GROWTH AND NUTRIENT ACCUMULATION AND EXPORT IN A SHORT-DAY ONION1

- New hybrid onions that are more productive are currently being cultivated. Information on growth and nutrient accumulation must thus be updated to assist in the refinement of existing recommendations, enabling the better exploitation of the productive potential of these new genotypes. We determined the growth of plants and the accumulation and export of nutrients of the short-day onion ‘Soberana’ established by direct seeding. The experimental design was a randomized block with four replicates and the treatments were evaluation times. Leaf number, tissue dry weight, nutrient accumulation, and the maximum daily rate of accumulation were evaluated throughout the growing cycle. The order of nutrient accumulation was (g plant -1 ) K (0.72) > Ca (0.38) > N (0.32) > S (0.14) > P (0.08) > Mg (0.06) and (mg plant -1 ) Fe (2.26) > Mn (1.43) > Cu (0.93) > Zn (0.91) > B (0.85). Macronutrient demand was highest between 61 and 148 days after sowing (DAS), and micronutrient demand was highest between 70 and 148 DAS.


INTRODUCTION
The onion is among the most economically important vegetables in Brazil.Approximately 57 000 ha were cultivated in 2015, producing 1 461 000 t at an average yield of 26 t ha -1 (IBGE, 2016).
The use of hybrids and new cultivation technologies can increase onion yield three-fold above the national average.These new hybrids have better bulb uniformity than open-pollinated cultivars and can thus tolerate high planting densities and are more productive (COSTA et al., 2002).They are also resistant to pests and diseases, adapted to different climatic conditions, and better able to use the available inputs.The plants are consequently better adapted for mass production, which affects their nutritional needs (PURQUERIO; SANTOS; FACTOR, 2016).Adequate plant nutrition can directly improve bulb health, quality and yield (KURTZ; ERNANI, 2010).Characterizing plant growth and nutrient uptake throughout the growing cycle is thus essential for crop planning to maximize productive potential (VIDIGAL et al., 2009;PURQUERIO, 2010).
Monitoring the uptake of nutrients can identify the periods of higher nutritional requirement and dry-matter production and provide reliable information on the most convenient times of fertilizer application, avoiding the possible deficiency or over-consumption of some nutrients (HAAG; MINAMI, 1988;FURLANI;PURQUERIO, 2010).Information about nutrient accumulation and export by onions under various cultivation conditions and for various genotypes should thus be used as references for defining the management of soil fertility.
Some information is available about the accumulation of nutrients by hybrids and open-pollinated onion cultivars in Brazil, from studies with hybrids Optima (PÔRTO et al., 2006) and Superex (PÔRTO et al., 2007); the cultivars Alfa Tropical (VIDIGAL; MOREIRA; PEREIRA, 2010), IPA 11, and Texas Grano 502 (AGUIAR NETO et al., 2014); and recently the hybrid Aquarius (MORAES et al., 2016) and the cultivar Bola Precoce (KURTZ et al., 2016).The results of these studies, however, indicated large differences in the quantities and proportions of nutrients accumulated by the plants due to the genotypic variations of each cultivar, environmental conditions, and yield increase (FERNANDES; SORATTO; SILVA, 2011).Constantly updating nutrient-uptake studies of new genotypes on the market throughout the growing cycle is therefore essential (FACTOR et al., 2018).This information is essential for onion crops to assist in the amendment of existing fertilizer recommendations.The aim of this study was thus to monitor plant growth and the accumulation and export of nutrients for 'Soberana' onions.
