Introduction

Resistance to mechanical damage in corn seeds is a factor that is present at different stages of the production process, such as the sowing, harvesting, drying, production and storage stages (^{Basu, 1994}; ^{Desai et al., 1997}). The seeds are affected because mechanical damage reduces the physical, physiological and sanitary qualities. On the other hand, the design of agricultural machines frequently takes into consideration the physical characteristics of the products that it processes in order to perform an efficient and effective job in relation to the consumption of energy (^{Mohsenin, 1986}).

Seed resistance to compression plays an important role in the operations performed by the industry, as well as the machine design (^{Mohsenin, 1986}), as it is deemed important for the machines used and the speed of the process to inflict as little damage as possible to the seed. Resistance to compression is also important in the genetic improvement to produce seeds that adjust to the machinery and to the processes, among other important factors. In addition to being subjected to compression during their handling and processing, the seeds are also subjected to impact. According to ^{Mohsenin (1986)}, in order to cause equivalent bruise damage in apple fruit, approximately 1.5 more times the impact energy is needed than compression; it is therefore considered that a corn seed compression test could significantly reflect its resistance to impact. ^{Tipler and Mosca (2005)} indicate that it is possible to determine the average force exerted during one collision; it is therefore considered possible to relate the strength and the displacement from one compression test with the strength and the displacement from an impact test through the use of the appropriate equation.

The results obtained by some authors suggest that the mechanical resistance of the seeds depends on several factors, among which the humidity content, ripeness and the seed variety can be found. ^{Mesquita and Hanna (1993)} found a minimal resistance with lower humidity contents in soy beans. Bilanski (1966) cited by ^{Mohsenin (1986)} found that the necessary energy in order to cause a fracture in corn, wheat and soy seeds increases proportionally to the humidity content. In regards to this, ^{Mohsenin (1986)} and Multon (1981) cited by ^{Foutz et al. (1993)} mention that the increase in the water content “linked” to the protein, starch and pentose matrixes made the seeds less resilient to deformation, but more resistant to fracture. On the other hand, King and Riddolls (1959) cited by ^{Mohsenin (1986)} found that as the humidity content of the seed rose, the visible damage decreased but so did the percentage of germination, which indicates that the assessment of mechanical damage did not reflect any physiological damage.

On the other hand, ^{Gaytán-Martínez et al. (2006)} found differences between the seed hardness in 21 corn varieties. This was explained through a function, the components being the flotation index and the density. The density could be explained through the size of the starch grain, crystallinity and percentage of endosperm.

Notwithstanding the studies done on the mechanical damage of the seed, we still need to research several aspects that can help us better understand this phenomenon, and thus we could decrease the mechanical damage and its effect on the seed quality.

The aim of this investigation is to study the fracture response of the corn seed considering its strength and displacement in a compression test, according to the texture of the endosperm and the humidity content, and to evaluate the effects on the physiological quality. Our objective is to prove the hypothesis that states that the fracture strength is superior in a phenotype with a greater percentage of crystalline endosperm and with a higher humidity content.

Materials and methods

The study was done from 2012 to 2013 in the materials laboratory of the Departamento de Ingeniería Mecánica Agrícola, at the Universidad Autónoma Chapingo (UACH) and in the laboratory of Análisis de Semillas of the Programa de Producción de Semillas, Colegio de Postgraduados.

In order to evaluate which type of endosperm requires greater strength, displacement (by deformation) and more energy at the fracture point by compression, corn seeds from three varieties were used with different types of endosperm: a) Cacahuacintle (floury); b) HS-2 hybrid (semi-floury); and, c) Popcorn (crystalline). The seeds used from the HS-2 hybrid (semi-floury, also known as semi-dented) and from the Cacahuacintle variety (floury) were produced in 2011 in Texcoco, State of Mexico. The Popcorn seed (crystalline) was acquired in the same year in a commercial house, which at the moment of its acquisition presented a germination of 97%, similar to the initial germination of the HS-2 hybrid (97%) and of the Cacahuacintle variety (95%); prior to the study, the humidity of the Cacahuacintle, HS-2 and Popcorn was 7.66 and 8.87% in a humid base (h.b.). The percentage of floury and crystalline endosperm for each variety was determined, as well as the percentage of pedicel, pericarp and embryo, with the objective of relating the strength, displacement and energy at the fracture point by compression with the type of endosperm.

In order to find out the effect of humidity on strength, displacement (by deformation) and energy at the fracture point by compression, two levels of seed humidity were evaluated: 12 and 20% ± 0.1 in humid base (h.b). To achieve these humidity percentages, the seeds were moistened with distilled water from 2 to 2.5% above the humidity desired and were then dried at room temperature until obtaining the moisture assessment (12 and 20%). The seeds were kept (no more than 4 h) in airtight containers until the moment of compression. After compression, the seeds were dehydrated under environmental conditions for 24 h, and placed in a stove at 30 °C for 20 to 24 h. Finally, the seeds were exposed to room temperatures until reaching a humidity of 9 to 10%.

