Ethephon Application on Hazelnut (Corylus avellana L.) Trees: Productive and Physiological Experience in a Temperate Climate Zone

Abstract

Chile contributes 4% of global hazelnut (Corylus avellana L.) production, mainly developed in temperate regions with high autumn rainfall and humidity during harvest, which can compromise nut quality and increase postharvest losses. Thus, synchronizing harvests has become a critical aspect for growers in the southern region of Chile. This study focused on the effects of ethephon (ETH) spraying on trees to optimize nut drop timing and assess its impact on yield optimization and its influence on vegetative growth and inflorescence activity. From the 2020/2021 to the 2022/2023 seasons, experiments were conducted on a commercial hazelnut orchard of Tonda di Giffoni (TDG) planted in southern Chile. Four ETH (0, 250, 500, 1000 mg L⁻¹) treatments were sprayed 15 days preharvest and denoted as ETHA (sprayed 2020/2021) and ETHB (sprayed twice, in 2020/2021 and 2021/2022). Nut drop synchronization was periodically monitored at 7, 15, 21, 28, and 35 days after application (DAA), along with industrial quality parameters (nut weight, kernel yield) and inflorescence activity. In the first season, ETH significantly synchronized nut drops, achieving optimal results at 15–28 DAA with ETH 250 and 500, while ETH 1000 induced earlier drops but reduced yields. Total nut yield varied among seasons and demonstrated consistent performance of ETH 250, identified as the most efficient treatment for balancing nut drop timing. Industrial parameters showed seasonal differences but no adverse effects on nut quality. Conversely, the inflorescence activity remained unaffected by ETH concentrations. ETHA and ETHB treatments influenced tree shoot length variably across three seasons, showing significant concentration and seasonal interaction effects. These results demonstrate that ETH effectively synchronizes hazelnut harvests under temperate conditions, reducing post-harvest losses and optimizing logistics without compromising yield or quality.

1. Introduction

The hazelnut (Corylus avellana L.) industry plays an important role in global agriculture, with production reaching 585,150 metric tons (t) of kernels (edible part) for the 2022/2023 productive season. Moreover, Chile contributes 4% of the world’s hazelnut production, positioning the country as the main off-season producer in the southern hemisphere [1,2,3]. For the 2022/2023 season, Chile supplied a total of 52,100 t [4], strengthening an industry that is certainly interesting and economically attractive. Currently, hazelnut plantations cover 36,393 hectares (ha) distributed between the O’Higgins and Los Lagos regions, with projections for them to reach 60,000 by 2030. In this context, La Araucanía region contains 8437 ha, approximately 33% of the total planted surface in Chile [2]. In southern Chile, the hazelnut harvest usually begins during the first days of March and finishes in wintertime during July [4]. The harvest starts when 40% of the nuts have dropped, and the second collection should be carried out when 95% of the nuts have totally dropped [5]. Tonda di Giffoni (TDG) is among the most highly appreciated Italian hazelnut cultivars, due to the roundness of its kernels and excellent processing quality [6]. Afterwards, the harvested nuts should be subjected to a drying process until they reach approximately 8% humidity [7]. Hence, it is crucial to know the ideal harvest time, particularly in temperate regions with autumn rains and relative humidities of around 80–90%. In La Araucanía, the rainy season typically begins around this time, with an average of approximately 112 mm of precipitation based on meteorological data from studied areas during the last four years [8]. This implies a higher energy demand for fruit drying and several losses for farmers and companies. Additionally, it is difficult to preserve the quality and condition of nuts, as well as the logistics of the harvest, including rejections by the industry. Consequently, harvest and post-harvest are critical moments, which, if not carried out with the appropriate care, can compromise the efforts made during the year to obtain good production in terms of fruit yield and quality defects. Mold and mechanical damage reduce hazelnut taste and decrease the quality and safety of the final products, as well as leading to lower profits for growers [9,10]. Little research has been performed on how to achieve greater efficiency in the hazelnut harvesting process in terms of shortening nut drop periods, which would result in a lower probability of humidity, availability of machinery and staff, coordination of nut delivery to the industry, and optimization of post-harvest labor in the orchards [10]. Therefore, it is necessary to perform strategies aimed at the reduction in and synchronization of harvest times in this species, adapted to growers from temperate areas of production.

Ethephon (ETH), an ethylene-releasing compound, has been widely studied for its effects on flowering, height, and branching in herbaceous perennials releasing ethylene, a gaseous plant growth regulator [11,12,13] that stimulates evenly ripe fruit, decreasing preservation time, minimizing post-harvest losses, and improving coloration in crops and fruit trees [14]. In Japanese plum (Prunus salicina), it has been reported that ETH spraying at 0, 100, 200, 300, and 400 mg L⁻¹ induced a higher fruit yield, except at higher concentrations, at an earlier harvest time, but with no effect on overall yield, and fruit quality parameters such as soluble solids, firmness, and titratable acidity were reduced during post-harvest time [15]. Similarly, Valleser and Valleser [16] studied the effects of ETH dosed at 0, 1000, 2500, and 5000 ppm on Cacao (Theobroma cacao) ripening, showing earlier ripening between three and five days after application in a fresh wound in pods. However, these experiences have no shown effects during several seasons on the harvest performance, vegetative growth, or reproductive behavior of nut crops of hazelnut planted in temperate areas. Therefore, it was necessary to conduct field experiments of the exogenous application of ETH on hazelnut trees to obtain a better understanding of the short-, medium-, and long-term implications on nut harvest, flowering behavior, vegetative growth, and off-year triggering, and the application rates of this phytohormone in production areas under temperate climate conditions. Therefore, our aim was to determine how ETH sprayed on hazelnut cultivar Tonda di Giffoni at preharvest affected the concentration of the nut drop for optimization of the harvest, as well as the vegetative and reproductive behavior during three seasons in Southern Chile.

2. Materials and Methods

The study was performed during the 2020/2021, 2021/2022, and 2022/2023 seasons on a commercial hazelnut orchard (Fundo Caracas-Frutícola Agrichile S.A.) located in Cunco (39°0′18.80″ S; 72°18′46.39″ W), La Araucanía region, Chile (Figure 1). The orchard was grown on an Andisol (Typic Placudand), with a flat topography and good drainage [17]. The orchard’s temperate weather conditions are shown in Figure 2, corresponding to temperatures (°C) and precipitation (mm) (data collected from the weather station at the experimental site). The tree material corresponds to 14-year-old trees of the TDG cultivar planted in a 5 m × 4 m frame (500 trees ha⁻¹) with multi-axis training. TDG exhibits intermediate tree vigor and moderate branching density, along with a semi-erect growth habit. It is characterized by having two to three predominant nuts per cluster, and early blooming season, and low chilling requirements for vegetative buds. Production management of fertilization was 90−110 N, 30−40 P, and 50−80 K (kg ha⁻¹), and pest and disease control were carried out conventionally 3–4 times per season. The orchard was irrigated weekly from November to March based on ETc; water was supplied by a drip irrigation system with a drip line irrigating each tree, with emitters supplying water at a rate of 2.0 L h⁻¹ spaced 1.0 m apart [7].

2.1. Treatments and Experimental Design

During the 2020/2021 and 2021/2022 seasons, in late February and early March (BBCH = 85 [18]), four ETH treatments were sprayed at a concentration of 0 (ETH 0), 250 (ETH 250), 500 (ETH 500), and 1000 (ETH 1000) mg L⁻¹ in a water volume of 1700 L ha⁻¹ (

Table 1. Ethephon concentration (mg L⁻¹), season, and spraying day for cv. Tonda di Giffoni hazelnut trees at the experimental site, Fundo Caracas, Cunco (39°00′ S, 72°31′ W), La Araucanía region, Chile.

Ethephon Concentration		Season
Treatment	(mg L−1)	2020/2021	2021/2022	2022/2023
		Administration day
ETH 0	0	28 February 2021	6 March 2022	N/A 1
ETH 250	250
ETH 500	500
ETH 1000	1000

¹ Non-application season.

