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Application of different nitrogen doses in fertirrigated cacao seedlings Aplicação de diferentes doses de nitrogênio em mudas de cacaueiro fertirrigadas Aplicación de diferentes dosis de nitrógeno en plántulas de cacao fertirrigadas

The Medicilândia municipality is the main dried cocoa beans producer in Pará state, however, researches are being carried out in relation to the seedlings production. The study objective was to study the cacao seminal seedlings growth under a drip irrigation system, receiving different nitrogen doses with fixed doses of phosphorus (4,5 g) and potassium (3 g) through fertirrigation. The experiment was realized under greenhouse conditions with four treatments and five repetitions in a design of randomized blocks, being each composed by seven plants. Thus, T1 = check, without fertilization; T2 = 2 g of N + P and K; T3 = 7 g of N + P and K; T4 = 12 g of N + P and K. The treatments were scientific from the 31st to the 156th day after sowing, where the values of height, collar diameter and leaves number were obtained fortnightly. On the 156th day, the plants were harvested and obtained the root length, fresh matter and aerial part dry matter and root and the nutritional contents from the leaf and root plant tissue. The plants obtain results with the highest values of height, collar diameter and leaves number in the T2 treatment.


Introduction
The Medicilândia municipality, located in the Transamazônica region, Pará state, is known by its high potential in the dried cocoa beans production for chocolate and cocoa butter manufacture. According to the Geography and Statistics Brazilian Institute (IBGE, 2019) the dried cocoa beans average production in the county in 2019 was around 44.738 tons, equivalent to more than 34% from paraense production.
Cocoa is typical of tropical climate and, therefore, very demanding in water, the crop demands high rainfall with about 1.200 mm per year, distributed in 100 mm Monthly (Souza et al., 2009).
Regarding to cocoa mineral nutrition, it is known that the requirement for nitrogen (N) and potassium (K) is high in the fruiting phase and contributes to the production increase, nevertheless, there are few studies in the seedlings production phase (Sodré et al. 2012;Almeida et al. 2014). In addition, for the cacao effective fertilization, the water correct management so that the nutrients are solubilized, since most part of these nutrients are absorbed through mass flow. At the same time, it is common leaching losses, causing damage to the environment and production costs increase (Silva et al., 2017).
The fertirrigation seeks to unify the benefits of water supply and nutrients supply to crops, with the purpose of reducing labor costs and enabling the fertilizers use in irrigated agriculture, as it increases the efficiency of its use (Coelho et al., 2010). This technique basically consists in the combined fertilizers usage with the irrigation process, in this way the manures are injected into the water forming what it is called "enriched irrigation water".
Thus, this work aimed to evaluate the cacao seminal seedlings growth under irrigation system located by drip, receiving different nitrogen doses with fixed phosphorus and potassium dosages through fertirrigation.

Methodology
The experiment was conducted from April 25th to September 28th 2019 at Alvorada Farm, located at Km 80 South of the Transamazônica highway (BR-230) towards Altamira-Itaituba, under coordinates 3°27'11" S and 52 °48'17" W and 270 meters of altitude ( Figure 1). The region presents humid tropical climate, with dry season, Am type, characterized by average annual precipitation above 1.500 mm and average temperature above 18 °C in the coldest month of the year (Koppen, 1936).
It was assembled a greenhouse type nursery 8 x 11 m and covered with agrofilm (200 microns) and and a shading screen (50%). For the irrigation system, it was measured the flow of 2 random drippers for each of the 6 lateral lines, checking the collected water volume in 10 minutes. It was found that the average flow of each individual dripper individually corresponds to 0,172 L/h and for the whole system it corresponds to 41,28 L/h, including the edges, with C.V. = 9,45%. The average operating pressure for the entire system was 2 mca.
At the beginning of each lateral line, it was placed a register that allowed the individual opening during the nutritional solution application, which it was prepared from the granulated fertilizer grinding and dissolved in water. The fertilizer injection into the system was done through the Venturi tube method in an easel. The daily water blade applied was equivalent to the local average daily potential evapotranspiration (ETo) for the referred month in which the experiment was located. For sowing and fertilizer applications the soil was raised to field capacity. The used seeds for the seedlings production were taken from a single adult plant (matrix) in fructification phase, which were germinated in sawdust until transplanted into plastic bags of 2,2 dm³.
The seedlings were conducted only with irrigation up to 30 days. The experimental design was in randomized blocks with four treatments and five blocks (repetitions). The treatments differ by the applied N doses, keeping phosphorus and potassium constant at the doses of 4,5 g of P and 3 g of K. Thus, T1 = check, without fertilization; T2 = 2 g of N + P and K; T3 = 7 g of N + P and K; T4 = 12 g of N + P and K. The nutrients doses correspond to 10%, 5% and 10% of the recommended Research, Society andDevelopment, v. 10, n. 11, e136101119396, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i11.19396 4 elements for the culture's first year, N, P and K, respectively, according to the used soil characteristics (EMBRAPA, 2007).
The seedlings were daily irrigated and the nutritional solution applications occurred weekly, being the apportioned doses so that they reached the defined value for each treatment at the end of the experiment. The used elements sources were urea, triple superphosphate and potassium chloride. The values of each repetition are an arithmetic average of 7 plants. The area representative sketch is illustrated in Figure 2. The evaluated parameters were: plants height; collar diameter; leaves number; root system length; root green matter weight and from the aerial part; root dry matter weight and from the aerial part; chemical analysis (macro and micronutrients) from the leaf and root plant tissue.
The data analyses were performed through the statistical analysis software Sisvar 5.6 being the results compared by the Scott-Knott test (1974) at 5% error probability. The doses influence levels in the treatments were compared through regression analysis.

