Introduction

Globally, onion is the fourth most economically important vegetable crop and the third in terms of production volume. China is the leading producer and in 2013 accounted for 26 % of global production, while Brazil was the ninth largest producer with only 2 % (^{Salvador, 2015}). In Brazil, onion is the third most important vegetable in terms of economic value, surpassed by potato (Solanum tuberosum) and tomato (Solanum lycopersicum) in production volume.

In onion production, variety selection is conditioned initially by the photoperiod and temperature requirements for the bulb formation process, which are specific to each genotype and characteristic of each producing region (^{Menezes & Vieira-Neto, 2012}). Its phenological cycle depends on the variety, climate, and production system. The growth period can be divided into four stages: pre-planting, vegetative growth, bulb formation and maturation (^{Duarte-Costa & Milanez-de Resende, 2007}).

For the selection of the planting area or region, genotype x environment interaction should be considered, relating mainly to the photoperiod and temperature, which are directly affected by the planting dates (^{Faria et al., 2012}).

In Brazil, according to ^{Oliveira, Mendonça, and Santos (2009)}, it is possible to cultivate two type of genotypes: the so-called “short day” ones (11-12 h daylight photoperiod) that can be established in any region of the country, and “intermediate day” ones (12-14 h daylight photoperiod) which are more suited to the southern region.

In relation to the temperature required by the crop, the ideal range is from 20-25 °C. High temperatures (above 32 °C) during the pre-planting stage of development can lead to formation of premature bulbs, which is a disadvantage. By contrast, exposure of plants to prolonged periods of low temperatures (below 10 °C) may induce premature flowering (bolting), which is highly undesirable when pursuing commercial bulb production rather than seed production (^{Vilela-de Resende, Bona-Pires, Pinheiro-Camargo, & Marchese, 2007}).

Onion breeding programs in Brazil have developed hybrids with tropical germplasm that can expand planting areas into regions where there is no history of commercial cultivation. The aim of this is to reduce supply fluctuations throughout the year in the domestic market by increasing production and thereby reducing imports of this crop.

Tocantins is a new agricultural frontier, where annual crops such as corn and soybean are established during the summer in the Brazilian cerrado (second-largest biome in Brazil), and from May to September, either watermelon or melon is grown by producers.

Warm temperatures during the day and moderate during the night can favor the differentiation of onion bulbs that contain tropical germplasm. However, the lack of technical information about the crop in the region makes it necessary to evaluate onion genotypes to create recommendations that may be useful to producers in the region.

In view of these facts, the aim of the present study was to determine the adaptation and yield potential of onion established on three planting dates in the town of Gurupi, Tocantins, Brazil.

Materials and methods

Three experiments were conducted in 2015 in the experimental olericulture plot at the Federal University of Tocantins (UFT for its initials in Spanish), located in Gurupi in the Central-South part of the state of Tocantins (11° 44’ 42” South latitude, 49° 03’ 05” West longitude, 276 masl). Based on the ^{Köppen (1948)} climate classification system, the region has an Aw climate, defined as tropical wet with a rainy season during the summer and a dry season in winter. The soil of the region is a red-yellow lithosol (^{Empresa Brasileira de Pesquisa Agropecuária [Embrapa], 2006}).

During the experiments the observed minimum and maximum temperatures were 16.5 and 38.7 °C, respectively; the mean temperature was slightly variable, between 24 and 29 °C. Rainfall and temperature were obtained from the UFT weather station at the Gurupi campus (Figure 1, ^{Instituto Nacional de Meteorología [INMET], 2015}).

Figure 1 Mean monthly values of minimum, maximum and average temperatures, relative humidity (RH) and total rainfall during the period of the experiments in Gurupi, Tocantins, Brazil, 2015. 

The onion hybrids used in the study were NUN 1205^®, Dulciana^® and Cimarron^®, all obtained from Nunhems Bayer CropScience Vegetable Seeds^®. All these hybrids have a cycle of 90-95 days in warm climates and a photoperiod of 11-12 h daylight. The hybrids were transplanted on May 26, June 26 and July 26. On all planting dates, a completely randomized block experimental design with four replicates was used. The experimental plot consisted of three rows separated by 0.15 m, with 60 plants spaced 0.10 m apart. The 20 plants in the middle row were used as the experimental unit.

