Effects of polyethylene film covering the greenhouse, nitrogen fertiliser form, and foliar nutrition on the yield and quality of lettuce

This study investigates the effects of two types of polyethylene films covering the greenhouse, differing in PAR transmittance, as well as two forms of nitrogen fertiliser (100% N-NO₃ vs. 57% N-NO³+ 43% N-NH₄), and the use of foliar nutrition on the quality and yield of lettuce grown in the spring and summer-autumn seasons. Lettuce was grown on rockwool in a film greenhouse divided into two parts, each covered with one type of film. Three times per season the plants were sprayed with a solution of molybdenum, benzyladenine, urea with molybdenum, urea with benzyladenine, and the three substances combined. The yields were examined for dry matter, nitrates, ammonia nitrogen, protein nitrogen, soluble sugars, vitamin C, and macronutrients (phosphorus, potassium, calcium and magnesium). In both seasons, lettuce cultivated in the greenhouse covered with film having a higher PAR transmittance resulted in heavier heads and contained significantly lower levels of nitrates, NH₄⁺, P and Mg. The effect of plot (type of film) on vitamin C content depended on the season. The form of nitrogen fertiliser influenced dry matter, vitamin C, protein nitrogen, P and Mg contents; in the summer-autumn season it also affected head weight. The plants grown on a nitrate medium produced a higher single head mass and accumulated a larger amount of nitrates than those cultivated on a nitrate-ammonia medium. No effects were observed of foliar nutrition with salt solutions on the yield and quality of lettuce in either season.