Introduction

The salicylic acid (AS) is a phenolic compound found naturally in many plant species and today is regarded as a plant hormone (^{Raskin, 1992}, ^1992a). Applications of low concentrations of AS mainly in seedling stage, regulate various physiological and biochemical process that affect the growth and development of plants (^{Larque-Saavedra and Martin-Mex, 2007}; ^{Hayat et al., 2010}). Studies in a variety of plants, have shown that AS increases the response to control pathogens and induces disease resistance (^{Rivas-San Vicente and Plasencia, 2011}) and stress (heat, water and salt) (^{Rivas-San Vicente and Plasencia, 2011}) (^{Joseph et al., 2010}); accelerates flowering (^{Martin-Mex et al., 2012}); regulates photosynthesis and respiration (^{Joseph et al., 2010}; ^{Hayat et al., 2010}); and increases the aboveground biomass (^{Villanueva-Couoh et al., 2009}) and root (^{Martin- Mex et al., 2013}).

In a review by ^{Vazirimehr and Rigi (2014)} published with different crops of agricultural interest (corn, wheat, soybean, cucumber and tomato) work, they found that the AS stimulates and increases the content of chlorophyll, flavonoids, minerals, leaf area dry weight of the plant and similarly regulates the rate of photosynthesis, water content in leaves and membrane functions. AS affects root growth (^{Gutierrez-Coronado et al., 1998}) inducing changes in the length, weight, perimeter, area (Villanueva-Cohuo et al., 2009; ^{Larque-Saavedra et al., 2010}) and morphology root (^{Machado-Echevarria et al., 2007}). This cascade of responses may be responsible for the increase in yield of fruits found in Lycopersicon esculentum Mill (^{Larque- Saavedra et al., 2010)}, Capsicum annuum (^{Elwan and El- Hamahmy, 2009}; ^{Sánchez-Chavez et al., 2011}), Carica papaya (^{Martin-Mex et al., 2012}) Capsicum chinense and Cucumis sativus (^{Martin-Mex et al., 2012}), to name a few examples.

Furthermore, particularly in grasses it has been reported that the imbibing wheat seeds in a solution of 0.01 mM of AS activity is incremented nitrate reductase, leaf number and fresh and dry matter per plant (^{Hayat et al., 2005}); while with 0.5 mM of AS greater accumulation of proteins, sugars and minerals in the plant under drought stress conditions (^{El Tayeb and Ahmed, 2010}) is presented. In this context ^{Tucuch et al. (2015)} mention that 1 uM of AS canopy sprinkled the seedling stage, there is a significant increase in fresh root weight, plant height and biomass and fresh whole ^{Lopez-Tejeda et al. (1998)} found that when applying AS (0.1 mM) on a leaf at the beginning of fertilization, the yield in seed production is raised to 15%. Corn some work done under salt stress conditions and non-saline, suggest that this molecule is involved in resistance to salt stress induces increased fresh biomass and total dry (^{Gunes et al., 2007}; ^{Khodary, 2004}) stimulates greater photosynthetic rate (^{Tucuch-Haas et al., 2015}).

Corn is one of the staples of the Mexican population (^{Morris and Lopez, 2012}), estimated that 59% of the energy required in everyday life are obtained from this grain (^{Sierra et al., 2004}). In the last years of this century, the population increase has led to the increased demand for cereal, so that yields are not enough, be imported approximately 30.5% of the total consumed in the country (^{Morris and Lopez, 2012}), which has led to the search for new alternatives that contribute to production.

With the above and the importance of culture background, the present study was carried out with seedlings of a race of corn Yucatan Peninsula, in order to evaluate the effect of spraying low concentrations of AS on the size of the root.

Metodology

For this investigation landrace seed known locally as Xmejen-naal (Nal-tel x Tuxpeño) provided by a farmer of the state they were used. The two independent experiments in this grass in order to evaluate the effect of spraying, with low concentrations of salicylic acid (AS) to the canopy in the seedling root length were established.

Both experiments were conducted during the month of September 2013 at the Center for Scientific Research of Yucatan, the first experiment was conducted in a quarter of plant growth, with a photoperiod of 12 h light/12 dark, with a photonic flow 150 μmol m^-2 s^-1 and temperature of 25 ± 2 °C. The seeds germinated and seedlings grown in perlite contained in foam cups with a capacity of 1 L. The second experiment was set in open sky conditions with an average temperature of 28 °C and rainfall of 13 mm. The seeds were germinated and seedlings grown in perlite contained in PVC pipe 4 cm diameter and 23 cm long. Seedlings of both experiments were watered in the morning with 20 mL plant^-1 day^-1 of distilled water.

The AS concentrations tested were 0.01, 0.1 and 1 μM of AS and control that was only applied water, it should be mentioned that under field conditions for treatment 0.01 μM, no data were collected. For the preparation of solutions AS the methodology described by ^{Gutierrez-Coronado et al. (1998)} he was followed. Sprays were made to the canopy of the seedlings when they presented the first full leaf with ligule, for five consecutive days between 8-9 h. Five or ten days after the last application of AS, seedlings of both experiments were harvested for measurements of root and shoot. Measuring root length and plant height was performed with a millimeter ruler; fresh root weight and total fresh biomass, with an analytical balance (Sartorius, BP221S); the volume of the root with a graduated cylinder using the method displacement; and finally stem diameter with a digital vernier (Truper, CALDI-6MP).

