1 Background

The most aspects of complementary medicines are medicinal plants [1,2,3]. Traditional medicinal plants can be incorporated with herbal mixtures or single plant application as basic ingredients of medicines in various ways to treat different diseases in an organic life [4, 5], because medicinal plants are important natural sources of valuable chemical constituents [6, 7]. Foliar nutrition application has tremendous benefits, but the most notable ones include: rapid regulation of nutrient deficiencies, fast growth response of plants to foliar treatments and improved fixation of K and P. It can be beneficial and more effective than soil application of nutrients, especially when there is not enough moisture in top soil, while it can be integrated with other agrochemicals such as insecticides. It can also be effective when stressful conditions such as drought and root diseases affect the crops and is a cost-effective practice to improve yield, yield attributes and quality parameters of crops since usually only small amounts are needed, and the nutrients penetrate the stomata or the cuticle of the leaf, and then enter the cells [8].

Foliar application of magnesium retrieved plant growth, carbohydrates status and nutrients of banana [9]. Foliar calcium application had positive effects on quality of blueberry fruits [10], and foliar fertilizers including calcium chloride, such as Fruton Calcium and FOLANX@ Ca29, caused the significant increase in the calcium content of the fruits and foliage of banana [11]. Foliar Se utilization can alleviate abiotic stresses such as high temperatures induce to higher yield [12]. The application of K2SO4 had capability to boost the freezing tolerance of grapevine and applied as a prophylactic tool to alleviate freezing injury during the winter [13]. Foliar application of zinc, boron, calcium and potassium had significant effects on yield and yield attributes of guava [14]. Foliar spray of boron in the form of boric acid had effective impacts on fruit weight of mandarin orange [15]. Foliar application of Tecamin max® stimulated vegetative growth of okra in limited irrigation method [16]. Wang and Galletta [17] reported that foliar silicon application had positive effects on strawberry plant metabolism. Selenium application should be considered as the appropriate candidate for spinach production [18]. Foliar application of boric acid and zinc sulfate increased both the physiological traits of sesame and promoted its tolerance to drought stress [19]. Boron foliar application and spray of zinc had positive impacts on growth, yield and quality of onion [20]. Foliar application with ZnSO4.7H2O improved Zn concentration in pakchoi leaves [21]. Foliar application of K in the form of KLMet-1 (K2SO4 together with lysine and lysine plus methionine) during nut filling is the best treatment to gain optimum yield and quality of pistachio [22]. Foliar Fe application was appropriate to inhibit Fe deficiencies in alkaline conditions in pepper cultivation [23]; moreover, foliar spray of Zn could be suggested for red pepper cultivation to increase plant development, yields and some quality properties in regions where Zn insufficiently is high [24]. Total chlorophyll, chlorophyll a and b, and total soluble sugars of olive notably improved as the boron application rate increased [25]. Boron foliar application promoted oleuropein levels and stimulated volatile compound composition in olive leaves [26]. Boric acid application positively influenced oil contents and its composition of olive [27]. Foliar application of phosphites inhibited the phosphate-starvation response in tomato plants [28]. Foliar spray of zinc can alleviate the adverse impacts of zinc deficiency of tomato plants [29], and foliar spray of K, Mg and Zn stimulated fruit number and yield of tomato plants [30]. The positive effects of foliar application of macro- and micro-nutrients on pomegranate have been reported in former studies [31,32,33,34,35].

Despite the benefits, the most important limitations of foliar applications are the cost of multiple applications when needed, foliar burn effects or scorching in high and inappropriate concentrations, while it may need specific requirements of climate conditions such as wind velocity, humidity and temperature, sticking agent and large leaf area to obtain good efficiency. This review article is aimed to introduce foliar application of nutrients in increasing final yield and chemical constituents of medicinal and aromatic plants as an important technique toward sustainable farming systems.

The literature search included review articles, randomized control experiments, analytical studies and observations, which have been gathered from different sources such as Google Scholar, Scopus, Science Direct and PubMed. Review of the literature was carried out using the keywords, foliar application, micro-nutrients, macro-nutrients, medicinal plants, aromatic plants, pharmaceutical benefits and natural products.

