Introduction

The deterioration is a process that occurs in relatively dry seed and is expressed during rehydration of it. In this regard, three hypotheses are suggested to describe the processes determining deterioration: 1) accumulation of deleterious products related to breaking of macromolecules that inactivate both enzymes and nucleic acids or the occurrence ofnon-functional membranes and accumulation of mutagenic substances; 2) deterioration caused by wear and tear, which is considered to be an increase in the use of organelles, cells and organs, causing overall reduction in operating capacity; and 3) somatic mutations that increase in frequency with age seed (^{Burris, 1983}).

The deterioration in seeds is related to their age, moisture content and storage conditions. The deterioration decreases the percentage of germination, growth rate of weak or low vigor seedlings, characteristics which are evident during the establishment of the seedling field (^{Veselova and Veselovsky, 2003}) and tolerance to adverse conditions. The symptoms of impaired seed include: abnormal growth, damage to the main structures of seedlings, loss of soluble compounds (due to excessive membrane permeability), reduced enzyme activity, oxidative damage to DNA and proteins, and production of toxic substances (^{Basavarajappa et al, 1991}).

To counteract the negative effects of the degenerative process in seeds, have employed various treatments with partial success in various species, such as chemical treatment before planting (pre-sowing) (^{Grzesik and Janas, 2014}) and pre hydration (pre-soaking) with water or growth regulators this to improve the germination capacity and vigor (^{Butola and Badola, 2004}; ^{Afzal et al., 2005}; ^{Herrera et al, 2011}).

In this regard, the imbibition is the process of taking water from the seed (^{Moreno et al., 2006}). This occurs by immersing the seeds in osmotic solutions or certain amounts of water during a period of time. Imbibition allows a greater number of seeds quickly reach the same level of humidity and activate the metabolic apparatus related to the pre-germination process (^{Burgas and Powell, 1984}). It is therefore, that through the process of seed imbibition in water or various solutions is feasible to improve their physiological quality through uniformity in germination percentage (^{Artola et al, 2003}; ^{Sánchez et al, 2007}). ^{McDonald (2000)}, mentions that small seeds like onion, celery, carrot, tomato, pepper and lettuce, osmotic conditioning is successful. ^{Balaguera et al. (2009)} conducted a study with seeds of tomato (Solanum lycopersicum L.) inhibited in water with different concentrations of gibberellic acid and reporting, statistical differences in leaf area, fresh and dry mass of leaves, stem and roots and root length. Based on the above, the objective of this study was to evaluate the effect of soaking in the physiological quality of tomato seeds.

Materials and methods

The study was conducted in March 2015 in the laboratory of Molecular Genetics Montecillo Campus Postgraduate College, Montecillo, Texcoco, Mexico. The seed of a commercial was used of cultivar of tomato of the house Hortaflor: Saladet aged naturally with an initial germination percentage of 60%.

A seed was applies a pretreatment (SPT) consisting of immersing a sample of five grams of seed follows: the seed sample was placed in a bag flannel, subsequently introduced to a polyethylene vessel filled with distilled water (with capacity of 2 L) was aerated vigorously with the aid of an air pump for aquarium ELITE® 802 (Rolf C. Hagen Inc.) and an airstone Biozon Fragor air at room temperature for 20 h. After pregerminative treatment, the seeds were dried on paper towel at room temperature for 18 hours to remove excess water. The control treatment (ST) was one where the seed was not subject to pre-treatment (SPT).

The seeds subjected to pre-treatment (SPT) and the control seed (ST) were germinated in Petri dishes on sanita paper moistened with 3.5 ml of distilled water and they were, placed in each treatment four replications of 25 seeds. Subsequently, they covered with the lid of the box and immediately placed in glass desiccators vacuum adjusted at 200, 400 and 600 mm Hg (vacuum) where they were kept 12 days under ambient laboratory conditions with a temperature of 25 ± 1 °C. The variables evaluated were: total germination percentage (PG), percentage of viability (VIA); weights of the dry matter of the aerial part (PMSA) and the radicle (PMSR) expressed in micrograms (mg) after dried in an oven at 70 °C for 72 h (^{ISTA, 2012}).

