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Linda Gaudreau, Josée Charbonneau, Louis-P. Vézina, and André Gosselin

Two cultivars (Karlo and Rosanna) of greenhouse lettuce were grown under different photosynthetic photon fluxes (PPF) and photoperiods provided by 400-W high–pressure sodium lamps. Natural light was compared to suppletmental lighting treatments providing either 50 or 100 μmol m-2-s-1 for photoperiods of 16, 20 or 24 h. Lettuce plants were grown in hydroponic gulleys using a standard nutrient solution. Plant fresh weights were measured every week for the duration of each culture grown between August 1989 and June 1990. The incidence of tipburn and the overall quality of the shoots were determined at the end of each crop. Leaf nitrate contents and nitrate reductase activity were measured for various lighting treatments. The highest fresh weight was obtained for the highest PPF and the longest photoperiod. However, these treatments were associated with a higher incidence of tipburn. Supplemental lighting reduced the leaf nitrate contents and affected the nitrate reductase activity.

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Linda Gaudreau, Josée Charbonneau, Louis-Philippe Vézina, and André Gosselin

`Karlo' and `Rosana', two Boston-type lettuce (Lactuca sativa L.) cultivars, were subjected to various light treatments in greenhouses equipped with one of two propane heating systems. Photoperiods of 16, 20, 24, or 24 hours for 2 weeks after transplanting and then 16 hours (24–16) and photosynthetic photon flux of 50 or 100 μmol·m–2·s–1 provided by supplementary lighting (high-pressure sodium vapor lamps) were compared to natural light during four experiments performed in greenhouses between Sept. 1989 and May 1990. Using supplementary lighting resulted in significant increases in biomass (≤270%), head firmness, and tipburn incidence and decreases in production cycle length (≈30%). Treatment effects were most pronounced during the months when natural-light levels were low. Fresh weights were higher for `Karlo' than `Rosana'; however, `Rosana' was less susceptible to tipburn than `Karlo'. In general, the radiant heating system resulted in earlier crop maturity and a higher incidence of tipburn than the hot-air system.