Seedling height

For the development and growth of seedlings, the substrate used is a fundamental factor, since it contributes to the quality of the seedling (^{Ortega, 2010}). Currently there are a lot of materials that can be used for the production of substrates and their choice will depend on the plant species to propagate, propagating type, sowing season, propagation system, cost, availability and characteristics of the substrate (^{Hartman and Kester, 2002}).

According to its elaboration, there are several types of substrates used for the production of seedlings, since in few occasions the soil is used as a substrate due to the difficulty in its handling, for which reason it is necessary to select a material that replaces it so much in its physical as well as chemical properties, such as cation exchange capacity, porous space, water retention capacity, pH and electrical conductivity (^{Valdéz and Benavides, 2013}).

Of the three substrates evaluated in this work for the production of stevia seedlings, the results indicated that there were significant statistical differences (p< 0.05) in the height of the seedlings between substrates. It can be seen in Figure 1 and 2, that substrates 2 and 3 based on mixture of moss and coconut fiber (11.4 and 10.8 cm in height respectively), showed a greater growth with respect to substrate 1 (6.8 cm) of commercial foam. This may be due to the fact that chemically inert substrates, such as synthetic foam (substrate 1) act only as a support for the plant, while the rest are also involved in adsorption processes and are chemically active as the substrates, 2 and 3 (a base of moss and coconut fiber) since it has been reported that they intervene in the fixation of nutrients (^{Pastor, 2000}).

Figure 1 Development of stevia seedlings 22 days after sowing in three different substrates. 

Figure 2 Height of the stevia seedling in three types of substrates. 

The substrate 1, because it is a solid, inert, single-component substrate with a low cation exchange capacity (CIC), only acted as support for the seedlings, which resulted in the lowest growth in this type of substrate.

The substrate 2 based on moss and vermiculite, has a great water absorption capacity and a better CIC. The moss absorbs up to 20 times its dry weight and the vermiculite which is a mineral composed of aluminum-iron-magnesium silicate, has a structure in plates, which generates a high proportion of surface area/volume and therefore a high capacity of moisture retention (^{Bunt, 1988}). These characteristics were adequate for the production of stevia seedlings under the conditions used in the experiment, which agrees with that reported by ^{Gaudig et al. (2014)}, who indicated that moss possesses adequate physical and chemical properties for the production of diverse seedlings.

The substrate of coconut residues (substrate 3) presented an adequate growth of stevia seedlings, probably because it is a substrate of homogeneous granular structure, with high total porosity, high aeration capacity and water retention, low bulk density, pH between 5 and 6 and highly stable physical structure (^{Jasmin et al., 2003}; ^{Arévalo et al., 2016}).

The concept of water quality for irrigation refers to water characteristics that may affect soil and crop resources in their long-term use Bosch et al. (2012). To evaluate the water quality for irrigation, three main criteria must be defined: salinity, sodicity and toxicity (^{Gholami and Shahinzadeh, 2014}; Asamoah et al., 2015).

To measure the quality of irrigation water, direct and indirect variables are considered. The direct variables to measure water quality for irrigation are (1) salinity; (2) sodicity; and (3) alkalinity and specific ionic toxicity. Indirect variables, also called dependent environment are (1) tolerance of crops to salinity; (2) tolerance of soils to salinity, sodicity and alkalinity; (3) irrigation management; and (4) climate (^{Aragües, 2011}).

For the present study, information was available on the indicators that measure salinity, such as electrical conductivity (CE) and total dissolved solids (SDT), on sodicity, such as sodium absorption ratio (RAS), alkalinity a through pH and toxicity such as the contents of chlorides, boron and sodium. Regarding the independent variables and because the sowing was carried out in a protected structure, in containers with substrates and the same water management, only the tolerance of S. rebaudiana to salinity was contemplated.

Among the indicators evaluated on water quality, it was found with respect to the pH of the irrigation water that in the three treatments it tends mainly to a water of neutral pH (7.11 the highest and 6.9 the lowest). Waters with very high or very alkaline pH, hinder the absorption of nutrients in plants as they are precipitated and very acid pH can solubilize very toxic elements such as aluminum (^{Arzola et al., 2013}). In the present study none of the waters presented high alkalinity problems (> 8).

With regard to the RAS ^{Lingaswamy and Saxena (2015)} suggest using the criterion proposed by Richards that classifies it as follows: < 10 excellent, 10 to 18 good, 18 to 26 doubtful, > 26 not recommended. The RAS in T1 is good and T2 and T3 is excellent. Therefore, in the case of the RAS, there was no problem, especially due to the type of irrigation with a garden shower and the use of substrates.