The design was completely randomized factorial arrangement where the factor A are seed treatments subjected to pre-treatment and the control factor and seeds B are vacuum conditions (VAC 200, VAC 400 and VAC 600) with four replications. The results of the response variables underwent analysis of variance using the Statistical Analysis System (^{SAS, 2000}) statistical software version 9.0, and the differences between treatments were estimated with the mean comparison test with Tukey 5% probability. Prior to analysis of variance, the ratio of germinated seedlings per day expressed as percentages, were transformed by function T= arcsine = Ay/100, where y is the value transform and T is the transformed variable value. In addition, relations were estimated between the different variables recorded in this study and were described by simple Pearson correlations.

Results and discussion

In the Table 1 shows the analysis of variance for the variables of physiological quality where present, significant differences (p≤ 0.001 and p≤ 0.05) for seeds subjected to pre-treatment (SPT) and control (ST) were recorded in the variables PG, VIA and PMSR while that for the three vacuum conditions (VAC) if there were significant differences (p≤ 0.001) except in PMSR. In the interaction of SPT seeds and ST by the vacuum conditions (VAC), no statistical significances were found in all response variables.

Table 1 Mean squares analysis of variance for physiological quality variables evaluated in tomato seeds. 

±PG= germinación total en porcentaje; VIA= porcentaje de viabilidad; PMSA= peso de la materia seca de la parte aérea; PMSR= peso de la materia seca de la radícula. **, *= significancia estadística al 0.001 y 0.05 de probabilidad; ns = no significativo. CV= coeficiente de variación. JDato transformado con arco seno.

Moreover as the comparison Tukey for the seeds subjected to pre-treatment (SPT) and control seeds (ST) was observed in Figure 1, that when using SPT these started faster germination, seeing reflected in VIA greater compared to ST. That is, for PG was taken 74.4% in SPT cm and 69.5% in ST; VIA 74.8% in SPT and 69.7% in ST. ^{Moreno and Jiménez (2013)} found that in tomato seeds embedded in potassium nitrate germination percentages of 87.6 to 96.19%, respectively. Furthermore, these results demonstrate that the imbibition process occurs is a very important in the intensity of metabolic activity (^{Burgas and Powell, 1984}), increase, being breathing the first activity to be detected in minutes and thus accelerates the process germination and seedling emergence (^{Bewley, 1997}; ^{Bewley and Black, 1994}).

Figure 1 Behavior of the total germination percentage (PG) and the percentage of viability (VIA) in tomato seeds subjected to pre-treatment and control. Values with the same letter in the figure are statistically equal (Tukey a= 0.05). 

^{Bewley (1997)}; ^{Sánchez et al. (2001)} mention that imbibed seeds in water, show three phases of the respiratory activity fully recognized. In phase i which is shorter in time, the seed absorbs water rapidly, starts breathing, is carried out repair genetic material and mitochondria and initiates protein synthesis from preformed messengers. During Phase II, the synthesis of proteins from messengers that are synthesized in the same phase II and new mitochondria are produced. At the end of this phase II seed completes the process of germination and emergency, in the phase III starts seedling development (posgerminativa phase). It is important to note that if the phases of imbibition arrived too prolong these affect the process of germination, emergence and seedling development (^{Bewley and Black, 1994}).

Concerning the effect of the treatment applied to the SPT and ST, it was observed that the use seed PT was greater PMSA and PMSR regarding the control seeds (ST) which indicates that the pre-treatment of imbibition significantly affected distribution biomass between shoot and root. This is very important because the initial force depends on the production of biomass per seedling (^{López et al, 2004}) and its distribution among organs that alters its quality (Figure 2).

