First of all, it is worthy to mention that amino acids are actually participated in all metabolic, regulatory, and physiological aspects of plant metabolism, and act as a source of carbon, nitrogen, energy, enzyme, and co-enzymes (Shekari and Javanmardi, 2017). In addition, amino acids are well known bio-stimulant that act as precursors of several plant growth regulators and improve plant growth via increasing photosynthesis (Amin et al. 2011), production of sugar, protein and transcription of mRNA, (Keutgen and Pawelzik, 2008), and activate plant metabolic processes. Earlier studies showed that amino acids application promoted uptake of nutrients, enhanced the photosynthesis of hot pepper (Sarojnee et al.,2009), flowering, fruit set, and fruit yield of tomato crops(Neeraja and Reddy, 2005). Another possible mechanism of amino acid could be related to promotionthe root growth which improve uptake of water and nutrient, leading toboost cell division and increase fresh and dry matter (Rouphael et al.,2017) and improve productivity of the yield (Rouphael et al., 2017).Moreover, the improvement of plant growth and crop yield due to amino acids application, may be attributed to their role in increases chlorophyll biosynthesis and photosynthesis rate (Souri et al., 2017; Souri and Hatamian, 2019; Abdelkader et al., 2023).It was noted that application of amino acid individually acts as a signaling component, i.e., increased antioxidant enzyme activity and nutrient uptake (Calvo et al.,2014).
L-methionine amino acid acts as source of carbon, nitrogen and sulphur and documented as source of plant growth regulator ethylene, which plays fundamental roles in various aspects of growth and photosynthesis (Pierik et al., 2006).
Regarding ethylene effect, it can be suggested that ethylene acts as growth stimulator at low concentrations (Fiorani et al., 2002), stimulated photosynthesis by moving photosynthates to the photosynthesis machinery and increased rate of photosynthesis (Khan et al. 2007)and leaf growth (Khan 2005).
Regarding nitrogen effect, nitrogen is one of the major plant nutrients that playscrucial function in enhancing the yield and quality of fruits as well as the growth and development of plants. It is also necessary for the synthesis of enzymes and is a constituent of proteins and nucleic acids (He et al., 2022).In addition, nitrogen plays an important role in photosynthesis and growth because of its necessary role in cell division and expansion, besides nitrogen is a constituent of chlorophyll, thylakoid proteins and enzymes of the photosynthetic process (Ahmad et al., 2014).
Regarding sulphur effect, Korsheed and Altememe (2019) mentioned that application of sulphur on onion plants had a significant impact on plant growth and enhanced quantitative and qualitative of onion yield. Since, the enhancement in vegetative growth of onion that occurred as a result of spraying with organic sulphur may be attributed to the increased up take of sulphur by the onion plants, which play essential role in increasing cell divisions in the meristematic areas (Omrani and Al-Ubaidy 2021).Moreover, Mahmoud and El-Tanahy (2022) added that sulphur application increased yield of onion bulb and improved its quality, particularly pungency and flavors.
Earlier reports indicated that L-methionine not only make nutrients available to plants but also act as signal transducing molecules (Teixeira et al.,2017),where small doses of L-methionine are sufficient for plant development, and can act as signals of several beneficial plant physiological processes(Khan et al.,2019). Ahmad et al.(2014) stated that the application of L- methionine greatly increased relative growth rate, total leaf area, net photosynthesis and leaf nitrogen concentration of wheat. They added that the increase in photosynthesis by the application of L- methionine at 30 mg kg− 1 soil may be attributed to the increase in stomatal conductance and higher photosynthates accumulation. Khan et al.(2019) showed that L-methionine can be used as a suitable substitute for fertilizers to increase lettuce plant growth and crop yield, since it significantly improved growth performance by 23.60% via applying L-methionine at the lowest concentrations (0.2 mg/L). They added that L-methionine increased the chlorophyll content of plants and contributes to energy conservation, thus enhancing the plant yield. Moreover, application of L-methionine influenced phytohormones (e.g., auxins and cytokinin), which increased the chloroplast development and chlorophyll content (Anne and Thomas 2015). Recently, Mehak et al. (2021) stated that exogenously applied L-methionine was effective in minimizing the malondialdeyhde and hydrogen peroxide contents, and increasing the plant growth by increasing the shoot length, shoot fresh weight, chlorophyll pigments, proline contents and activity of antioxidant enzymes in sunflower plant grown under drought. They proposed using L-methionine as one of the efficient plant growth regulators under stressful environments.