). The commercial product of the ETH source was Ethrel^® 48SL ((2-chloroethyl) phosphonic acid 48% w/v) from Bayer S.A.^® The concentration evaluated was applied in a single sprayed administration 15 days before the scheduled harvest, using a back sprayer of 17 L (Cifarelli 1200, Cifarelli S.p.A-Voghera, Italy) with an output of 5 L min⁻¹. The start of the harvest for this cultivar in the studied area was estimated to be from 13 March to 15 March for the first season. Consequently, the ETH spraying was carried out on 28 February 2021. For the second season (2021/2022), the experiment was divided into two groups of trees. The first ones were kept without a new spraying of ETH (ETHA), and the second group was sprayed again under the same concentration and program described above (ETHB) on 6 March 2022. Finally, for the 2022/2023 season, no applications were performed in either group of trees; however, the monitoring of nut drop behavior and inflorescence activity (glomerulus and catkins) of the trees was continued.

2.2. Nut Drop Synchronization, Industrial Quality, and Condition

Each season, nuts were harvested at intervals of 7, 15, 28, and 35 days after application (DAA). After each harvest, the nuts were gently transported to the Laboratory of Physiology and Plant Nutrition for Fruit Trees to determine the harvest humidity in a thermobalance (Bel Engineering Mod. Air-thermo G163L, BEL Engineering® s.r.l., Santiago, Chilie) and dried in a forced-air oven (40 °C) for 72 h to stabilize the fruit at 6% humidity [7,19,20]. For evaluation of quality and condition, 40 nuts per replication were described by shape, whole nut and kernel weight, diameters, shell thickness, condition report, blank nuts, and kernel yield (%) according to the descriptors for hazelnut [21]. In their study, Ferrão et al. [22] and Gavilán-CuiCui et al. [20] employed the following equations to calculate three parameters—the nut roundness index (NRI), kernel roundness index (KRI), and kernel yield (%)—as follows:

$$\mathrm{NRI}={\displaystyle \frac{(\mathrm{nut}\mathrm{width}+\mathrm{nut} \mathrm{thickness})}{(2\times \mathrm{nut} \mathrm{length})}}$$

$$\mathrm{KRI}={\displaystyle \frac{(\mathrm{kernel} \mathrm{width} + \mathrm{kernel} \mathrm{thickness})}{(2\times \mathrm{kernel} \mathrm{length})}}$$

$$\mathrm{Kernel} \mathrm{yield}\left(\%\right)={\displaystyle \frac{\mathrm{kernel} \mathrm{weight}}{\mathrm{nut} \mathrm{weight}}}\times 100$$

2.3. Inflorescence Activity and Shoot Length Monitoring

The activity of glomerulus (FF) and catkins (MF) was monitored, as well as the vegetative growth, from May to February, with FF and MF every 15 counted days, as reported by von Bennewitz et al. [23], with minor adaptations on tagged shoots in the northeast (NE), northwest (NWT), and south (S) directions. On the same shoots, the annual length (SL) was measured at a frequency of 30 days. For the 2020/2021 season, the monitoring started from the end of the harvest (BBCH = 57) until the pre-harvest of the following season (BBCH = 81–85), between May 2021 and February 2022. For the 2022/2023 season, the same activity was performed between May 2022 and February 2023.

2.4. Evaluation of Premature Drop of Catkins

During the three seasons, a systematic collection was performed on catkins that dropped to the ground to evaluate the incidence of ETH concentration on premature dropping. Each set of catkins was properly labeled according to the treatment to which they were exposed. Subsequently, the weight (g) of each set was recorded using an analytical balance (Snug III-3000, Jadever Weightech, Inc., Vaughan, ON, Canada).

2.5. Statistical Analysis

A randomized experimental design was performed with four ETH treatments, considering four replications. Each replication consisted of three hazelnut trees (n = 48). Similarly, a three-way analysis of variance (ANOVA) was used, where the factors to be compared were season (S), treatment (T), and DAA for nut drop synchronization, industrial quality, and condition. For comparison of means, a Tukey post-hoc test was performed at p ≤ 0.05. For the inflorescence activity analyses, a two-way ANOVA was used. This process allowed the collection of detailed data, laying the foundations for an analysis of the factors that could influence the premature drop of this reproductive structure. For industrial quality, a principal component analysis (PCA) was performed. All statistical analyses were performed with the free software R©, version 4.3.3 (R Development Core Team 2008).

3. Results

3.1. Weather Conditions

The orchard experienced a persistent precipitation period from March to July, with an average accumulation of 1200 mm yr⁻¹ across all studied seasons (Figure 2). The highest temperature resulted in a mean of 36.2 ± 1 °C recorded in February from 2020 to 2023. For the spraying time of ETHA and ETHB treatments, the mean temperatures were registered as 16.0 ± 1 and 13.9 ± 1 °C for the 202020/21 and 2021/2022 seasons, respectively. Seasonal consistency was observed annually, presenting high precipitation and colder temperatures during autumn and winters, whereas unusually warm summers were observed in this area, characterized by high maximum temperatures, which could be related to a higher ETH performance.

3.2. Synchronization of Fruit Drop and Industrial Yield

3.2.1. Synchronization of Nut Drop

Figure 3 shows the distribution of the nut drop (%) under the evaluated ETHA treatments at 7, 15, 28, and 35 DAA for the 2020/2021 and 2021/2022 seasons. A significant triple interaction was found when a higher amount of nuts was harvested at 15 and 28 DAA for both seasons, highlighting ETHA 500 as the most efficient treatment. Particularly, in 2020/2021, ETHA 1000 showed the highest nut drop (59%) at 15 DAA, whereas ETHA 250 followed closely, with 51%, in comparison to ETHA 0, which showed a delayed drop at 28 DAA (57%) (p ≤ 0.05). For the 2021/2022 season, the nut drops were slightly lower, with ETHA 250, 500, and 1000 achieving a 76% total nut drop between 15 and 28 DAA, while ETHA 0 resulted in a 66% nut drop (p ≤ 0.05). Conversely, ETHA 500 and 1000 showed a significantly lower nut drop at 35 DAA, exhibiting 35% of total nut yield in comparison to ETHA 0.

An experiment replication was performed for the 2021/2022 and 2022/2023 seasons (Figure 4), where an increase in nut drops over time was observed, for which the peak values were obtained at 7 DAA (2021/2022) after ETHB 250, 500, and 1000, resulting in 80, 77, and 77%, respectively, in comparison to the ETHB 0 treatment (40%) (p ≤ 0.05). On the other hand, the 2022/2023 nut drop patterns between ETHA and ETHB showed non−significant differences at all harvest times.

Regarding total yield (kg ha⁻¹) per treatment (

Table 2. Total nut yield (kg ha⁻¹) for ETHA and ETHB treatments (0, 250, 500, and 1000 mg L⁻¹) for the 2020/2021, 2021/2022, and 2022/2023 seasons in Cunco, La Araucanía region. Data represent the mean of four replications ± standard error. Different letters indicate significant statistical differences (p ≤ 0.05) among treatments within each ETH spray program. NS: not significant, * p ≤ 0.05.

Treatment	Season
	2020/2021				2021/2022				2022/2023
	Nut Yield (kg ha−1)
ETHA 0	3377.50	±	70.19	b	1956.25	±	84.49	b
ETHA 250	3776.88	±	111.78	a	1715.63	±	156.45	b
ETHA 500	3027.13	±	79.40	c	2325.00	±	143.04	a
ETHA 1000	3572.50	±	74.02	ab	1715.63	±	105.03	b
ETHB 0					2432.50	±	74.48	b	3105.00	±	63.57	b
ETHB 250					2705.00	±	83.93	a	3666.88	±	50.66	a
ETHB 500					2182.50	±	132.98	c	3251.25	±	119.17	b
ETHB 1000					2345.00	±	116.09	bc	3288.75	±	84.90	b
T	NS				NS				NS
S	*				*				*
T × S	NS				NS				NS

T: treatment; S: season.