Figure 2 Weights of the dry matter of the aerial part (PMSA) and the radicle (PMSR) evaluated in tomato seeds subjected to pre-treatment and control. Values with the same letter in the figure are statistically equal (Tukey a= 0.05). 

In comparing means for vacuum conditions (VAC) was observed, that by subjecting the seed in the VAC 200 were obtained the highest values of PG and VIA with 82.9% in PG and 83.9% in VIA compared, with treatment VAC 600 which had 51.2% of PG and VIA. According ^{Artola (2002)} and ^{Artola et al. (2003)} as the vacuum increases, the capacity of seed germination is decreased (Table 2). These results demonstrate that oxygen is one of the main factors affecting the germination process, the substrate being required for respiration to produce energy, which is necessary for germination occurs (^{Taylor, 1997}).

Table 2 Comparison of means for treatment under vacuum (VAC) in physiological quality variables evaluated in tomato seeds. 

± PG= germinación total en porcentaje; VIA= porcentaje de viabilidad; PMSA= peso de la materia seca de la parte aérea; PMSR= peso de la materia seca de la radícula. Medias con la misma letra en cada columna son estadísticamente iguales (Tukey α= 0.05).

As for the weight values of dry matter of different plant structures ranged from 0.009 to 0.024 mg for PMSA and 0.004 to 0.006 mg for PMSR being also the vacuum condition VAC 200 which had the highest values in these two parameters, so it can be considered that VAC test was appropriate to differentiate the seeds for its vigor (Table 2).

As for the generated combinations seed pre-treatment without pretreatment under vacuum (SPT x VAC) through variables PG, VIA, PSMA and PMSR, increased response was obtained in the pre-treated seeds without pretreatment VAC 200 being the optimum condition to establish the condition of seed vigor tomato. The variation observed in combinations for PG was 49.9 to 83.3%, VIA49.9 to 84.6%, PMSA of 0.007 to 0.024 mg and PMSR of 0.003 to 0.007 mg (Table 3).

Table 3 Comparison of means for treatment under vacuum (VAC) in physiological quality variables evaluated in tomato seeds. 

±PG= porcentaje de germinación total; VIA= porcentaje de viabilidad; PMSA= peso de la materia seca de la parte aérea; PMSR= peso de la materia seca de la radícula. Medias con la misma letra en cada columna son estadísticamente iguales (Tukey α= 0.05).

Based on the results shown in Table 3, it follows that between seed pre-treatment without pre-treatment under high vacuum (VAC 600) respiratory activity decreased this due to lack of oxygen being the main cause of the drastic reduction in germination, viability and accumulation of dry matter of the tomato seedlings.

Moreover, by applying the correlation coefficient Pearson to data obtained in this investigation it was found that, the total germination percentage (PG) is positively correlated with the variables weight of the dry matter of the aerial part (PMSA) (r = 0.81; p≤0.0001) and radicle (PSMR) (r = 0.33; p≤0.06). These results demonstrate that the type of statistical analysis to the variables evaluated in the physiological quality underwent, the sensitivity and accuracy of the pre-treatment of imbibition and the vacuum condition, the important criteria to differentiate seed lots based on their potential value for sowing seed were the PG and PSMA. In this regard, ^{Thomson (1979)} indicates that the higher seed germination will result in larger seedlings have a higher dry matter accumulation when germinating under favorable conditions (^{Pérez et al., 2006}). This response is explained based on a high energy output in the form of ATP due to a high content of protein in the mitochondria (^{McDaniel, 1973}).

Conclusions

The seed imbibition subjected to pretreatment in water germinated faster than seeds without imbibition. By vacuum test VAC 200 could differentiate between tomato seed lots vigor by the condition of their seeds being this an important criterion to determine the potential value of the seed for planting. The pretreatment of imbibition and in combination with the condition of VAC 200 accentuate differences to ascertain the condition of seed vigor tomato.