Effect of Ni and Urea on Growth, Nitrate and Nutrients Concentration in Lettuce (Lactuca sativa) Grown in Hydroponics

The N source used in commercial hydroponic culture of vegetables is mainly NO3-N. The rate of NO3- uptake is usually high, particularly in leafy vegetables and it can be markedly increased when the NO3- supply in the environment is high. An abundant N supply leads to a high NO3-N absorption and accumulation in plants. When NO3- rich vegetables are consumed, various harmful effects on human health may occur such as met-hemoglobinemia (Blue Baby Syndrome) and cancer. Keeping levels of NO3- below limits of FAO seems to be impossible without changing conventional fertilizer application techniques. The suitability of urea for the cultivation of field crops has been well documented. Urea is used as the main source of N fertilizer for crops grown in soil. Its use as N source for crops grown under the hydroponic system has yet to be evaluated. To hydrolyze urea, the enzyme urease requires Ni as a component. Substitution of urea for commonly used N03-N fertilizers in hydroponic culture of vegetables would not only enable to avoid excessive accumulation of N03- in plants but would also reduce the cost of production. Leafy vegetable crops, such as lettuce and spinach, contain large amounts of N03-N. Therefore, it is important to reduce N03- concentrations in hydroponically grown with lowest negative effects on yield.
The experiments were carried outin greenhouse hydroponicsResearchFaculty of Agriculture, University of Tabriz in randomized complete block designwithtwo factors ureaatfivelevels of 0,25, 50, 75and100milligrams perliter(U0, U25,U50, U75, U100)andnickelattwo levels of0and2mg per liter (Ni0, Ni2)ofnickelsulfate(NiSO4)in4replicatesusinglettuce(Lactuca sativa cv. Siyahoo). Plants fed with the modifiedHoagland solutionorhalf theconcentration. Treatments added to nutrient solution when plants were in four leaf stage. Plants were harvested 50 days after treatment. Different organs (leaves, stems and roots) were separated and each separate simultaneous freshweight wasmeasured. Dry weight of organs wasmeasured afterit was oven-dried at 80ºCfor 72h. Leavesoven-dried andthenpowdered, and weredigested(usingacid) tomeasure theelements. Extracts from thedigestionmethodwere used for determination ofnickelusingDimethylglyoximemethod.Spectrophotometer used to cover the wavelength at 530nm. Potassium was measured by Flame Photometer410.Totalnitrogenwas measuredbyKjeldahlmethod.Thehomogeneouspowders of dried leaves with hot water were extractedwithnitratemeter(Horiba, Japan)and they were used to measuretheirnitrate content. Analysis was performed usingthe Software Statistical Package for the Social Science (SPSS) v. 16.0. Individual treatment means were compared with a Duncan’s test to determine whether they were significantly different at the 0.05 probability.
U50treatedwith 1.8 fold increasecompared with thecontrol groupshowed thehighestfresh weight. The yield increased with increasing concentration to 50 mg/l urea, butat higher urea concentrations, 50 mg/l,yieldsignificantlydecreased, althoughitwas significantlyhighercompared to control. .Enhanced growth and yield in two levels of U25 and U50were coerced. It was duo tohydrolysis urea with the help nickel stored in seed endosperm and also contamination application of nickel fertilizers in nutrient solutionsthat led to release of urea nitrogen.The highestandthelowest concentration ofnickelinleaveswith11-fold increase,were observedatconcentrations ofU50andU100, respectively. Dilution phenomenon occurred with increasingurea concentrationmore than U50.Nickelconcentration inleaveswassignificantlyincreased that this is theopposite offresh weightanddry weight. In U50 treated K concentration was 1.6-fold higher compared to control. With increasing urea concentration more than U50,K concentration decreased. Applyingthe Ni, 8 percent decreased K concentration in leaf tissues. With increasing urea innutrientsolution, totalnitrogenconcentration of leaf tissuealsoincreased,so that theplantsU100 have 1/1-foldmoretotalnitrogencomparedwithU25plants. Theinteractions betweenthe treated also showed that plantsU0Ni2compared to control(U0Ni0), have 1.2-fold moretotalnitrogen. Concentration ofnitrateinplantsleavesthat showednickelwas 1.2 fold highercompared to the plants withoutnickel.Interactive effectofureaconcentration andapplication ofnickelalso showedthat treated plants hadU0Ni0 2.6-fold more nitratethanU0Ni2plantsleaftissues.
Urea was hydrolyzed in low and middle concentrations (U25 and U50) and led to increasing yield. An inhibition of NO3- uptake can result from the action of Ni on H+ ATPase pump, though it can also affect the carrier of Hﳓ랳⦩ꊻ. Moreover, proteins of the NO3- uptake system contain -SH groups, and due to that they are sensitive to heavy metals including Ni.