A crop composed primarily of leaves, iceberg lettuce responds greatly to nitrogen fertilization, as well as being influenced by the planting time. Both factors affect yield and post-harvest quality.
The planting time is a fundamental factor in the cultivation of iceberg lettuce, as it is a plant that is greatly influenced by environmental conditions, as it adapts better to regions with a mild climate. The ideal temperature conditions for its development are 23 ºC during the day and 7 ºC at night. Very high temperatures can cause burning of the edges, forming heads that are not very compact, and also contribute to the occurrence of calcium deficiency, known as “tip-burn”.
As it is a crop basically made up of leaves, lettuce responds greatly to nitrogen fertilization and organic fertilizers. The supply of nitrogen to the plant favors the production of fundamental biomolecules, such as proteins and amino acids, in addition to being the constituent of chlorophyll molecules. Nitrogen deficiency slows down plant growth and induces absence or poor head formation; older leaves become yellowish and fall off easily. However, when applied too much, in top dressing, in the last third of the cycle, the cultivars that form heads have less firmness, which could be detrimental to commercialization. Research reports a negative relationship between plants in normal conditions of available nitrogen and deficient plants, with a reduction in leaf fresh mass in conditions of nutrient limitation.
For this reason, the objective of this work was to evaluate the effects of nitrogen doses in coverage and growing seasons on the productive characteristics and post-harvest quality of iceberg lettuce, under conditions in the south of Minas Gerais.
The experiment was conducted in the municipality of Três Pontas, Minas Gerais, at an altitude of 870m, located at 21º22'00" south longitude and 45º30'45'' west longitude, in soil classified as Distroferric Red Oxisol with a clayey texture.
The experimental design used was randomized blocks, in subdivided plots, with four replications, using the Raider cultivar. The plots consisted of planting seasons (autumn and winter) and the subplots consisted of nitrogen doses in addition to the dose applied by the producer of 60 kg/ha of nitrogen (0kg/ha, 60kg/ha, 120kg/ha and 180 kg /there is). Urea was used as a nitrogen fertilizer, which was applied as top dressing at 10, 20 and 30 days after transplantation at 40%, 30% and 30%, respectively, of the evaluated dose. The doses of urea coverage per plot per plant were previously diluted in pure water, applying 10 mL of the solution, laterally to each plant.
Soil preparation consisted of plowing, harrowing and raising the beds to 0,20 m in height. The basic planting fertilizer carried out by the producer consisted of 1,5 kg/ha of formulated 02-16-08 and 1000 kg/ha of simple superphosphate. After the fertilizers were incorporated into the soil, two drip lines were installed in each bed, with emitters spaced every 30 cm and with a flow rate of 1,5 L/h. Fertilization carried out through daily fertigation totaled 30 kg/ha of N and 60 kg/ha of K2 O, using urea and potassium chloride as sources.
The plots consisted of beds with four rows 2,1m long spaced 0,30m apart, with 0,35m between plants. The central lines formed the useful area, removing two plants at each end. A protective structure was installed across the entire area, consisting of high tunnels 2,0m high, covering two beds per tunnel, made of galvanized iron tubes, covered with low-density transparent plastic film, added with anti- UV, 100 µm thick, with the beds covered with black plastic mulching film, 4m wide and 35 µm thick. The seedlings were grown in multicellular trays of 288 cells each, filled with commercial substrate.
The crop was kept clean through manual weeding, and the phytosanitary control adopted was the standard method used by the producer.
The transplantation of 25-day-old seedlings was carried out on 10/03/2003 (autumn) and 13/05/2003 (winter), and the harvests were carried out on 28/04/2003 and 18/07/2003, respectively, when the plants were fully developed and the total and commercial fresh mass (head) (g/plant) and commercial head circumference (cm) were evaluated. Post-harvest conservation was carried out on a sample of two commercial heads of lettuce, evaluated at 7, 14, 21 and 28 days, in a cold room at 5 ± 2 °C and relative humidity of 90 % ± 2 %, using notes (grade 1: extremely deteriorated commercial heads; grade 2: deteriorated commercial heads; grade 3: moderately deteriorated commercial heads; grade 4: slightly deteriorated commercial heads and grade 5: commercial heads without deterioration), using three evaluators and taking the average of the grades obtained. The collected data were subjected to analysis of variance and regression based on the polynomial model at a 5% probability level.
The total and commercial fresh mass of the aerial part showed significant effects for nitrogen doses, independently, with linear increases with increasing nitrogen doses (Table 1). The doses applied were insufficient to achieve maximum productivity of commercial fresh mass per plant. With regard to total and commercial fresh mass during planting seasons, winter cultivation was significantly superior with yields of 988,8g/'plant and 670,1 g/plant, respectively. The circumference of the commercial part (head) showed a linear increase with increasing nitrogen doses in winter cultivation. In autumn, a dose of 100,7 kg/ ha-of N promoted the largest circumference (42,0 cm) (Table 1).
Post-harvest conservation in a cold room at 7, 14, 21 and 28 days after harvest did not show significant effects for planting times, presenting at 7 and 14 days perfectly preserved with grades ranging from 4,1 to 5,0 deteriorating yourself later. Similar results were observed for nitrogen doses for the 7th and 14th day evaluation, which obtained scores ranging between 4,0 and 5,0. With regard to post-harvest conservation at 21 and 28 days, interaction effects were observed, obtaining linear effects for winter cultivation with the increase in N doses and in autumn, maximum conservation in N doses was estimated. 114,2 kg/ha of N in coverage for 21 days and 100,8 kg/ha of N for 28 days after harvest (Table 1), that is, higher doses of N promoted better post-harvest conservation of iceberg lettuce. This characteristic is of great importance because the final product is processed and stored in cold rooms for subsequent distribution. Therefore, greater conservation of the product after harvest is desirable and of considerable relevance. Good vegetative development of iceberg lettuce, culminating in excellent head formation and compactness, probably explains the better post-harvest conservation promoted by nitrogen fertilization.
Based on the results obtained in the present work, it can be inferred that the cultivation of iceberg lettuce is viable in the two growing seasons tested and that the total and commercial fresh mass were higher in winter cultivation and increased linearly with increasing doses. of nitrogen. Regarding post-harvest conservation, at 21 and 28 days, in winter cultivation, positive linear effects were observed with the increase in nitrogen doses and in autumn quadratic models were adjusted with points of maximum conservation at doses of 114,2, 100,8kg/ha and XNUMX kg/ha of N in top dressing, respectively.
Geraldo Milanez de Resende, Jony Eishi Yuri, José Hortêncio Mota, Embrapa Semiárido
Article published in issue 79 of Cultivar Hortaliças e Frutas
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