Although sugarcane is a very rustic plant, the economic viability of its agro-industrial production is gradually harmed as environmental characteristics become more adverse (VITTI et. al, 2005).

The increasingly intensive exploitation of the soil, in areas that are not very favorable or even in regions that are more suitable for growing sugar cane, has generated problems in crops, mainly linked to the removal of micronutrients from the soil, without the necessary replacement of these elements that are fundamental to productivity (DIAS et al., 2016).

Many agronomic techniques are used in sugarcane production, including the choice of varieties suitable for the soil and climate, soil conservation and chemical correction, pest and weed control, among others (VITTI et al., 2005).

These soils require improved management to obtain more economically favorable results. To this end, in addition to correcting soil acidity with liming, NPK fertilization and crop rotation with legumes such as soybeans, peanuts or the use of green manures, the application of micronutrients is essential (MELLIS and QUAGGIO, 2015).

Most tropical soils have low availability of micronutrients, mainly zinc and boron, caused by the low natural fertility of these soils, high degree of weathering and also by inadequate management, such as excessive liming (or superficial incorporation of limestone) and even fertilization (phosphate) (DIAS et al., 2016).

Tokeshi (1991) states that in systems with sugarcane cultivation, the micronutrient is influenced, in addition to the correction of soil acidity, by the varieties of sugarcane, the available water and the activities of microorganisms.

Micronutrients in sugarcane cultivation

Plant nutrients can be defined as those without which plants do not complete their life cycle, irreplaceable by others, and directly involved in plant metabolism (BECARI, 2010). They play vital roles in plant metabolism, as they are responsible for metabolic and/or phenological processes and enzyme activators (MELLIS and QUAGGIO, 2015).

According to Fageria et al. (2002), cited by Adorna (2011), micronutrient deficiency is present all over the world due to the increase in the demand for micronutrients by intensive management practices and adaptation of highly productive cultivars, which may have a greater requirement for micronutrients; increased crop production in marginal soils with low levels of essential nutrients; greater use of concentrated fertilizers with lower amounts of micronutrients; decreased use of animal manure, compost and crop residues, the use of soils with low native reserves, and the involvement of natural and anthropogenic factors that limit adequate availability for plants and create imbalances between nutrients.

Casarin et al. (2001) report that in soils with low fertility or that are exploited for many years, the occurrence of micronutrient deficiency in sugarcane becomes even more aggravated.

The productivity gains provided by micronutrients in sugarcane go beyond the simple fact of producing more and increasing the profitability of the crop. There is currently a great demand for the production of sugarcane ethanol in the country, which has caused the area occupied by the crop to grow every day. However, this expansion has generated criticism from the international community, which has come to consider Brazil as the main culprit for the global food crisis, claiming that sugarcane has occupied areas previously destined for grain production (MELLIS; QUAGGIO; CANTARELLA, 2008).

Malavolta (1982), cited by Vitti et al. (2005) presented the quantities of micronutrients extracted and exported by sugarcane crops, which are shown in Table 1.

Table 1. Extraction and export of micronutrients for the production of 100 t of stalks (Vitti, 2005).

Although extracted in smaller quantities, the micronutrients that may present the greatest limitations for the productivity of sugarcane crops in Brazil are Cu and Zn (ORLANDO FILHO et al., 1994). For Mellis et al. (2008), the limiting ones are B, Cu, Zn, Mn and Mo.

Orlando Filho et al. (2001) warn that sugarcane often presents the phenomenon of “hidden hunger” in relation to micronutrients. In other words, the plant does not show the visible characteristic symptoms of micronutrient deficiency, but the deficiency exists, economically limiting productivity.

Vitti and Mazza (2002) found micronutrient levels below those suitable for sugarcane in leaf and soil samples, mainly B and Zn, in the regions of Piracicaba and Araçatuba, although which soils were not specified.

Anderson and Bowen (1992) comment that soils cultivated with sugarcane have adequate iron levels, but deficiency of this micronutrient may occasionally occur in soils with pH in the alkaline range.