The seedlings were produced in 200-cavity expanded polystyrene trays, with each cavity containing commercial Plantmax® substrate (composed of equal parts of pine bark, charcoal, peat moss and vermiculite). The trays were kept in shade houses under sprinkler irrigation. The seedlings were transplanted 30 days after sowing (das).

According to the crop requirements, fertilizer was applied 30 and 50 days after transplanting (dat) with 75 kg∙ha^-1 of N and 75 kg∙ha^-1 of P. Sprinkler irrigation and weed, pest and disease control were carried out according to crop recommendations (^{Duarte-Costa & Milanez-de Resende, 2007}).

In the three experiments, harvesting was carried out when the plants showed advanced signs of senescence, such as yellowing and dry leaves, and when more than 80 % presented soft stem and their leaves were bent or inclined. After harvest, the bulbs were “cured” (dried before storage) for 30 days; then the remains of the roots and aerial parts of the bulbs were removed.

The characters evaluated in the field were:

The evaluated post-harvest characteristics were:

Data were submitted to an individual analysis of variance, followed by a joint analysis. The means were compared with the Tukey test (1949), P ≤ 0.05. Statistical analysis was carried out using SISVAR software version 5.3.(^{Ferreira, 2008}).

Results and discussion

The analysis of variance showed significant effects among the hybrids on the evaluated characters, with the exception of PH and pH, revealing genetic variations among them. In the case of dates, only YIELD, PH, ABW, and BD were affected (P ≤ 0.05, Table 1).

In May (first evaluation), there were significant differences in yield between evaluated dates and hybrids. NUN 1205 presented higher yields (54.22 t∙ha^-1, Table 2); however, in June greater similarity was shown between Cimarron and Dulciana with 54.90 and 58.05 t∙ha^-1, respectively (Table 2).

When the transplant was performed in July, yields decreased, probably due to the occurrence of temperatures (between 15 and 25 °C) higher than the suitable range for these varieties. The highest temperatures, recorded in September at the beginning of bulb differentiation, favor fast formation and maturation (smaller bulbs), resulting in reduced yields (^{Souza & Resende, 2002}). The Cimarron hybrid presented the lowest yield (27.90 t∙ha^-1), being even lower than the average yield in Brazil (28.03 t∙ha^-1 ,^{Instituto Brasileiro de Geografia e Estatística - Levantamento Sistemático da Produção Agrícola [IBGE - LSPA], 2015}).

For each planting date, plant height was similar among the hybrids. On the second date, NUN 1205 presented the greatest height (58.99 cm), differing statistically from the others. In relation to the June transplant, the highest averages (between 51.40 and 58.99 cm, Table 2) of all hybrids was observed.

For average bulb weight, hybrids differed only with respect to the May transplant date, where Cimarron obtained the lowest weight (106.75 g, Table 2). In June, when high commercial prices occur due to the demand for onion to make fast food (^{Baier et al., 2009}), the highest means, 183.00 and 193.50 g, were recorded in Cimarron and Dulciana, respectively.

For the July transplant, there was a decrease in average bulb weight (Table 2), which in the Cimarron hybrid fell from 183 g∙bulb^-1 in June to 93 g∙bulb^-1. In this case, high relative humidity caused by rainfall at the end of the cycle could have caused the cessation of bulb growth.

^{Quartiero et al. (2014}) evaluated onion hybrids in Guarapuava-PR and found a significant reduction in average bulb weight at late transplant dates. This may have been due to variations in photoperiod and temperature during the year causing alterations in characteristics related to bulb yield and premature bulb formation (^{Sirtoli, Furlan, & Rodrigues, 2010}). However, the photoperiod in the experimental area had no significant alterations, so temperature was the factor that contributed the most to the modifications observed.

In the May transplant, no significant statistical differences in pseudostem diameter were observed. This parameter is important since it is related to the photosynthetically active part and the onion bulb. When the plant is exposed to short periods of extreme cold (<6 °C), the pseudostem tends to increase in diameter. For the evaluated dates, the pseudostem had values between 1 and 2 cm in diameter (Table 2), values similar to those found by ^{Kunz et al. (2009}) and considered acceptable to the market.

