Treating Garlic Seed Cloves with Negative Temperatures Increases the Productivity of Noble Garlic in the Brazilian Semi-Arid Region

Abstract

The treatment with low temperatures, for a certain period, stimulates the accumulation of phytohormones in garlic seed cloves, which influence the growth and development of the plants. Thus, this work aimed to evaluate the effects of garlic seed cloves under negative temperatures for different periods on the development and productivity of noble garlic cultivated in the Brazilian semi-arid region. Two experiments were carried out and analyzed separately in the municipalities of Mossoró and São Miguel, RN, Brazil, in which three temperature ranges (−5 to −3 °C, −2 to 0 °C and 1 to 3 °C) and three periods (50, 55, and 60 days) were tested. Low-temperature conditioning of the garlic seed cloves was carried out in a temperature-controlled freezer. Relative humidity during treatment ranged from 60 to 70%. Seed cloves submitted to temperatures below zero provided the differentiation of cloves, even in unfavorable edaphoclimatic conditions for cultivating noble garlic. Seed cloves conditioned with negative temperatures increased the productivity of garlic grown in Mossoró, but with a lower average yield than garlic produced in São Miguel. Garlic seed cloves treated at negative temperatures in São Miguel provided garlic productivity equivalent to the yield of garlic grown in the states of the southern region. In addition, it produced class 6 and 7 bulbs, which have greater commercial value and acceptance by consumers. The results demonstrated the potential of garlic seed cloves treated with temperatures below zero to increase the production of noble garlic in the semi-arid region of the country.

1. Introduction

Garlic (Allium sativum L.) is one of the world’s most produced and consumed vegetables [1], having great economic and social relevance for small producers in Brazilian family farming. Additionally, human food consumption stands out for its nutritional value and can be consumed in nature or as condiments and industrialized products. In addition, it has medicinal properties in preventing various cardiac and circulatory diseases.

In 2021, Brazil produced 167 thousand tons of garlic, with an average yield of 12,797 kg ha⁻¹ [2]. According to the National Association of Garlic Producers, the Brazilian population consumes 300,000 tons of this vegetable per year (1.5 kg habitant⁻¹ year⁻¹) [3]. When analyzing the amount of garlic produced and consumed in Brazil, it is necessary to import around 133 thousand tons from other countries, almost 45% of the garlic consumed, to meet domestic demand. The other 55% are imported mainly from China and Argentina [4].

Culturing this vegetable in places where production is not very expressive constitutes one of the alternatives to increase garlic production in Brazil and reduce external dependence. However, to fully develop, noble garlic requires low temperatures and a long photoperiod [5], with mild winters and sunny, dry summers [6]; that is, climatic factors directly influence productivity and the final economic result of the crop [7]. Thus, cultivation in regions with high temperatures, such as the country’s semi-arid region, has limitations for developing garlic plants.

In Brazil, treating the garlic seed cloves with low temperatures expanded the regions and planting times of noble garlic cultivars, previously restricted to the country’s south. During treatment, the bulb (seed clove) is stored in a cold chamber with temperatures ranging from 3 to 5 °C for 40 to 60 days and relative humidity between 70% and 80% [4]. The exposure of seed cloves to low temperatures alters their hormonal balance [8], stimulating the accumulation of hormones that induce morphological changes [9] and promoting plant development in the field.

With this treatment, it was possible to produce garlic in places not possible before [7], for example, in the Brazilian semi-arid region [9,10], whose thermo-photoperiodic conditions are unfavorable for crop development. Several studies report the effect of garlic seed clove treatment using temperatures between 0 and 5 °C on garlic plants’ growth, development, and productivity [4,11,12,13]. Treatment with negative temperatures also favors garlic development, providing greater plant height, bulb ratio, and number of leaves [5], with significant increases in the production and quality of bulbs [14]. Given this, the treatment of garlic seed cloves with temperatures below zero potentially increases the production of noble garlic, favoring the development of the culture.

However, depending on where garlic is grown, the physiology and morphological variability of the plants are affected, as well as the productivity and quality of the bulbs [15]. By reducing the treatment temperature, we expect to reduce the duration of the treatment, maintaining or improving the benefits of this technique on the productive performance of the crop, among them the increase in productivity and the diameter of the bulbs to commercial standards. Thus, this work aimed to evaluate the effects of treating garlic seed cloves using negative temperatures for different periods on the development and productivity of noble garlic cultivated in the Brazilian semi-arid region.

2. Materials and Methods

2.1. Location and Characteristics of the Experimental Areas

Garlic seed cloves were planted simultaneously from March to October 2022 at Mossoró and São Miguel municipalities in Rio Grande do Norte, Brazil. In Mossoró (5°03′37″ S, 37°23′50″ W, altitude 72 m), the seed cloves were planted at the Rafael Fernandes Experimental Farm of the Universidade Federal Rural do Semi-Árido (UFERSA), located in the district of Lagoinha. Temperature, relative humidity, and precipitation were monitored through meteorological stations near the study area. The average temperature was 25.8 °C, with a minimum of 22.4 °C and a maximum of 28.1 °C, with an average relative humidity of 80% (Figure 1). According to the Köppen classification, the climate in the region is of the BSh type, that is, dry and very hot, with an annual precipitation of around 670 mm [16]. The soil in the experimental area was classified as ARGISSOL RED-YELLOW [17].

