PRECISION TOOLS FOR IRRIGATION MANAGEMENT OF TOMATO SEEDLINGS

Seedling production is important to vegetable productions, however, irrigation planning for seedling production is usually inefficient due to the lack of information about water consumption by the seedlings, which decreases the quality of the seedlings. The objective of this work was to evaluate the use of hydrogel in the substrate and determine the appropriate irrigation management for the production of tomato seedlings (Solanum lycopersicom), using an automated irrigation system. The experiment was conducted in a protected environment at the Federal University of Viçosa, Brazil. Tomato seedlings were grown in polyethylene trays on substrates with, and without hydrogel. The seedlings were irrigated using an automated micro sprinkler system. Irrigation treatments were chosen considering different crop evapotranspiration (ETc) and reference evapotranspiration (ETo), and the use of timer. The height, root length, stem base diameter, root and shoot dry weights, seedlings were evaluated. The use of the hydrogel for the production of tomato seedlings resulted in seedlings with higher final heights, larger stem base diameter, greater shoot dry weight, larger crown area projection, and greater shoot relative growth rate.


INTRODUCTION
Seedling production is an important stage, since good quality seedlings better express their genetic potential, and directly affect the plant yield. Environments used for seedling production usually shelter different crops, and water management is standardized for all crops because of the practical difficulties of applying different water regimes for each crop (REGHIN et al., 2007).
Appropriate water managements promote seedling development and water saving, since only the amount of water required by the plants is applied. The use of practices for water use efficiency through the application of appropriate irrigation depths and frequency are essential for the success of irrigated agriculture (VALNIR JÚNIOR et al., 2017).
The use of automated irrigation systems optimizes the use of water resources for agricultural production. These systems can be important tools to avoid application of excessive irrigation depths and reduce labor costs. Thus, the adoption of these techniques that use automatic control of irrigation by producers is important to improve water use efficiency (SANTOS et al., 2015). According to Lino et al. (2017), the use of resistive sensors to monitor soil moisture coupled to an Arduino platform can be efficient to determine irrigation time.
The use of hydrogels in agriculture to improve productivity rates is increasing in recent decades. Hydrogels are water-retaining polymers that can be mixed with soil or substrate to alter their physical and chemical properties, and increase water retention and nutrient absorption (FAGUNDES et al., 2015).
The use of soil conditioners can alter soil (or substrate)-water-plant-atmosphere relationships by modifying the water use of the system. This can provide greater water security to the crop, greater water use efficiency, and higher seedling quality, however, the value of these parameters, and improvements that could be reached are little studied (MARQUES; CRIPA; MARTINEZ, 2013).
Considering the need of techniques to promote water use efficiency in greenhouse nurseries, the objective of this study was to evaluate the use of hydrogel in the substrate and determine the appropriate irrigation management for the production of tomato seedlings (Solanum lycopersicom), using an automated irrigation system. wherein Ab is the area of a tray (0.1352 m²); da is the water density (1000 g dm -3 ); j is the number of weighed trays (5); and Δmi is the weight (g) of water retained in each sampled tray.

MATERIAL AND METHODS
The WRS for seedling production is defined as the point between field capacity and dried substrate. A different reference was adopted in the present work, considering the point between field capacity and original conditions of the substrate. Thus, the mean water depth applied to the substrate was 4.40 mm. pH in water, and KCl and CaCl 2 at 1:2.5 ratio; P, and K extracted by Mehlich-1; Ca, Mg, Alextracted by KCl 1 mol L -1 ; H + Al extractor by calcium acetate 0.5 mol L -1 at pH 7.0; SB = sum of exchangeable bases; CTC (t) = effective cation exchange capacity; CTC (T) = cation exchange capacity at pH 7.0; V% = saturation by bases; m% = saturation by aluminum.
The rate of the hydrogel used in the experiment (Hydroplan EB, Hydroplan, São Paulo, Brazil) was 5.3 grams of hydrogel per liter of substrate (SANTOS et al., 2015). A microsprinkler irrigation system was used in the experiment; it was evaluated according to its working conditions using the Christiansen uniformity coefficient (CUC; %), according to Equation 02, 2 wherein qi is the flow rate in the collector (L h -1 ); n is the number of collectors; qm is the mean flow rate of n collectors (L h -1 ).
A system of acquisition and processing of meteorological data, and a control of the experimental irrigation, consisting of a microcontroller (ATMega 2560, Atmel, San Jose, USA) developed in an Arduino ATMega board, a real time clock, a memory card module, an air temperature sensor, and a relative humidity and global radiation sensor (pyranometer), were used to more efficiently determining the irrigation depth to be used.
An accumulated water depth (IRN) (Eq. 03 and Eq. 04) corresponding to the crop evapotranspiration (ETc) between two consecutive irrigations was considered for the irrigation management. The drain was considered null in the model assuming that the irrigations were carried out with application intensity (IA) below the water infiltration rate into the substrate (BERNARDO et al., 2019).
The microcontroller evaluated the average hourly air temperature (Tar), relative humidity (Uar), global radiation (Rg), and ETo. Then, the hourly ETc and the accumulated depth (IRNac) were calculated for each treatment. The ETc was determined using the crop coefficient for the experimental conditions, ALEMAN, C. C. et al.  Plant height, root length, and root and shoot dry weights were evaluated in six evaluations during the development of the seedlings. Stem base diameter was evaluated in a split-plots arrangement at 25 days after emergence (DAE).
A complete randomized experimental design was used, in a 4×2 factorial arrangement, consisting of four irrigation managements and two substrates. The means were compered using the Tukey's test at 5% significance level.

