PHOTOPERIOD AND PHOTOSYNTHETIC PHOTON FLUX INFLUENCE GROWTH AND QUALITY OF GREENHOUSE-GROWN LETTUCE

'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 mu mol.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 (less than or equal to 270%), head firmness, and tipburn incidence and decreases in production cycle length (approximate to 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.