Functions of the main micronutrients and symptoms of deficiencies

Micronutrients play vital roles in plant metabolism, forming part of compounds responsible for metabolic processes and enzyme activation, acting on growth, tillering, disease resistance, quality and productivity (VITTI et al., 2005).

BORON (B)

Boron is responsible for root development and sugar transport (VITTI et al., 2005). This element is directly related to calcium metabolism, responsible for the proper formation of the cell wall (VITTI et al., 2005). Boron is an essential element for plant growth, as it participates in several processes, such as ion absorption, carbohydrate transport, lignin and cellulose synthesis, as well as nucleic acids and proteins (ALLEONI; CAMARGO; CASAGRANDE, 1998).

Figure 1. New leaves showing wrinkling (VITTI et al., 2005).

B is an extremely important micronutrient for sugarcane, especially when related to sucrose accumulation (SIQUEIRA, 2014). According to the author, the promising effect of B on sucrose accumulation may be related to its function in the formation of the cell wall and in the structure of the conducting vessels (DIAS et al., 2016).

Martello (2016) studied the effect of boron deficiency on two sugarcane varieties grown in nutrient solution and found that the studied varieties behaved differently when faced with low availability of B in the nutrient solution, with one being more tolerant than the other in relation to boron deficiency. However, for both, the deficiency reduced the length and surface of the roots and increased the root diameter of the sugarcane plants. There was a reduction in the length of internodes and plant height. It was also found that dry matter production was increased in plants under B deficiency and the concentration of nutrients in the aerial part, except Ca, was reduced. (DIAS et al., 2016).

The same author comments that new studies are needed to refine the B recommendation for sugarcane, not only with regard to the rate to be applied, but also to investigate the fate of the micronutrient in the plant (DIAS et al., 2016).

COPPER (Cu)

Copper is absorbed by plants in ionic form (Cu⁺²) (SOBRAL and WEBER, 1983). It is one of the most important micronutrients for sugarcane, acting as an activator of several enzymes (TAIZ; ZEIGER, 2004), such as polyphenol oxidase, which is of great importance in the respiratory processes of growing plants (BONNER, cited by JACINTO et al. 1964).

Figure 2. Copper deficiency in sugarcane leaf blade (ANDERSON and BOWEN, 1992).

IRON (Fe)

Dias et al. (2016) state that Fe deficiency is temporary, making rooting more superficial during a period in which the shoots are being nourished by reserves from the seedling stalk or by the roots of the ratoons. At this stage, the sugarcane presents interveinal chlorosis, forming longitudinal parallel streaks across the entire length of the leaf. In more severe cases, the leaves turn white and have dry tips. Although it is a very common symptom in sugarcane fields, there is no need to supply sugarcane with this micronutrient, as these symptoms disappear with the full establishment of the roots, which find Fe available in abundance in Brazilian soils. Even so, Alvarez and Wutke (1963) found that the addition of 2 kg ha^-1 of Fe provided an increase of 9% in sugarcane productivity.

Figure 3. Chlorotic-whitish plant due to iron deficiency (VITTI et al., 2005).

MANGANESE (Mn)

Manganese is the second most required micronutrient by sugarcane crops (BENNET et al., 2013) and its lack can cause problems for sugarcane crops, mainly low productivity.

Symptoms of Mn deficiency initially occur in the youngest leaves and are characterized by the appearance of longitudinal chlorotic streaks in the interveinal spaces; these streaks are progressive and become necrotic, drying the interveinal spaces of the leaf blade, over time (SOBRAL and WEBER, 1983).

Figure 4. Yellow streaks along the veins (VITTI et al., 2005).

ZINC (Zn)

Among the essential micronutrients for sugarcane production, zinc can be considered one of the most important, as its deficiency has been frequently observed (VALE et al. 2008).

Tokeshi, 1991 observed that in plants older than six months, there is a slight shortening of the internodes, interveinal chlorosis and more pronounced yellowing from the margin to the central vein, when the blade next to it normally remains green.