In São Miguel, the seed cloves were planted at Fazenda SGagro (6°16′01″ S, 38°31′45″ W, altitude 602 m). The average temperature throughout the experiment was 23.8 °C, varying between 20.4 and 27.1 °C, with an average relative humidity of 70% (Figure 1). Meteorological stations near the experimental area monitored climatic data. According to the Köppen classification, the climate in the region is type Aw, with a clear dry season in winter, from May to October, and a rainy season in summer, from November to April, with annual precipitation around 960 mm [16]. The soil of Fazenda SGagro was classified as HAPLIC CAMBISOL [17].

Before the implementation of the experiments, soil samples were taken in the experimental areas at a depth of 0 to 20 cm for the chemical and physical characterization of the soil (

Table 1. Chemical and physical characterization of soils in experimental areas, at 0–20 cm depth, in Mossoró and São Miguel, RN, 2022.

pH	EC	OM	P	K+	Ca2+	Mg2+	Al3+	H + Al	SB	T	V	Sant	Clay	Silt
H2O	dS m−1	g kg−1	mg dm−3		cmolc dm−3						%	%
Mossoró
6.2	0.2	8.5	8.4	39.1	1.3	0.6	0.0	0.7	2.0	2.7	74.1	90.0	5.0	5.0
São Miguel
7.33	0.56	18.9	23.6	375.7	6.84	2.18	0.0	0.43	10.02	10.45	95.88	53.8	31.0	15.2

EC = electrical conductivity; OM = organic matter; SB = sum of bases; T = CEC at pH 7.0 = cation exchange capacity; V = base saturation.

), according to the methodology described by Teixeira et al. [18].

2.2. Treatments and Experimental Design

The experimental design was complete randomized blocks in a 3 × 3 factorial scheme with four replications: three temperatures to treat garlic seed cloves (−5 to −3 °C, −2 to 0 °C, and 1 to 3 °C), with three periods (50, 55, and 60 days). The experimental plot consisted of six rows of plants, spaced 0.20 × 0.10 m, with a total area of 1.8 m². The four central rows were considered useful for data collection, disregarding the two plants at each end of the plot.

According to the treatments, low-temperature treatment of the garlic seed cloves was applied in a temperature-controlled freezer using a thermostat. Relative humidity during treatment ranged from 60 to 70%. The periods were established by placing the bulbs in the freezer at different times so that planting occurred on the same date. We studied the cultivar Ito (noble garlic) produced on the Fazenda SGagro, São Miguel, RN, Brazil.

2.3. Installation and Conduction of Experiments

Soil preparation consisted of plowing and harrowing, followed by raising the beds. Fertilization was performed based on soil analysis and recommendation by Holanda et al. [19], using at planting, in the Mossoró experiment, 15 t ha⁻¹ of organic compost and 180 kg ha⁻¹ of P₂O₅, and in the São Miguel experiment, 15 t ha⁻¹ of organic compost, 180 kg ha⁻¹ of P₂O₅, 20 kg ha⁻¹ of N, 40 kg ha⁻¹ of K₂O, 112 kg ha⁻¹ of Ca, 73 kg ha⁻¹ of S, 12 kg ha⁻¹ of Zn and 1.65 kg ha⁻¹ of B.

In topdressing, fertilization was carried out weekly via irrigation water from 22 to 90 days after planting (DAP). A drip irrigation system consisting of three hoses was installed in each block, with emitters spaced every 0.30 m and a flow rate of 1.5 L h⁻¹. We applied 79.0 kg ha⁻¹ of N, 60.0 kg ha⁻¹ of K₂O, 26.0 kg ha⁻¹ of S, 14.3 kg ha⁻¹ of Mg, 5.0 kg ha⁻¹ of Ca, 13.5 kg ha⁻¹ of Zn, 2.1 kg ha⁻¹ of B and 0.60 kg ha⁻¹ of the commercial formulation Rexolin^® BRA (11.6% K₂O, 1.28% S, 0.86% Mg, 2.1% B, 0.36% Cu, 2.66% Fe, 2.48% Mn, 0.036% Mo and 3.38% Zn).

After treatment, the seed bulbs were threshed and sorted by size in sieves with meshes of 10 × 20 mm (size 2), 8 × 17 mm (size 3), and 5 × 17 mm (size 4). [20]. To ensure excellent plant uniformity, size 2 and 3 cloves were planted in the first three blocks and size 4 cloves in block four. The cloves were planted at a depth of 2 to 3 cm.