Regarding chemical composition of the yielded crop, the application of amino acid on plant play an important role in the synthesis of plant compounds such as enzymes, proteins, amines, secondary metabolites, phenolic compounds and flavonoids, which regulate different plant processes (Haghighi et al.,2022).In addition, the plants supplied with amino acids generally have higher amount of proteins, sugars, and other nutrients (Souri et al., 2017).It is well-known that amino acids are the precursors of phenolic compounds (Amin et al.,2011).Mobini et al. (2014) demonstrated that amino acids spraying have positive effects on protein, phenolic and flavonoid compounds. Recently, administration of amino acidat 300 mg/L can be advised to enhance cabbage growth, productivity, biochemical features, and nutritional value processes (Haghighi et al., 2022).
Regarding role of Nps,it is well known that crops can slowly absorb nutrients from traditional fertilizers; meanwhile nano-fertilizers provides high surface areas to volume ratio leading to a greater number of active sites for biological activity (Feregrino-Pérez et al., 2018). Moreover, nano-fertilizers were more efficient and effective than the conventional fertilizers due to their positive effects on the growth, development and nutritional quality of different crops (Ali and Al-Juthery, 2017). Recently, Swathi et al. (2022) mentioned that leaf stomatal apertures facilitate the uptake of Nps and their entrance into leavesas a result of foliar application (Swathi et al. 2022).According to Mala et al. (2017), nano-fertilizer application promoted germination and biochemical features of Vigna radiata in comparison with traditional fertilizer since nano-fertilizers provided the crop plant with more nutrient availability that accelerating the rate of reaction or synthesis process and thereby enhanced the nutritional parameters of the plant. Hemerly (2016) reported that the nitrogen application in nano form increased the plant growth, and yield quality by enhancing protein contents, and absorption of essential nutrients. Moreover, application of NPs at appropriate concentration have beneficial effects on plants including increases of yield quality, and decrease of oxidative stress (Lu et al., 2020; Li et al., 2020)but sometimes high concentrations have adverse effects on crops (Tighe-Neira et al.,2018).NPs also showed significant increments on carotenoids, phenolics compound,indoletotal sugar, and amino acid contents of coriander and maize (Khater 2015; Moghaddam and Madani, 2016). Phenolic compounds are known as antioxidant compounds, which exhibit free radical scavenging activity due to the free hydroxyl group in their structure (Kowalska et al.2014). Therefore, the increased accumulation of total phenolic and flavonoids, indicated a positive effect of this L-methionine and its nano-particle on antioxidant capacity of onion plants.
Considering the importance of RAPD and ISSR markers for the study of genetic diversity in various genera and species of plants, very little work has been done in different species varieties and cultivars of onion (Mukherjee et al., 2013).So, this work is an attempt to study the effect of foliar spraying with methionine and methionine Nps on onion plants via changes in the reproducible RAPD-DNA fragments.
Hence, the changes in the reproducible RAPD-DNA fragments that detected as a result to the effect of foliar spraying with methionine and methionine nano-particles on onion plants were shown in Tables (7 and 8) and illustrated in Figure (3). Sudhaa et al.(2019)and Mansour et al.(2020) studied the genetic similarity and diversity between onion cultivars using molecular markers of RAPD. They found 52 and 32 total amplified polymorphic fragments respectivelythat produced via using RAPD primers.