The contact between nutrient and leaf is made mainly through foliar fertilization. In foliar fertilization, nutrients are applied in aqueous solution and need to enter the cell (cytoplasm, vacuole, or organelles) to perform their functions, as a nutrient is considered absorbed when inside the cell. For this, there are two barriers to over-come, namely the cuticle/epidermis and the membranes, plasmalemma, and tonoplast.
The epidermis and cuticle cover the upper and lower leaf surfaces. The cuticle, which is the outermost part, is of a complex chemical nature, formed by waxes, cutin, pectin, and cellulose. It is water-permeable. The epidermis provides wetting and hydrophilic properties. Foliar uptake, like root uptake, comprises a passive phase (cuticular penetration) and an active phase (cellular absorption).
(a) Passive – Consists of a non-metabolic process in which the nutrient applied to the foliar surface crosses the upper or lower cuticle (Fig. 3.1), occupying the AFS (apparent free space) formed by the cell wall, intercellular spaces, and the external surface of the plasmalemma. For cuticular uptake, solute molecules must have a diameter of less than 4–5 nanometers.
Epicular waxes
Cutina, pectin, Intracuticular waxes Cuticle Parede celular
Plasma membrane
Cuticle Epidermal cells
H2O
With adjuvants
Without adjuvants
Mesophilic cells
Fig. 3.1 Foliar structure from a cross-section of the leaf blade with cuticle detail
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It would be the nutrient entry in the leaf apoplast. The decreasing hydrophilic order is, namely pectin> cutin> cuticular waxes. We note that, due to its structure in the form of overlapping scales and not continuous as previously thought, there is some space where solutions can pass.
We also note that some substances are capable of undoing some chemical bonds between the units of the cuticular waxes when applied to the leaf surface. The rup-ture of these bonds results in some openings in the cuticle, facilitating the penetra-tion of solupenetra-tions. This phenomenon is known as facilitated diffusion. Urea is an example of substance that promotes these changes, which is why it is often used in foliar spraying. Thus, urea stands out as an additive to increase the uptake rates of cations and anions (Freire et al. 1981).
(b) Active – The nutrient is effectively absorbed after overcoming the cuticle, pass-ing through the membranes (plasmalemma and/or tonoplast) of epidermis and mesophyll cells. Afterwards, the nutrient reaches the symplast, where it is metabolized or transported between cells through cytoplasmic projections (plasmodesmata) to the phloem. Thus, nutrients are transported through long distances as in the root, although there are no Casparian strips on leaves.
However, unlike roots, the phloem can be transported through apoplast on leaves. We note that this is a slow metabolic process, which occurs against a concentration gradient and requires energy supply (ATP). It is the occupation of the leaf symplast. Active nutrient uptake is moderated by a specific carrier.
In the phloem, nutrients are redistributed in forms that differ from the ones they were absorbed, such as P (hexosphosphate), N (amides), S (elemental or organic S), and micronutrients Cu, Fe, Mn, and Zn (organic, as chelates) (Malavolta 2006).
External and internal factors affecting nutrient uptake by leaves
As seen in root uptake, foliar uptake is also influenced by several external (envi-ronment) and internal (plant) factors.
External Factors
Among external factors influencing foliar uptake, the following are considered: the contact angle of the solution and leaf, the temperature and humidity, solution con-centration and composition, and light.
The contact angle between solution and leaf surface is related to increased or decreased wetting of the leaf by the solution. Thus, the more the solution is spread, increased is its contact with the foliar surface and increased its possibility of uptake.
The temperature and humidity of the environment determine the drying speed of the solution applied to the foliar surface. Thus, high temperatures or low relative humidity of the air (<60%) facilitate the evaporation of the solution, contributing to decrease its permanence on the foliar surface and reduce the possibility of uptake.
The solution concentration to be applied must take into account the possibility of evaporation, and an overly concentrated solution may damage leaves. Thus, when preparing a solution, it is necessary to consider the actual conditions of its evapora-tion prior to its applicaevapora-tion, based on air temperature and humidity data.