The irrigation system used was a micro sprinkler, with emitters spaced every 3.0 m and a flow rate of 27 L h⁻¹. The irrigation depths were estimated, considering the type of soil, stage of crop development, and the region’s climate. During the clove differentiation period in Mossoró (30 DAP) and São Miguel (35 DAP), the irrigation depth was reduced by 50% for ten days to reduce the incidence of sprouting. Nitrogen fertilization was also suspended during this period, with the same purpose of reducing plant overgrowth.

During experiments, manual weeding was carried out to control spontaneous vegetation and spraying with products based on dithiocarbamate, difenoconazole, azoxystrobin, and copper oxychloride for the prevention and control of purple spot and bacterial diseases. To control pests such as thrips and caterpillars, insecticides based on chlorfenapyr, imidacloprid, and beta-cyfluthrin were used.

Harvesting was carried out when approximately 2/3 of the leaves of the garlic plants were yellow or dry. The Mossoró experiment occurred at 78 DAP for treatments with positive temperatures, 102 DAP for treatments with negative temperatures, and in São Miguel, 90 and 126 DAP for treatments with positive and negative temperatures.

The curing process was initiated in the field, where the plants remained for three days. Subsequently, the plants were placed in a room and cured in the shade for 12 days. The cleaning was done to clean the bulbs, eliminating the roots, leaves, and dry and dirty tunics.

2.4. Variables Analyzed

The variables analyzed were percentage of plants with sprouting, counted at harvest, by the ratio between the number of over-grown plants and the number of normal plants; percentage of plants with formed bulbs, estimated after cleaning the bulbs, from the ratio between the number of plants with formed bulbs and the total number of plants; number of bulblets per bulb, performed after cleaning, in ten commercial bulbs of the useful area of the plot; classification of bulbs based on cross-sectional diameter into class 3 (>32 to 37 mm), class 4 (>37 to 42 mm), class 5 (>42 to 47 mm), class 6 (>47 to 56 mm) and class 7 (>56 mm) [21]; the bulbs of each class were weighed, and the data were expressed as a percentage of each class in relation to the commercial production of bulbs; commercial productivity was obtained considering the bulbs not sprouted and with a transversal diameter > 32 mm; non-commercial productivity was obtained regarding the undifferentiated, sprouted bulbs with a transversal diameter < 32 mm; and total productivity was obtained by the sum of commercial and non-commercial productivity.

2.5. Statistical Analysis

Data were submitted to analysis of variance for each experiment separately, with means compared by Tukey Test (p ≤ 0.05), with the aid of the statistical program SISVAR^®, version 5.4 [22]. The data normality was confirmed before performing the Tukey test to guarantee results validity.

3. Results

3.1. Differentiation of Cloves and Crop Cycle

The differentiation of cloves in Mossoró occurred between 30–40, 40–50, and 50–60 DAP, for the temperatures of 1 to 3 °C, −2 to 0 °C and −5 to −3 °C, respectively. This was observed in São Miguel around 35–45, 50–60, and 60–70 DAP for the same temperatures. Among the treatments studied, those with negative temperatures stood out in both experimental sites, postponing the onset of bulbar differentiation and consequently increasing the crop cycle. Bulbs subjected to negative temperatures originated plants that remained longer in the field, about 24 (Mossoró) and 37 days (São Miguel) longer than plants whose bulbs were submitted to positive temperature (

Table 2. Description of the beginning and end of clove differentiation and the noble garlic cycle, cv. Ito obtained from seed cloves stored at low temperatures for different periods. Mossoró and São Miguel, RN, 2022.

Site	Temperature (°C)	Period (Days)	Differentiation (Days)		Cycle (Days)
			Start	End
Mossoró	−5 to −3	50	50	60	102
		55
		60
	−2 to 0	50	40	50	102
		55
		60
	1 to 3	50	30	40	78
		55
		60
São Miguel	−5 to −3	50	60	70	126
		55
		60
	−2 to 0	50	50	60	126
		55
		60
	1 to 3	50	35	45	90
		55
		60

).

3.2. Sprouting Percentages and Plants with Formed Bulbs

For the sprouting, a significant interaction existed between period and temperature in both experimental sites (

Table 3. Percentage of sprouted plants and bulbs formed from noble garlic, cv. Ito obtained from seed cloves stored at low temperatures for different periods. Mossoró and São Miguel, RN, 2022.

