The Effect of Nitrogen Form on Chemical Composition and Yield of Lettuce (Lactuca sativa L.) Cultivars in Soilless Culture

Ammonium (NH4+) and nitrate (NO3-) ions are the two main forms of nitrogen (N) for plants. But, they influence differently on growth and chemical composition of plants. The effect of N form on plant growth depends on plant species, development stage of plant, pH, and temperature, ratio of NH4+ /NO3- and nitrogen level of nutrient solution. Lettuce is one of the leafy vegetables that has been cultivated in soilless culture in many greenhouses in the world. This plant can respond well to NH4+ nutrition, but the information about optimum NH4+/NO3- ratio in the nutrient solution and respond of lettuce cultivars to partial replacement of NO3- by NH4+ is scarce. Application of nitrification inhibitors such as 3, 4-dimethylpyrazole phosphate (DMPP) with ammonium fertilizers lead to high N-use efficiency as well as reducing denitrification and leaching losses. Nitrification inhibitors are compounds that delay the biological oxidation of ammonium to nitrite by depressing the activity of Nitrosomonas bacteria. This study was conducted to elucidate the effect of nitrogen form (N-NO3− and N-NH4+) and the use of a nitrification inhibitor (DMPP) on chemical composition and yield of lettuce (Lactuca sativa L.) cultivars in research greenhouse of Shahrekord University.

A factorial experiment using completely randomized design was carried out with two factors of NH4+/NO3- ratio (0:100, 15:85 with and without DMPP, 30:70 with and without DMPP) and lettuce cultivars (Teresa and California) with three replications under hydroponic conditions. Lettuce plants were grown in 1.7 L plastic pots (one plant per pot) and the substrate used was mixture of cocopeat + perlite with ratio of 2:1 (v/v). Different nutrient solutions were applied by hand two or three times per week to obtain a leaching fraction of 5 to 20%. After seven weeks plants were harvested, fresh weight of shoots and roots were determined and plants were dried in an oven at 60 °C. Then, dry weight of shoots and roots were measured and plants were ground for nutrient analysis including of P, K, Ca, Mg, Fe, Mn, Zn and Cu.   

The results showed that application of nutrient solution with NH4+/NO3- ratio of 30:70 in California cultivar and 15:85 in Teresa cultivar led to significant increase shoot P concentration compared with the 0:100 of NH4+/NO3- ratio (40 and 13%, respectively). This was due to synergistic effect of NH4+ on the uptake of P by roots. In both Teresa and California cultivars, replacing 30% NO3- in the nutrient solution with NH4+ resulted to significant decrease shoot K concentration (27.3 and 14.8% in Teresa and California cultivars, respectively) as well as shoot Ca concentration (42.0 and 31.1% in Teresa and California cultivars, respectively) compared with the 0:100 of NH4+/NO3- ratio. This decrease is related to antagonistic effects of NH4+ on the uptake of K and Mg by roots. In Teresa cultivar, increasing the NH4+/NO3- ratio to 15:85 led to the meaningful increase of shoot Fe (97%), Mn (68%) and Zn (54%) concentration in comparison with 0:100 of NH4+/NO3- ratio. But, in California cultivar shoot Mn concentration increased (65%) with 30% replacement of NO3- by NH4+. This means that changing NH4+/NO3- ratio in the nutrient solution is an excellent approach to control the relative uptake of cations and anions by the plant. The greatest quantity of shoot fresh weight in Teresa (334 g pot-1) and California (435 g pot-1) cultivars were obtained from 0:100 and 15:85 of NH4+/NO3- ratios, respectively. The current study indicates that the lettuce cultivars respond differently to the form of N supply. There is a genotypic variability in the ability of plants to supply carbon skeletons for NH4+ assimilation in the roots. Thus, California cultivar is a genotype sensitive to enhanced ammonium nutrition and Teresa cultivar is a genotype insensitive to enhanced ammonium nutrition. Increasing the NH4+/NO3- ratio to 30:70 led to the meaningful decrease (42%) of root fresh weight in comparison with nutrient solution without NH4+. Application of nitrification inhibitor DMPP with the NH4+/NO3- ratios of 15:85 and 30:70 had not significant effect on the shoot fresh and dry weight as well as the concentration of P, K and Cu in the shoot of both lettuce cultivars in comparison to these ratios without DMPP.

The results suggest that the NH4+/NO3- ratios of 0:100 and 15:85 can be recommended for production of Teresa and California lettuce cultivars under the conditions of the present study, respectively.