2 Main text

2.1 Foliar application on different plants

One of the most important sustainable management in enhancing crop yield and quality parameters is foliar application, which can also supplement soil fertilization, when oleuropein are utilized to the soil, and absorbed by roots and transfer to aerial parts. Foliar application of KH2PO4, CaCl2 or NH4NO3 enhanced heat tolerance of creeping bentgrass by slowing leaf senescence and enhancing photosynthetic activities, and inhibition of lipid peroxidation, and keeping the scavenging capability may alleviate the negative impacts [36]. Potassium chloride foliar application to mature trees of cocoa may lead to increase in dry bean weight, bean weight, post and husk weight [36]. Urea foliar application at various stages, namely, tillering, anthesis, jointing and grain filling had positive impacts on plant height, the number of seeds per spike, grain weight and protein content of winter wheat [37]. A simple and sustainable technique to screen plant is foliar application which tolerate and accumulate As in Chelidonium majus cultivation [38]. For bio-fortification of soybean grains, Li2SO4 and LiOH are good sources [39]. Foliar treatment with FeSO4 in eggplant lead to significant increase in quantum yield of photosystem (PS II), SPAD index, photochemistry (Fv/Fm) and performance index (PI) values of both old and young leaves [40]. The size of nodules may significantly increase after molybdenum foliar application as well as maintaining a longer duration of effective N2 fixation [41]. Zn foliar application boosted the number of pods, pod shelling and yield and a hundred seed mass and seed zinc [42]. Foliar application of K and P alleviated the adverse impacts of salinity on plant growth and stimulated the concentrations of these nutrients in the grain and leaves of sorghum [43]. The supplementary Fe application mitigated Zn concentration in both roots and leaves of plant grown at high Zn [44]. Foliar application of Zn + Fe led to the maximum yield with a mean of 2613.84 kg ha−1 of soybean yield [45]. Foliar application of calcium and kinetin at the floral bud and full flowering stages did not significantly influence carbon dioxide (CO2) assimilation, final yield and yield attributes of soybean [46].

Manganese foliar application may improve seed yield and the lint content of cotton [47]. Foliar application of silicon can significant stimulate resistance in wheat plants to drought condition, especially at the tiller and anthesis stages via increasing chlorophyll content, and keeping relative water content as well as maintaining cellular membrane integrity [48]. Foliar application of copper, iron, magnesium, manganese and zinc significantly improved their leaves, corresponding concentrations of aquaponic-treated tomato seedlings [49]. Foliar application of silicon has positive impacts on strawberry plant metabolism. Foliar application of the combination of salicylic acid and sodium nitroprusside to roots or leaves is more effective than its utilization to roots or spraying to leaves [50]. The effective influence of manganese sulfate on leaf proline content, and the significant impacts of zinc on stem proline, maximum root, oleic, linoleic and linolenic unsaturated fatty acids, and chlorophyll b is reported [51]. Foliar application P at low rates might promote mid-season P deficiency in winter wheat, and that may lead to higher P use efficiencies [52]. Combined foliar application of B3+ and K+ indicated higher plant growth and productivity of cotton at various level of salinity [53]. Foliar application of 1000 ppm Zinc (Zn) and 350 ppm Boron (B) can markedly increase seed yield and quality of safflower grown under terminal drought [54]. Application of urea (5000 mg L−1) proved to be the most effective in boosting fresh and bulb dry weights and improved mineral uptake [55]. Zn foliar application in the form of complexed with amino acid caused positive increase of fresh mass of a hundred nut, total nut yield and spilled nuts of pistachio [56]. Foliar supplied of N and Zn are synergistically effectual on Zn bioavailability of winter wheat [57]. The glutathione content of vineyard was significantly improved after foliar organic nitrogen application [58]. Foliar urea application improved protein but higher concentrations reduced sugars and phenolics of escarole (Cichorium endivia L. var. latifolium) [59]. Selenite application at tuber bulking stage was effective for Se-rich potato production [60]. Zn and Se application improved photosynthesis and growth of Cd-stressed wheat and alleviated Cd and boosted Zn and Se concentrations in wheat, as well as up-regulating the antioxidant defense of wheat under Cd stress [61]. Foliar Zn application diminished Cd content in tobacco leaves while foliar Se application increase it; however, foliar Zn treatments were not as effective as soil amendments in decreasing Cd content of leaves [62]. Foliar nitrogen application helped to decrease the negative impacts of water-logging in Lulo plants (Solanumquitoene cv. septentrionale) [63]. Boron and zinc foliar application significantly increase the percentage of the olive oil, but in the warm climates, boric acid treatment improved olive oil quality such as increasing unsaturated fatty acid and reducing saturated fatty acid [64]. Sulfur nanoparticles had notable impacts on tomato growth by foliar spraying, at 200 ppm increased tomato growth compared to the control treatment [65]. Significant and positive influence was achieved by foliar application of manganese chloride and manganese on physiological characteristics of green gram at 200 and 300 mM salinity level which was appropriate in alleviating the adverse impacts of NaCl [66]. Foliar nutrition of magnesium oxide (MgO) nanoparticles had positive and significant effects on macronutrient concentration of cotton plants such as magnesium, potassium, phosphorus and nitrogen [67].