Site	Period (Days)	Sprouting (%)			Plants with Formed Bulbs (%)
		Temperature (°C)			Temperature (°C)
		−5 to −3	−2 to 0	1 to 3	−5 to −3	−2 to 0	1 to 3
Mossoró	50	3.42 ± 1.36 Aa	2.14 ± 0.42 Ab	0.00 ± 0.00 Aa	72.31 ± 4.40 Bb	81.06 ± 1.27 ABa	90.52 ± 0.46 Aa
	55	1.70 ± 0.70 Aa	0.00 ± 0.00 Ab	0.00 ± 0.00 Aa	83.96 ± 2.69 Ba	79.92 ± 3.13 Ba	97.86 ± 0.44 Aa
	60	2.09 ± 1.05 Ba	7.80 ± 3.23 Aa	0.00 ± 0.00 Ba	79.41 ± 4.56 Bab	74.00 ± 2.65 Ba	98.18 ± 1.82 Aa
FT		6.51 **			39.49 **
FP		4.14 **			3.62 *
FPxT		4.24 **			2.77 *
CV (%)		121.98			6.46
São Miguel	50	6.24 ± 1.21 ABb	10.46 ± 4.20 Aa	0.86 ± 0.86 Ba	81.50 ± 2.55 Bb	86.97 ± 3.43 Ba	100.00 ± 0.00 Aa
	55	16.23 ± 3.27 Aa	9.01 ± 2.39 Aa	0.00 ± 0.00 Ba	84.26 ± 4.06 Bb	85.29 ± 3.94 Ba	100.00 ± 0.00 Aa
	60	4.25 ± 1.46 Ab	3.03 ± 0.83 Aa	0.42 ± 0.42 Aa	94.02 ± 0.88 Aa	84.05 ± 2.74 Ba	100.00 ± 0.00 Aa
FT		13.70 **			30.40 **
FP		5.70 **			1.42 ns
FPxT		3.44 *			2.80 *
CV (%)		75.79			5.61

Equal letters do not differ statistically by the Tukey Test. Lowercase letters compare columns, and uppercase letters compare rows. ns = not significant; * = Tukey test at 5% probability; ** = Tukey test at 1% probability; F_T = F calculated for temperature; F_P = calculated F for period; F_PxT = F calculated for interaction of the temperature with period; CV = coefficient of variation.

). In the Mossoró experiment, the highest percentage of plants sprouted was observed in the treatment with a temperature of −2 to 0 °C for 60 days. No sprouted plants were observed in treatment with positive temperature, regardless of the period. In São Miguel, a higher percentage of sprouting occurred in treatments with temperatures below zero, with the highest value verified at −3 to −5 °C for 55 days.

A significant interaction was observed between the temperature and period for the number of plants with formed bulbs in Mossoró and São Miguel (Table 3). In both locations, the highest percentages of plants with formed bulbs, regardless of the period, were observed when the seed cloves were treated with positive temperatures. Seed cloves treated at −5 to −3 °C for 55 days (Mossoró) and 60 days (São Miguel) also provided high amounts of plants with formed bulbs, 84 and 94%, respectively.

3.3. Number of Cloves per Bulb

Low temperatures influenced the number of cloves per bulb. In São Miguel, the positive temperature provided bulbs with the most cloves (10.8 bulblets per bulb). In Mossoró, the highest number of cloves per bulb was observed in treatments at negative temperatures (

Table 4. Number of bulbs per bulb of noble garlic, cv. Ito obtained from seed cloves stored at low temperatures. São Miguel and Mossoró, RN, 2022.

Site	Temperature (°C)	Number of Cloves per Bulb
Mossoró	−5 to −3	6.93 ± 0.11 a
	−2 to 0	7.05 ± 0.25 a
	1 to 3	6.13 ± 0.12 b
FT		8.14 **
FP		0.83 ns
FPxT		0.81 ns
CV (%)		9.1
São Miguel	−5 to −3	10.06 ± 0.19 b
	−2 to 0	10.20 ± 0.20 ab
	1 to 3	10.80 ± 0.17 a
FT		5.04 *
FP		2.85 ns
FPxT		1.14 ns
CV (%)		5.87

Equal letters in the columns do not differ statistically by Tukey Test. ns = not significant; * = Tukey test at 5% probability; ** = Tukey test at 1% probability; F_T = F calculated for temperature; F_P = calculated F for period; F_PxT = F calculated for interaction of the temperature with period; CV = coefficient of variation.

).

3.4. Bulb Productivity and Classification

The temperature and period interaction in both locations influenced the commercial and total bulbs’ yield. For non-commercial productivity, there was a significant interaction of factors in Mossoró and temperature and period separately in São Miguel (

Table 5. Commercial, non-commercial, and total productivity of noble garlic, cv. Ito obtained from seed cloves stored at low temperatures for different periods. Mossoró and São Miguel, RN, 2022.

