Effect of season and fertilization on lettuce

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 23ºC during the day and 7ºC at night. Very high temperatures can cause burning of the edges, form poorly compact heads 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.

Experiment

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 60kg/ha of nitrogen (0kg/ha, 60kg/ha, 120kg/ha and 180kg/ha ). Urea was used as nitrogen fertilizer, which was applied as top dressing ten, 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 10ml of the solution, laterally to each plant.

Soil preparation consisted of plowing, harrowing and raising the beds to a height of 0,20 m. The basic planting fertilizer carried out by the producer consisted of 1,5 kg/ha of formulated 02-16-08 and 1.000 kg/ha of simple superphosphate. After the fertilizers were incorporated into the soil, two lines of dripping tubes were installed in each bed, with emitters spaced every 30cm and with a flow rate of 1,5L/h. Fertilization carried out through daily fertigation totaled 30kg/ha of N and 60kg/ha of K₂O, 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 2m high, covering two construction sites per tunnel consisting of galvanized iron tubes, covered with low-density transparent plastic film, added with anti-UV, of 100µm thick, with the beds covered with black plastic film mulching, 4m wide and 35µm 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.

Transplantation of 25-day-old seedlings was carried out on 10/3/2003 (autumn) and 13/5/2003 (winter), and harvests were carried out on 28/4/2003 and 18/7/2003, 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 seven, 14, 21 and 28 days, in a cold room at 5°C ± 2°C and relative humidity of 90% ± 2%, using of grades (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 removing 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.

Preliminary results

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 higher with yields of 988,8g/plant and 670,1g/plant, respectively. The circumference of the commercial part (head) showed a linear increase with increasing nitrogen doses in winter cultivation. In autumn, the dose of 100,7kg/ha^- of N promoted the largest circumference (42,0cm) (Table 1).

Post-harvest conservation in a cold room at seven, 14, 21 and 28 days after harvest did not show significant effects for planting times, being perfectly preserved at seven and 14 days with grades varying from 4,1 to 5, deteriorating later. Similar results were observed for nitrogen doses for the evaluation at seven and 14 days, which obtained scores ranging between 4 and 5. With regard to post-harvest conservation at 21 and 28 days, interaction effects were observed, obtaining for winter cultivation linear effects with increasing N doses and in autumn maximum conservation was estimated at doses of 114,2kg/ha of N in coverage for 21 days and 100,8kg/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 was higher in winter cultivation and increased linearly with the increase in 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.

Table 1 - Average values ​​and regression equations for total and commercial fresh mass, dry mass, post-harvest conservation and head circumference of iceberg lettuce as a function of nitrogen doses and planting times. Três Pontas (MG)

Characteristics		Seasons de planting
		Winter	Autumn
Total fresh mass (g plant-1) **		988,8 to	764,9 b
Commercial fresh pasta (g plant-1)*		670,1 to	450,5 b
Regression equations
Total fresh dough	Y = 782,6750 + 1,0458X** R2 = 0,90
Commercial fresh pasta	Y = 474,2375 + 1,2185X** R2 = 0,71
Head circumference (cm)	Y (Winter) = 38,7700 + 0,0355X** R2 = 0,96
	Y (Autumn) = 39,7375 + 0,04471X - 0,000222X2**R2 = 0,70
Post-harvest conservation at 21 days	Y (Winter) = 2,9750 + 0,0025X* R2 = 0,70
	Y (Autumn) = 1,7412 + 0,0313X - 0,000137X2**R2 = 0,95
Post-harvest conservation at 28 days	Y (Winter) = 0,8975 + 0,0066X** R2 = 0,80
	Y (Autumn) = 1,0425 + 0,0119X - 0,000059X2**R2 = 0,85

Characteristics

Seasons de planting

Winter

Autumn

Total fresh mass (g plant^-1) **

988,8 to

764,9 b

Commercial fresh pasta (g plant^-1)*

670,1 to

450,5 b

Regression equations

Total fresh dough

Y = 782,6750 + 1,0458X** R² = 0,90

Commercial fresh pasta

Y = 474,2375 + 1,2185X** R² = 0,71

Head circumference (cm)

Y (Winter) = 38,7700 + 0,0355X** R² = 0,96

Y (Autumn) = 39,7375 + 0,04471X - 0,000222X²**R² = 0,70

Post-harvest conservation at 21 days

Y (Winter) = 2,9750 + 0,0025X* R² = 0,70

Y (Autumn) = 1,7412 + 0,0313X - 0,000137X²**R² = 0,95

Post-harvest conservation at 28 days

Y (Winter) = 0,8975 + 0,0066X** R² = 0,80

Y (Autumn) = 1,0425 + 0,0119X - 0,000059X²**R² = 0,85

Means followed by the same letter in the lines do not differ from each other, using the Tukey test at a 5% probability level ** and * Significant at 1% and 5% probability, using the F test.

Click here to read the article in Revista Cultivar Hortaliças e Frutas, issue 79.