3.1 Introduction
The solution composition is another aspect to be considered, as each chemical element in the solution has a different uptake rate (Table 3.1) and there are relatively fast-absorbing nutrients (50% of the nutrient applied to the leaf), such as N (0.5–36 h), and slow-absorbing nutrients, such as Fe and Mo (up to 20 days). Uptake rates vary in each nutrient as data are obtained under different experimental condi-tions (Malavolta 1980). Recent research, such as on S, are in accordance with the data shown in Table 3.1, that is, approximately 33% of the sulfur applied to the first trifoliate leaves of bean plants was absorbed in the period of seven days (Oliveira et al. 1995). Regarding B (in citrus), researchers observed that the highest absorp-tion efficiency occurred after 16 hours of spraying (Boaretto 2006).
There are differences in foliar uptake related to the chemical nature of the ion (cation or anion) and even to the accompanying ion. Regarding the chemical nature of the ion, we observe that the pores in the cuticle contain negative charges (polyga-lacturonic acids), implying increased uptake of cations in relation to anions as repulsion occurs. Thus, NH4+ uptake rate is higher than that of NO3−. Regarding the accompanying ion, studies indicate that Mg applied to apple tree leaves has increased uptake when the accompanying ion is in the form of chloride compared to nitrate or sulfate due to variation in solubility and hygroscopicity among these salts (Allen 1960).
Therefore, each nutrient has specific characteristics during uptake, with different speeds of entry into the plant and after its uptake and different mobility (nutrient transport from leaves to other organs through the phloem), varying from element to element as previously discussed (Table 3.1).
Regarding the so-called partially mobile nutrients, recent studies with citrus using the isotopic technique indicate low efficiency of these nutrients in plant nutri-tion. Boaretto et al. (2003), based on the results obtained, concluded that foliar fer-tilization with micronutrients is efficient to supply Zn, Mn, and B to sprayed leaves, but is insufficient to change the content of these micronutrients in new leaves born after foliar spraying in orange trees. The results indicate that less than 10% of the contents of Zn and Mn deposited on the foliar surface of orange trees are absorbed,
Table 3.1 Uptake rate of
nutrients applied to the leaves Nutrient Time for 50% of total absorption N – Urea (CO-NH2)2 0.5–36 h
P – H2PO4− 1–15 days
K – K+ 1–4 days
Ca – Ca2+ 10–96 h
Mg – Mg2+ 10–24 h
S – SO42− 5–10 days
Cl – Cl− 1–4 days
Fe – Fe-EDTA 10–20 days
Mn – Mn2+ 1–2 days
Mo – MoO42− 10–20 days
Zn – Zn2+ 1–2 days
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being insufficient to increase the micronutrient contents of leaves receiving foliar fertilization. Less than 1% of the contents of Zn and Mn deposited on leaves are transported to the parts of orange trees grown after foliar fertilization, being insuf-ficient to significantly alter leaf contents of these micronutrients in these parts. We note that the translocated amount is small (1%) and does not impress the radio-graphic film in contact with the new parts of leaves (Boaretto et al.2003, Fig. 3.2).
Fig. 3.2 Radioautography. Leaves 1, 2, and 3 received 54Mn (a); Leaves that received 65Zn (b), and New branch that developed after 65Zn was applied (c). Leaves were outlined to locate the branch in the radiographic film
3.1 Introduction
Solution pH: The solution can modify the pH of the foliar surface, changing cuticle permeability, increasing uptake rate from the beginning of the process, and increasing nutrient availability in the solution. In this sense, Rosolem et al. (1990) found that the N of solutions with decreased pH (3.0–4.0) was absorbed more quickly than that of solutions with increased pH (6.0–7.0), reaching 50% of the N applied after 5.5 and 11.5 hours, respectively.
Swanson and Whitney (1953) observed increased phosphorus uptake by bean plant leaves from solutions with decreased pH value when studying phosphate sources with variable pH. This was probably due to easier uptake of the H2PO4 ion found in acidic pH. Similar results were obtained by Oliveira et al. (1995), who observed increased S uptake by bean plant from sources with decreased pH values.
The fastest K uptake was obtained at pH 3 and when the application was performed in the form of phosphates or citrates. For urea, the highest uptake rate occurs from pH 5–8, and the lowest from 6 and 9 (Castro et al. 2005).
Light is another factor to be considered, as it participates in the photosynthetic process, producing indispensable energy for the active uptake phase. This energy source is inexistent in the dark, decreasing uptake rates.
(a) Internal Factors
Among internal factors, that is, related to the plant, we considered the humidity of the cuticle, foliar surface, age, and ionic internal state.
The humidity of the cuticle is important for the pathway of the chemical element in the passive uptake phase. Considering that element diffusion is part of this dynamics, a minimum humidity level is indispensable for its occurrence. Dehydrated cuticles in wilted leaves are practically impermeable.