Site	Period (Days)	Temperature (°C)
		−5 to −3	−2 to 0	1 to 3
		Commercial productivity (t ha−1)
Mossoró	50	2.88 ± 0.30 Aa	3.25 ± 0.44 Aa	1.41 ± 0.48 Bb
	55	3.45 ± 0.22 Aa	3.58 ± 0.40 Aa	1.38 ± 0.42 Bb
	60	3.36 ± 0.64 Aa	2.75 ± 0.24 Aa	2.85 ± 0.52 Aa
FT			18.25 **
FP			1.81 ns
FPxT			4.18 *
CV (%)			22.49
		Non-commercial productivity (t ha−1)
Mossoró	50	1.44 ± 0.07 Aa	1.65 ± 0.20 Aab	1.72 ± 0.19 Aa
	55	1.51 ± 0.12 Aa	1.37 ± 0.19 Ab	1.78 ± 0.07 Aa
	60	1.54 ± 0.24 ABa	1.95 ± 0.16 Aa	1.20 ± 0.16 Bb
FT			0.86 ns
FP			0.13 ns
FPxT			4.21 *
CV (%)			18.80
		Total productivity (t ha−1)
Mossoró	50	4.32 ± 0.20 Aa	4.90 ± 0.30 Aa	3.14 ± 0.30 Bb
	55	4.96 ± 0.14 Aa	4.95 ± 0.30 Aa	3.16 ± 0.30 Bb
	60	4.90 ± 0.43 Aa	4.70 ± 0.12 ABa	4.05 ± 0.12 Ba
FT			36.77 **
FP			2.87 ns
FPxT			2.81 *
CV (%)			10.19
		Commercial productivity (t ha−1)
São Miguel	50	6.68 ± 0.31 ABb	7.63 ± 0.26 Aa	6.27 ± 0.27 Ba
	55	6.26 ± 0.69 Ab	6.15 ± 0.39 Ab	5.96 ± 0.28 Aa
	60	8.14 ± 0.23 Aa	7.58 ± 0.55 Aa	6.04 ± 0.49 Ba
FT			8.01 **
FP			8.10 **
FPxT			2.98 *
CV (%)			10.33
		Total productivity (t ha−1)
São Miguel	50	7.64 ± 0.14 Ab	8.48 ± 0.16 Aa	6.52 ± 0.21 Ba
	55	7.64 ± 0.62 Ab	7.32 ± 0.26 Ab	6.29 ± 0.17 Ba
	60	8.71 ± 0.26 Aa	8.34 ± 0.46 Aa	6.33 ± 0.40 Ba
FT			36.78 **
FP			5.32 *
FPxT			2.79 *
CV (%)			7.24

Equal letters do not differ statistically by the Tukey Test. Lowercase letters compare columns, and uppercase letters compare rows. ns = not significant; * Tukey test at 5% probability; ** Tukey test at 1% probability; F_T = F calculated for temperature; F_P = calculated F for period; F_PxT = F calculated for interaction of the temperature with period; CV = coefficient of variation.

and

Table 6. Non-commercial productivity of noble garlic, cv. Ito obtained from seed cloves stored at low temperatures and different periods. São Miguel, RN, 2022.

Site	Temperature (°C)	Non-Commercial Productivity (t ha−1)
São Miguel	−5 to −3	0.97 ± 0.14 a
	−2 to 0	0.92 ± 0.09 a
	1 to 3	0.29 ± 0.05 b
	Period (days)	Non-commercial productivity (t ha−1)
	50	0.69 ± 0.13 ab
	55	0.96 ± 0.15 a
	60	0.54 ± 0.10 b
FT		17.70 **
FP		5.40 *
FPxT		1.62 ns
CV (%)		42.69

Equal letters do not differ statistically at 5% probability by the Tukey Test. Lowercase letters compare columns, and uppercase letters compare rows. ns = not significant; * Tukey test at 5% probability; ** Tukey test at 1% probability; F_T = F calculated for temperature; F_P = calculated F for period; F_PxT = F calculated for interaction of the temperature with period; CV = coefficient of variation.

).

In Mossoró, the highest total and commercial yields were achieved at negative temperatures. Seed cloves submitted with temperatures from −5 to −3 °C and −2 to 0 °C for 55 days resulted in the highest profits, total and commercial, with an average of 4.96 and 3.58 t ha⁻¹, respectively. The lowest non-commercial productivity (1.20 t ha⁻¹) was observed when garlic was subjected to positive temperatures for 60 days. However, treatment with −2 to 0 °C temperature for 55 days also provided low non-commercial productivity (1.37 t ha⁻¹) (Table 5).

Similar to what happened in Mossoró, negative temperatures increased the productivity of garlic cultivated in São Miguel, with higher marketable (8.14 t ha⁻¹) and total (8.71 t ha⁻¹) yields observed in treatments with temperatures from −5 to −3 °C, associated with a time of 60 days (Table 5). In garlic cultivated in São Miguel, the temperature increase in the treatments decreased non-commercial productivity, with a lower average observed in the treatment with positive temperature (0.29 t ha⁻¹). Additionally, the periods influenced the non-commercial productivity of garlic cultivated in São Miguel, with lower production of bulbs classified as non-commercial (0.54 t ha⁻¹) observed at 60 days period treatment (Table 6).

Garlic seed cloves treated at low temperatures influenced the commercial productivity of each class in both experimental sites. In garlic cultivated in São Miguel, an isolated significant effect was observed for classes 3, 4, and 6 (

Table 7. Productivity of each class about commercial productivity of noble garlic, cv. Ito obtained from seed cloves stored at low temperatures. São Miguel, RN, 2022.

