Calcium Nitrate Dose and Application Period in American Lettuce ( Lactuca sativa L . )

The aim of this study was to evaluate the yield of American lettuce subjected to different dosages of calcium nitrate on two application schedules. The experiment used four replicates of a 2 × 5 factorial randomized complete block design, with two application schedules (Schedule 1: 50% of the dose at transplanting and 50% at 20 days after transplanting; Schedule 2: 50% at 10 days and 50% at 20 days after transplanting) and five doses (0, 150, 300, 450 and 600 kg ha). The following variables were evaluated: the number of inner and outer leaves, head height and diameter, head height/diameter ratio, compactness, stem diameter, relative index of chlorophyll, commercial production and nitrogen (N) and calcium (Ca) content in the inner and outer leaf. The data were subjected to analysis of variance (F test, with Tukey test for comparison of the means) for the application schedule and regression analysis for the calcium nitrate dose. The application of calcium nitrate positively influenced the nutritional characteristics of American lettuce in the 2 schedule and the dose of 470 kg ha presented better production.


Introduction
Lettuce (Lactuca sativa L.), among the leafy vegetables, is the most commercialized in Brazil.It has a high content of vitamins and has a large amount of minerals (Santi et al., 2013).It is consumed by all social classes and is even a raw material for some fast food centres.This vegetable has great social and economic importance and is cultivated by small and medium producers located near major cities.
Commercial cultivars are categorized by their leaf characteristics and whether they form a cabbage-like head.The American type presents characteristically crisp, consistent leaves with prominent ribs, forming a compact head.It is highly resistant to transport and recommended for the preparation of sandwiches, since it resists contact with hot foods better than other cultivars (Filgueira, 2013).
A balanced mineral fertilization is necessary for commercial crops, and potassium (K), nitrogen (N), calcium (Ca) and phosphorus (P) are the most absorbed elements in lettuce fertilization.Lettuce has high demand for N, and N deficiency slows plant growth and induces poor head formation and yellowing of older leaves (Olfati et al., 2009).When Ca is not available at appropriate levels, a physiological disturbance called 'tip burn' may occur, which is more pronounced in hot regions (Beninni et al., 2003;Cometti et al., 2004).Soil characteristics, fertilizer application timing, climatic conditions and soil management practices are mentioned as some of the factors influencing plant nutrient dynamics (Santos et al., 2010a).N is a nutrient that requires adequate management in the lettuce crop because it easily leaches from the soil and because the crop absorbs a larger amount of N in the final stage of the cycle.According to Costa et al. (2010), two aspects are fundamental to the management of N fertilization: the source and the parcelling of the doses to reduce losses by volatilization and leaching.
(Raphanus sativus L.) root production.Martínez et al. (2013) concluded that calcium nitrate provided a higher yield of tomato (Lycopersicum esculentum) fruits.Cortez et al. (2009) concluded that increased calcium nitrate doses provided better results in lettuce production.Other studies point to improved lettuce production when calcium nitrate is used to supply the N and Ca (Chohura & Kolota, 2011).
In this context, the aim of this study was to evaluate the productivity of the American lettuce fertilized with calcium nitrate in different doses and in two application schedules.

Location and Installation of the Experiments
The experiment was carried out in an open field from March to June 2014 in the municipality of Catalão, Goias state (GO), at 18°09′49.6″S, 47°18′56.4″W and approximately 787 metres of altitude.The climate of the region, according to the Köppen-Geiger classification (Cardoso et al., 2014), is defined as tropical climate (Aw) with a dry season in winter.

Statistical Design and Plant Materials
The experimental design was a randomized complete block factorial 2 × 5, two application schedules (Schedule 1: 50% of the dose at transplanting and 50% at 20 days after transplanting; Schedule 2: 50% at 10 days and 50% at 20 days after transplanting) and five doses (0, 150, 300, 450 and 600 kg ha -1 ).The source of calcium nitrate used consisted of 14% N and 18% Ca.
The plots measured 1.0 × 1.2 m, containing four rows with four plants each, arranged in a spacing of 0.25 × 0.30 m, totalling 16 plants per plot.The height of the beds was 20 cm.For the evaluations, four central plants were used in each plot.The lettuce cultivar used was Lucy Brown, which has a 75-day cycle and is a large plant with thick leaves that protect the head.It is light green in colour and has good weight compactness and high tolerance to bolting.
The seedlings were produced in a protected nursery in expanded polystyrene trays with 200 cells.A commercial substrate was used for sowing, with two pelleted seeds being placed in each cell.After sowing, the seeds were covered with a thin layer of substrate, irrigated and taken to the nursery.Seedlings were thinned 10 days after emergence, leaving only one plant per cell.Watering in the nursery period was carried out three times per day by a micro sprinkler system.In the nursery phase, preventive treatment for pest and disease control and fertilization with monoammonium phosphate were carried out via irrigation.
The preparation of the experimental area consisted of ploughing, disking and manual turnover of the beds.Planting fertilization was carried out based on the soil analysis, according to the recommendation suggested by Filgueira (2013).At the time of planting 60, 300 and 90 kg ha -1 of N, P 2 O 5 and K 2 O, respectively, were applied.Transplanting was performed when the seedlings reached four leaves.Soon after transplanting, the area was irrigated.Irrigation management was carried out based on the daily evaporation rate, using a class A tank, with approximately 5 mm day -1 from a conventional sprinkler irrigation system.Crop treatments were performed as recommended for this crop.

