Foliar Nutrition with Organic Acids

Foliar Nutrition with Organic Acids: Comparison

Please note this is a comparison between Version 2 by Dean Liu and Version 1 by Mohunnad Massimi.

As a result of global warming related to the development of industry and agriculture, the proportion of atmospheric carbon dioxide has increased, and temperatures have risen to un-precedented levels. As a result, heat stress, aridity, and salinity in soil has increased, leading to significant research focused on soil deterioration and reduced agricultural productivity. Therefore, it is necessary to provide the means to maintain crop productivity. Agricultural research is seeking novel solutions that guarantee stability and increase the production and quality of crops, including innovative models for feeding crops using non-traditional methods, the most important of which is nourishing plants via their leaves to ensure the cessation of their soil consumption. It is considered an integrated pest-control method, and the technique could be included in plant nutrition. Foliar nutrition has been shown to be a perfect substitute for providing secondary nutrients and micronutrients to plants; however, it cannot be substituted for the fertigation or the fertilization of maintain the soil’s macronutrients (nitrogen, phosphorus, and potassium). This study shed light on the most important research, conclusions, and generalizations on the technique of foliar feeding using organic, especially for tomato and pepper plants.

crop quality
foliar feeding
integrated pest management

1. Salicylic Acid Foliar Nutrition Experiments in Tomatoes and Peppers

The foliar application of salicylic acid and salicylic acid/KMnO₄ had neither a significant effect on tomato fruit yield, the content of soluble sugars in fruits, nor the nutritional status of leaves in terms of macro- and micro-nutrients. Following the foliar application of salicylic acid and salicylic acid/KMnO₄, tomato fruits had a higher concentration of ascorbic acid and a lower buildup of phenolic compounds and free amino acids. There was no effect from the exogenous foliar application of salicylic acid on the prevention of fruit production decline due to high salt stress ^[1]. However, it was proved that to recover the lowered growth characteristics of tomato plants under the salinity stress of sodium chloride (100 mmol dm⁻³ NaCl), the most effective methods of salicylic acid application were leaf pretreatment, root pretreatment, and leaf treatment. The concentration used in each method was salicylic acid (2.17 mmol dm⁻³) ^[2].

However, the severity of vascular browning and leaf yellowing were significantly reduced in tomato plants treated with root (0.2 mmol dm⁻³) or leaf foliar spray (0.2 mmol dm⁻³) with salicylic acid and inoculated with Fusarium oxysporum f. sp. lycopersici (one of the soil-borne fungal pathogens of tomato wilt) ^[3].

During low-temperature periods on fall plantations, foliar spraying for sweet peppers with salicylic acid at 2.17 mmol dm⁻³ and chelated zinc at 1.087 mmol dm⁻³ were utilized to boost the ultimate production and fruit quality of sweet pepper plants. With 2.17 mmol dm⁻³ salicylic acid, plus 1.087 mmol dm⁻³ chelated zinc, the maximum values of the early, marketable, and total yields, as well as the physical characteristics of sweet pepper fruits, were obtained, followed by the results found with 1.087 mmol dm⁻³ salicylic acid with 2.17 mmol dm⁻³ zinc ^[4]. It was reported that three red sweet pepper cultivars grew faster, yielded more, and had better fruit nutritional quality after the foliar application of humic or salicylic acids. Depending on the cultivar, salicylic acid (32.61 mmol dm⁻³) increased fruit weight, flesh thickness, and total yield. Salicylic acid was found to be more efficient than humic acid in ^[5]. It was concluded that spraying 2 sweet pepper cultivars with salicylic acid at 2.17 mmol dm⁻³ or 1.087 mmol dm⁻³ boosted production more than that of the untreated plants ^[6].

In a field experiment, Bacillus amyloliquefaciens strain 5B6 (phyllosphere bacteria) was shown to protect pepper plants against cucumber mosaic virus (CMV) by boosting the defense priming through salicylic acid and jasmonic-acid signaling ^[7]. As compared to the effects on the controls, spraying 1.8 mmol dm⁻³ salicylic acid enhanced sweet pepper plant height, stem diameter, fruit number, weight, length, diameter, vitamin-C content, total soluble solid content, and fruit production ^[8]. As compared to the untreated controls, the foliar treatment of 0.20 mmol dm⁻³ salicylic acid (30 days after transplantation), followed by 0.00010 mmol dm⁻³ Epibrassinolide (EBR) (60 days after transplantation), dramatically improved bell pepper growth and yield characteristics. By enhancing growth characteristics (plant height, spread, and leaf area), as well as photosynthetic efficiency, these treatments alleviated heat stress on bell pepper growth ^[9]. The results from these studies are summarized in (Table 1).