0.2% foliar spray of NiSO4.7H2O markedly improved growth, yield and Cu, Mn, Fe and Zn status in barley [68]. Zn foliar application decreased Cd and increased Zn in rice grain, stimulated rice leaves, antioxidant system, increased enhanced Cd chelation onto cell wall of rice leaves, and maybe utilized as an additional measure to manage Cd accumulation in rice grains [69]. Under high As stress, foliar silica sol promoted As accumulation in cell walls of roots of rice [70]. Sulfur foliar application was appropriate method for alleviating Cd in rice and improving the grain yield, it can also decrease Cd toxicity on PSII reaction center and improved photosynthetic efficiency [71]. Foliar Si and Se application can alleviate Cd in soft wheat, decreased Cd-induced oxidative stress by increasing antioxidative systems, up-regulated influx transporter and down-regulate influx transporter expression [72]. Foliar application of selenium was found effective for agronomic biofortification of wheat, and the appropriate dose of Se for increase yield and biofortification was 21 g ha−1 [73]. The maximum flavonoid content of grape was related to iron foliar application with significant impact on grape physic-chemical characteristics [74]. Foliar Zn application increased the reduction impacts of Cd in rice after lime application, as well as significant increase in the transfer ration of Cd from rice straw to the grain [75]. Zn-amino acid complexes (ZnAAC) and ZnSO4 foliar spray increased both yield and quality of grain wheat, and ZnAAC was a good agent to boost grain Fe, Zn and protein concentration [76]. NPK nanoparticle fertilizer foliar application indicated a significant increase in yield ad morphological traits [77]. The significant increase in antioxidant activity system of winter wheat and positive impacts on the uptake of nutrients under drought stress was reported after application of selenium [78]. In sunflower plants, Si fertilizer had a great ability to attenuate sodium toxicity [79]. Si foliar application decreased Cd uptake and translocation, enhanced Cd compartmentation onto root cell walls; moreover, both Se and Si foliar application increased SOD in plant tissues [80]. Foliar application of potassium significantly influenced antioxidant activity and phenolic compounds in fig fruit, and potassium treatment is proved to increase fig quality [81]. NaHS and CaCl2 combined application increased Ni tolerance by decreasing oxidative stress, improved growth, alleviated Ni stress by increasing the nitrate reductase activity in the leaves and enhancing the protein content [82].

Selenium application at the rate of 25 g ha−1 can be applied as an appropriate agent to increase Se content for consumption in broccoli production, as well as enhancing the major organic compounds of Se, like Se-methylselenocysteine (Se-MeSeCys), selenocystine (SeCys2), and selenomethionine (SeMet) [83]. Foliar boron application had significant influence of total dry matter of tomato and beet [84]. Zinc nanofertilizers foliar application enhanced vegetative trains such as internode length, plant height, shoot and root fresh and dry weight, the number of pods, grain weight, zinc content in grain of pinto bean cultivars [85]. Foliar Mn and Zn application can increase the seed yield and quality of safflower under drought stress condition [86]. Selenium foliar application (1 μM) and nano-Se (2 μM) significant enhanced leaf area, chlorophyll content, growth and yield of pomegranate [87]. Combined application of nitrogen (142.8 kg ha−1) and K2O (1.15 kg ha−1) with mepiquat chloride (0.048 + 0.021 kg ha−1) were reported as important treatment to increased seed yield, seed vigor and seed viability of cotton plants [88]. Zinc nanoparticle (ZnO NPs) and biochar significantly improve biomass of corn, alleviated the Cd content, enhanced the Zn concentration and decreased the oxidative stress and improved the enzyme activities [89]. Silicon foliar spray increased lignin biosynthetic enzyme activity, up-regulated lignin biosynthetic genes, increased photosynthesis by enhancing chlorophyll content and stomatal conductance, mitigated the drastic shading impacts on stem stability and could be beneficial in reducing soybean lodging [90]. B application noticeably enhanced maize growth through increasing photosynthetic capacity, tissue-B content, water-status, up-regulation of antioxidative defense-system; furthermore, mitigated negative impacts of B on corn was also reported [91]. Zinc foliar application (100 mg L−1) and molybdenum (40 mg L−1) stimulated growth and yield characteristics of sugar beet through balancing translocation and nutrients uptake [92]. Application of Ca, B and GA3 increased quantitative and qualitative characteristics of pomegranate fruit, enhanced peel integrity and decreased sun-burning and cracking of pomegranate and increased fruit chemical composition and peel moisture [93]. Foliar spray of urea (2%) at flowering, and flowering + pod formation stages increased Fe and Zn concentration of chickpea’s grain [94].