In both experiments a completely randomized statistical design with seven replicates per treatment, each group contained repetition of a plant was used. The results were analyzed by ANOVA and when statistical differences were detected, the comparison of means was performed by the method of Tukey (p= 0.05) with the SAS statistical package.

Discussions

The data length, fresh weight and volume of the root of both experiments are presented in Figure 1. As shown AS favored length increased in both cases compared to control. Under controlled conditions (growth chamber), concentration 1 μM of AS significantly increased length in 30.6% compared to control, whereas treatment of 0.1 μM of AS increase the length in 19.3% and treatment of 0.01 μM of AS no effect was detected. Moreover, treatment of 1 μM of AS 7.4% favored in fresh root weight compared with the control, although this difference was not significant. The volume of the root- treated seedlings 0.1 μM of AS increased 24.7% and 1 μM of AS 14.9% compared to control.

Figure 1 Effect of different concentrations of salicylic acid to corn seedlings sprayed on the length, fresh weight and root volume. Each block is the average of seven individuals. Means with different letters mean they are significantly different. 

In the open sky conditions AS increase the root length significantly by 18% when 0.1 μM of AS and 10% was sprayed, although not significantly, with 1 μM treatment of AS compared to the control. The fresh root weight (Figure 1) was significantly increased up to 35% with 0.1 μM of AS compared to control, with 1 μM of AS favored it by 19%, which was not significant. No significant differences in the root volume in any of the treatments were found AS although the stimulation pattern by the effect of AS compared to the control was maintained. Values of 15.5 and 13% increase over control for the effect of 1 μM and 0.1 μM of AS were recorded.

It can be seen in the graphs the effect that the AS on root growth in this grass although responses bioassay shows us a slightly different pattern in the dose response curve. The effect found in the present study, in both experiments, consistent with what has already been reported in other species as Glycine max (L.) (^{Gutierrez-Coronado et al., 1998}), Lycopersicum esculentum (^{Larque-Saavedra et al., 2010}), Triticum aestivum (^{Arfan et al., 2007}), Zea mays under salt stress conditions (^{Khodary, 2004)}, Catharanthus roseus (^{Echevarria-Machado et al., 2007}) and Chrysanthemum morifolium (^{Villanueva-Couoh et al., 2009}). Although unknown the mechanism of action of how the AS applied in the canopy seedling stimulates root development, published data indicate that the AS promotes cell division apical root meristem (^{Shakirova et al., 2003}), as well as increasing the size of the cap and the emergence of lateral roots (^{Echevarria-Machado et al., 2007}), which could happen in this experiment.

Other variables that were estimated in the canopy of seedlings in the two experiments reported in previous paragraphs are presented in Table 1. It can be seen that the plant height increases by 22.1% with the treatment of 1.0 μM of AS in whereas with 0.1 μM of AS this increase it was 23% compared to control. Similarly stem diameter was stimulated in 12.3% and 19% compared to control treatments with 1.0 and 0.1 μM of AS respectively, differences however, were not significant. Unlike the above variables, the fresh weight of biomass of seedlings showed a significant increase in the concentration highlighting 0.1 μM of AS with an increase of up to 42% in the experiment developed under field conditions. This effect was not detected in the experiment developed in the growth room.

Table 1 Spray with different concentrations of salicylic acid in the diameter, height and total fresh biomass of corn seedlings grown in growth chamber (CC) or in open air (CA). 

The results of the effect of AS in the canopy of the plants reported in this study are consistent with published by ^{Khodary (2004)}, who reported significant differences in these variables for corn developed in conditions of salinity in response to applications 10^-2 Mof AS. The same effect has been reported in other species as Carica papaya (^{Martin-Mex et al., 2012}), Lycopersicum esculentum (^{Larque-Saavedra et al., 2010}) and Chrysanthemum morifolium (^{Villanueva-Couoh et al., 2009)}, which reported increases in height and stem diameter. However it should be noted that our data differ from those published by ^{Khan et al. (2003)}, who report that found no significant differences in overall height and biomass fresh corn sprinkled with 1.0 and 0.01 μM of AS in plants grown in soil under greenhouse conditions.

The results of this study mainly related to the effect on length and root volume, organ of vital importance for the whole plant organism, support the proposal that foliar applications of AS in seedling stage, favor a greater absorption of the ions soil because the scanning area of the plant is greater, an approach that was published by ^{Larque-Saavedra and Martin-Mex (2007)}, who mentioned that one of the mechanisms by which bio-productivity increases with AS, is through modification of the root. Similarly it is supported by data published by ^{Gunes et al. (2007)}, and Ahmed ^{El Tayeb (2010)}; ^{Sanchez-Chavez et al. (2011)} and ^{Fahad and Bano (2012)} who have reported increases in the content of essential ions in the tissues of plants treated with AS.

These results confirm the importance of this molecule as a plant growth regulator and highlights its potential use to increase bio-productivity of grasses, as has suggested ^{Larque-Saavedra and Martin-Mex (2007)}.

Conclusion

Spraying of low concentrations of salicylic acid to increase corn seedlings rooted size, regardless of the culture conditions.