The maximum, mean, and minimum air temperatures during the experimental period were 27.4,18.4, and 10.6 °C, respectively (Figure 1A).Total rainfall was 270.4 mm (Figure 1B).The experimental design was randomized blocks with four replicates.The treatments were evaluation periods 36, 50, 64, 78, 92, 106, 120, 134, and 148 days after sowing (DAS).Each block consisted of a bed 30 m long and 1.1 m wide.Two additional beds were prepared as borders along the length of the plots.The total experimental area was 270 m 2 .
We used the hybrid onion Soberana (Agristar), characterized by round and yellow bulbs approximately 60.0 mm in diameter and a medium weight of 165.0 g.The onion is planted in March and April in SP and in April and May in southern Brazil (AGRISTAR, 2015).The onions were mechanically planted on 02 April 2014 in beds in four lines with row and plant spacings of 0.30 and 0.04 m (25 plants m -1 ), respectively.The plants were thinned 22 DAS to one plant every 0.08 m (12 plants m -1 , 320 000 plants ha -1 ).The plants were watered by drip irrigation with one line between onion rows and 20 cm between emitters.Phytosanitation was performed as needed.
Samples were collected at intervals of 14 days beginning on 07 May (36 DAS).The numbers of leaves and bulbs, root dry mass (DM), nutrient accumulation, and yield were evaluated.Diagnostic shoots were collected at 78 DAS for the analysis of tissue nutrient contents (TRANI; RAIJ, 1997).
We collected 200 and 80 plants from the two central lines of each plot in the first and second evaluations, respectively, 10 plants from the third to seventh evaluations, and four plants from the eighth and ninth evaluations.Six plants were left as a border for the subsequent collections.The last evaluation was at 148 DAS, when more than 60% of the plants were ready for harvesting (leaves clicked down).
The collected plants were washed with water and detergent and separated into leaves, bulbs, and roots, which were then dried in a forced-air circulation oven at 60 °C to a constant dry weight.The dry material was weighed and chemically analyzed to determine the nutrient (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) contents of the tissues (leaves, bulbs, and roots) as described by Malavolta, Vitti and Oliveira (1997).The root system was collected in a soil volume of approximately 0.2 × 0.2 × 0.2 m.Nutrient accumulation was calculated by multiplying the content of each nutrient in each tissue (leaves, bulbs, and roots) by the DM of each tissue.The total accumulation of each nutrient in the plant was determined by the sum of the accumulations in the tissues.The data for nutrient accumulation were analyzed using a non-linear three-parameter regression model defined by the best statistical fit (F test) and the coefficient of determination (R 2 ).SigmaPlot 12.5 (Systat Software, USA) was used for the analyses.
Nutrient daily accumulation rates (NDARs) were obtained by the difference between adjusted accumulations for two consecutive days.The inflection points of the adjusted curves corresponded to the times of maximum NDARs.The periods of highest accumulation of dry mass and nutrients were determined by the minimum and maximum curve points in sigmoidal models calculated using the method described by Venegas, Harris and Simon (1998).The export of nutrients was calculated by multiplying the nutrient accumulations in the bulb at 148 DAS by the total number of plants ha -1 (320 000).