), ETHA 250 and 1000 showed maximum yields during the 2020/2021 season, in which the nut harvest reached over 3500 kg ha⁻¹ in comparison to ETHA 0 and 500, which reached 3000 kg ha⁻¹ (Figure 3). A nut yield reduction was observed under all treatments for the 2021/2022 season, with nut yields with ETHA 0 achieving the highest yield, at around 2500 kg ha⁻¹, followed by ETHA 250, at around 2200 kg ha⁻¹. ETHA 500 and ETHA 1000 showed the lowest yields, with ETHA 1000 dropping to about 1500 kg ha⁻¹, indicating a reduction compared to the previous season.

Total yield followed a downward trend as the concentration of ETHB increased. In 2021/2022, ETHB 0 and ETHB 250 yielded the highest values, with 2432 and 2705 kg ha⁻¹, respectively, while ETHB 500 showed a lower yield (2182 kg ha⁻¹). In 2022/2023, the absolute values for yield increased compared to the previous season. ETHB 250 and ETHB 1000 exhibited yields of 3667 and 3288 kg ha⁻¹, respectively. For total yield, significant differences were found only between seasons. However, no significant season–treatment interactions or differences between treatments (p ≤ 0.05) were observed, which is particularly relevant given the lack of effect of ETH spraying on trees.

3.2.2. Industrial Parameters

For the 2020/2021 and 2021/2022 seasons,

Table 3. Industrial quality of nuts and kernels at 7, 15, 28, and 35 DAA per treatment (T) with ETHA 0, 250, 500, and 1000 mg L⁻¹ for the 2020/2021 and 2021/2022 seasons (S). Data represent the mean of four replications ± S.E (n = 40). Cunco, La Araucanía region. Different letters indicate statistical differences (p ≤ 0.05) among treatments within DAA. NS, not significant; * p ≤ 0.05.

		2020/2021												2021/2022
DAA	Treatment	Nut Weight				Kernel Weight				Shell Thickness				Nut Weight				Kernel Weight				Shell Thickness
		(g)								(mm)				(g)								(mm)
7	ETHA 0	2.28	±	0.10	b	0.85	±	0.05	c	1.35	±	0.08	a	2.54	±	0.07	b	1.16	±	0.03	a	1.34	±	0.04	ab
	ETHA 250	2.35	±	0.11	b	0.95	±	0.04	c	1.35	±	0.13	a	2.41	±	0.08	b	1.16	±	0.03	a	1.43	±	0.05	ab
	ETHA 500	2.25	±	0.02	b	0.95	±	0.04	c	1.18	±	0.10	ab	2.38	±	0.07	b	1.09	±	0.04	ab	1.31	±	0.04	ab
	ETHA 1000	2.23	±	0.11	b	0.92	±	0.07	c	1.21	±	0.12	ab	2.27	±	0.08	bc	1.03	±	0.05	b	1.29	±	0.04	ab
	Mean	2.28	±	0.08		0.92	±	0.05		1.27	±	0.11		2.40	±	0.08		1.11	±	0.04		1.34	±	0.04
15	ETHA 0	2.51	±	0.05	ab	1.19	±	0.03	ab	1.27	±	0.11	ab	2.55	±	0.08	b	1.14	±	0.06	ab	1.57	±	0.05	a
	ETHA 250	2.57	±	0.09	a	1.18	±	0.06	ab	1.24	±	0.14	ab	2.51	±	0.07	b	1.10	±	0.05	ab	1.41	±	0.14	ab
	ETHA 500	2.40	±	0.06	b	1.15	±	0.05	b	1.21	±	0.12	ab	2.40	±	0.05	b	1.10	±	0.03	ab	1.25	±	0.04	b
	ETHA 1000	2.50	±	0.07	ab	1.14	±	0.04	ab	1.31	±	0.15	ab	2.78	±	0.24	a	1.13	±	0.04	ab	1.54	±	0.04	a
	Mean	2.49	±	0.07		1.16	±	0.04		1.26	±	0.13		2.56	±	0.11		1.12	±	0.04		1.44	±	0.07
28	ETHA 0	2.70	±	0.04	a	1.26	±	0.02	a	1.29	±	0.12	ab	2.54	±	0.08	b	1.10	±	0.06	ab	1.39	±	0.05	ab
	ETHA 250	2.62	±	0.09	ab	1.24	±	0.04	a	1.25	±	0.11	ab	2.54	±	0.09	b	1.15	±	0.04	ab	1.51	±	0.04	a
	ETHA 500	2.42	±	0.05	b	1.13	±	0.03	b	1.08	±	0.09	b	2.44	±	0.06	b	1.14	±	0.05	ab	1.49	±	0.05	a
	ETHA 1000	2.54	±	0.06	ab	1.17	±	0.03	ab	1.21	±	0.12	ab	2.28	±	0.09	c	1.02	±	0.06	bc	1.53	±	0.04	a
	Mean	2.57	±	0.06		1.20	±	0.03		1.21	±	0.11		2.45	±	0.08		1.10	±	0.05		1.48	±	0.05
35	ETHA 0	2.37	±	0.07	b	1.05	±	0.05	bc	1.34	±	0.13	a	2.49	±	0.08	b	1.07	±	0.06	ab	1.50	±	0.05	ab
	ETHA 250	2.28	±	0.11	b	1.05	±	0.04	b	1.07	±	0.10	b	2.44	±	0.09	b	1.07	±	0.05	ab	1.35	±	0.05	ab
	ETHA 500	2.68	±	0.08	a	1.20	±	0.07	a	1.30	±	0.08	ab	2.35	±	0.06	bc	1.07	±	0.05	bc	1.28	±	0.04	b
	ETHA 1000	2.59	±	0.10	ab	1.18	±	0.03	ab	1.26	±	0.11	ab	2.20	±	0.07	c	0.96	±	0.06	c	1.20	±	0.03	b
	Mean	2.48	±	0.09		1.12	±	0.05		1.24	±	0.11		2.37	±	0.07		1.04	±	0.05		1.33	±	0.04
Significance	S	NS				NS				*				NS				NS				*
	DAA	*				*				NS				*				*				NS
	T	NS				NS				NS				NS				NS				NS
	S × DAA	NS				NS				NS				NS				NS				NS
	S × T	NS				NS				NS				NS				NS				NS
	DAA × T	NS				NS				NS				NS				NS				NS
	S × DAA × T	NS				NS				NS				NS				NS				NS

shows nut and kernel weight, as well as shell thickness, determined at 7, 15, 28, and 35 DAA under ETHA treatments. The first season showed a significant effect of DAA on nut weight at 15, 28, and 35 DAA, reaching 8.4, 11.2, and 8.9% respectively. Similarly, kernel weight was linked with DAA, exhibiting greater values at 28 DAA in comparison to 7 (23.3%), 15 (3.3%), and 35 DAA (6.7%) (p ≤ 0.05). Regarding to shell thickness, no significant differences among ETHA and DAA were found. However, statistical differences between seasons were determined, showing thicker nutshell for 2021/2022.

Table 4. Industrial quality of nuts and kernels at 7, 15, 28, and 35 DAA per treatment (T) with ETHB 0, 250, 500, and 1000 mg L⁻¹ for the 2021/2022 and 2022/2023 seasons (S). Data represent the mean of four replications ± S.E (n = 40). Cunco, La Araucanía region. Different letters indicate statistical differences (p ≤ 0.05) among treatments within each spray program. NS, not significant; * p ≤ 0.05.