Table 1. The most important studies in salicylic acid foliar spraying in tomatoes and peppers.

Treatment *	Concentration	Impact of Foliar Application
Salicylic Acid ^[2]	2.17 mmol dm⁻³	Restored the reduced growth characteristics of tomato plants subjected to the salinity of sodium chloride stress (100 mmol dm⁻³ NaCl)
Salicylic Acid ^[3]	0.2 mmol dm⁻³	The severity of vascular browning and leaf yellowing was significantly reduced in tomato plants treated with a salicylic acid leaf foliar spray and inoculated with Fusarium oxysporum f. sp. Lycopersici (soil-borne fungal pathogen of tomatoes wilt)
Salicylic Acid + Chelated Zinc ^[4]	2.17 mmol dm⁻³ + 1.087 mmol dm⁻³	To increase the quantity and quality of sweet pepper fruits, foliar spraying with salicylic acid and chelated zinc could be used
Salicylic Acid ^[5]	32.61 mmol dm⁻³	Red sweet pepper cultivars with increased fruit weight, flesh thickness, and total yield
Salicylic Acid ^[6]	2.17 mmol dm⁻³ or 1.087 mmol dm⁻³	Sweet pepper plant production was increased
Salicylic Acid ^[8]	1.8 mmol dm⁻³	Increased the number, weight, length, and diameter of sweet pepper fruits, as well as their vitamin-C content, total soluble solid content, and fruit production
Salicylic Acid ^[9]	0.20 mmol dm⁻³	Salicylic acid (30 days after transplantation) followed by Epibrassinolide (EBR) 0.00010 mmol dm⁻³ (60 days after transplantation) increased bell pepper yield, photosynthetic efficiency, and heat tolerance

* Measurement conversions are as follows: 1 mg L⁻¹ equals 0.0217391 mmol dm⁻³, 1 g L⁻¹ equals 21.7391304 mmol dm⁻³, and 1 µM equals 0.001 mmol dm^−3.

2. Tomato Foliar Application in Humic Acid, Fulvic Acid, and Gibberellic Acid Trials

The foliar spraying of the gibberellic acid GA₃ (0.01 mmol dm⁻³) as a growth regulator exhibited a growth-promoting impact on unstressed tomato seedlings and was successful in enhancing the salinity of the sodium-chloride tolerance of tomato seedlings, up to 25 mmol dm⁻³ NaCl, with foliar treatments ^[10]. It was concluded that humic acid sprays with a concentration of 434.78 mmol dm⁻³ could be used successfully to improve tomato growth and yield ^[11]. The results reported by Kazemi revealed that spraying tomatoes with humic acid (0.65 mmol dm⁻³) and calcium chloride (15 mmol dm⁻³), either alone or in combination (0.65 mmol dm⁻³ humic acid + 15 mmol dm⁻³ calcium), had a substantial effect on vegetative and reproductive growth, as well as the chlorophyll content. Calcium (15 mmol dm⁻³) + humic acid (0.65 mmol dm⁻³) foliar sprays resulted in the highest vitamin C, yield (25.36 t ha^–1), fruit firmness, and lowest blossom end-rot incidence (5%) ^[12]. Researchers have tested individual and combined foliar sprays of humic acid (86.96 mmol dm⁻³), fulvic acid (869.57 mmol dm⁻³), and chelated calcium (54.35 mmol dm⁻³) on tomato plants 4 times (after 2, 4, 6, and 8 weeks post-transplanting). All foliar sprays of humic acid, fulvic acid, and calcium, either individually or in combination, boosted vegetative growth, production, and fruit quality ^[13][14]. Furthermore, these therapies reduced the prevalence of blossom end-rot in tomato fruits ^[13][14].

In recent years, there has been an increased focus on the performance of humic acid-based products, particularly potassium humate. Humustim, a foliar fertilizer, has been particularly useful in tomatoes (Table 2).

Table 2. Impact of foliar application trials of humic acid, fulvic acid, and gibberellic acid applied to tomato plants.