The strength, displacement (by deformation) and energy were assessed at the fracture point by compression in the seeds using the study factors previously mentioned (variety and humidity), applying compression according to the methodology recommended by the ^{ASAE (2005)} as indicated in the following paragraph. Subsequently, the compression energy necessary for the fracture of the HS-2 hybrid obtained in this study was compared to the impact energy for the same case reported by ^{Mancera et al. (2007)} with the objective of estimating the degree of similarity between both types of energy.

In order to understand the effect of the fracture caused by the compression of seeds on the physiological quality, two compression levels were assessed: control (without compression) and fracture. To apply the compression to fracture, each seed was placed in a resting position with the embryo facing upwards between flat and polished surfaces (^{ASAE, 2005}) at a displacement crosshead speed of 1 mm/ min in an Instron® universal testing machine model 3385H (Instron, Corp., Norwood, MA, USA) with a cell charge of 250 kN (kilonewton). The displacement of the crosshead stopped when a change in slope was observed characteristic of a fracture in the “strength-displacement” curve or when noise caused by the rupture of the seed was detected. For the latter, a microphone placed close to the support of the seed (bottom plate) and connected to a computer with a sound-amplifier of 36 Db (decibels) was used, along with digital noise cancelling and acoustic echo. The volume of the speakers was adjusted to a sufficiently audible level, with the help of the sound level graphic in order to prevent discarding low intensity sounds.

Experiment design and statistical analysis

In order to describe the seed texture for each variety by using the seed components, a completely random DECA experimental design was used with one study factor: seed variety. Afterwards, an analysis of variance and a comparison of averages (Tukey, 0.05) between varieties for each seed component were carried out. To know the effect of humidity and the type of endosperm on the strength, displacement (by deformation) and energy to the fracture point by compression and on the damage index, a DECA with two study factors was used: the variety (Cacahuacintle, HS-2 and Popcorn) and humidity content in the seed (12 and 20% ± 0.1 h.b.). Then, an analysis of variance and a comparison of averages were performed for each of them (Tukey, 0.05).

In order to know the effect of humidity, the type of endosperm and the compression level on the physiological quality, a DECA was used with three study factors: variety (Cacahuacintle, HS-2 and Popcorn), humidity content in the seed (12 and 20% ± 0.1 h.b.) and compression level (control and fracture). Afterwards, an analysis of variance and comparison of averages were done (Tukey, 0.05) for the main effects. The analysis of variance and the comparison of averages were done with the help of the ^{SAS version 8 software (1999)}.

Evaluated variables

Seed components. The dry biomass weight of the seed components was determined using the dissection method described by Salinas and Vázquez (2006). Three repetitions of 25 seeds were weighed and then moistened with water at 70-80 °C for 15 min. Subsequently, the pedicel, pericarp, embryo and endosperm were separated with the help of a scalpel, determining their dry biomass weight through the methodology described by the ^{ASABE (2006)}, which consists in drying the samples on the stove at 103 °C for 24 h. The samples used for this evaluation were independent to the ones used for the rest of the evaluations, as this evaluation was destructive.

Strength and displacement by compression. For each treatment, the information was analyzed corresponding to the strength (S) and displacement (D) collected by the BlueHill 2.0® software (^{Instron®, 2006}). The energy (E) was determined by the calculation of the approximate area under the curve (E=DxF/2). The methodology followed in order to carry out the compression is mentioned above. Each treatment was evaluated in samples of 30 seeds with six repetitions.

Damage index. With the help of a stereoscopic microscope, a morphological characterization of external damage was done in samples of 45 seeds, in three repetitions of fifteen seeds. The seeds were numbered as follows: 1) without fracture (WF); 2) with tissue detachment, severe fracture (SF); 3) with base fracture (B), which corresponds to the zone of the first proximal quarter of the seed regarding the thread; 4) in the medium region of the seed (M); 5) in the region of the embryo and scutellum (ES); and, 6) in the crown (C), which corresponds to ¼ distal seed (Figure 1). Subsequently, the type and location of the damage were categorized, assigning a severity level to each category: WF= 0, FC=1, FB and FM=2 and SF and FE=3. The most elevated values will reflect more severe damage and therefore lower physical and physiological seed qualities. The sub-samples used for this evaluation came from the samples where the strength and displacement by fracture were determined and reintegrated to the samples where the physiological quality was determined, since this evaluation was not destructive.

Figure 1. Damage zones in the seed: fracture in base B, comprised by the first quarter of the longitude; in the medium part M, comprised between 25 and 75% of the longitude; in crown C, corresponding to the distal quarter of the seed’s longitude; and in the region over the embryo and scutellum ES, in the ventral side. 