RESULTS AND DISCUSSION
The number of leaves increased from the beginning of the growing cycle until harvest (12 leaves) at 148 DAS (Figure 2).Number of leaves is a phenological characteristic that can be used instead of DAS to monitor plant development over time.It can thus be used to plan nutrient distribution during growing seasons and in regions where environmental conditions affect the duration of the growing cycle (MORAES et al., 2016).
The accumulation of DM (leaves, bulbs, and roots) by the plants was low until 64 DAS, representing only 7% (2.4 g plant -1 ) of the total of 36.8 g plant -1 at 148 DAS (Figure 2).DM accumulation was highest from 81 to 148 DAS, at 85% (31.4 g plant -1 ) of the estimated total.Plant DM increased significantly when 'Soberana' had eight leaves, so we inferred that the plants required a vegetative canopy with 72% of the leaves present at the end of the growing cycle to initiate the largest increase in DM.The estimated foliar DM accumulation was highest (17.0 g plant -1 ) at 148 DAS.Accumulation was only 12% (2.1 g plant -1 ) of the maximum estimated by 73 DAS.DM accumulation then intensified until 117 DAS, totaling 76% (15.1 g plant -1 ).The accumulation then slowed and tended to stabilize.Similar results of slow foliar DM accumulation until 70 DAS were reported for 'Optima' and 'Superex', with subsequent acceleration until 110 DAS and then stabilization (PÔRTO et al., 2006(PÔRTO et al., , 2007)).
N accumulation by 'Soberana' was highest (0.32 g plant -1 ) at 148 DAS.The period of highest accumulation from 61 to 135 DAS corresponded to 81% of the total (0.26 g plant -1 ) (Figure 3A).This period comprised the intensification of foliar (73-117 DAS) and bulb (after 106 DAS) DM accumulation.The plants at this stage had a greater demand for nutrients, probably due to the increase in the metabolic activity associated with cell division for forming new tissues (BREWSTER, 1994).The estimated P accumulation was highest (0.08 g plant -1 ) at 148 DAS.The plants accumulated 79% (0.06 g plant -1 ) of the total from 72 to 129 DAS, the period of the highest demand (Figure 3A).P plays a key role in cell division, sexual reproduction, and plant metabolism, so it is essential for the growth of roots and shoots (THOMAZELLI et al., 2000).
The estimated Mg accumulation (0.06 g plant -1 ) was highest at 148 DAS.The requirement for Mg was highest from 83 DAS until the end of the cycle and was equivalent to 84% (0.05 g plant -1 ) of the highest estimate (Figure 3A).The maximum requirement began during the acceleration of foliar DM accumulation (73-117 DAS) and could be attributed to the participation of Mg in the structure of chlorophyll and the activation of enzymes in photosynthetic reactions (EPSTEIN;BLOOM, 2006).
The estimated accumulation of the micronutrient B was 0.85 mg plant -1 at 148 DAS.The requirement began after 81 DAS and continued to increase until 148 DAS (Figure 3B).The plants accumulated 86% (0.73 mg plant -1 ) of the total during this period (bulbification).The most important B functions are associated with cellular structure (EPSTEIN;BLOOM, 2006).We therefore inferred that B participated in the formation of the bulb peal, causing a demand for this nutrient until the end of the growing cycle.
Cu accumulation (0.93 mg plant -1 ) was highest at 148 DAS.The period of its highest requirement (59% of the total, 0.55 mg plant -1 ) began at 112 DAS, later than those for the other micronutrients, and its accumulation continued to increase until harvest (Figure 3B).Cu accumulation increased throughout the growing cycle and was highest at the end, similar to 'Alfa Tropical' (VIDIGAL; MOREIRA; PEREIRA, 2010).
Mn accumulation was highest from 70 DAS until harvest, with an 86% (1.23 mg plant -1 ) increase relative to the highest estimated value (1.43 mg plant -1 ) (Figure 3B).Mn plays an important role in photosynthesis (EPSTEIN;BLOOM, 2006), so its maximum requirement begins shortly before the acceleration of foliar DM accumulation (73 to 117 DAS), as also observed for 'Aquarius', in which 84% of the accumulated Mn was in the leaves (MORAES et al., 2016).
Nutrients with daily accumulation rates can be distributed throughout the growing cycle so they can be available at times of high demand, thereby avoiding deficiencies and surpluses.The NDARs for the various nutrients were notably not highest at the same time, because the need for each nutrient varies with plant growth and tissue formation.Determining NDARs and identifying the periods of maximum requirement for each genotype is thus important for correct plant nutrition.The nutrients extracted by 'Soberana' (kg ha -1 ) at the end of the growing cycle are shown in Table 1.Some nutrients are returned to the soil by foliar decomposition, and some are removed in the bulbs (export).Quantifying the accumulation of nutrients in the harvested tissues (export) is thus important for evaluating their removal from the crop area.

Figure 1 .
Figure 1.Maximum, mean, and minimum air temperatures (A) and rainfall (B) from April to August 2014.

Figure 2 .
Figure 2. Accumulation of dry mass (DM) and number of leaves (NL) for the 'Soberana' onion during the growing cycle.The error bars represent mean standard errors.

Figure 3 .
Figure 3. Accumulation of macronutrients (A) and micronutrients (B) for the 'Soberana' onion during the growing cycle.The error bars represent mean standard errors.

Figure 4 .
Figure 4. Daily accumulation rates of macronutrients (A) and micronutrients (B) for the 'Soberana' onion during the growing cycle.

Table 1 .
Nutrient extraction by plants, export in bulbs, and percentage of the export as a function of extraction (E/E) for the 'Soberana' onion at the end of the growing cycle (148 DAS).