		2021/2022												2022/2023
DAA	Treatment	Nut Weight				Kernel Weight				Shell Thickness				Nut Weight				Kernel Weight				Shell Thickness
		(g)								(mm)				(g)								(mm)
7	ETHB 0	2.55	±	0.08	a	1.14	±	0.05	a	1.57	±	0.05	a	2.27	±	0.07	ab	1.14	±	0.04	ab	2.50	±	1.36	a
	ETHB 250	2.46	±	0.08	a	1.16	±	0.03	ab	1.27	±	0.04	b	2.38	±	0.05	a	1.14	±	0.05	a	1.24	±	0.17	b
	ETHB 500	2.49	±	0.07	ab	1.22	±	0.04	ab	1.26	±	0.04	b	2.28	±	0.07	a	1.18	±	0.08	a	1.22	±	0.15	b
	ETHB 1000	2.43	±	0.08	b	1.09	±	0.05	b	1.35	±	0.05	b	2.21	±	0.07	ab	1.02	±	0.05	ab	1.25	±	0.15	b
	Mean	2.48	±	0.08		1.15	±	0.04		1.36	±	0.05		2.28	±	0.07		1.12	±	0.06		1.55	±	0.46
15	ETHB 0	2.53	±	0.07	ab	1.12	±	0.04	a	1.33	±	0.05	b	2.34	±	0.06	a	1.20	±	0.03	ab	1.26	±	0.21	b
	ETHB 250	2.64	±	0.06	a	1.20	±	0.03	a	1.38	±	0.05	b	2.31	±	0.08	ab	1.20	±	0.03	ab	1.20	±	0.14	b
	ETHB 500	2.47	±	0.05	ab	1.16	±	0.04	ab	1.32	±	0.04	b	2.11	±	0.08	a	1.17	±	0.03	a	1.16	±	0.14	b
	ETHB 1000	2.57	±	0.08	b	1.16	±	0.05	ab	1.34	±	0.05	b	2.34	±	0.07	ab	1.21	±	0.02	ab	1.28	±	0.18	b
	Mean	2.55	±	0.07		1.16	±	0.04		1.34	±	0.05		2.28	±	0.07		1.19	±	0.03		1.22	±	0.17
28	ETHB 0	2.48	±	0.09	ab	1.07	±	0.07	a	1.50	±	0.05	a	2.35	±	0.04	ab	1.19	±	0.02	ab	1.22	±	0.18	b
	ETHB 250	2.47	±	0.06	ab	1.12	±	0.05	a	1.40	±	0.05	ab	2.47	±	0.07	b	1.27	±	0.03	a	1.26	±	0.18	b
	ETHB 500	2.22	±	0.09	b	1.01	±	0.07	b	1.40	±	0.06	ab	2.37	±	0.05	a	1.22	±	0.02	ab	1.22	±	0.20	b
	ETHB 1000	2.28	±	0.09	b	0.96	±	0.06	b	1.42	±	0.05	ab	2.23	±	0.07	ab	1.15	±	0.04	ab	1.16	±	0.22	b
	Mean	2.36	±	0.08		1.04	±	0.06		1.43	±	0.05		2.36	±	0.05		1.21	±	0.03		1.22	±	0.20
35	ETHB 0	2.23	±	0.09	ab	0.97	±	0.06	ab	1.40	±	0.05	ab	2.22	±	0.07	ab	1.16	±	0.03	b	1.30	±	0.23	b
	ETHB 250	2.30	±	0.07	a	1.03	±	0.07	a	1.40	±	0.11	ab	2.27	±	0.06	ab	1.15	±	0.04	a	1.25	±	0.19	b
	ETHB 500	2.19	±	0.11	b	0.91	±	0.08	b	1.41	±	0.03	b	2.22	±	0.05	b	1.15	±	0.02	ab	2.52	±	1.44	a
	ETHB 1000	2.31	±	0.10	b	0.98	±	0.07	b	1.39	±	0.05	b	2.25	±	0.06	ab	1.16	±	0.03	ab	1.31	±	0.21	b
	Mean	2.26	±	0.09		0.97	±	0.07		1.40	±	0.06		2.24	±	0.06		1.16	±	0.03		1.59	±	0.52
Significance	S	*				*				*				*				*				*
	DAA	NS				NS				NS				NS				NS				NS
	T	NS				NS				NS				NS				NS				NS
	S × DAA	NS				NS				NS				NS				NS				NS
	S × T	NS				NS				NS				NS				NS				NS
	DAA × T	NS				NS				NS				NS				NS				NS
	S × DAA × T	NS				NS				NS				NS				NS				NS

shows effect of ETHB on both nut and kernel weight, as well as shell thickness, in harvested nuts at 7, 15, 28, and 35 DAA during the 2021/2022 and 2022/2023 seasons. These industrial components were significantly raised during the first season in comparison to the values determined for 2022/2023.

In the 2020/2021 season, the highest number of wrinkled kernels was observed in harvested nuts from all ETHA treatments compared to those kernels evaluated during the harvest performed in the second season (

Table 5. Wrinkled, moldy, blank, and double kernels detected at 7, 15, 28, and 35 DAA under ETHA (0, 250, 500, and 1000 mg L⁻¹) for the 2020/2021 and 2021/2022 seasons. Cunco, La Araucanía region (n = 40).

Treatment	DAA	Season
		2020/2021				2021/2022
		Wrinkled	Moldy	Blank	Double	Wrinkled	Moldy	Blank	Double
		(%)
ETHA 0	7	20.00	2.50	0.00	5.00	2.50	5.00	5.00	2.50
	15	5.00	7.50	0.00	2.50	7.50	7.50	7.50	15.00
	28	0.00	0.00	0.00	0.00	0.00	6.67	6.67	0.00
	35	2.50	10.00	0.00	0.00	2.50	10.00	5.00	10.00
	Mean	6.88	5.00	0.00	1.88	3.13	7.29	6.04	6.88
ETHA 250	7	37.50	2.50	7.50	2.50	0.00	5.00	5.00	0.00
	15	5.00	5.00	0.00	2.50	0.00	10.00	7.50	5.00
	28	2.50	2.50	2.50	0.00	7.50	15.00	2.50	30.00
	35	0.00	2.50	2.50	0.00	2.50	10.00	10.00	5.00
	Mean	11.25	3.13	3.13	1.25	2.50	10.00	6.25	10.00
ETHA 500	7	15.00	7.50	5.00	7.50	2.50	2.50	2.50	5.00
	15	2.50	0.00	0.00	0.00	7.50	10.00	0.00	0.00
	28	2.50	2.50	0.00	2.50	7.50	15.00	0.00	25.00
	35	5.00	0.00	0.00	2.50	0.00	2.50	2.50	5.00
	Mean	6.25	2.50	1.25	3.13	4.38	7.50	1.25	8.75
ETHA 1000	7	15.00	12.50	10.00	5.00	2.50	10.00	5.00	5.00
	15	12.50	0.00	0.00	2.50	5.00	10.00	0.00	30.00
	28	0.00	0.00	0.00	5.00	2.50	7.50	5.00	32.50
	35	0.00	0.00	5.00	2.50	0.00	5.00	10.00	15.00
	Mean	6.88	3.13	3.75	3.75	2.50	8.13	5.00	20.63

). However, the incidence of this quality defect decreased considerably (8.75%) towards the next evaluated season. Contrarily, the incidence of mold found inside nuts from ETH treatments and DAA doubled for 2021/2022, whereas blank and double nuts tended to increase during the second evaluated season. Nevertheless, we cannot determine whether the treatments were conducted due to these quality defects.

For ETHB during the 2021/2022 season (

Table 6. Wrinkled, moldy, blank, and double kernels detected in hazelnuts harvested at 7, 15, 28, and 35 DAA under ETHB (0, 250, 500, and 1000 mg L⁻¹) for the 2021/2022 and 2022/2023 seasons. Cunco, La Araucanía region (n = 40).