The foliar surface is an important internal factor, as the upper and lower foliar surfaces have some distinct anatomical aspects.
In the literature, it has been indicated that foliar uptake of nutrients occurs pref-erentially on the foliar surface, where the cuticle is thinner (such as the lower or shaded leaf surface) and the largest number of stomata are found. In this surface with an increased number of stomata there are many guard cells, which have a high amount of pores. In addition, the cuticular wax composition of guard cells is less resistant to the solute passage (Karabourniotis et al. 2001). The entry of ions through the stomatal cavity is improbable as the entry of liquids is insignificant due to its architecture (Ziegler 1987), presence of cuticle lining (although thin), and CO2, O2, or water vapor (positive pressure), which can also prevent the passage of solution.
Foliar age is important as there is increased cuticle development with maturation and aging of leaves, increasing solution penetration resistance and difficulty in the uptake process.
Plant age affects nutrient uptake rate and growth rate, which normally follow a sigmoidal curve where we expect increased plant response to the nutrient applica-tion in the linear part of the curve.
The ionic internal state (nutritional status), within limits, regulates the amount of elements to be absorbed, as seen for root uptake. The higher the concentration of chemical elements in the leaf cells increased the difficulty to absorb new elements.
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Finally, we note that plant nutrition with foliar application of nutrients should always be used as a complement to soil fertilization (Fig. 3.3).
Besides nutrients, amino acids have been indicated for foliar spraying, but there are doubts on their uptake and use in plants. Amino acids can play different roles in plants, such as stress-reducing agents, nitrogen source, and hormone precursors (Maeda and Dudareva 2012). It is important to better understand the mechanisms that amino acids use as biostimulants (hormonal and antioxidant action) and not as a source of nutrients.
General Aspects of Foliar Spraying
Foliar application with formulas like N-P-K has been negative, except for N-Urea, for the following reasons:
• Large amount of nutrients required by plants at the beginning of their development.
• Small foliar area at the beginning of the crop.
• Leaf burning problems.
• Among the many P and K forms studied, few adapt to the foliar application.
• Cost of operation.
Besides problems related strictly to compatibility, the presence of one nutrient in the solution can negatively affect the uptake of another, especially in multinutrient solutions.
Foliar fertilization has some advantages, such as:
• The high rate of utilization of the nutrients applied to the leaves by plants.
• Correction of some short-term micronutrient deficiencies.
• Possibility of applying micronutrients along with pesticides.
We note that the foliar application of nutrients requires a series of essential pre-cautions for maximum efficiency, such as:
• Foliar application of nutrients cannot be used as a replacement of soil fertiliza-tion but as a complement, as previously stated.
Fig. 3.3 Nutrient application via soil and leaf
3.1 Introduction
• Foliar application of macronutrients does not sufficiently increase the foliar tis-sue, as plants have high macronutrient demands, consequently not having signifi-cant effects on production. Therefore it would not be advantageous to use this technique for these nutrients.
• Foliar application of micronutrients has their low requirement by plants as a positive point, requiring small amounts to be applied; the negative point would be the decreased mobility in the plant, that is, it will remain in increased quantity in leaves that received application. Thus, with the appearance of new leaves, there may be a repetition of deficiency symptoms. The frequency of application of micronutrients may improve their efficiency.
• The water used must be clean, as the presence of impurities such as clay can cause reactions with nutrients, reducing their action.
• The pH value of the solution must be controlled.
• The use of appropriate application technology, such as well-regulated equipment (specific spray nozzles, pressure, bar height), ensures increased homogeneity and reduced drift.
• Favorable environmental conditions, such as temperature below 30 °C, air rela-tive humidity above 50%, wind below 3 m/s, and high probability of rain.
The use of adhesive spreader or surfactant is important to increase the solution/
leaf contact surface and consequently increase uptake.
The use of humectant is important to delay drying of the solution by decreasing the deliquescence point of the solution on the foliar surface, maintaining the nutri-ent in ionic form for a longer time. These substances (e.g., sorbitol) are mandatory for foliar fertilization, as it is often performed in a limiting environmental condition and especially as they ensure adequate cuticular uptake of the nutrient.
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R. de Mello Prado, Mineral nutrition of tropical plants, https://doi.org/10.1007/978-3-030-71262-4_4