RESULTS AND DISCUSSION
The means of the microclimate variables of the study region ( Figure 1) were as expected (ALLEN et al., 1998), with maximum global radiation near the noon, showing sunny days; average hourly air temperatures lower at night, and higher (above 20 °C) between 9:00 a.m. and 7:00 p.m.; maximum mean air relative humidity between 0h and 8h, decreasing with increasing global radiation, reaching the minimum (35%) around 4h. The average reference evapotranspiration calculated with these variables presented a profile similar to the global radiation, with a maximum of approximately 0.65 mm h -1 at 12 h. Microclimate variables (mean hourly global radiation, mean air temperature, mean relative air humidity, and reference evapotranspiration) during the tomato seedling production cycle.
The water was balanced in real time during the experiment, considering the interactions within every 60 minutes. Figure 2 shows the balance adopted for the timer (with, and without hydrogel), Kc (with hydrogel), Kc (without hydrogel), 0.50ETo (with, and without hydrogel), and 1.00 ETo (with, and without gel) treatments during the 25 days of the cycle.  Table 3 shows the hourly ETo, hourly ETc, accumulated water depth (IRNac), and applied water depth (IRapl), and the frequency and number of irrigations of each treatment.   The interaction between the factors DAE, management, and substrate was significant for plant height (Table 4). Regarding the seedlings produced without hydrogel, no significant difference in plant height between the 4 managements was found up to the 22 DAE; in the final evaluation (25 DAE), the management 0.50ETo resulted in higher plant heights when compared to the treatments with timer, and Kc managements. The treatment with 1.00ETo remained with intermediate plant heights. The height of seedlings is a important parameter for analyse the quality of seedlings. However, this parameter is important to predict seedling uniformity and estimate their development in the field (TITTONELL; GRAZIA; CHIESA, 2002).
Different heights of seedlings were found at 11 DAE with the use of hydrogel, and at 22 DAE without hydrogel. The use of hydrogel affected the seedling precocity. Similar result was found during the vegetative development of sweet pepper plants with the use of hydrogel (TITTONELL; GRAZIA; CHIESA, 2002). Navroski et al. (2016), and Bernardi et al. (2012) evaluated the use of hydrogel (6 g L -1 ) for the production of eucalyptus seedlings and found that the use of hydrogel favored the development of the seedlings, reducing the requirement of nutritional supplementation in about 20%.
The interaction between the factors DAE, management, and substrate was not significant for root length (Table 5), but there were significant interactions between the factors DAE and substrate, and DAE and management for this variable. There was a limitation in root development in depth, because the seedlings reached the lower part of the tray cell, and the roots had reduced contact zone with the substrate, hindering root aeration, and water availability (WALTHIER et al., 2016). ALEMAN, C. C. et al. The insufficient conditions for root development generated dense and shallow root systems, and caused the death of the meristem root. Therefore, secondary root development occurred rather than deep development.
No significant differences in root length during the cycle for the managements and treatments with, and without hydrogel were expected. Seedlings treated with hydrogel had different root lengths at the beginning (9 DAE) and end (25 DAE) of the cycle. At 9 DAE, the difference was probably due to the uneven germination over the first 5 days, which included the interval between sowing and emergence. At 25 DAE, the difference found was between the irrigation managements; the treatments with timer presented an higher mean than that with 0.50ETo.The stem base diameter (SBD) was measured only at the 25 DAE, which corresponds to the last day of the molting cycle (Table 6). Table 6. Stem base diameter of tomato seedlings produced with, and without hydrogel in the substrate, and subjected to different irrigation managements.
Means followed by the same uppercase letters in the columns, or lowercase letters in the rows do not differ by the Tukey test at the 5% probability level.
The SBD of seedlings grow in the substrate with hydrogel were lower for the timer, Kc, and 1,00ETo treatments when compared to the 0.50ETo; and without hydrogel, no difference SBD was found when comparing the 4 managements.
Although studies show no consensus about the directly effect of SBD on seedling quality, some studies correlate SBD with tomato productivity (HERNÁNDEZ et al., 2017), and others associate SBD with other variables that result in good quality seedlings (COSTA et al., 2012).
The use of substrate with hydrogel resulted in seedlings with greater SBD in all managements. This result indicate that the irrigation depth applied to the substrate with hydrogel was readily available to the seedlings. Navroski et al. (2016) found larger SBD when using gel on the substrate or soil, and explained this result by the improved water retention and use by plants.
The root dry weight of the seedling with hydrogel was higher than that of those without hydrogel in the substrate at 15 DAE, in all irrigation managements (Table 7). This result indicates more root development in the substrate with hydrogelnot associated to their growth in depth, which was limited due to the tray cell area, but to the development of secondary roots. Navroski et al. (2016) associated the development of these roots with the effect of the use of hydrogel. Secondary roots are associated with water absorption, thus, seedlings with high numbers of these roots tend to have better resistance to water deficit. This is desirable when transferring the seedling to the field. Walthier et al. (2016) found that low water availability and aeration space may be factors that affect negatively root dry weight.
The shoot dry weight of the seedlings (Table 8) at 25 DAE was significantly lower in the 0.50ETo when compared to the Kc, and 1.00ETo managements. The probable increased water and nutrient retention on the substrate promoted by the hydrogel (FAGUNDES et al., 2015) reduced the water deficit effects of the different irrigation management.
Water deficit and nutritional deficiency affected seedlings in treatments with hydrogel only in the last evaluation (25 DAE), denoted by of their shoot dry weight, and slight purple coloration of the seedlings, typical of phosphorus deficiency. Means followed by the same uppercase letters in the columns, or lowercase letters in the rows do not differ by the Tukey test at the 5% probability level.

CONCLUSIONS
The use of hydrogel in the substrate is an essential factor to make early morphological evaluations in tomato seedlings.
The use of substrates with hydrogel, and automate irrigation system make possible to transplant the tomato seedlings at 25 days after emergence.