Treatment	DAA	Season
		2021/2022				2022/2023
		Wrinkled	Moldy	Blank	Double	Wrinkled	Moldy	Blank	Double
		(%)
ETHB 0	7	7.5	7.5	7.5	15.0	2.5	5.0	0.0	5.0
	15	5.0	7.5	7.5	7.5	0.0	0.0	0.0	0.0
	28	3.3	13.3	6.7	3.3	0.0	0.0	0.0	0.0
	35	0.0	7.5	5.0	5.0	2.5	2.5	2.5	0.0
	Mean	3.96	8.96	6.67	7.71	1.25	1.88	0.63	1.25
ETHB 250	7	2.5	2.5	5.0	7.5	0.0	5.0	2.5	0.0
	15	0.0	12.5	0.0	7.5	2.5	2.5	5.0	2.5
	28	2.5	12.5	0.0	2.5	0.0	0.0	0.0	0.0
	35	0.0	12.5	10.0	0.0	2.5	2.5	2.5	2.5
	Mean	1.25	10.00	3.75	4.38	1.25	2.50	2.50	1.25
ETHB 500	7	0.0	12.5	5.0	7.5	2.5	7.5	0.0	2.5
	15	0.0	5.0	0.0	5.0	0.0	5.0	10.0	0.0
	28	2.5	7.5	12.5	0.0	0.0	0.0	0.0	2.5
	35	2.5	2.5	20.0	2.5	2.5	2.5	0.0	2.5
	Mean	1.25	6.88	9.38	3.75	1.25	3.75	2.50	1.88
ETHB 1000	7	2.5	0.0	10.0	10.0	12.5	2.5	2.5	2.5
	15	0.0	7.5	5.0	0.0	0.0	5.0	2.5	2.5
	28	10.0	2.5	10.0	5.0	0.0	0.0	0.0	0.0
	35	5.0	7.5	15.0	0.0	2.5	2.5	0.0	0.0
	Mean	4.38	4.38	10.00	3.75	3.75	2.50	1.25	1.25

), in terms of wrinkled kernels, the highest rate was recorded for ETHB 1000 at 28 DDA. However, for the 2022/2023 season, the incidence of the above−mentioned parameter decreased. The mold incidence for ETHB showed the highest value at 28 DDA (13.3%), although for the following season there was also a downward trend, with ETHB 500 at 7 DDA (7.5%) being the highest value. In general, the 2021/2022 season presented higher values in terms of condition, with 20% double kernels under ETHB 500 at 35 DDA and 15% under ETHB at 0 to 7 DDA.

The quality features of nuts for DAA and ETHA in the 2020/2021 and 2021/2022 seasons are shown in

Table 7. Quality features of kernel yield (%), nut roundness index (NRI), and kernel roundness index (KRI) at 7, 15, 28, and 35 DAA per treatment (T) under ETHA 0, 250, 500, and 1000 mg L⁻¹ for the 2020/2021 and 2021/2022 seasons (S). Data represent the mean of four replications ± S.E (n = 40). Cunco, La Araucanía region. Different letters indicate statistical differences (p ≤ 0.05) among treatments within each spray program. NS, not significant; ** p ≤ 0.001.

		2020/2021												2021/2022
DAA	Treatment	Kernel Yield				NRI				KRI				Kernel Yield				NRI				KRI
		(%)												(%)
7	ETHA 0	40.57	±	1.64	b	1.02	±	0.01	a	0.98	±	0.06	a	43.86	±	1.24	ab	0.95	±	0.01	a	0.84	±	0.04	bc
	ETHA 250	36.15	±	3.19	c	0.94	±	0.01	b	0.83	±	0.08	b	42.95	±	2.12	ab	0.93	±	0.02	ab	0.89	±	0.03	abc
	ETHA 500	41.93	±	2.24	ab	0.96	±	0.03	b	0.90	±	0.06	ab	44.00	±	2.74	ab	0.92	±	0.01	b	0.91	±	0.03	ab
	ETHA 1000	39.15	±	2.63	b	0.97	±	0.01	b	0.91	±	0.02	b	40.20	±	1.63	b	0.93	±	0.01	a	0.84	±	0.04	bc
	Mean	39.45	±	2.43		0.97	±	0.02		0.90	±	0.05		42.75	±	1.93		0.93	±	0.01		0.87	±	0.04
15	ETHA 0	40.55	±	3.32	b	0.99	±	0.03	ab	0.92	±	0.06	ab	41.55	±	2.55	ab	1.00	±	0.02	a	0.93	±	0.06	ab
	ETHA 250	43.49	±	0.96	ab	0.92	±	0.02	c	0.91	±	0.02	b	41.96	±	2.24	b	0.95	±	0.02	a	0.82	±	0.04	c
	ETHA 500	45.86	±	1.63	a	0.92	±	0.01	c	0.94	±	0.02	ab	43.41	±	1.53	a	0.94	±	0.02	a	0.90	±	0.04	bc
	ETHA 1000	43.12	±	1.53	ab	0.96	±	0.02	b	0.94	±	0.02	ab	41.56	±	0.99	ab	0.94	±	0.01	ab	0.87	±	0.03	bc
	Mean	43.25	±	1.86		0.95	±	0.02		0.93	±	0.03		42.12	±	1.83		0.96	±	0.02		0.88	±	0.04
28	ETHA 0	44.54	±	1.51	ab	0.97	±	0.02	b	0.95	±	0.05	ab	43.65	±	1.11	a	0.97	±	0.02	a	0.92	±	0.05	ab
	ETHA 250	43.72	±	1.51	ab	0.95	±	0.01	bc	0.89	±	0.01	b	45.42	±	0.76	a	0.95	±	0.01	a	0.97	±	0.02	a
	ETHA 500	45.33	±	2.45	ab	0.94	±	0.01	bc	0.92	±	0.02	ab	46.72	±	1.28	a	0.93	±	0.01	ab	0.95	±	0.04	a
	ETHA 1000	42.26	±	1.34	b	0.93	±	0.01	c	0.88	±	0.01	b	45.61	±	0.99	a	0.96	±	0.01	a	0.98	±	0.04	a
	Mean	43.96	±	1.71		0.94	±	0.01		0.91	±	0.02		45.35	±	1.03		0.95	±	0.01		0.95	±	0.04
35	ETHA 0	41.93	±	3.19	ab	0.96	±	0.01	bc	0.83	±	0.08	b	41.20	±	2.14	ab	0.95	±	0.02	a	0.89	±	0.07	abc
	ETHA 250	44.00	±	2.24	ab	0.96	±	0.01	b	0.92	±	0.02	ab	43.06	±	2.66	ab	0.92	±	0.01	b	0.87	±	0.05	bc
	ETHA 500	41.56	±	1.05	bc	0.94	±	0.02	bc	0.94	±	0.06	ab	44.57	±	1.05	a	0.93	±	0.01	ab	0.85	±	0.04	bc
	ETHA 1000	43.49	±	1.11	ab	0.93	±	0.02	bc	0.95	±	0.02	a	40.09	±	1.47	b	0.92	±	0.01	b	0.83	±	0.02	bc
	Mean	42.75	±	1.90		0.95	±	0.02		0.91	±	0.05		42.23	±	1.83		0.93	±	0.01		0.86	±	0.04
Significance	S	NS				NS				**				NS				NS				**
	DAA	**				NS				NS				**				NS				NS
	T	NS				NS				NS				NS				NS				NS
	S × DAA	NS				NS				NS				NS				NS				NS
	S × T	NS				NS				NS				NS				NS				NS
	DAA × T	NS				NS				NS				NS				NS				NS
	S × DAA × T	NS				NS				NS				NS				NS				NS

. Kernel yield (%) under the ETHA 500 treatment exhibited the highest value at 15 DAA, reaching up to 45.86 ± 1.63 in 2020/2021 and 46.72 ± 1.28 in 2021/2022 at 28 DAA. The NRI showed less variability throughout treatments and DAA, with averages close to 0.95 ± 0.02. Only ETHA 0 at 7 DAA with 1.02 ± 0.01 for 2020/2021 is highlighted. For the same parameter in 2021/2022, the values increased at 15 and 28 DAA in all treatments. However, ETHA 1000 is notable for 35 DAA. Regarding the KRI, the highest values in 2020/2021 were observed at 7 and 35 DAA for ETHA 0 and ETHA 1000 (0.98 ± 0.06 and 0.95 ± 0.02, respectively), but the KRI’s high values for ETHA 250, 500, and 1000 were concentrated at 28 DAA (0.97 ± 0.02, 0.95 ± 0.04, and 0.98 ± 0.04, respectively) in 2021/2022. The statistical significance analysis indicates significant differences in kernel yield for DAA and the KRI for each season, while the NRI did not show significant differences overall at p ≤ 0.05.

Table 8. Quality features of kernel yield (%), nut roundness index (NRI), and kernel roundness index (KRI) at 7, 15, 28, and 35 DAA per treatment (T) under ETHA 0, 250, 500, and 1000 mg L⁻¹ for the 2021/2022 and 2022/2023 seasons (S). Data represent the mean of four replications ± S.E (n = 40). Cunco, La Araucanía region. Different letters indicate statistical differences (p ≤ 0.05) among treatments within each spray program. NS, not significant; ** p ≤ 0.001.

