As cultivars can differ distinctly, we included two cultivars in the experiment. For each treatment, cultivar, and replicate, we measured the concentration of flavonoid glycosides and phenolic acids, assessed head mass, number of leaves, and dry matter content. To sum up, we wanted to investigate three hypotheses with this experiment: (I) Cool-cultivated lettuce http://www.selleckchem.com/products/epz-6438.html plants contain higher concentrations of phenolic compounds than warm-cultivated ones. Experiments were conducted in growth chambers to strictly separate the effects of temperature from radiation because they are known to strongly interact (Løvdal et al.,
2010). Red Oak Leaf and red Lollo lettuce (L. sativa L. var. crispa L. cv. Eventai RZ and L. sativa L. var. crispa L., cv. Satine, respectively; RijkZwaan, De Lier, The Netherlands) differ regarding their recommended greenhouse cultivation schedule: The seed company recommends red Oak Leaf from fall to spring, throughout the winter (November to April), while for Lollo Rosso cultivation in late fall and spring is advised. The seeds were sown in rockwool cubes, Pexidartinib kept at 10 °C for 2 days for germination and subsequently grown in a conventional
greenhouse until the experiment started. When plants had developed four true leaves (5 weeks old) and weighed about 0.9 g they were transferred into growth chambers (Yorck, Mannheim, Germany) where they were grown using deep flow technique, in four growth chambers simultaneously. The nutrient solution was prepared according to Sonneveld and Straver (1988) and exchanged and analyzed every week. In two chambers, the air temperature was 20 °C during daytime and 15 °C at night (warm treatment), whereas it was 12/7 °C (day/night) in the other two (cool treatment). Relative humidity was approximately 80%. Radiation was supplied by high-pressure sodium discharge lamps SON-T PLUS 400 W (Philips, Amsterdam, The Netherlands). The light cycle consisted of four elements: Methane monooxygenase 11 h of darkness, 0.5 h of dawn, 12 h of light and another 0.5 h of twilight. During the light phase, the mean photosynthetic photon flux density (PPFD) was 247 μmol m−2 s−1,
during dusk and dawn, respectively, only some of the lamps were switched on, resulting in a mean PPFD of 95 μmol m−2 s−1, as measured with a portable light meter Li-250 (Li-COR Inc., Lincoln, Nebraska, USA). Hence, the plants intercepted a daily light integral of 11.4 mol m−2 day−1. Plants cultivated for 13 days intercepted a total light integral of 148 mol photosynthetically active radiation (PAR), while those cultivated for 26, 39 and 52 days intercepted 296, 445, and 593 mol PAR m−2 s−1, respectively. To elucidate harvest dates at which the plants cultivated in different temperatures will have reached a comparable growth stage (based on head mass and number of leaves) we used the concept of “sum of temperatures”.