Treatment *	Concentration	Impact of Foliar Application
Gibberellic Acid GA₃ ^[10]	0.01 mmol dm⁻³	Foliar treatment improved tomato seedling salinity of sodium chloride tolerance up to 25 mmol dm⁻³ NaCl
Humic Acid ^[11]	434.78 mmol dm⁻³	Foliar humic acid sprays were used successfully to improve tomato growth and yield
Humic Acid + Calcium ^[12]	0.65 mmol dm⁻³ + 15 mmol dm⁻³	Foliar tomato spray produced the most chlorophyll, vitamin C, yield (25.36 t ha⁻¹), fruit firmness, and had the lowest incidence of blossom end rot (5%)
Humic Acid, Fulvic Acid, Chelated Calcium Solutions ^[13]	86.96 mmol dm⁻³ 869.57 mmol dm⁻³ 54.35 mmol dm⁻³	All foliar sprays of humic, fulvic acid, and calcium, used four times (after 2, 4, 6, and 8 weeks post-transplanting), either individually or in combination, increased vegetative growth, production, and fruit quality. In addition, the prevalence of blossom end rot in tomato fruits was reduced

* Measurement conversions are as follows: 1 M equals 1000 mmol dm⁻³, and 1 ml L⁻¹ equals 1000 mg L⁻¹ and equals 21.7391304 mmol dm⁻³. Each 1% equals 10,000 mg L⁻¹ and equals 217.39 mmol dm^−3.

3. Foliar Application of Humic and Ascorbic Acid Trials in Peppers

A study conducted by Karakurt found humic acid treatment had a considerable effect on the total chlorophyll content in organically produced peppers, primarily on the chlorophyll-b content. The highest total chlorophyll concentration was found with a foliar application of 434.78 mmol dm⁻³. As compared to the controls (0 mmol dm⁻³), the foliar humic acid treatment resulted in significant increases in the mean fruit weight as well as the early and total yields ^[15].

In an experiment that compared different spray treatments for pungent pepper, it was concluded that spraying humic acid (10.87 mmol dm⁻³) and zinc (10.87 mmol dm⁻³), or spraying zinc (10.87 mmol dm⁻³) and boron (4.34 mmol dm⁻³), together, were the most promising treatments for improving the physiological and biochemical qualities, respectively, of peppers, in a study based on average values ^[16]. Adding humic acid (3260.87 mmol dm⁻³) to the foliar fertilizer at a rate of 0.01 t ha⁻¹ yielded the highest seedling height, stem diameter, number of leaves, both shoot fresh-and-dry weights, and root dry weight, as well as macronutrient percentage (NPK%) ^[17]. Under cold conditions, the foliar application of Biomin aminochelate (an organic aminochelate fertilizer) improved chili pepper growth and quality attributes, followed by Humifolin fertilizer (a humic-acid-based fertilizer). The foliar application of Biomin and Humifolin resulted in higher values for leaf area, leaf number, chlorophyll index, root-and-shoot biomass, and leaf concentrations of soluble sugars, nitrogen, potassium, calcium, and zinc ^[18].

When comparing the effects of foliar applications on sweet peppers, seaweed extract at 54.35 mmol dm⁻³ and yeast extract at 108.7 mmol dm⁻³ recorded the highest significant values of most plant parameters, such as plant height, number of leaves, number of branches, leaf area, fresh-and-dry weights, and the chemical constituents of leaves, such as the chlorophyll (chlorophyll a, chlorophyll b, and total chlorophyll a + b), nitrogen, phosphorus, and potassium percentages. In addition, spraying 32.61 mmol dm⁻³ humic acid ranked second and considerably enhanced various parameters, such as the number of branches, fresh-and-dry weights, and leaf area. Plants treated with chicken manure and sprayed with either seaweed extract at 54.35 mmol dm⁻³ or yeast extract at 108.87 mmol dm⁻³, in the presence of biofertilizers over two seasons, produced the most significant results in terms of plant metrics and chemical contents ^[19]. Chelators (humic acid: HA1 (0 mmol dm⁻³) and HA2 (10.87 mmol dm⁻³)) and micronutrients (manganese: Mn1 (0 mmol dm⁻³); Mn2 (10.87 mmol dm⁻³) and molybdenum: Mo1 (0 mmol dm⁻³); Mo2 (2.17 mmol dm⁻³)) as foliar applications showed that HA2Mn2 and HA2Mo2 had significant results in all variables, suggesting that it could improve the quality of the green pungent pepper by increasing carbohydrate contents, antioxidant constituents, and antioxidant activities ^[20].

Another study conducted by Khazaei and Estaji found that the foliar spray of ascorbic acid (1 mmol dm⁻³) considerably enhanced the shoot fresh weight, root dry weight, antioxidant characteristics, ascorbate, polyphenol oxidase, and ascorbate peroxidase, in plants under drought stress ^[21]. The results of these studies are outlined in Table 3.