		2021/2022												2022/2023
DAA	Treatment	Kernel Yield				NRI				KRI				Kernel Yield				NRI				KRI
		(%)												(%)
7	ETHB 0	41.55	±	2.55	a	1.00	±	0.02	a	0.93	±	0.06	ab	49.97	±	0.62	b	0.96	±	0.02	a	0.97	±	0.09	ab
	ETHB 250	44.29	±	1.97	a	0.96	±	0.02	ab	0.88	±	0.04	b	47.07	±	0.35	c	1.01	±	0.02	a	0.87	±	0.05	b
	ETHB 500	43.76	±	1.54	a	0.95	±	0.03	ab	0.92	±	0.04	b	51.50	±	3.72	a	0.98	±	0.01	a	0.91	±	0.06	ab
	ETHB 1000	40.90	±	1.00	a	0.95	±	0.01	ab	0.86	±	0.02	b	45.00	±	2.41	c	0.98	±	0.02	a	0.98	±	0.09	ab
	Mean	42.62	±	1.77		0.96	±	0.02		0.90	±	0.04		48.38	±	1.77		0.98	±	0.02		0.93	±	0.07
15	ETHB 0	43.73	±	1.47	b	0.96	±	0.03	a	0.90	±	0.05	ab	49.26	±	1.05	a	0.97	±	0.02	a	0.82	±	0.04	b
	ETHB 250	45.53	±	0.81	b	0.96	±	0.01	a	0.98	±	0.01	a	47.98	±	0.85	b	1.00	±	0.01	a	0.81	±	0.06	b
	ETHB 500	43.99	±	2.96	b	0.93	±	0.01	ab	1.11	±	0.16	a	46.87	±	2.11	b	0.96	±	0.02	a	0.95	±	0.02	ab
	ETHB 1000	46.36	±	0.52	a	0.96	±	0.01	a	1.00	±	0.02	a	47.86	±	1.48	b	0.97	±	0.02	a	0.81	±	0.05	b
	Mean	44.90	±	1.44		0.95	±	0.01		1.00	±	0.06		47.99	±	1.37		0.97	±	0.02		0.85	±	0.04
28	ETHB 0	40.57	±	2.89	c	0.96	±	0.00	a	0.91	±	0.09	ab	50.82	±	0.42	a	0.96	±	0.01	a	0.84	±	0.08	b
	ETHB 250	45.61	±	1.54	a	0.94	±	0.02	ab	0.94	±	0.03	ab	49.57	±	2.15	a	0.96	±	0.01	a	0.90	±	0.08	ab
	ETHB 500	43.69	±	0.87	b	0.94	±	0.01	ab	0.85	±	0.05	b	51.17	±	0.57	a	0.97	±	0.01	a	0.88	±	0.09	ab
	ETHB 1000	39.20	±	1.36	d	0.93	±	0.02	b	0.84	±	0.02	b	49.20	±	1.14	a	0.96	±	0.02	a	0.84	±	0.06	b
	Mean	42.27	±	1.66		0.94	±	0.01		0.88	±	0.04		50.19	±	1.07		0.96	±	0.01		0.87	±	0.08
35	ETHB 0	41.41	±	0.88	b	1.00	±	0.01	a	0.95	±	0.02	a	48.15	±	2.26	b	0.93	±	0.00	b	0.86	±	0.09	ab
	ETHB 250	43.05	±	3.47	a	0.93	±	0.02	ab	0.89	±	0.09	ab	51.58	±	1.94	a	0.99	±	0.02	a	0.88	±	0.02	b
	ETHB 500	37.31	±	4.33	c	0.93	±	0.01	ab	0.78	±	0.07	b	51.90	±	0.84	a	0.95	±	0.02	ab	0.92	±	0.03	ab
	ETHB 1000	39.31	±	1.54	c	0.95	±	0.01	a	0.86	±	0.03	b	47.72	±	2.69	b	0.97	±	0.01	ab	0.79	±	0.04	b
	Mean	40.27	±	2.56		0.96	±	0.01		0.87	±	0.05		49.84	±	1.93		0.96	±	0.01		0.86	±	0.05
Significance	S	**				NS				NS				**				NS				NS
	DAA	NS				NS				NS				NS				NS				NS
	T	NS				NS				NS				NS				NS				NS
	S × DAA	NS				NS				NS				NS				NS				NS
	S × T	NS				NS				NS				NS				NS				NS
	DAA × T	NS				NS				NS				NS				NS				NS
	S × DAA × T	NS				NS				NS				NS				NS				NS

shows a significant effect of ETHB in both seasons only for the three parameters. In 2021/2022, for fruit weight (g), ETHB 250 showed interesting values at 7, 15, and 35 DDA, with 2.46 ± 0.08, 2.64 ± 0.06, and 2.30 ± 0.07, respectively. Another notable value was for ETHB 0 at 7 DDA (2.55 ± 0.08). Kernel weight (g) showed the greatest increase at 15 DDA, while ETHB 250 (1.20 ± 0.03) also presented the highest values at 28 and 35 DDA. The shell thickness (mm) showed significant differences between seasons and presented the highest values for ETHB 250 in 2021/2022 (2.64 ± 0.06) and ETHA 500 for 2022/2023 (2.52 ± 1.44).

Principal component analysis (PCA) was conducted for the ETHA industrial parameters in the 2020/2021 and 2021/2022 seasons, which revealed two principal components: PC1, which explained 25.95% of the total variability, and PC2, which explained 28.50% (Figure 5). The blue arrows represent several variables and their contribution to these components. The variable “kernel yield” was more associated with PC2, while “DAA” and “treatment” were mainly aligned with PC1. On the other hand, “KRI” was negatively associated with PC1 and PC2, and “NRI” had influence on the variability explained by PC2. The directions and lengths of the arrows indicate the correlation between the variables and the principal components, showing how each variable contributed to the total variability and its relationship with the others. In Figure 5b, a contour map shows the relationship for ETHA between the ETH treatment (ETHA 0, 250, 500, and 1000) on the X axis and the 2020/2021 and 2021/2022 seasons on the Y axis, with the colors representing the kernel yield (%). The colors in the sidebar indicate that the kernel yield varied from 41.5% (blue, lowest yield) to 44.5% (red, highest yield). On the left side of the graph, where the treatment is low, the kernel yield is low (blue colors). As treatment increases (moving to the right on the graph), kernel yield increases, moving through greens and yellows until it reaches its maximum value (red) near the ETHA 500 treatment. After this point, kernel yield decreases slightly, evidenced by the transition from red to yellow and then to green. Season showed a minor impact on the variability of kernel yield compared to treatment, as most of the changes in yield are exhibited on the X axis (Figure 5b). In Figure 5c the graph represents a relationship between the applied treatment, DAA, and kernel yield (%). It was determined that yield varied significantly depending on these variables, suggesting that there is an optimal combination of treatment and harvest time in terms of kernel yield. The principal component analysis plot (Figure 6a) for the industrial parameters of ETHB in the 2021/2022 and 2022/2023 seasons reveals an interesting relationship between the analyzed variables. A strong interaction was determined between harvest and treatment for the last two evaluated seasons. Likewise, kernel yield and KRI showed a positive correlation, indicating that both indicators could be influenced by common factors. In addition, there was some opposition between these two groups of variables. However, in both experiments (ETHA and ETHB), they showed similar behaviors.

3.3. Inflorescence Activity and Shoot Length

The ratio of glomerulus per catkin obtained from the four different treatments (ETHA 0, 250, 500, and 1000) during the months of May to August (2020/2021 and 2021/2022) is shown in

Table 9. Inflorescence activity expressed in ratio of glomerulus/catkins determined during from May to August for ETHA 0, 250, 500, and 1000 mg L⁻¹ for the 2020/2021 and 2021/2022 seasons (S). Cunco, La Araucanía region. Data represent the mean of four replications ± S.E. Different letters indicate statistical differences (p ≤ 0.05) among treatments within each spray program. NS, not significant.

