Effects of Different Ratios of Urea:Nitrate in Nutrient Solution on the Growth and Nitrate Accumulation of Red French Lettuce in Soilless Culture

آب و خاک

Article 5, Volume 38, Issue 2 - Serial Number 94, May and June 2024, Pages 239-252   

Document Type: Research Article

DOI: 10.22067/jsw.2024.86415.1372

Authors

S. Naseri; Sh. Kiani^* ; H.R. Motaghian

Soil Science and Engineering, Faculty of Agriculture, Shahrekord University, Shahrekord, Iran

Abstract

Introduction
Urea is one of the nitrogen chemical fertilizers for vegetable production in soil. But it is seldom used in soilless cultures. Leafy vegetables such as Lettuce (Lactuca sativa L.) contain high levels of nitrate and attempts have been made to reduce the nitrate concentration in this crop for human consumption. Using reduced forms of nitrogen, i.e. urea, is one of the applied strategies for reducing nitrate accumulation in lettuce. Little information is available concerning urea as a source of nitrogen for production of leafy vegetables such as lettuce in soilless culture. This experiment was conducted to investigate the effect of different ratios of urea:nitrate in nutrient solution on the growth indices, yield and nitrate accumulation of red French lettuce (Lactuca sativa L. cv. Lolla Rossa) in soilless culture.
 
Materials and Methods
A hydroponic experiment using completely randomized design was carried out with seven ratios of urea:nitrate in nutrient solution and four replications in the research greenhouse of Shahrekord University. Urea:nitrate ratios in nutrient solution were: 0:100, 10:90, 20:80, 30:70, 40:60, 50:50 and 60:40. Lettuce seedlings were grown in 2 L plastic pots (one plant per pot) containing mixture of cocopeat + perlite at the ratio of 2:1 (v/v) and were manually fertigated with nutrient solutions on a daily basis. Four weeks after transplanting, lettuce plants were harvested and fresh weights of shoot and root were determined. Plant growth indices including of plant height, plant diameter, leaf length, leaf width, leaf number, leaf greenness index and leaf brix level were measured. After measuring the growth indices, the leaves were grouped separately according to leaf numbers 1-10=outer leaves, >11= inner leaves. The samples were dried in an oven at 60 °C and were ground. Nitrate concentrations in samples were determined calorimetrically using a spectrophotometer at a wavelength of 410 nm. Analysis of variance was performed using SAS software version 9.4. Means comparison was conducted using least significant difference test at 0.05 probability level.
 
Results and Discussion
The results indicated that application of different ratios of urea to nitrate in nutrient solution had not significant effect on the lettuce growth indices including of plant diameter, leaf length, leaf width, leaf number, leaf greenness index and leaf brix level in comparison with 0:100 of urea:nitrate ratio. Also, root and shoot fresh weights were not affected by urea:nitrate ratio in nutrient solution. The greatest quantity of shoot fresh weight (141 g per plant) was obtained with a 50:50 urea:nitrate ratio. However, this was not significantly different from the shoot fresh weight (125 g per plant) observed when urea was not included in the nutrient solution. Shoot nitrogen concentration (except for plants nourished with a 50:50 urea:nitrate ratio) was not affected by increasing the urea:nitrate ratio in the nutrient solution. The results revealed that application of urea in nutrient solution effectively provided the nitrogen requirement of lettuce. This indicates that lettuce plants can efficiently hydrolyze urea and use it efficiently as a nitrogen source. Application of urea in the nutrient solution led to significant decrease in the nitrate concentration of lettuce root (P< 0.05). Moreover, increasing urea:nitrate ratio in nutrient solution resulted in significant decrease of the nitrate concentration of outer leaves, inner leaves and all leaves of lettuce (P< 0.01). The highest and lowest nitrate concentration in inner, outer and all leaves of lettuce were obtained in plants nourished with 0:100 and 50:50 urea:nitrate ratio in nutrient solution, respectively. Application of urea:nitrate ratio of 50:50 led to the meaningful decrease of nitrate concentration in root (43%), outer leaves (41%), inner leaves (44%) and all leaves (43%) of lettuce in comparison with 0:100 of urea:nitrate ratio. Urea had a repressive effect on nitrate influx and decreased its uptake by plants. Also, after urea uptake by plant root, it is first degraded by cytosolic ureases and then ammonium is incorporated via the GS-GOGAT (Glutamine Synthetase- Glutamine α-OxoGlutarate Amino Transferaze) cycle. Therefore, application of urea in nutrient solution can lead to the reduction of nitrate accumulation in plants.
 
Conclusion
Based on the shoot fresh weight and nitrate concentration in lettuce leaves, replacing 50% of nitrate in nutrient solution with urea is recommended for red French lettuce production in hydroponic culture under the conditions of the present study. Compared to other nitrogen fertilizers, urea has a lower price and its application in nutrient solution is useful in reducing production costs.

Keywords

Hydroponic; Leafy vegetables; Methemoglobinemia; Nitrogen form

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