Height
Through the average comparisons analysis by Scott-Knott (1974) (Table 4), the treatments T1 and T2 presented higher average height in relation to the other higher at 58 and 72 days of age. At 86 days, the corresponding dose to the T2 treatment provided greater height. At 156 days of age, the plants from treatment T2 were the ones that presented the highest height values, with an average of 60,28 cm, while the plants from treatment T4 presented the lowest values, with an average of 44,62 cm. Followed values by equal letters in the same column do not differ with each other at p<0.05 by the Scott-Knott test (1974). Source: Authors.
It was observed in this research a faster development in the cacao seedlings height than those found by Santos (2019), for seedlings with the same age. The obtained values for height in this work are also considerably superiors than those found by Souza Júnior and Carmello (2008), who observed the cocoa clonal seedlings behavior under different urea doses, where they reached a maximum height of 14,86 cm with application via substrate and 13,33 cm via leaf. It is noteworthy that the It is noteworthy that the worked doses by the referred authors were lower than those used in this research. In Figure 3, it is possible to notice the visual difference between the treatments. The nitrogen has a structural function in the plant, therefore, essential for its vegetative growth. In addition to participating in the plant vital processes such as photosynthesis and respiration, it has an important role in the ionic absorption from the other elements and in the growth and cell differentiation of the plant tissues (Ferreira, 2019).
In Figure 4, it is shown a quadratic regression graphic showing the N doses influence on plants height at 156 age days.  Source: Authors.
The regression model shows an increasing behavior up to the optimal dose of 2 g per plant, which corresponds to an approximate height of 55,96 cm.
The nitrogen fertilizers excessive usage salines the soil, which can restrict the plants growth, since the mineral ions exceed the nutrient adequate zone, making it difficult the absorption of others or reach levels that come to limit the water availability (Taiz; Zeiger , 2017), a fact that may have contributed to the plants height reduction at higher dosages.

Collar diameter
The average values from the collar diameter are shown in Table 2. It is noticed that the treatments T1 and T2 were the ones that stood out in relation to the others from 86 age days, enduring until the experiment end, where they presented values of 11,13 mm and 11,46 mm, respectively.  (2008), who obtained increments, with a fall tendency with higher doses. However, the obtained values in this work are similar to those found by Santos (2019) for the same seedlings age, who obtained collar diameter values varying between 6,3 and 7,5 mm at 120 DAE (days after emergency).

Figure 5:
Cacao seedlings collar diameter at 156 age days in N doses function.
Source: Authors. Silva et al. (2015) also observed negative effects for the stem diameter variable, in guava rootstocks, attributing this behavior to a nutritional imbalance caused by N excess or a physiological disorder triggered by the urea excess in the water.
For Gonçalves et al. (2015), within the same species, the plants with higher CD present more chances to survive, as they show formation capacity and of new roots growth.
Increasing N doses promoted less seedling development in this work, the same was observed by other authors working with fruit species and native species from Amazon (Silva et al., 2015;Bezerra, 2018).
This study is not enough to affirm that nitrogen is the responsible for the deleterious effect caused on cacao seedlings, however this behavior may have been externalized due to the used high amount of this nutrient for the seedling vegetative stage from the studied culture.

Leaves number
In Figure 6, it is shown a quadratic regression graphic where it shows the N doses influence on the plants leaves number at 156 age days. Although the treatments did not have statistically deferred from each other in terms of leaves number, higher N doses provided yellowing followed by burning in some leaves and reduced size. This demonstrates that even though the plant of the same species does not alter its leaves release, the disturbance in the supply of some essential element causes alterations in the crop normal development, resulting in yellowing symptoms and leaf area reduction (Trevisan et al., 2020). The obtained values for APDM were higher than those found by Dias et al. (2020), which evaluated the cocoa seedlings development with organic fertilizer source supply, however, the RDM values were lower. This confirms that the nutrients supply from chemical sources through fertirrigation stimulates the aerial part rapid development, since these are readily available to plants, while the supply through organic sources especially favors the root development, mainly by the soil physical characteristics improvement.

