Twelve cultivars of leaf-type lettuce (Lactuca sativa var. crispa) were selected for the study. This type of lettuce forms open heads with loose leaves that do not close to cover younger leaves. Six green-colored cultivars originated from Semo, a.s., Czech Republic (Dubagold, Zlatava, and Zoltan) and Bejo Zaden B.V., Netherlands (Aleppo, Biondonna, and Kiribati); six red-colored cultivars were also from Semo (Dubared, Roden, and Rosaura) and Bejo Zaden (Carmesi, Oakly, and Spectation). The experiments were performed in the spring period (April, May). Lettuce seeds were sown in plastic pots and germinated under standard laboratory conditions (ca. 21 ℃ , 12-hour photoperiod). After germination, the lettuce plants were transplanted into a growth chamber (air-conditioned box model MC1750 (Snijders Scientific, Tilburg, Netherlands) and grown under 14/10 h (day/night) photoperiod, 21/18 ℃ temperature, 60% humidity, and 250 µmol.m ^-2.s^-1 light intensity. The commercial peat substrate (Klasmann, Germany) was used (pH 6.0, nutrient content: N: 220 mg/L, P₂O₅: 110 mg/L, K₂O: 220 mg/L, Mg: 80 mg/L). Approximately at the stage of second, fully expanded, true leaf the plants were transplanted to 0.5 l pots and kept at the same conditions for seven days to recover. After recovery, the plants were transferred into one of the three experimental conditions described below. Plants were watered regularly to avoid drought stress. Considering a high level of nutrients in the substrate and a short duration of the experiments, no additional nutrition was applied to plants.The experiments were held at SAU in Nitra (48° 19′ 7″ N, 18° 4′ 55″ E, 144 m asl). To distinguish the effects of UV radiation from other environmental factors such as temperature, humidity, and light intensity, the plants were grown in three different environments: 1. direct sunlight (outdoor conditions with high UV), 2. under clear glass (outdoor conditions with low UV), and 3. greenhouse (indoor conditions with low UV).Plants grown under direct sunlight conditions were placed into a vegetation cage (a walk-in cage surrounded by the thin wire mesh from the top and side to protect the experimental plants against birds and animals) and exposed to almost unrestricted sunlight and ambient temperature and humidity. Plants were watered as needed to achieve a fully hydrated state. Temperature outdoors was monitored.Plants grown under clear glass were placed in similar environmental conditions as those cultivated under direct sunlight, but were grown in the glass shelter constructed from clear glass sheets (thickness of 8 mm). The clear glass sheets were positioned such as to eliminate UV light coming to plants from the south and above. The backside (oriented to the north) of this glass shelter was covered by the plastic-coated wire mesh not impeding the flow of air, so the temperature and other conditions were almost identical to fully open outdoor conditions. Temperature outdoors and under the glass sheets was occasionally compared using hand-held thermometers, showing only insignificant differences. The glass cover lowered the intensity of photosynthetically active radiation (PAR) by 10-15% at noon due to absorbance and reflectance of radiation by the glass. The overhang of the glass shelter, wire mesh from the north part as well as composition of buildings from the north west and east direction was very favorable to prevent excessive access of diffuse UV radiation. Thus, despite UV radiation is not fully eliminated, its level represents only a small fraction compared to the direct UV radiation incident to plants exposed to direct sunlight outdoors.Plants grown indoors were placed in a regular greenhouse constructed from clear glass that eliminated approximately 15-20% of PAR intensity at noon. Light intensity in the greenhouse reached almost 1,000 µmol photons m^-2 s^-1 during sunny days, therefore it can still be regarded as fully saturating or excessive radiation, similar to that at outdoor conditions. Temperature in the greenhouse was lowered during the day by the automated ventilation, but air vents were closed during the night. Temperature in the greenhouse was substantially higher than outdoors (environmental conditions 1 and 2). During the experiment, the night temperature in the greenhouse ranged between 15 and 20 ℃ , whereas the daily maximum temperature oscillated mostly between 20 and 32 ℃ . The maximum temperature of 35 ℃ was reached during a few of the warmest days. In each environment, plants were grown in the randomized complete block design, with weekly rotations of plant positions. Four healthy, well-developed plants from each cultivar were selected for analyses from each of the three environments. Non-destructive analyses started 30 days after sowing and continued for another 30 days. The complete above-ground parts of the plants were harvested at the end of the experiment (60 days after sewing) and were used for destructive analyses.Comparison between environments is based on the assumption that the plants grown under the glass sheets outdoors were exposed to similar light and low UV conditions as the plants in the greenhouse, but the temperature conditions were similar to those in direct sunlight outdoors. By comparing accumulation of phenolic compounds in plants grown in the three environments, we could distinguish the effects caused by UV radiation from those caused by the temperature.

Increased accumulation of total phenolics, flavonoids, anthocyanins, and phenolic acids was observed in direct sunlight conditions outdoors as compared to the greenhouse conditions with low UV radiation, but elevated day and night temperatures. The level of UV radiation played a dominant role in the accumulation of flavonoids, anthocyanins and methoxycinnamic acid; while temperature was a major factor affecting concentrations of phenolic acids, mostly rosmarinic, p-anisic and vanillic acid. The concentrations of compounds estimated with the non-invasive fluorescence excitation ratio method were highly consistent with those obtained by standard analytical approaches.