Season	Treatment	May				June				July				August
2020/21	ETHA 0	0.14	±	0.04	a	0.19	±	0.05	ab	0.06	±	0.03	b	0.63	±	0.09	a
	ETHA 250	0.25	±	0.13	b	0.16	±	0.12	b	0.18	±	0.10	b	0.38	±	0.18	b
	ETHA 500	0.08	±	0.06	c	0.38	±	0.11	a	0.62	±	0.14	a	0.78	±	0.15	a
	ETHA 1000	0.02	±	0.01	c	0.04	±	0.03	c	0.12	±	0.06	b	0.63	±	0.18	a
2021/22	ETHA 0	0.14	±	0.04	a	0.19	±	0.05	a	0.06	±	0.03	b	0.63	±	0.09	a
	ETHA 250	0.02	±	0.02	b	0.20	±	0.13	a	0.00	±	0.00	b	0.90	±	0.10	a
	ETHA 500	0.00	±	0.00	c	0.06	±	0.05	b	0.05	±	0.05	b	0.55	±	0.16	a
	ETHA 1000	0.00	±	0.00	c	0.00	±	0.00	b	0.00	±	0.00	b	0.60	±	0.16	a
Significance	S	NS				NS				NS				NS
	T	NS				NS				NS				NS
	S × T	NS				NS				NS				NS

. For the 2020/2021 season, ETHA 500 showed higher ratio values in a major part of the studied months (except May) compared to the other treatments, while ETHA 0 and 1000 showed values lower than the mean for all the months studied. Conversely, in the 2021/2022 season, the treatments did not show significant differences for the months of July and August. However, during the months of May and June, significantly lower values (almost 0) were observed for ETHA 500 and 1000 in comparison to ETHA 0.

In both seasons, the month of August showed greater variations according to treatment, although without significant differences, except for ETHA 250 during the 2020/2021 season (0.78 ± 0.15). This effect was also observed in the 2021/2022 season, but without significant differences among the treatments evaluated, suggesting an evaluation over more productive seasons for greater representativeness.

Table 10. Inflorescence activity expressed in ratio of glomerulus/catkins determined from May to August for ETHA 0, 250, 500, and 1000 mg L⁻¹ for the 2021/2022 and 2022/2023 seasons (S). Cunco, La Araucanía region. Data represent the mean of four replications ± S.E. Different letters indicate statistical differences (p ≤ 0.05) among treatments within each spray program. NS, not significant, * p ≤ 0.05.

Season	Treatment	May				June				July				August
2021/22	ETHB 0	0.14	±	0.04	a	0.19	±	0.05	b	0.06	±	0.03	b	0.63	±	0.09	a
	ETHB 250	0.13	±	0.07	a	0.18	±	0.09	b	0.02	±	0.02	b	0.64	±	0.15	a
	ETHB 500	0.10	±	0.04	a	0.23	±	0.11	b	0.00	±	0.00		0.60	±	0.16	a
	ETHB 1000	0.11	±	0.07	a	0.46	±	0.29	a	0.00	±	0.00		0.13	±	0.13	b
2022/23	ETHB 0	0.06	±	0.03	b	0.27	±	0.09	a	0.68	±	0.21	a	0.91	±	0.09	a
	ETHB 250	0.05	±	0.02	a	0.30	±	0.09	a	0.89	±	0.22	a	0.92	±	0.08	a
	ETHB 500	0.01	±	0.01	b	0.09	±	0.05	b	0.29	±	0.14	b	0.50	±	0.17	a
	ETHB 1000	0.01	±	0.01	b	0.21	±	0.11	a	0.19	±	0.11	b	0.67	±	0.14	a
Significance	S	*				NS				NS				NS
	T	NS				NS				NS				NS
	S × T	NS				NS				NS				NS

illustrates the ratio of glomerulus to catkins in the four different treatments identified as ETHB (0, 250, 500, and 1000) during the months from May to August in two studied seasons. For the first evaluated season (2021/2022), ETHB 1000 showed the highest ratio during June, while for May and July the treatments showed no significant differences. This undoubtedly reflects a low interaction between glomerulus and catkins (p ≤ 0.05). However, during August, ETHB 0, 250, and 500 showed higher ratio values, exceeding up to five times the means obtained during the previous months (0.62).

For the 2022/2023 season, the differences were more pronounced between the months of May and June, with ETHB 250 resulting in higher interactions during the month of May. For July, ETHB 0 and 250 showed higher ratios, while in August there were no significant differences between the treatments evaluated, with interesting results for ETHB 0 and 250, Figure 7 provides a more graphic view of inflorescence behavior.

Figure 8 represents the shoot length (cm) over three consecutive seasons (2021, 2022, and 2023) in relation to different ETHA concentrations and evaluated in three cardinal orientations (CO) of the orchard: northeast (NE), northwest (NWT), and south (S).

For 2021, treatments of ETHA 250 and 1000 showed the highest shoot growth (30–35 cm), mostly for the NWT and S orientations. Consequently, for 2022, a generalized decrease in shoot growth was observed and resulted in ranges of 10 to 15 cm (mean) for most of the evaluated treatments (highlighting ETHA 0 and 250). Finally, for 2023, the results obtained were relatively homogeneous, showing a maximum growth (20–25 cm) for ETHA 1000 in the S orientation. This pattern suggests that both ETH spraying and environmental conditions associated with CO may influence the development of vegetative trees, highlighting the positive impact of higher doses in favorable seasons.

Concerning the shoot length (Figure 9) resulting from the first season studied (2022), higher growth was obtained for ETHB 250 (mean 6.68 ± 1.89), although no significant differences were obtained compared to the same season in relation to the evaluated parameters. However, for the 2023 season, higher shoot growth values were obtained for ETHB 500 (18.63 ± 4.82), resulting in a significant interaction (p ≤ 0.05) of the treatments evaluated in relation to the productive year studied. Furthermore, in both years, cardinal orientation had an impact, with NWT showing the highest shoot length for ETHB 500, with 10.02 ± 2.25 and 18.63 ± 4.82 in 2022 and 2023, respectively (p ≤ 0.05).

3.4. Premature Drop of Catkins

Figure 10a shows the catkins that fell during the 2020/2021 and 2021/2022 seasons, revealing a greater fall for ETHA 250 and and 1000 (37.35 and 38.34%) during the 2021/2022 season, while the lowest values (−5.93%) of fallen catkins were obtained for ETHB0 in the same evaluated season. However, for 2020/2021, no significant data were obtained regarding the decrease in the number of fallen catkins.

On the other hand, Figure 10b shows similar behavior in relation to the studied treatments, highlighting a higher number of fallen catkins under ETHB 1000 for the 2021/2022 season and under ETHB 500 in the 2022/2023 season.

4. Discussion

In our study about ETH spraying in preharvest, climatic conditions at the experimental site were similar to those reported by Meriño−Gergichevich et al. [7], which indicate climatic stability over the years, with rainy winters and high temperatures at the beginning of the harvest. Our experiment was performed for first time during March 2021, with 100 mm of precipitation, and similar behavior was observed in following years (2022 and 2023). Higher maximum temperatures (36.2 ± 1 °C) between ETH spraying and harvest (7, 15, and 28 DAA) times were determined. In fact, several abiotic factors are known to induce ETH synthesis, such as tempertures; however, in our experiment, ETHA 0 showed a lower nut drop at 7 and 15 DAA compared to other tretaments, indicating that ETH production was not triggrered by high temperatures in the orchard. Djanaguiraman and Prasad [24] reported enhanced ETH synthesis (2–3 times) in the leaves and pods of soybean plants (Glycine max L.) subjected to a high−temperature regime (28/38 °C) compared to plants under an optimal temeprature regime (18/28 °C). This higher ETH accumulation implied a reduction in the number of seeds per pod and, consequently, the yield plant⁻¹. Whereas several authors have studied the negative consecuences of increased ETH activity on yield and quality losses, our study did not show negative effects on hazelnuts sprayed and harvested under a typical temperature regime in March. Moreover, Poór et al. [25] indicated the key role of ETH on thermobalance through stress−related proteins, helping to maintain the functionality and stability of cells in plant organisms stressed by high temperatures.