Root length, fresh matter and dry matter
The lower statistically average observed for fresh matter and dry matter from treatment T4 can be explained by the large consumption of carbohydrates by the plant, since the NH4+ assimilation in high amounts requires large amounts of carbohydrates (Souza & Fernandes, 2018).
The results show a difference between the check and the treatment 4 for the evaluated parameters. These results partially corroborate with those found by Ramos et al. (2014), who did not found any difference for the roots dry mass of cacao seedlings subjected to different concentrations of N and K.
In Figure 7, the quadratic regression graphics are presented that show the N doses influence on RL, APFM, RFM, APDM and RDM at 156 age days. It can be observed a decrease from the variables according to the doses increase. A possible explanation for this effect may be the fact that the urea on the soil rapidly transforms into ammonium (NH4 + ), so that, in increasing concentrations, the seedlings might have absorbed a greater amount of N in the ammoniacal form, causing a toxic effect to the plant as seen by Silva et al. (2015) and Silva et al. (2014), when they evaluated the increasing N doses effect on the dry matter of guava and jackfruit seedlings, respectively.
The N absorption in excess in the ammoniacal form changes the water balance in plants, reducing the water flow from the roots to the plant aerial part, causing wilting (Souza & Fernandes, 2018).
Although the APDM did not diverged between the treatments, the achieved results for this parameter were considerably higher than those found by Souza Júnior and Carmello (2012). This difference may be linked to the N higher amount used, however, other factors such as container, substrate and propagation form (sexual or asexual) may also have contributed to the observed differences.

Nutrients accumulation
The plant nutritional content from the leaf and root components presented oscillation in function of the treatments, showing antagonistic and synergistic interactions from N doses with some elements (Table 4). As shown by the analysis, it is noted that there was the following relation of average concentration from macronutrients of N > K > Ca > Mg > P > S for both leaves and root. The N, P and K, Mg, S and Zn concentrations are similar between the root and leaf components, whereas Ca, Mn and B had higher concentrations on the leaf than on the root, while Fe and Cu showed higher concentrations on the root than on the leaf.
In Table 5, it is shown the leaf's nutritional levels classification according to its appropriate range for the cocoa crop, in accordance with Souza Júnior et al. (2012). As shown by the analysis, the check was the treatment that presented the highest normality in relation to the nutrients adequate levels, while the other treatments had changes in these levels.
It is noteworthy that all the treatments that used fertirrigation showed nutrients equal ranges, with the exception of N, which was the only nutrient applied in different doses.
Analyzing each nutrient separately, it is observed that N had a marked accumulation in the root, as the doses increase.
Despite of showing an increasing behavior on the leaves, it remained in average levels in the treatments T1 and T2, highlighting that the 2g dose was not enough to leave its content above the adequate for the concentration normal range on the leaf tissue.
The P presented high considered values in the treatments with fertirrigation, possibly due to the phosphorus high concentrations present in the application solution. The K presented in low levels for all the treatments, however it was observed its increment as N doses were increased, possibly due to its important action in the N assimilation enzymes activation when NH4 + is in toxic levels in the plant tissues (Souza & Fernades, 2018), having its absorption also increased.
Ca, Mg, Fe and Cu were the most affected nutrients by the treatments. The Ca, despite of presenting adequate values in relation to its content on the leaves, it was strongly affected in the root. The Ca decrease consequences in the roots can be seen visually ( Figure 4) and also in the RFM and RDM values (Table 3), since this nutrient plays an important function on the cell wall and on the growth tissues. Souza and Fernandes (2018) report that the Ca absorption by the roots can be decreased by high concentrations of K + , Mg 2+ , and N-NH4 + . These results corroborate with those obtained by Ramos et al. (2014), who also observed a decrease in Ca and Mg content with the increase from K content on the leaves. obtaining Mg positive accumulation with the N doses increase in acerola seedlings, in this work the Mg nutrient content decreased as N doses were increased, being this behavior observed both in the root and on the leaves. This behavior may possibly be related to the K effect present in the nutritional solution or to a secondary effect of the N doses that promoted greater K absorption, causing the Mg content to reduce, similarly to what was observed by Ramos et al. (2014). The magnesium content reduction can be attributed to competitive absorption between these two elements (Viecelli, 2017). The Mg together with N are the only soil nutrients that are chlorophyll constituents, and its requirement in the cocoa crop is higher than that required by most crops (Souza & Fernandes, 2018).
The Cu values showed an average level in the control treatment, while for the other treatments it was observed a positive increment in the roots and a negative incremente on the leaves. Its deficiency on the leaves can be attributed by this element be one of the main nutrients affected by the NH4 + presence (EMBRAPA, 2020). On the other hand, Fe, despite of a smaller difference between the treatments, it showed average levels only in the control treatment, while for the other treatments this level was high. It is likely that the high concentration of this nutrient in the roots be in function of the soil residues presence, being this behavior also observed for the Mn, which may be related to its abundance in most soils and association with Fe (Souza & Fernandes, 2018).

Conclusion
The N supply via fertirrigation up to 2g dose per plant promoted greater increments in height, collar diameter, fresh and dry root matter and aerial part.
Increasing N doses modify the nutritional absorption of some essential elements, altering their content in the plant tissue and promoting lower biometric parameters development.