Physiological quality. The physiological quality was evaluated by the standard germination test, emergence speed of the radicle and the dry biomass weight of the seedling components. The germination test was done using the “between paper” methodology, which consists in wrapping the seeds in paper towels and hydrating them inside polyethylene bags (25 x 35 cm) in a germination chamber at 25 ± 5 ºC; after 7 days the (%) of the number of normal seedlings was calculated (^{ISTA, 2005}). The standard emergence speed and germination were evaluated with a sample of 90 seeds which were grouped into 3 repetitions of 30 seeds per treatment.

Both the evaluation of the emergence speed of the seedling and the standard germination test were done on the same experimental units. The number of seeds with root emergence was counted on a daily basis during seven days and the root emergence speed index was calculated with the equation proposed by Maguire (1962):

Where: VG= root emergence speed; X1, X2 y Xn= percentage of seeds with root emergence, daily for seven days; Y1, Y2 and Yn= number of days since the sowing of seeds [a dimensional].

The dry biomass weight of the seedling components (seed remnants, aerial part and root) was determined on day nine through the methodology described by the ^{ASABE (2006)}.

Results and discussion

Seed components

There was a statistical significance in the percentages of pedicel (p≤ 0.05), pericarp (p≤ 0.01), embryo (p≤ 0.01), soft endosperm (p≤ 0.01), hard endosperm (p≤ 0.01) and total (p≤ 0.01) between the seed varieties (Table 1). The seed from the Cacahuacintle variety showed the highest percentage of dry biomass by: embryo (11.5%), soft endosperm (76.2%) and pedicel (2.5%), even though the percentage of the pedicel was not statistically different than that of the HS-2 pedicel biomass (2%). Conversely, the Popcorn seed had the highest percentages of dry pericarp biomass (9.3%) and hard endosperm (65.2%), while the HS-2 seed presented the highest dry biomass percentage of total endosperm (82.3%, Figure 2). According to Sema et al. (2008), the smaller seeds had a higher proportion of pericarp than the bigger seeds, which explains the pericarp difference between Popcorn (smaller seed), HS-2 and Cacahuacintle (bigger seed).

Table 1. Mean squares of the seed components in the Cacahuacintle, HS-2 and Popcorn varieties. 

Fuente de variación	Grados de libertad	Pedicelo (%)		Pericarpio (%)		Embrión (%)		Endospermo suave (%)		Endospermo duro (%)		Endospermo Total (%)
Variedad	2	0.4728	*	21.7414	**	7.5917	**	2936.03	**	2883.49	**	2.6808	**
Error	6	0.0553		0.0565		0.1067		3.84		3.79		0.0217
CV (%)		10.91		3.82		3.20		4.70		4.89		0.18
R2		0.74		0.99		0.96		0.99		0.99		0.98

Figure 2. Seed components of the Cacahuacintle, HS-2 and Popcorn varieties. The difference of each component between varieties is indicated by minuscule letters (Tukey, 0.05). 

Effect of seed variety and humidity on the damage index

Some seeds classified in accordance to their index damage for the evaluation are shown in Figure 3. The variety had a significant effect (p≤ 0.05) on the damage index, while the humidity content of the seed (Table 4). The HS-2 variety presented the highest damage index and the Popcorn variety presented the lowest (Table 4).

Figure 3. External damages in the Cacahuacintle seed: A) seed without fracture; B) with severe fracture; C) fracture on the base; D) fracture on the middle part; E) fracture on the crown; and F) fracture on the embryo. 

Table 4. Mean squares and main effects of the study factors: variety and humidity content of the seed on the damage index. 

Fuente de variaición	Grados de libertad	Índice de daño
Variedad (V)	2	17 438.61	*
Humedad (H)	1	1 670.81	NS
V*H	2	1 452.27	NS
Error	12	3 246.65
CV (%)		20.73
R2		0.50
Efectos principales
Variedad
HS-2		327.78	a
Cacahuacintle		276.67	ab
Palomero		220.01	b
DMS 0.05		87.7610
Humedad (%)
12		265.1	a
20		284.45	a
DMS 0.05		58.5240

Conclusions

The seed of the Cacahuacintle corn variety with floury endosperm presented the lowest resistance to the compression force and the most displacement for its fracture. The Popcorn variety with crystalline grain needed more compression strength than the other two varieties for its fracture, even though it was not statistically different than the HS-2 variety of semi-floury grain in regards to the displacement for its fracture in the compression test. There was no statistical difference in the compression force required for the seed fracture between the seed humidity of 12 and 20%, but with 20% humidity there was a greater displacement. The variety with the highest damage index was HS-2, which had the highest percentage of endosperm and germination; the latter may be due to the strength of the seed. Humidity had no significant effect on the damage index, but it had an effect on the percentage of germination.