Site	Temperature (°C)	Class 3 (%)	Class 4 (%)	Class 5 (%)	Class 6 (%)	Class 7 (%)
São Miguel	−5 to −3	10.06 ± 1.54 b	18.91 ± 1.89 b	41.21 ± 2.74 a	28.10 ± 3.79 a	1.73 ± 0.78 a
	−2 to 0	9.62 ± 0.83 b	14.71 ± 1.14 b	39.04 ± 2.27 a	35.61 ± 3.02 a	1.01 ± 0.74 a
	1 to 3	18.12 ± 2.05 a	33.19 ± 3.13 a	43.68 ± 2.95 a	5.02 ± 1.44 b	0.00 ± 0.00 a
FT		9.01 **	20.06 **	0.62 ns	40.61 **	2.16 ns
FP		0.07 ns	0.33 ns	0.01 ns	0.07 ns	3.02 ns
FPxT		0.44 ns	0.37 ns	0.71 ns	1.06 ns	1.31 ns
CV (%)		43.83	33.63	24.75	37.83	223.91

Equal letters in the columns do not differ statistically at 5% probability by Tukey Test. ns = not significant; ** = Tukey test at 1% probability; F_T = F calculated for temperature; F_P = calculated F for period; F_PxT = F calculated for interaction of the temperature with period; CV = coefficient of variation.

). In Mossoró, this occurred for classes 3, 4, and 5 (

Table 8. Productivity of each class about commercial productivity of noble garlic, cv. Ito obtained from seed cloves stored at low temperatures. Mossoró, RN, 2022.

Site	Temperature (°C)	Class 3 (%)	Class 4 (%)	Class 5 (%)
Mossoró	−5 to −3	59.50 ± 3.85 a	30.04 ± 2.51 b	10.46 ± 2.51 a
	−2 to 0	68.61 ± 4.45 a	25.19 ± 1.66 b	6.20 ± 1.66 ab
	1 to 3	42.25 ± 6.32 b	54.24 ± 1.70 a	3.51 ± 1.70 b
FT		7.74 **	12.84 **	3.61 **
FP		0.12 ns	0.01 ns	0.99 ns
FPxT		1.24 ns	1.45 ns	0.91 ns
CV (%)		29.34	41.22	94.86

Equal letters in the columns do not differ statistically by Tukey Test. ns = not significant; ** = Tukey test at 1% probability; F_T = F calculated for temperature; F_P = calculated F for period; F_PxT = F calculated for interaction of the temperature with period; CV = coefficient of variation.

).

In the São Miguel experiment, the treatments under positive temperatures favored higher concentrations of class 3, 4, and 5 commercial bulbs. Negative temperatures promoted primarily class 6 and 7 garlic commercial bulbs and high percentages of class 5 bulbs (Table 7).

For garlic grown in Mossoró, the highest concentrations of class 3 (68.61%), 4 (54.4%), and 5 (10.46%) bulbs were observed at temperatures −2 to 0 °C, 1 to 3 °C, and −5 to −3 °C, respectively. By increasing the diameter of the bulbs, there was also a decrease in the concentrations of commercial bulbs in the treatments with negative temperatures (Table 8).

4. Discussion

Garlic seed cloves treated at negative temperatures increased the crop cycle in both experimental sites. The time that the crop remained in the field was even longer for garlic grown in São Miguel. In this case, the high altitude and the mild temperatures throughout the cultivation cycle favored the development of the plants when compared with the Mossoró experiment, which presents opposite climatic conditions that are more stressful than the garlic crop.

The increase observed in the garlic crop cycle in the present study, in treatments with negative temperatures, may be unfavorable from an economic point of view since it increases production costs due to greater investment in irrigation, fertilization, agricultural pesticides, hand labor, and other factors. However, with the plants remaining longer in the field, there is a greater accumulation of photoassimilates, with the formation of larger bulbs, which can provide greater productivity and economic return. This result is particularly interesting for the garlic grown in Mossoró, which had an accelerated plant growth cycle due to climatic conditions.

It is important to point out that garlic seed cloves treated with temperatures from 0 to 10 °C for two months before planting accelerate the plant cycle in the field [5]. According to Dufoo-Hurtado et al. [8], exposing seed cloves to low temperatures alters their hormonal balance and promotes early plant development. This precocious development is even more accelerated when garlic is cultivated in regions with high temperatures throughout the cultivation cycle, as observed in Table 2 for the experiment in Mossoró, about garlic grown in São Miguel.

Overall, in both experiments, negative temperatures increased the susceptibility of garlic plants to sprouting, especially for the garlic grown in São Miguel. Sprouting is considered one of the main factors that reduce the productivity of noble garlic and, consequently, its commercial value. Several factors have been related to garlic sprouting, such as photoperiod, temperature, cultivar, excess nitrogen, irrigation, as well as the regulatory action of gibberellin [11,23,24,25,26,27].