Evaluated Characteristics
The harvest was performed when 80% of the plants in the experimental area were at harvest point to 51 days after transplanting; that is, when the plants had a completely closed head.The variables evaluated were height and head diameter, performed with the aid of a graduated ruler; stem diameter, determined by measuring with a digital calliper; head diameter × height ratio, determined by dividing the head height by the diameter, compactness, obtained by noting the scale of resistance (no head = 0, soft = 1, medium = 2, firm = 3) of the head to hand pressure, performed by the same person; the number of outer leaves after discarding the dead and senescent leaves, leaving only the heads of lettuce; relative index of chlorophyll, with readings being taken with a chlorophyll meter (ClorofiLog CFL 1030, Falker) on the mature leaves of four plants in each plot; number of outer and inner leaves, determined by marking and counting all the outer and inner leaves of the head; and the wet and dry weight of outer and inner leaves, with the leaves being weighed on a precision scale and then being dried in a forced air oven at 65 °C until constant weight.
The N and Ca contents of the outer and inner leaves were also quantified using four newly mature leaves at the time of head formation from each plot, following the method of Malavolta (1997).To measure commercial production, heads with commercial classification were weighed on a precision scale.

Statistical Analysis
The data were subjected to analysis of variance (F test) for the application schedule, and regression analysis for the calcium nitrate dose.The statistical analyses were performed using the statistical analysis program Sanest (Zonta et al., 1987).

Growth Variables and Yield
Table 1 shows the averages for the application schedule factor on the characteristics of height and head diameter, stem diameter, height ratio and diameter of the head, compactness and relative index of chlorophyll.The characteristics of head height and diameter, stem diameter, head height × diameter ratio and relative index of chlorophyll did not vary significantly in relation to the timing of the calcium nitrate application.
Table 1.Mean values of head height (HH), head diameter (HC), stem diameter (SD), head height × head diameter ratio (HH/HC), head compaction (HC) and relative index of chlorophyll (RIC) due to calcium nitrate application timing.Catalão, GO, Brazil Note.Means followed by the same letter in the column, for each factor studied, do not differ from one another according to the Tukey test at 5% probability.
The first application schedule showed significantly better compactness.Compactness also significantly improved as the calcium nitrate dose increased (Figure 1A).American lettuce head compactness is related to the processing yield.Consumers prefer plants with large and heavy heads, which occur in plants with a more compact head and provide higher yield during processing.During processing, more compact heads facilitate leaf chopping, thus increasing the industrial yield (Yuri et al., 2002).
Figure 1      The values of Ca in the outer leaves were fitted by a quadratic regression with a minimum point of 348 kg ha -1 (Figure 3B).This can be explained by the increase in wet weight; the cells became more turgid, thus increasing the internal water content and diluting the Ca.
N is a mobile element in the plant, being important mainly in the synthesis of pigments and amino acids.Ca, however, is a practically immobile element in the plant, being found mainly in regions that are in full vegetative growth (Tsialtas et al., 2016).Due to these mobility characteristics, top-dressing fertilization provided higher leaf contents, allowing better absorption, due to the more developed root system and, consequently, adequate mobilization of the nutrients by the plant (Maillard et al., 2015).
In commercial production, the observed increase may be related to the of calcium nitrate when the root system is more developed, taking advantage of the N.According to Melgar et al. (1991), the N availability can be increased through split application during the growth period of the plants since this parcelling improves N absorption by the plants and reduces losses from leaching because the plant root system is more developed.Cortez et al. (2009) evaluated lettuce production as a function of fertilization with calcium nitrate in a nutrient solution and concluded that increasing the doses provided better production.The mean values for the variable were similar to but lower than those observed by Resende et al. (2005Resende et al. ( , 2009)), who used higher doses of N per ha -1 .
When commercial production was evaluated with the application of calcium nitrate, a significant effect was observed for the doses, with data being fitted to a quadratic linear regression with a maximum point of 470 kg ha -1 (Figure 3C).According to Nascimento et al. (2017), different sources of N can be used for crisp lettuce at the discretion of the producer according to cost.Silva et al. (2017) observed an increase in the production of carrots with the use of calcium nitrate.N in the form of nitrate is available immediately for absorption and is readily metabolizable by the plant.Urea, for example, requires approximately 25 days to make N available (Rogeri et al., 2015).Calcium nitrate is commonly used as top-dressing to supply both N and Ca requirements (Filgueira, 2013).

Conclusions
The application of calcium nitrate positively influenced the nutritional characteristics of American lettuce in the 2 nd schedule and the dose of 470 kg ha -1 presented better production.

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Figure 2. I (C) in Ame