The CE is an indicator of the total dissolved salts in the water and its classification is a subject that has been much discussed and questioned (^{Tartabull and Betancur, 2016}) since, to consider its effects, not only should this isolated indicator be taken, but also the texture of the medium, precipitation and tolerance to the salinity of the crop. For the case of the waters of T2 and T3 they were within the range of 0.2 to 2 considered as suitable for agricultural irrigation and the water of T1 with restrictions.

Another indicator closely related to the CE is the SDT, and according to the classification by the degree of restriction of use as irrigation water, it is considered that a water with SDT of <450 mg L^-1 has no restriction of use, waters with 450-2,000 mg L^-1 of SDT have a slight or moderate restriction and waters with SDT of > 2 000 mg L^-1 present a severe restriction ^{Cortes et al. (2009)}.

Due to this, it was expected that the seedlings irrigated with water with low CE and low content of SDT were the best, but did not show such behavior. This could be due to various factors such as the content and type of salts in the irrigation water, since the purified water due to its purification process had a sodium absorption ratio of 10.82 (high according to what is recommended for water from irrigation of <10) and interchangeable sodium percentage of 2.2, higher than well water with 3.91 and 0.81 of sodium absorption ratio and interchangeable percentage of sodium respectively. Another aspect that influences is that S. rebaudiana, is considered a plant moderately sensitive to salinity (^{González et al., 2016}), which allows it to support saline waters like those of T1, without an apparent damage in the development of the seedlings.

The height of the seedlings presents significant statistical differences (p< 0.05) with respect to the quality of irrigation water, agreeing with that reported by various authors, who have observed changes in the growth of the stem of different plants by varying the salinity of the plant. Water, mainly a decrease in growth due to an increase in salinity (^{Pares and Basso, 2013}; ^{Ruiz et al., 2014}).

It was observed that the height of the seedlings that received water from T1 (9.12^a cm) and T2 (10.24^b cm) were statistically different (p< 0.05); however, they were equal to seedlings irrigated with water with T3 (9.71^ab cm) (Figure 3).

Figure 3 Height of stevia seedlings in three water qualities. 

Due to these results, sodium played an important role in the growth of the seedlings since, although SDT and CE were low in the water of T3, the percentages of sodium absorption were adequate in the intermediate water quality, in addition It is possible that the cuttings require a certain amount of minerals to develop properly, making the irrigation water of T2 has been better.

Analyzing the response of the height of the seedlings with water from T1, it was observed that from day 3 of sowing the cutting, significant statistical differences were found (p< 0.05) between the type of substrates, being less the development of seedlings in substrate 1 (Table 2). This same behavior was observed on day 12 and 22, as well as with the seedlings obtained with irrigation water from T2 and T3. The best development of the seedling with substrate 2 and substrate 3 was very marked on the different sampling days and under any type of water quality. However, the best performance was of substrate 2 under the quality of water with medium SDT at 22 days after planting the cuttings.

Analyzing the results of heights for water quality in the growth time in the different substrates, the cuttings were observed on the 22^nd day of the cuttings, a significant effect (p< 0.05) of the type of water, finding a better development in the seedlings with water of T2 and T3.

Root length

The development of the root of the seedlings was influenced by the type of substrate (p< 0.05) with substrate 2 being the best, followed by substrate 3 and finally substrate 1 (Figure 4). On day 12 of development, the length of the roots, presented an average of 4.4 cm and 5.8 cm 22 days after planting the cuttings. Due to its characteristics the moss-based substrate allows a better rooting and an optimal development of the seedlings. It has been, reported that it has good physical and chemical characteristics and besides being good for stevia, it proves to be efficient for the production of forest and vegetable plants (^{García et al., 2001}; ^{Rangel et al., 2002}).

Figure 4 Root length of stevia seedlings in three types of substrate. 

Although the quality of the water had a significant effect on the height of the seedlings, the same effect on the length of the roots was not found being 5.0, 5.1 and 5.1 cm for T1, T2 and T3 respectively.

When analyzing the size of roots individually, it was found that with water from T1 and T3, substrate 2 and 3 were the best (Table 3); however, water from T2 was better on substrate 2. On day 22 regardless of water quality, substrate 3 (5.9 ^b cm) and substrate 2 (6.1^b cm) were better than substrate 1 (5.1^a cm).

Based on these results, it was observed that the quality of the water did not affect the size of the roots; however, although it was not measured, it was perceived that the secondary roots were more abundant in the seedlings irrigated with water of T2 and T3.