2.2 Foliar application of nutrients on medicinal and aromatic plants

Foliar application has tremendous beneficial effects on yield and yield attributes of medicinal and aromatic plants and herbs [94, 95]. Foliar applied urea at full-flowering and pod-setting gave acceptable outcomes but the appropriate stage of utilization was first flowering, full flowering and pod setting [95]. Foliar application of methanol had remarkable impacts on essential oil and physiological characteristics of Lavandula stoechas L. in NaCl salinity stress environment [96]. Zinc and iron foliar application had effective impact on agronomic traits in Alyssum desertorum, Borago officinalis, Calendula officinalis and Thymus vulgaris [97]. Zinc application had significant effects on the physiological characteristics of Lavandula stoechas L. plants growing under salinity stress conditions [98]. Silicon foliar application influenced green color index of orchids, nutrient absorption and improving concentrations of Si of Phalaenopsis and Dendrobium orchids [99]. Photosynthesis and grapy yield was increased by spray of mixture of Cu-Fe and alleviated the Cu accumulate in soil and plant [100]. Some of the most important examples of positive effect of foliar application of nutrients on aromatic and medicinal plants are indicated in Table 1.

Table 1 The most notable impacts of foliar application of nutrients on medicinal and aromatic plants
Full size table

3 Conclusions

Foliar application of nutrients is the best technique of sustainable agriculture and horticultural production systems to improve both quality and quantity of medicinal and aromatic plants. Foliar application of nutrients basically micronutrients is an age-old practice to rectify the deficiency symptoms. Foliar application may lead to quick growth of seedling after germination in annual crops, during tillering and grain filling stages, during flowering and fruit-set in deciduous crops, during rapid fruit expansion in many types of fruit crops and during bulking-up process in bulb or tuber crops. The positive effect of foliar application of Fe and Zn on anise and chamomile, Se on atractylodes, water spinach and tea, coffee and stevia, magnesium on banana, Zn on basil, saffron and fenugreek, calcium on blueberry, P and S on castor bean, iron on costmary, cooper, magnesium and zinc sulfate on damask rose, N and P on fennel and mustard, Zn, B, Ca and K on guava, Fe, Cu, Mn and Zn on lemon balm, B on mandarin orange and olive, sodium chloride on mint, B and Zn on onion and sesame, Fe on pepper, Fe on peppermint, K on pistachio, Ni on pot marigold, Si, Zn and K on spearmint, Se on spinach, Sin on strawberry, K and Ca on thyme, K, Zn and Mg on tomato have been reported in various studies. Foliar application is an appropriate technique of feeding medicinal plants, fruits and herbs by utilizing liquid fertilizer directly to their leaves, and essential elements can be absorb into the leaves directly, which is the proper method for small amounts of both micro- and macro-nutrients. Foliar applications of nutrients have tremendous benefits on medicinal and aromatic plants, but the most important advantages are increase resistance to insect, pests and diseases, increase drought tolerance, boost soil salinity tolerance and enhance resistance to physiological disorders, it may have positive effect for the immobilized nutrients like iron, better and rapid nutrient absorption in different plant growth stages, especially at early crop growth stages. The positive effects of feeding plant via foliar fertilizers directly to the leaves are reported in many studies, therefore foliar application can be considered as the best tool to give supplemental doses of both minor and major nutrients, stimulants and even plant hormones. Foliar application is advisable where additional benefits from the plant’s activity can be obtained. The big advantage of foliar treatment is that they can address an urgent need within a relatively short time, and they are therefore specially efficient as a preventive and in some cases curative treatments. Foliar treatment is not generally intended to replace soil or fertigation applications, but to complement them. More researches are needed to survey the impacts of nutrient foliar application on final yield and quality of medicinal and aromatic plants and herbs.