The effect of foliar application of urea, Mo and BA on nitrate metabolism in lettuce leaves in the spring and summer-autumn seasons

‘Melodion’ butter lettuce was grown on rockwool slabs in a film tunnel in the spring and summer-autumn seasons. The following experimental factors were investigated: foliar application with a solution of urea (1%), Mo (1 mg dm^-3) and benzyladenine (5 mg BA dm^-3) and light conditions in the film tunnel (films covering two parts of the tunnel differed in light permeability for PAR radiation). The nitrate metabolism was studied before application, three days after treatment and on the harvest day (seven days after application). Higher light intensity stimulated nitrate reductase activity, ammonium ions and soluble sugars, while fewer nitrates were observed in lettuce leaves. In the spring season, the least nitrate ions occurred as a consequence of urea + Mo nutrition, and when BA was added in the summer-autumn. Moreover, a positive effect on soluble sugar content was also observed when BA was used. In the spring, BA stimulated nitrate and nitrite reductase activities in the leaves of lettuce that had received less light.