Table 3. Impact of tests of humic and ascorbic acid foliar application in peppers.

Treatment *	Concentration	Impact of Foliar Application
Humic Acid ^[15]	434.78 mmol dm⁻³	The total chlorophyll-b content in organically grown peppers increased as well as mean fruit weight and early total yield
Humic Acid and Zinc or Zinc and Boron together ^[16]	10.87 mmol dm⁻³ and 10.87 mmol dm⁻³,10.87 mmol dm⁻³ 4.35 mmol dm⁻³	Enhanced the physiological and biochemical properties of pungent pepper
Humic Acid added to the foliar fertilizer ^[17]	3260.87 mmol dm⁻³	The highest pepper seedling height, stem diameter, the number of leaves, shoot fresh-and-dry weights, root dry weight, and (nitrogen-phosphorus-potassium %) were produced
When combined with drought stress, it improved sweet pepper shoot fresh weight, root dry weight, antioxidant characteristics, ascorbate, polyphenol oxidase, and ascorbate peroxidase

* Measurement conversions are as follows: 1 g L⁻¹ equals 21.7391304 mmol dm⁻³, and 1 ml L⁻¹ equals 1000 mg L⁻¹ and equals 21.7391304 mmol dm⁻³. Each 1% equals 10,000 mg L⁻¹ and equals 217.39 mmol dm^−3.

4. Foliar Application of Growth Regulators in Peppers

Although the gibberellic-acid (GA₃) and abscisic-acid (ABA) treatments (2.17 mmol dm⁻³) reduced sweet pepper yield, GA₃ (0.70 mmol dm⁻³) increased plant height and the levels of tyrosine, phosphate, sulfate, iron, and phosphorus while decreasing glucose and fructose. As compared to the control plants, the foliar spraying of indole-3-acetic acid (IAA) (0.70 mmol dm⁻³) had no effect, whereas plants treated with ABA had lower levels of sucrose but higher levels of iron. When sprayed every two weeks, GA₃ dramatically improved the quality of sweet pepper fruits while not affecting total yield ^[22]. On unstressed sweet pepper seedlings, a foliar treatment of GA₃ (0.01 mmol dm⁻³) exhibited a growth-promoting effect and was successful in increasing the salinity tolerance of sodium chloride up to 50 mmol dm⁻³ NaCl ^[9].

The effect of foliar applications of different concentrations of morphactin on the pepper root mycoflora (Capsicum annuum) was investigated. In response to increasing morphactin concentrations, the root mycoflora was shown to diminish. This effect was attributed to a change in the root exudate pattern in response to the foliar administration of the chemical, as well as a retardation of lateral root growth. Morphactins are a class of synthetic growth regulators (fluorene-9-carboxylic acid derivatives) that have been shown to limit or modify new plant growth ^[23]. The foliar application of growth regulators in peppers are shown in (Table 4).

Table 4. Impact of foliar application of growth regulators on pepper.

Treatment *	Concentration	Impact of Foliar Application
Abscisic Acid (ABA) ^[22]	2.17 mmol dm⁻³	Plants had lower sucrose levels and higher iron levels
Gibberellic Acid (GA₃) ^[22]	0.70 mmol dm⁻³	Plant height and tyrosine, phosphate, sulfate, iron, and phosphorus levels increased while glucose and fructose levels decreased
Indole-3-Acetic Acid (IAA) ^[22]	0.70 mmol dm⁻³	No effect
Biomin (0.2%) and Humifolin (0.2%) ^[18]	43.478 mmol dm⁻³ 43.478 mmol dm⁻³	Chili peppers with higher values for leaf area, leaf number, chlorophyll index, root, and shoot biomass, and soluble sugar, nitrogen, potassium, calcium, and zinc concentrations in the leaves
GA₃ ^[9]	0.01 mmol dm⁻³	Salinity sodium chloride tolerance increased up to 50 mmol dm⁻³ NaCl	Seaweed Extract (2.5 ml L⁻¹) + Yeast Extract (5 g L⁻¹) ^[19]	54.35 mmol dm⁻³ + 108.7 mmol dm⁻³	Most sweet pepper plant parameters and chemical constituents of leaves, such as chlorophylls (chlorophyll a, chlorophyll b, and total chlorophyll a + b), nitrogen, phosphorus, and potassium percentages, had the highest significant values
Morphactin ^[23]	0.217 or 2.17 or 21.7 mmol dm⁻³	Limited plant growth	Humic Acid and Manganese or Humic Acid and Molybdenum ^[20]	10.87 mmol dm⁻³ 10.87 mmol dm⁻³ 10.87 mmol dm⁻³, 2.17 mmol dm⁻³	Increased the carbohydrate content, antioxidant constituents, and antioxidant activities and quality of green pungent pepper
Ascorbic Acid ^[21]	1 mmol dm⁻³

* Measurement conversions are as follows: 1 mg L⁻¹ equals 0.0217391 mmol dm^−3, and 1 M equals 1000 mmol dm^−3.