On the other hand, according to Wiman and Andrews [26], the effects of preharvest ETH treatments were evident approximately 14 days after spraying on hazelnut cultivars Jefferson and Yamhill. These authors reported that treatments of 750 and 1000 ppm sprayed on Yamhill showed a peak nut drop (90%) almost three weeks after application, although no differences between treatments were found. However, nut drop was over 90% in Jefferson when ETH was sprayed in trees at 500, 750, and 1000 ppm in comparison to the control. However, ETHA spraying on the TDG studied in southern Chile demonstrated higher efficiency, with ETH adminsitered at 250 and 500 ppm during the 2020/2021 season. Instead, for 2021/2022 ETHA, the synchronization of nut drop was similar in all treatments throughout the assayed harvest times. In concordance with Valleser and Valleser [16], cacao pod ripening accelerated in response to higher concentrations (2500 to 5000 ppm) of ETH treatment, taking two days after applying the highest concentration of ethephon (5000 ppm).

Our work is the first approach to ETH application and evaluation during three consecutive seasons in fruit crops. When we sprayed the trees (ETHB) for the second time in 2021/2022, concentrations of 250, 500, and 1000 mg L⁻¹ showed at 7 DAA, with a similar rate of 78% of dropped nuts among the treatments. Spraying mature Dashehari mango fruits with an aqueous solution of ethrel (600 ppm) ensured faster and uniform ripening in four days [27]. In addition, some studies performed on Thompson Seedless table grapes suggested that ETH at 1445 and 2890 mg L⁻¹ promoted pre−harvest abscission, fruit quality, and ETH residue of grapes. Both ETH treatments strongly induced abscission, causing > 90% pre−harvest abscission [28], indicating that intermediate concentrations determined the benefits of synchronizing harvesting periods. In a two−season study, Wang et al. [29] reported that a high ETH concentration (4000 mL ha⁻¹) improved cotton (Gossypium arboreum) yield and earliness, while some quality features were also improved with even higher concentrations (6000 mL ha⁻¹).

One of the most important industrial parameters in the hazelnut industry is kernel yield. We evaluated this parameter at all harvest times, and our results showed that kernel weight was increased by harvest time but that ETH treament did not exert effects on this parameter. In fact, our findings showed kernel yields similar to those reported by Meriño-Gergichevich et al. (2021) [7], Gavilan-Cuicui et al. [22], Escobar-Hernández et al. [30], and other authors such as Erdogan and Özdemir [31], who indicated that TDG reached an average of 48.36% kernel yields in Chile and Turkey. Von Bennewitz et al. [23] presented that the same cultivar in central Chile reached an average of 44.27%. The above is also related to the results reported by Meriño et al. [7], who indicated that in the same geographical area of the study (Cunco, Chile), kernel yields were around 43.27%, contrary to the results reported by Manterola−Barroso et al. [19] for the same location but in the 2018/2019 season, indicating that kernel yield reached 38.14%. Additionally, in both studies, kernel weight was not related to a thicker shell in nuts. When we analyzed shell thickness, the season showed an effect on this structure. In relation to the NRI and KRI, seasonal implications are attributed to obtaining more rounded nuts and kernels.

Our findings suggested seasonal effects on the condition parameters of nuts. In terms of the percentages of moldy, blank, wrinkled, and double kernels for the three evaluated seasons, the numbers are similar to those reported by Gavilan-CuiCui et al. [22] for the TDG cultivar planted in the same orchard for the 2020/2021 and 2021/2022 seasons. Studies have focused on determining the relationships between fruit cluster drops and the development of the ovary, ovule, and embryo in commercial hazelnut cultivars under different nutritional and irrigation strategies, considering local weather variability [32,33,34,35]. Blank nuts, which show normal exterior size but lack an edible kernel in the shell, are an important reason for poor hazelnut yields [36]. Our work in the Fondecyt 11160762 project identified rates of up to 15% of blank nuts in plantations of the cultivar TDG (nine−year−old plants). In the middle of summer (January and February), this physiological occurrence is caused by insufficient soil moisture [37,38] and nutritional deficiencies [39]. Moreover, fruit cluster drops that occurred in summer (prior to harvest) were mostly due to nutritional deficiencies [40]. These abnormal nuts are rejected by the industry, and several agronomical strategies are followed in conventional hazelnut orchards to minimize these disorders, such as nutrient and water management, assited pollination, biostimulants, and growth regulators during the fruit set stage [7,41]. However, ETH applicatons are not included as part of these strategies. In this context, many factors, such as temperatures, irrigation, low availability of nutrients, and hormonal imbalance, have been associated with these dissorders, particularly during preharvest at the nut set and growth phenological stages [7,42,43]. In our study, ETH spraying was performed when many of these conditions were provoked in the nuts prior to harvest within the evaluated season.

In agreement with von Bennewitz et al. [23], the cultivar TDG exhibits a marked time lag in the maturation of its inflorescences, with catkins being the first to open in early June in central Chile. This physiological behavior, known as protandric dichogamy, is manifested after a vernalization period that ranges from 271 to 417 cold hours. Traditionally, La Araucanía possesses relatively favorable weather conditions for hazelnut crop, such as high relative humidity (75–80%), annual rainfall of 1200 mm, mean temperatures of between 13 and 14 °C, and a minimum of 700 h of chill temperatures (0–7.2 °C), which are required to break bud dormancy [33]. Hayashi et al. [11] reported delayed female flowering in three herbaceus species (Echinacea purpurea, Monarda didyma, and Physostegia virginiana) up to nine days after three ETH applications (1000 mg L⁻¹) concomitantly with a lower vegetative growth; however, the authors declared an increase in the number of inflorescences. Pasqualotto et al. [44] mentioned that temperature has a significant impact on the growth of hazelnut shoots. According to this study’s findings, the threshold temperature range (T−50%) that triggers the start of radial growth at these sites varied between 13.1 and 16.6 °C, with an average of 14.6 °C. The ratio of glomerules/catkins was followed between May and August for all evaluated seasons, and our findings indicated that a higher value (~1.0) was found in last month of winter (August) under all ETH concentrations sprayed in 2021 and 2022 for ETHA and ETHB.

5. Conclusions

There is little knowledge regarding the application of ETH on hazelnut traits planted in temperate zones such as southern Chile. Our findings confirmed that the application of ETH promoted the synchronization of nut drop, concentrating at 15 and 28 DAA. Moreover, the application via asfoliar spraying did not show a negative influence on subsequent seasons, which allowed for a reduction in fruit exposure to rain or humidity during harvest time. However, no influence of the application of ETH on the industrial yield of TDG trees was detected during the three evaluated seasons. As for the industrial parameters, such as kernel yield, NRI, and KRI, it was confirmed that doses of 500 mg L⁻¹ favored kernel yield, with 28 DAA being the time that presented the highest kernel yield, with 51.9%.

ETHA concentrations affected tree shoot length over three seasons. In 2021, ETHA 250 and 1000 doses achieved the highest growth, while a significant decrease was observed in 2022. However, in 2023, more uniform growth was recorded, especially with ETHA 1000 in the S cardinal orientation. For ETHB in 2022, ETHB 250 achieved remarkable shoot length, although without significant differences. However, in 2023, ETHB 500 exhibited significantly higher growth, with a significant interaction between treatments and seasons. In addition, the NWT cardinal orientation showed the greatest shoot length in both years, highlighting the relevance of ETH concentration and orientation conditions on shoot development.

Regarding the relationship between glomerules and catkins, although both structures were partially synchronized between May and July, ETH sprayed at 250 and 500 mg L⁻¹ showed the closest interaction between July and August, with the latter being the point at which synchronization reached its highest point.

Finally, this research represents the first insights into ETH use and its effects on productive components as a practical tool for hazelnut growers in temperate areas, optimizing harvest procedures and resources. However, several physiological and biochemical responses are being studied with the goal of merging those two different effects and the quality of the produced nuts.