Figure 5. Wide chlorotic band on the leaf blade and red spots on the leaves in new leaves (BECARI, 2010).

MOLYBDENUM (Mo)

Mo is essential for plants that utilize nitrate (N^–NODE₃ ^–) as one of the nitrogen sources, improving the efficiency of nitrogen fertilization and the production of sucrose by sugarcane. This micronutrient is a component of the nitrate reductase enzyme responsible for the conversion of nitrate (N^–NODE₃^–) to nitrite (N^–NODE₂) which is later converted into amino acids (VIDOR and PERES, 1988).

Molybdenum increases the efficiency of nitrogen fertilization and sucrose production. It is essential for nitrogen metabolism in plants that use soil nitrate and/or atmospheric nitrogen as a source of this nutrient (VITTI et al., 2005).

Figure 6. Molybdenum deficiency in sugarcane and molybdenum deficiency in sugarcane (ANDERSON and BOWEN, 1992).

NICKEL (Ni)

Despite being an essential micronutrient for sugarcane, symptoms of Ni deficiency are difficult to detect in plants and are often confused with symptoms of Mn and Fe deficiency. Plant toxicity is more common due to this element (MELLIS and QUAGGIO, 2013). Other symptoms of Ni deficiency in plants are reduced leaf size and altered shape, dark green region at the leaf tips, leaf apical necrosis, curvature and wrinkling of the leaf apical region, and in more severe cases, absence of laminar development (REIS, 2014).

SILICON (Si)

Silicon is an element involved in functions related to transpiration, capable of concentrating in the epidermis of leaves, forming a physical barrier to the invasion of fungi inside the cells, also hindering the attack of sucking and chewing insects (KORNDORFER, 2004).

Silicon, although not considered an essential element for plants, is the element most absorbed by sugarcane, followed by potassium, nitrogen, calcium and magnesium (TISDALE et al., 1985) and once silicon is absorbed by sugarcane, it is found on the edges of the leaves in the form of amorphous silica, and can develop negative charges (EPSTEIN, 1999).

Figure 7. Characteristic symptoms of silicon (Si) deficiency on the abaxial (a) and adaxial (b) surfaces of sugarcane leaves. (KORNDORFER, 2004)

Responses of sugarcane crops to micronutrient application

In his master's dissertation in 2010, Becari carried out 7 treatments consisting of fixed doses of micronutrients, applied only in the planting furrow and evaluated the results with individual fertilizations of each micronutrient (Zn, Mn, Cu, B, Mo) and a complex with the 5 micronutrients, in addition to the control treatment and concluded that the application of Zn and Mo, in the planting furrow was able to increase the foliar content of these micronutrients.

The same author also observed that the application of micronutrients (Zn, Mn, Cu, B and Mo) in low fertility soils increased the agricultural and industrial (sugar) productivity of the sugarcane plant. In addition, Zn was the micronutrient that provided the greatest productivity gains (20 t ha^-1 in joint analysis) in plant cane.

In 2010, Vasques and Sanches evaluated the efficiency of using micronutrients in sugarcane crops and concluded that the use of micronutrients provides increases in sugarcane productivity. The best results were obtained when applications were made via the cuttings plus the foliar application.

Considering that the best results were obtained through applications via cuttings, in addition to foliar applications, ILSA BRASIL presents a range of solutions that can be alternatives for greater crop profitability, such as ILSAMIN AS, which has 3% of Nitrogen in its formulation, in addition to 7.5% of molybdenum, a micronutrient that stimulates the vegetative development of the plant, as it is linked to nitrogen metabolism. Boron, responsible for root development and sugar transport, can be found for foliar fertilization in ILSAMIN BORON+B (4.0+5.0), in addition to the benefits associated with nutrients, ILSA BRASIL liquid fertilizers have a range of amino acids (21), coming from their GELAMIN matrix.

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Authors:

Agr Eng. Dr. Angélica Schmitz Heinzen

Agricultural Eng. Msc. Thiago Stella de Freitas

Agricultural Engineer Tuíra Barcellos