower, as compared to GA and other priming agents. These findings are supported by those of Farahmandfar et al. (2013), who compared the effects of sodium chloride, SA, GA and hydro-priming on chlorophyll content, dry weight, and length of plumule and seedling radicle of Trigonella foenum graecum. They found that seed treatment with salicylic acid enhanced the physiological parameters of fenugreek seedlings more effectively than gibberellic acid and other treatments. Our results are also in line with those of Razzaq et al. (2013), who studied the effects of seed priming with salicylic acid (50, 100, 150 ppm), abscisic acid (50, 100, 150 ppm) and ascorbic acid (10, 20, 30 ppm) on membrane stability index, chlorophyll content, relative water content (RWC) root-shoot ratio and yield in wheat cultivars, and found that salicylic acid delivered pronounced and better effects than other chemicals. Similarly, El-Hai et al. (2009) investigated the effect of exogenous and endogenous application of citric acid and salicylic acid against charcoal rot, observing various physiological parameters of sunflower (chlorophyll content, stem diameter, flower head diameter, seed plant yield, and 100-seed weight). All treatments with antioxidants reduced the incidence of charcoal rot disease, and physiological parameters were significantly increased in sunflower crops under field conditions. The studies undertaken by Youssef et al. (2017) found that exogenous application on sunflower plants with SA (0.7 and 1.4mM) played a vital role in increasing yield, head diameter, plant height, total seed per head, and chlorophyll content (a,b), while also mitigating abiotic stress at higher concentration. A similar trend was found in the present study, with our results indicating that physiological parameters are enhanced with the application of priming agents at a higher concentration. Exogenous application (acibenzolar-S-methyl and exopolysaccharides) has been shown to improve physiological properties and stimulate the defense mechanism in tomato against bacterial spots (Blainski et al., 2018).
Presents results were also strengthen by the findings of (Kalpana et al., 2015) regarding the agronomical parameters. They reported that plant height, dry weight, and chlorophyll content increased in wheat plants primed with KCl, KNO3, and GA against biotic stress, and among them GA was found superior to other priming agents. Similar findings were reported by Arun et al. (2017), who investigated the effects of seed priming with CaCl2, GA, ZnSO4, and MG (NO3)2 on different physiological parameters in cowpea cultivars. The results indicated that seed treatment with GA enhanced all physiological parameters more efficiently as compared to other priming agents. Langeroodi and Noora (2017) reported a significant increase in physiological parameters (chlorophyll content, relative water content, and seed yield) by applying gibberellic acid as seed treatment in soybean under water stress. An increment was noted in physiological parameters (root shoot length, dry matter, relative water content) by foliar application of SA in maize against TMV in comparison to control (AA et al., 2019).