In the Master’s thesis, we studied the influence of UV-A on photosynthetic activities of the green and red lettuce (Lactuca sativa L.). In the experimental part, performed in a greenhouse, in the period from April to June 2019, we used two varieties of the green lettuce, 'Kyrio' and 'Hawking', and the red variety 'Oakly'. In two separate experiments we exposed the plants to the following light regimes: the control group was under 12 h photoperiod illuminated only with the basal lighting of blue and red light with PPFD 400 μmol m-2 s-1; while the UV1 and UV2 treatments received additional UV-A light with PFD 1,6 μmol m-2 s-1 (λmax = 399 nm), for a period of 4 or 8 hours daily, respectively. The effect of UV-A on the lettuce growth was observed only for the 'Hawking', where the fresh mass decreased by 17,3 % under 8 hours of daily exposure. On the other hand, the effect in the case of the red lettuce 'Oakly' was significant but small - with negligible ecological relevance and in the case of the 'Kyrio' there was no response. The influence of additional lighting with UV-A on the chlorophyll was observed only in one of two experiments (reduction) and would have to be checked with more accurate chemical analyses. The irradiated plants did not show signs of radiation stress, but plants of all treatments had reduced photochemical efficiency, which we attributed to temperature stress in the heated greenhouse and possible fluctuations in the amount of water for irrigation. The net photosynthesis of UV2 plants was higher than in controls. Since the same trend is indicated for photochemical efficiency, it is to presume that this increase is due to the enhancement of protection mechanisms in the photosynthetic apparatus. It is less likely that photosynthetic rate is increased due to the additional energy of blue light, present in the spectrum, radiating by the UV-A diodes since plants received sufficient PPFD.
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