5. Experiments of Foliar Application of Amino Acids in Tomatoes and Peppers

Because of the differences in their free-amino-acid composition, the effect of commercial items on the iron (Fe) nutrition of tomato seedlings varies greatly depending on their origin. A product comprising animal-derived amino acids appeared to be poisonous and had negative impacts on iron nutrition. Exogenous treatments of the product (through roots or foliar applications) containing plant-derived amino acids, however, boosted plant growth and improved the iron nutritional levels of tomato seedlings cultivated with lime-induced iron deficiency, especially when the product had been applied directly to roots ^[24]. In another experiment, foliar treatments were applied, and it was discovered that applying the amino acids together (in this case, aspartic acid and glutamic acid) had better results than applying them separately. Tomato plants were poisonous when 15 mmol dm⁻³ of alanine was applied, and this toxicity was not reversed by applying an aspartic + glutamic + alanine mixture at the same time ^[25]. Another study found that an aminochelate foliar spray on tomatoes was superior, as compared to a soil application, in terms of the vitamin-C concentration and the total soluble solids in fruits; however, the soil application of aminochelate outperformed the foliar spray. Aminochelate therapy, particularly via foliar application, greatly increased plant growth, biomass production, and the yield in moderately calcareous soil ^[26]. Two tomato cultivars were administered a foliar spray of an organic amino acid (proline) at a concentration of 0.22 mmol dm⁻³. Heinz-2274 had a 63.5% increase in the above-ground biomass, but the Rio Grande had only a 38.9% increase ^[27].

The foliar spraying of sweet pepper plants with 1.087 or 2.17 mmol dm⁻³ of folic acid and a mixture of methionine, lysine, and cysteine resulted in the highest total protein and total sugars in the dry-weight leaves. In addition, the foliar spraying of sweet pepper plants with a 1.087 mmol dm⁻³ folic acid mixture, including lysine and cysteine amino acids, increased flowering and reduced fruit shedding by 17.2%. Finally, a foliar application of 1.087 mmol dm⁻³ folic acid combined with a mixture of methionine, lysine, and cysteine amino acids resulted in the highest significant averages of fruit weight, diameter, dry weight, total soluble solids, and vitamin-C content ^[28]. A bio-stimulant treatment involving a ratio of glutamic acid + glutamine/aspartic acid corrected an imbalance induced by the pepper mosaic virus (PepMV), improving all measured parameters, as compared to those of uninfected plants ^[29].

A study performed by Khan found showed that, under hydroponic circumstances, the foliar application of amino acids and rockweed (Ascophylum nodosum) seaweed extract improved the growth and production of two bell pepper cultivars while maintaining biochemical fruit quality under long-term cold storage. A total of 65.22 mmol dm⁻³ of amino acids and 86.96 mmol dm⁻³ of seaweed performed well in terms of vegetative and reproductive parameters while 108.70 mmol dm⁻³ of amino acids and 43.48 mmol dm⁻³ of seaweed treatment maintained improved biochemical fruit quality under cold storage ^[30].

As compared to the controls, the combination of 130.44 mmol dm⁻³ seaweed extract and 17.39 mmol dm⁻³ amino acids had the highest values for plant height, the number of branches, and the percentage of dry matter of shoots in sweet pepper cultivars ^[31]. Treatments with glutathione and arginine, especially at 2.17 mmol dm⁻³, had a stronger boosting impact on hot pepper plants due to its beneficial effect on yield, endogenous growth promoters, ascorbic acid, anthocyanins, tannins, phenolic compounds, carbohydrate, protein, and amino acid levels in yielding fruits, as compared to treatments with tryptophan ^[32]. The foliar application of amino acids to bell pepper plants increased the fruit diameter and length. Fruit morphologic alterations were also caused by high levels of urea ^[33]. The most prominent results are displayed in Table 5.

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