In two successive pot experiments with lettuce cv. ‘Królowa Majowych’, conducted in a phytotron, this study investigated the effect of cool white fluorescent light (FRS) at a PPFD 200 μmol·m^-2·s^-1 and red-blue LED light at a PPFD of 200 and 800 μ mol·m^-2·s^-1 on photosynthesis, yield, leaf area, SLA, and the content of photosynthetic pigments, total N and nitrates. Experimental plants were grown in sphagnum peat supplemented with full-strength Hoagland’s solution at the beginning of the experiment. 10 days after plants were pricked out, 4 experimental series were made which differed in the form of N supplied to the growing medium at a rate of 420 mg (2N): 1) Hoagland’s solution (control); 2) Hoagl + 2N-NO₃; 3) Hoagl + 2N-NH₄; 4) Hoagl + 2N-NH₄/NO₃. The obtained results showed that the lettuce leaf yield under FRS light was distinctly higher than under LED light at a PPFD of 200, and in particular at 800 μmol·m^-2·s^-1. Besides, the leaves grown under FRS light showed a significantly thinner leaf blade (SLA) and a lower content of photosynthetic pigments, total N and nitrates. The photosynthetic rate was higher under LED light relative to FRS light. Different nutrition of plants with N-NO₃, N-NH₄ and N-NH₄/NO₃ had a similar effect on the yield and analysed traits of lettuce leaves, regardless of the type of light and the level of irradiation with LED light. LED lamps seem to be a very promising light source for plants, but they require further research on how to adapt the spectral distribution of light to their requirements.

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