The lower rates of sprouted garlic cultivated in Mossoró observed in the treatments with positive temperature possibly occurred due to the reduction in irrigation performed only at the time of differentiation of the bulbs in these treatments. According to Gabriel et al. [28], one of the alternatives to reduce plant sprouting, used by garlic producers in Brazil, consists of suspending irrigation during the bulbing period, causing water stress.

In São Miguel, the super-sprouting of the plants behaved similarly to what was observed in the Mossoró experiment. However, at the beginning of the differentiation of the cloves of the treatments with positive temperature, the reduction of irrigation was unfeasible due to the rains registered in the region (Figure 1). This ended up leading to the occurrence of sprouted plants in the treatments with positive temperatures (Table 3). In addition, the region’s high altitude and mild temperatures throughout the growing season may also have contributed to higher percentages of plants with super-sprouting in this location.

The higher percentages of plants with formed bulbs indicate garlic adaptability [9]. The results obtained in this research suggested an adaptation of the Ito cultivar to the edaphoclimatic conditions of São Miguel when the seed glove was subjected to positive temperatures in pre-planting since 100% of the plants formed bulbs (Table 3). However, negative temperatures increased commercial productivity in both locations, even with fewer plants with formed bulbs (Table 5). This result indicates that seed cloves treated with temperatures below zero can be used compensatively, meeting the edaphoclimatic requirements of the garlic crop.

The number of cloves per bulb is an intrinsic characteristic of each cultivar; that is, it has a genetic character and may be influenced by environmental factors. According to Lucena et al. [9], the number of bulblets per bulb varies according to the cultivar’s adaptability to each location’s climatic conditions. In this case, high temperatures during the differentiation process influence the number of bulbs per bulb, as observed in garlic grown in Mossoró, which had a lower number of bulbs than the experiment in São Miguel. Thus, the results obtained in this research suggest that the climatic conditions of the municipality of São Miguel, characterized by higher altitude and mild temperatures, about Mossoró, favored the greater number of cloves per bulb.

The consumer market prefers bulbs of a larger size with a smaller number per bulb, an important characteristic for commercialization since they allow for higher commercial prices [10]. However, the smaller number of cloves does not guarantee greater value for the production due to the commercial depreciation and cloves with a lower weight.

The productivity values confirmed the results previously obtained with other variables, making it possible to observe a significantly higher production in the treatments with temperatures below zero. In our study, the productivity of both experiments was below the national average (12.8 kg ha⁻¹) [2]. However, the productivity of garlic cultivated in São Miguel, observed in the treatments with negative temperatures, is equivalent to the average yield of garlic produced in the southern region, with productivity ranging from 4.5 to 9.8 kg ha⁻¹ [2]. Luz et al. [14] also obtained high productivity values with the Ito cultivar, applying temperatures below zero in the pre-planting treatment of seed cloves.

The treatments with negative temperatures originated plants that remained longer in the field. This occurs because the treatments of seed cloves at low temperatures alter their hormonal balance [8], stimulating the accumulation of hormones that induce morphological changes [9] and promoting plant development in the field. Thus, temperatures below zero provided the necessary physiological stimuli for the garlic plants to reach a good productive potential, even in unfavorable conditions for cultivation, as observed in the Mossoró experiment.

With plants spending more time in the field, there is greater absorption of water and nutrients and, consequently, greater liquid photosynthesis, resulting in a greater accumulation of mass [29], thus justifying the higher productivity observed in treatments with negative temperatures (Table 5). For Ahmed et al. [30], plants with greater production potential translocate more nutrients and photoassimilates from leaves and stems to bulbs. According to Luz et al. [5], seed cloves treated with negative temperatures favor garlic development, providing a greater plant height, bulbar ratio, and number of leaves.

The average temperatures of the locations throughout the cultivation cycle also directly influenced the growth and development of the garlic crop. Cultivation in regions with higher temperatures, as observed in garlic grown in Mossoró, which presented an average temperature of 25.8 °C throughout the experiment, reduces the time for the beginning of bulbification and the total cycle of the crop (Table 2), resulting in low productivity (Table 5). In São Miguel, conversely, the milder temperatures throughout the crop development, with an average of 23.5 °C, increased the garlic cycle, thus obtaining greater productivity (Table 5).

The culture of noble garlic requires low temperatures and a long photoperiod [5], with mild winters and sunny and dry summers [6], to fully develop. Climatic factors condition the crop, productivity, and final economic result [7]. According to Atif et al. [31], a small increase in temperature (1 to 2 °C) accelerates the bulbing process, reducing garlic yield. This indicates that, depending on the climatic conditions in which garlic is grown, the combination of temperature and period has different effects on the responses of garlic plants.

In both experiments, the non-commercial productivity was mainly composed of unformed bulbs, which did not show differentiation from the bulbs. It is important to point out that the treatments with negative temperatures provided greater commercial productivity in Mossoró and São Miguel (Table 5), even with fewer plants with formed bulbs (Table 3). This result highlights the potential of the treatments with sub-zero temperatures to increase garlic productivity. According to Luz et al. [14], as the temperature of the treatments of the garlic seed cloves increases (−3 to −1 °C to 1 to 3 °C), storage for 50 days, the productivity of bulbs in the discard class is increased, which demonstrates the great advantage of using negative temperatures.