In terms of number of leaves, observed values ranged between 4.81 for Dulciana and 7.12 for Cimarron (Table 2). The results found in this study are similar to those reported by ^{Cecílio-Filho, Marcolini, May, and Barbosa (2010)} in San José del Rio Pardo-SP, where six and seven leaves were obtained. An unusual characteristic of the onion is that when the growth phase of the bulb begins the growth of the leaves is interrupted. For this reason, the size of the bulb at harvest is highly associated with the size and number of plant leaves; therefore, it is important to maintain a large number of healthy leaves with good development to ensure the production of large bulbs that are suitable for the market (^{Oliveira, 2015}).

According to the methodology proposed by ^{Dhumal, Datir, and Pandey (2007)} to classify the concentration of pyruvic acid, onions are classified on the basis of pungency as low pungency/sweet if the bulbs have values from 0-3 μmol pyruvic acid∙g^-1; medium pungency for 3-7 μmol pyruvic acid∙g^-1 and high above 7 μmol pyruvic acid∙g^-1. The concentrations of pyruvic acid in the hybrid Cimarron were 8.48, 8.91 and 7.87 μmol pyruvic acid∙g^-1, in May, June, and July, respectively (Table 2). These values corresponded to the category of high concentrations.

On the other hand, medium concentrations were obtained by NUN 1205 for all dates, while Dulciana presented medium values in June and high ones in July (Table 2). According to ^{Crowther et al. (2005)}, onions with a low concentration of pyruvic acid are desirable when they are destined to consumption in natura, this being an essential prerequisite for this type of market. However, ^{Souza et al. (2008)} assert that there is a market preference for the consumption of onions with a medium to high concentration of pyruvic acid, which is a classification similar to that observed in the present study.

Among all the post-harvest characteristics of onion, the concentration of pyruvic acid is one of the most important (^{Vilela-de Resende et al., 2010}). The chemical composition and sensory characteristics of the bulb’s taste, color and smell depend mainly on genetic and environmental factors, crop management, nutrition and planting date (^{Randle, 1997}).

A high concentration of pyruvic acid is desirable for the industrialization of onions; this is due to the fact that aromatic compounds are lost during the dehydration processes (^{de Rezende-Chagas, Milanez-de Resende, & Pereira, 2004}).

The pH did not vary significantly between the evaluated dates and hybrids, with the exception of NUN 1205, which had pH values of 4.81 in May and 5.12 in July (Table 2). These values are similar to those found by ^{Schunemann, Treptow, Lopes-Leite, and Vendruscolo (2006)} and ^{Vilela-de Resende et al. (2010)}. In relation to bulb shape, the hybrids did not differ significantly in the transplant dates (60-80 mm, Table 2).

Another important characteristic in the classification of the collected product is the transverse diameter. ^{Souza and Resende (2002)} note that the Brazilian consumer market prefers bulbs of 40-80 mm in size. In the present study, the three hybrids presented values within that range (48.6-68.7 mm, Table 2).

Dulciana presented an average diameter of 55.1 mm in July, differing significantly from the other dates, whereas Cimarron in May had the smallest bulb diameter (48.6 mm). Very Large bulbs are not commercially desirable, obtaining lower prices than those of class III (^{de Oliveira-Rodrigues, Costa-Grangeiro, de Negreiros, da Silva, & Novo-Júnior, 2015}).The ideal genotype for onion cultivation is the one with the highest yield associated with the highest percentage of bulbs belonging to class III (51 and 60 mm) and class III mature (61 and 70 mm), which are preferred by housewives (^{Silva, Teixeira, & Amado, 1991}) and receive the best remuneration during the commercialization process (^{Torre-Figueiredo et al., 2011}).

In the present study, all bulbs had a class III mature rating, which means that the state of Tocantins has the potential for onion production and has an adequate photoperiod (between 11 and 12 hours) for the formation of good quality bulbs for the market.

Conclusions

In the region of Gurupi, Tocantins, Brazil, yields similar to those of other onion-producing regions were observed; therefore, onion production can be recommended in this region.

The most suitable month for transplanting onion in this region is June, which is when the highest yields were obtained. However, the May and June transplants also achieved adequate yields with bulb classifications that satisfy market demands.