The highest yields of non-commercial bulbs, observed in treatments with negative temperatures (−5 to −3 °C and −2 to 0 °C) and in the shortest times (50 and 55 days) (Table 6), are directly related to the lowest percentages of bulbs with clove differentiation, and to the highest amounts of plants sprouted, also observed in the same treatments (Table 3). Sprouting and undifferentiated bulbs negatively influence the garlic crop, reducing productivity and increasing non-commercial bulb production. Other authors have also observed a decline in commercial production due to increased sprouting [4,10,26].

Some studies have reported different results from those found in the present study. According to Resende et al. [4], with an increasing storage time (30–50 days) of garlic seed cloves treated at low temperatures, there is an increase in the incidence of sprouted plants, impairing the commercial quality of Caçador and Quitéria cultivars. Lopes et al. [10] evaluated the effect of periods and planting times of garlic seed cloves treated at low temperatures and also observed that the production of non-commercial garlic increased by increasing the storage time (50–65 days) associated with the planting time “5th of June”.

In garlic grown in São Miguel, seed cloves treated at low temperatures provided higher concentrations of class 5 commercial bulbs due to the local environmental conditions, which are more favorable for garlic cultivation. The São Miguel experimental area is located in a higher altitude region (602 m), with mild temperatures throughout the development of the garlic crop (Figure 1B). These conditions positively influenced the production of bulbs with larger diameters (Table 8).

In Mossoró, the production of commercial bulbs was concentrated in the smaller classes (Classes 3 and 4), probably because of the high temperatures during cultivation (Figure 1A) associated with the low altitude of the experimental area (72 m). These environmental conditions are more stressful for garlic plants. However, for class 5 garlic plants, negative temperatures provided more commercial bulbs than seed cloves treated with positive temperatures (Table 7). This is a promising result for the garlic grown in Mossoró, showing the potential of subzero temperatures to increase the production of larger-diameter bulbs.

The difference in production between the locations may be due to the Ito cultivar’s physiological response to each location’s edaphoclimatic conditions. In garlic cultivated in São Miguel, the foundation fertilization, the natural fertility of the soil, and the favorable climate contributed to larger-diameter bulbs with a larger diameter. According to Luz et al. [5], the same cultivar may behave differently depending on the growing region. Other factors, such as soil fertility and irrigation management, can also influence the final performance of the crop [10].

The influence of climate on garlic yield was reported by Feitosa et al. [32] when evaluating ten garlic cultivars managed in different locations, with altitudes ranging from 30 to 1.100 m. According to these authors, all cultivars had lower productivity at lower altitudes. Lucena et al. [9], working with garlic in the semi-arid region of Brazil, also stated that the development and production of this vegetable are compromised at low altitudes.

In general, seed cloves treated with temperatures below zero increased the production of noble garlic in both sites. These results indicate the potential of this technique, associated with sub-zero temperatures, to bring higher yields to the garlic crop in the study regions, with positive effects on the developmental characteristics and yield of the bulbs. Other authors have reported increased garlic production by treating the seed cloves below-zero temperatures [5,14].

The mild temperatures throughout the cultivation cycle of the garlic produced in São Miguel, associated with temperatures below zero, provided the necessary physiological stimuli for the plants to reach a productive potential equivalent to the garlic grown in the states of the southern region. These conditions allowed the production of bulbs with a larger diameter (classes 6 and 7), which have greater commercial value and consumer acceptance. This is a positive result since several works in the semi-arid region of Rio Grande do Norte report garlic production of only classes 3, 4, and 5 [9,10,33].

On the other hand, the edaphoclimatic conditions of Mossoró reduced the garlic crop cycle, meaning that the plants did not have enough time to accumulate biomass. This directly influenced the total yield of the bulbs, with results lower than those found in São Miguel. Thus, one must consider the need for further research evaluating the adaptability of the Ito cultivar and other varieties of noble garlic to the growing conditions in Mossoró to increase the productive yield. Additional studies using temperatures below zero in the pre-planting treatment of seed cloves should also be carried out in other regions of the Brazilian semi-arid region, which have high altitudes, thus confirming this technique’s potential to increase the productivity of noble garlic.

5. Conclusions

Seed cloves treated with negative temperatures increase the productivity of noble garlic; however, at low altitudes (72 m), productivity is lower than garlic produced at altitudes above 600 m, which presents an average yield like those in the southern region of Brazil. In addition, class 6 and 7 bulbs are produced in higher altitude conditions, a desirable characteristic due to their greater commercial value and consumer acceptance. One must consider the need for further research using negative temperatures on the performance of the Ito cultivar and other varieties of noble garlic grown at low altitudes. Additional studies should also be carried out in other high-altitude regions and, thus, confirm the potential of negative temperatures in increasing the productivity of noble garlic.